Self-Healing Mechanisms in Geopolymer Composites: Microstructural Characterization and Performance Metrics

Abstract

Self-healing geopolymer composites represent a frontier in sustainable construction materials, capable of autonomous crack repair through intrinsic chemical reactions or extrinsic agent delivery, restoring structural integrity without external intervention. The main aim of this review is to comprehensively analyze self-healing mechanisms, performance characteristics, and further research directions in this area. The article draws from the state-of-the-art literature review with critical evaluation, enriched by representative case studies. The following issues are discussed: (1) Microstructural mechanisms of self-healing, including intrinsic (continued geopolymerization of unreacted precursors, Ca-rich phases forming CSH-like products, carbonation-induced precipitation) and extrinsic (microcapsules, vascular networks, mineral admixtures like Na₂SiO₃ or MgO), and their microstructural evolution during healing. (2) The currently used methods of investigating self-healing and their limitations. (3) The challenges, such as deep crack healing (>500 μm), multiple healing cycles, and environmental robustness. (4) Applications, including durable pavements and 3D-printed elements prone to shrinkage cracking. (5) Future directions emphasizing hybrid mechanisms, in situ monitoring (acoustic emission, ultrasonic testing), and life-cycle assessment. The main findings show that self-healing geopolymers advance circular economy principles by extending service life and minimizing repair costs in low-carbon infrastructure.

1. Introduction

Self-healing technologies have been successfully implemented across a wide range of materials, including polymers, metals, ceramics, and cementitious systems, using both intrinsic mechanisms (based on reversible bonding) and extrinsic approaches (e.g., capsule-based systems) [1].

Self-healing in cement-based materials has emerged as an effective strategy to autonomously repair cracks and extend service life, particularly in aggressive environments such as marine infrastructure [2]. Furthermore, self-healing technologies are increasingly recognized as a key component of sustainable concrete systems, as they reduce maintenance demands, enhance durability, and contribute to extending the service life of low-carbon construction materials [3].

Concretes exhibit limited autogenous self-healing, mainly through continued hydration and carbonation processes, which has led to the development of more effective autonomous systems such as bacteria-based, microcapsule-based, and reactive additive approaches [1]. In addition to bacteria-based self-healing systems, recent research has increasingly focused on enzyme-induced calcium carbonate precipitation (EICP) as an alternative biomineralization strategy. Unlike microbially induced calcium carbonate precipitation (MICP), which relies on the metabolic activity of living microorganisms, EICP utilizes free urease enzymes to catalyze urea hydrolysis and induce CaCO₃ precipitation within cracks. Recent studies have demonstrated that EICP can effectively promote crack sealing, improve water-tightness, partially restore mechanical properties, and enhance the long-term durability of cementitious materials [4,5,6].

In this context, geopolymers represent a promising emerging class of cementitious materials, combining the inherent autogenous healing potential of inorganic binders with the possibility of integrating advanced autonomous strategies such as microcapsules or reactive precursors to enhance durability and crack-sealing performance.

The self-healing mechanism in cementitious materials is strongly governed by the chemical composition of the matrix. In OPC systems, the presence of Ca²⁺ ions promotes carbonation and the formation of CaCO₃ and C–S–H phases, whereas geopolymer systems favor the formation of aluminosilicate gels (e.g., N-A-S-H) and continued geopolymerization due to their alkali-rich environment [7]. In conventional cement-based materials, self-healing mainly occurs through continued cement hydration and carbonation products that fill and seal cracks. In contrast, geopolymers heal primarily through secondary geopolymerization, the formation of new aluminosilicate gels, and, in some cases, microbiologically induced healing processes [8].

The main differences in self-healing phenomena between geopolymers and cement-based systems are presented in

Table 1. Self-healing mechanisms in geopolymers vs. cement-based systems.

Category	Geopolymers	Cement-Based Systems
Dominant self-healing type	Autogenic (intrinsic)	Autogenic + Autonomous (engineered)
Primary healing products	N-A-S-H gel; C-A-S-H (when Ca is present)	CaCO3 (calcite); secondary C-S-H
Fundamental mechanism	Re-polymerization and precipitation of aluminosilicate gels	Continued hydration + CaCO3 precipitation
Key ion species	Na+, OH−, Si, Al	Ca2+, CO32−, OH−
Dominant reaction type	Gel formation (network reconstruction)	Crystallization (carbonate precipitation)
Role of water	Activates dissolution and ion mobility	Enables hydration and carbonation
Effective crack width range	Typically ≤ 100–200 μm (system-dependent)	Autogenic: ≤150 μm; Autonomous: up to ~1 mm
Kinetics	Often slower, controlled by alkaline chemistry	Faster under moisture and CO2 availability
Role of calcium (Ca)	Critical for C-A-S-H formation; limited healing without Ca	Essential (main source of Ca2+ for healing)

.

Not all additives commonly used in cement-based materials are equally effective in geopolymers. Borçato and Medeiros-Junior [9] investigated the self-healing performance of metakaolin-based geopolymers incorporating a crystalline admixture, an expansive agent, and hydrated lime as a calcium source [9]. All of these additives are effective for self-healing concrete materials. Their results showed that another situation is in the case of geopolymers, self-healing was negligible in calcium-poor mixtures, whereas geopolymers containing 10% hydrated lime exhibited partial crack closure through the formation of calcium-based healing products [9]. The crystalline admixture enhanced self-healing and matrix densification, while the expansive agent proved ineffective and even reduced the healing efficiency of the geopolymer system [9].

In the case of other additives such as superabsorbent polymers (SAP), they behave differently in geopolymers than in conventional cement-based materials because geopolymer pore solutions contain much higher concentrations of dissolved ions and multivalent species [10]. As a result, the selection and dosage of SAP, as well as the amount of additional curing water, must be specifically tailored to the chemical environment of the geopolymer matrix.

Although self-healing is increasingly recognized as a key advantage of geopolymer materials, research on its mechanisms and long-term performance remains relatively limited [11,12]. Combining the inherently low permeability of geopolymers with self-healing mechanisms could provide one of the most promising approaches for improving corrosion resistance and extending the service life of next-generation construction materials [13].

A lot of work stresses that further investigation of geopolymer systems is necessary, especially in the area of autonomous self-healing. While extensive research has focused on the application of bacteria in cement-based concrete, studies investigating the effects of bacterial strains in geopolymer concrete remain limited [12,14]. Nodehi et al. [15] also noticed that although geopolymers do not currently exhibit self-healing performance comparable to bacterial concretes, they are considered a highly promising matrix for the development of future self-healing technologies [15]. Their excellent durability, chemical stability, and compatibility with bacterial or encapsulated healing systems make them attractive candidates for next-generation smart construction materials [15].

Despite the growing interest in self-healing geopolymer materials, the current state of knowledge remains fragmented and significantly less developed than that of ordinary Portland cement (OPC)-based systems. Most published studies focus on individual healing strategies, specific precursor systems, or short-term laboratory observations, while comprehensive analyses integrating microstructural mechanisms, healing products, performance recovery, and practical applications are still scarce. In addition, the influence of geopolymer chemistry, environmental exposure, crack characteristics, and healing-agent type on healing efficiency has not yet been systematically synthesized.

Another important limitation of the existing literature is the lack of standardized evaluation methods and long-term field validation. Reported healing efficiencies are often assessed using different crack widths, curing conditions, and performance indicators, making direct comparison between studies difficult. Furthermore, the long-term durability of autonomous healing systems, particularly bacteria-based and capsule-based approaches, remains insufficiently understood under realistic service conditions.

The main aim of this review is to provide a comprehensive and critical analysis of self-healing mechanisms in geopolymer composites, with particular emphasis on their microstructural characteristics, governing factors, healing efficiency, and performance recovery. The review synthesizes current knowledge on both intrinsic (autogenous) and extrinsic (autonomous) self-healing approaches and evaluates their potential to enhance the durability and service life of geopolymer materials.

The scope of the review includes:

• Fundamental mechanisms of self-healing in geopolymer systems, including continued geopolymerization, gel precipitation, carbonation, and biomineralization;
• Autonomous healing strategies based on microcapsules, bacteria, mineral admixtures, polymers, and hybrid approaches;
• Relationships between geopolymer chemistry, microstructure, environmental conditions, and healing performance;
• Characterization and monitoring methods used to evaluate healing processes, including microscopy, diffraction-based techniques, and non-destructive testing;
• Assessment of healing efficiency through mechanical, durability-related, and crack-closure performance metrics;
• Current and potential applications of self-healing geopolymers in infrastructure, energy systems, marine environments, and additive manufacturing;
• Identification of current challenges, knowledge gaps, and future research directions, including hybrid healing concepts.

Overall, this review aims to establish a comprehensive understanding of the state of the art in self-healing geopolymer composites and to provide guidance for the development of next-generation sustainable construction materials.

2. Methodology

The methodology adopted in this study is based on a critical review of the scientific literature, complemented by an analysis of selected case studies relevant to the research topic. In the initial stage of the investigation, a systematic search strategy was implemented, whereby combinations of two predefined keywords were used: “geopolymer” and “self-healing” to identify and retrieve relevant publications from the Scopus database. The results were searched in “Title, abstract, keywords” to avoid articles that only slightly mentioned the research area. The provided search returned 151 documents (Figure 1). The Scopus database was selected as the primary source of literature because of its broad coverage of peer-reviewed publications in engineering, materials science, and construction-related research fields. Although other databases, such as Web of Science, also index relevant literature, a substantial overlap exists between these databases for journals in the areas of geopolymers, cementitious materials, and self-healing technologies. Since the objective of this study was to provide a critical and thematic review rather than a fully systematic literature review, Scopus was considered sufficient to identify the major research trends, key publications, and recent developments in the field while avoiding redundancy associated with duplicate records from multiple databases.

Figure 1a presents the annual publication output, indicating a relatively limited research activity between 2011 and 2019, with only a small number of studies published each year. A pronounced increase is observed after 2020, which demonstrates the rapidly growing interest in self-healing materials and related technologies. Figure 1b shows the subject area distribution reveals that Engineering (35.9%) and Materials Science (29.3%) are the dominant research domains, highlighting the strong focus on material design, performance, and engineering applications. Smaller but significant contributions from Environmental Science, Physics, Energy, and Earth Sciences indicate the multidisciplinary character of self-healing material research and its relevance to sustainability-oriented technologies.

The geographical distribution of publications is presented in Figure 1c; it indicates that China is the leading contributor, followed by the United States and India, highlighting the strong international interest in self-healing materials research. The results also demonstrate significant contributions from countries across Asia, Europe, the Middle East, and Oceania, reflecting the global and collaborative nature of this research field. Figure 1d shows the analysis of publication types reveals that research articles dominate the literature (55.6%), indicating that the field is primarily driven by original experimental and theoretical studies. Conference papers, conference reviews, and review articles together account for a substantial proportion of publications, emphasizing the dynamic development of the field and the continuous dissemination of emerging research findings.

In order to systematically visualize the intellectual structure and thematic distribution of the research field addressed in this article, VOSviewer software (version 1.6.20; Centre for Science and Technology Studies, Leiden University, Leiden, The Netherlands) was employed. The analysis was conducted on an Excel database containing 151 publications identified and exported from the Scopus database (Figure 2).

The VOSviewer keyword co-occurrence map illustrates the main research themes and relationships within the self-healing materials field, with “self-healing” emerging as the most prominent and highly connected keyword. The network is organized into several thematic clusters, linking self-healing technologies with topics such as durability, microstructure, bacteria, calcium carbonate precipitation, microcapsules, corrosion resistance, and sustainable development. The dense connections between nodes indicate strong interdisciplinary interactions, particularly between cementitious materials, geopolymer research, microbiologically induced healing, and advanced characterization techniques such as SEM and EDS. The large size of keywords such as “self-healing concrete”, “calcium carbonate”, “bacteria”, “durability”, and “microstructure” reflects their high frequency of occurrence and central importance in the literature. Overall, the visualization demonstrates that current research is increasingly focused on enhancing the durability and sustainability of construction materials through both biological and non-biological self-healing approaches.

From the retrieved publications, the authors identified and selected the studies deemed most relevant to the thematic scope of the individual sections of this article. Particular emphasis was placed on recent scientific contributions, with the literature review primarily focusing on publications published between 2016 and May 2026. A few previous publications shown in the Scopus database (2011–2015) were not strictly connected with the topic of review and described the topic of geopolymers and the topic of self-healing separately or, for example, employed a geopolymer coating as a protective shell for granules containing bacterial spores and nutrients intended for use in self-healing concrete [17].

Given the rapid expansion of research in this field after 2020 and the increasing importance of sustainable, low-carbon construction materials, there is a clear need for an updated and critical review that consolidates current knowledge, identifies the most effective healing mechanisms, and highlights the key challenges preventing large-scale implementation of self-healing geopolymer technologies. Such a review can provide a scientific framework for future research and accelerate the development of durable and resilient geopolymer-based infrastructure.

During the preparation of this work, the authors used Copilot (AI) to assist with figures creation and reformulate some sentences and Grammarly (a spell-checking tool) for grammar check. After using this tool/service, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.

3. Crack Formation Mechanisms

Geopolymers, similarly to conventional concrete, are prone to cracking during their service life. These cracks facilitate the ingress of water, chlorides, sulfates, and corrosive gases, which accelerates material degradation and reduces the durability of structures; therefore, self-healing technologies aim to autonomously seal cracks and restore the material’s protective function [18]. Self-healing mechanisms can mitigate the progression of deterioration, seal cracks, and significantly extend the service life of structures [19].

Han et al. [20] emphasize that fly ash–slag geopolymers are highly susceptible to drying shrinkage, which can be four to six times greater than that of conventional cement paste and significantly increases the risk of cracking during service. Due to their low tensile strength, limited ductility, brittleness, and proneness to microcrack formation and propagation, these geopolymer materials require efficient self-healing technologies to maintain long-term durability and structural performance [20].

Some research shows that geopolymers have a natural tendency to self-heal. Yuan et al. [21] pointed out that certain compositions achieve nearly complete autogenous healing when crack widths are below approximately 50 μm, allowing substantial recovery of material performance. However, for cracks wider than 50 μm, the naturally formed healing products are generally insufficient to completely fill and seal the crack, which may promote alkali leaching and durability deterioration [21].

In the context of mitigating temperature-induced cracking, self-healing systems may perform a slightly different role. The internal insulation system proposed by Zhao et al. [22] prevents thermal stress. It is based on a geopolymer refractory material containing a controlled combination of closed-cell and open-cell porosity, which allows limited molten salt penetration and the formation of a stable frozen-salt barrier within the insulation layer. This internal insulation also helps mitigate thermal stresses by acting as a thermal buffer between the molten salt and the tank wall, thereby reducing temperature gradients and temperature inhomogeneities within the structure. By lowering the shell temperature and creating a more gradual thermal profile across the insulation layers, the system reduces the risk of fatigue damage, thermal cracking, and stress-related failures in molten-salt storage tanks. Furthermore, the reduced thermal fluctuations contribute to improved long-term structural stability and enhance the operational reliability of thermal energy storage systems operating at elevated temperatures [22].

Crack formation in geopolymer materials may result from various physical, mechanical, thermal, and environmental actions, each producing distinct damage patterns and microstructural changes. Since self-healing efficiency is closely related to crack morphology and transport phenomena within the matrix, different failure modes can activate different healing pathways and lead to diverse healing outcomes.

Table 2. Failure modes vs. self-healing response in geopolymers.

Failure Mode	Damage Characteristics	Activated Self-Healing Process	Healing Efficiency	Microstructural Response
Shrinkage cracking	Fine distributed microcracks	Autogenic gel formation	High	Rapid gel sealing, pore blocking
Mechanical cracking	Wider localized cracks	Diffusion-controlled precipitation	Moderate	Partial crack filling
Thermal cracking	Network of cracks	Transport-driven healing	Variable	Non-uniform gel distribution
Chemical degradation	Matrix dissolution	Competing dissolution/precipitation	Low	Microstructure degradation dominates
Freeze–thaw damage	Microcrack network	Limited healing	Low–moderate	Partial pore blocking

presents a classification of typical failure modes in geopolymer systems and their corresponding self-healing responses, highlighting the dominant healing mechanisms and resulting microstructural modifications.

Effective crack control is fundamental to self-healing, as narrow crack widths (≈54 μm) enable efficient autogenous repair processes. This can be achieved through the use of polyvinyl alcohol (PVA) fibers, which promote multiple fine cracking via fiber bridging and ensure conditions favorable for crack sealing and healing [23].

From the point of view of crack propagation, the addition of fibres seems to be very promising. This was confirmed by an investigation provided by Ozel et al. [24]. The hybrid combination of polyethylene (PE) and nylon fibers promoted the formation of multiple fine microcracks instead of a few large cracks, which is essential for efficient self-healing. Thanks to the fibres, this process of self-healing was very effective because the average crack width was only about 106 μm, despite the use of relatively coarse recycled concrete aggregate with a maximum particle size of 2 mm. Nylon fibers additionally contributed through their hydrophilic nature, retaining and gradually releasing moisture that supported continued geopolymerization. As a result, the fiber system enhanced crack control and created favorable conditions for the accumulation of healing products within the cracks [24].

A similar effect can also be obtained with usage microcapsules. The embedded microcapsules modify the crack propagation path by promoting crack deflection and branching, which dissipates fracture energy and prevents the formation of large interconnected cracks. Additionally, after capsule rupture, newly formed geopolymer gels bridge crack faces and wrap around pores and damaged regions, effectively sealing defects and limiting further crack growth and permeability increase [25].

The self-healing process in geopolymers evolves through several distinct kinetic stages, from the dissolution of reactive species and ion migration to gel formation and long-term microstructural stabilization. Each stage contributes differently to crack sealing and durability recovery [7,8,26].

Table 3. Kinetic regimes of autogenic self-healing in geopolymers.

Stage	Time Scale	Controlling Factor	Dominant Process	Role in Self-Healing Efficiency
Initial	hours–1 day	Dissolution kinetics	Release of Si, Al, Na species from matrix	Provides precursors necessary for healing reactions
Early	1–7 days	Ion transport (diffusion)	Migration of dissolved species into cracks	Governs ability to reach crack zone → critical for small crack healing
Intermediate	7–28 days	Nucleation and precipitation	Formation of N-A-S-H/C-A-S-H gel in cracks	Main stage of crack filling and sealing
Late	>28 days	Densification and restructuring	Gel maturation and pore refinement	Improves long-term durability and permeability reduction
Long-term (aging)	months	Structural stabilization	Possible zeolite-like phase formation	May enhance stability but limited additional crack closure

presents the characteristic kinetic regimes of autogenic self-healing in geopolymer materials.

Zhao et al. [27] demonstrated that the majority of crack-healing activity occurred during the first seven days of exposure to the healing environment, indicating a rapid early-stage self-healing process. Although crack closure and property recovery continued beyond the first week, the healing rate gradually decreased with increasing healing time [27].

Wu et al. [28] noticed that the age at which cracks formed had a significant influence on the self-healing performance of geopolymer composites, with specimens pre-cracked at 3 days showing substantially better healing than those cracked at 28 days. Early-age cracks promoted the formation of larger amounts of C-(A)-S-H gel, calcium carbonate (CaCO₃), and ettringite, resulting in greater crack closure and higher tensile stiffness recovery. In contrast, specimens cracked at 28 days exhibited a lower increase in C-(A)-S-H content and ettringite was no longer detected, indicating a reduced capacity for further geopolymerization and self-healing [28].

4. Classification of Self-Healing Mechanisms in Geopolymers

4.1. Intrinsic Self-Healing Mechanisms

4.1.1. Main Mechanism of Intrinsic Self-Healing

Intrinsic self-healing, also referred to as autogenous self-healing, is a mechanism in which crack repair occurs through processes inherently present within the geopolymer matrix, without the addition of external healing agents. In geopolymer materials, healing is mainly associated with continued geopolymerization, dissolution–precipitation reactions, and the formation of additional phases, such as N-A-S-H, C-(N)-A-S-H, or others within microcracks. The effectiveness of this mechanism depends on the availability of unreacted precursors, moisture, and suitable environmental conditions.

Figure 3 presents a schematic representation of a typical intrinsic self-healing mechanism in geopolymer systems, illustrating the most frequently observed healing processes. However, it should be emphasized that self-healing may proceed through various alternative and/or overlapping mechanisms, depending on the geopolymer composition and the surrounding environmental conditions.

Intrinsic (autogenic) self-healing mechanisms in geopolymers originate from processes naturally occurring within the material, without the need for externally introduced healing agents. These mechanisms are primarily associated with continued geopolymerization, dissolution–precipitation reactions, and the formation of new gel phases or carbonation products within cracks. Although the fundamental principles differ from those observed in OPC systems, both material groups rely on the availability of reactive constituents and favorable environmental conditions [8,29].

Table 4. Intrinsic mechanisms in self-healing—comparison between geopolymers and cement-based systems.

Mechanism	Geopolymers	Cement-Based Systems
Unreacted binder reaction	Residual aluminosilicate dissolution + gel formation (N-A-S-H)	Continued cement hydration (C-S-H formation)
Precipitation-based healing	Aluminosilicate gel precipitation in cracks	CaCO3 precipitation (dominant mechanism)
Environmental activation	High pH pore solution drives reactions	Moisture + CO2 exposure drives carbonation
Efficiency limitation	Limited by ion mobility and Ca availability	Limited to small cracks (<150 μm)

compares the main autogenic self-healing mechanisms operating in geopolymer and OPC-based materials.

Nevertheless, the effectiveness of autogenic healing in geopolymers is highly system-dependent, and its long-term contribution to durability enhancement remains insufficiently understood due to the variability of geopolymer compositions and testing conditions.

4.1.2. Basic Strategies

The most well-documented self-healing mechanism in geopolymers is autogenous healing, which is driven by the continued geopolymerization of previously unreacted aluminosilicate particles. When moisture is available, these residual particles react further and generate new geopolymeric gel phases that progressively fill and seal cracks within the matrix [11].

One of the first studies in this area was provided by Liu et al. [30]. The geopolymer was based on class F fly ash activated with an 8 M NaOH solution, and a geopolymer–drilling mud hybrid containing 20% synthetic-based mud was also investigated. The self-healing mechanism was attributed to the continued geopolymerization of previously unreacted particles upon exposure to pore fluids, resulting in the formation of additional geopolymeric reaction products that sealed microcracks and restored the internal structure of the material. The main result was that damaged geopolymer specimens not only recovered their mechanical performance after re-curing but in some cases achieved more than 120% of the reference compressive strength, whereas Portland cement showed limited recovery and retained lower strength after damage [30].

Also, the autogenic mechanism of self-healing was observed by Wu et al. [28]. The primary self-healing mechanism involved the activation of unreacted geopolymer particles by Na₂SO₄, leading to continued geopolymerization and the formation of additional C-(A)-S-H gel, together with precipitation of CaCO₃ and, at early ages, ettringite within the cracks. These reaction products progressively filled and sealed microcracks, making the mechanism particularly valuable for marine, hydraulic, and tidal-zone structures exposed to sulphate-rich wet–dry environments [28].

Similar observations were made by Kan et al. [31]. The geopolymer was produced from a blend of high-calcium fly ash, low-calcium fly ash, and metakaolin, activated with NaOH and sodium silicate, and reinforced with PVA fibers to achieve strain-hardening and multiple microcracking behavior. The self-healing ability relied on the material design itself, where PVA fibers controlled crack widths to below about 25 μm, creating favorable conditions for autogenous healing. The self-healing mechanism was attributed to the continued geopolymerization of unreacted fly ash and metakaolin particles, leading to the formation of new amorphous aluminosilicate gel phases rich in Si, Al, and O that progressively filled and sealed the microcracks. Unlike cement-based systems, no significant calcite formation was observed, and healing was dominated by geopolymeric reaction products. The main result was the excellent recovery of mechanical performance after healing: crack numbers decreased substantially, tensile strain capacities remained above 3.2%, and several healed specimens even exhibited higher ultimate tensile strengths than uncracked control specimens, demonstrating highly effective autogenous self-healing [31].

In turn, Guo and Yang [32] produced a geopolymer composite from fly ash and steel slag activated with a sodium silicate/NaOH solution and reinforced with PVA fibers, with the optimum composition containing 21 wt.% steel slag. The self-healing capability was primarily associated with the strain-hardening behavior induced by the PVA fibers, which generated a dense network of narrow microcracks, while the calcium-rich steel slag provided reactive species for further hydration and geopolymerization. The healing mechanism involved the precipitation of Na₂CO₃ through carbonation, together with the formation of C–S–H gel and additional N–A–S–H gel inside the cracks, leading to gradual crack filling and sealing during air curing. The main result was the excellent autogenous self-healing performance of the composite: pre-cracked specimens showed substantial crack closure and a 57% increase in tensile strength after healing, confirming effective recovery of mechanical properties [32].

Ulugöl et al. [33] investigated geopolymer composites produced entirely from construction and demolition waste (CDW) precursors, including waste clay bricks, roof tiles, hollow bricks, concrete rubble, glass, and, in some mixtures, ground granulated blast furnace slag (GGBS). The materials were activated using NaOH, while selected mixtures also contained Na₂SiO₃ and Ca(OH)₂ to evaluate the influence of calcium and silicate availability on self-healing behavior. The self-healing mechanism was found to rely on a synergy of carbonation and secondary geopolymerization. During wetting–drying cycles, cracks were progressively filled by carbonate products, particularly CaCO₃, while unreacted aluminosilicate precursors continued to geopolymerize, generating new binding phases inside the damaged zones. Microstructural analyses confirmed that CaCO₃ was the main long-term healing product, whereas Na₂CO₃ appeared as a transient early-age product. In addition, the presence of Si, Al, and Na within healed cracks provided evidence of ongoing geopolymerization and the formation of new geopolymeric gels. The main result of the study was that 100% CDW-based geopolymer composites exhibited effective autogenous self-healing, with many microcracks becoming almost completely sealed after wetting–drying cycles. Slag incorporation further enhanced the stability and effectiveness of the healing process by promoting the formation of CaCO₃ and providing a more durable self-healing response [33].

In turn, Zheng et al. [34] investigated autogenous self-healing in fly ash-based geopolymers, focusing on permeability recovery through continued geopolymerization of unreacted particles and the formation of additional N-A-S-H gel within damaged zones. They confirmed that self-healing leads to a genuine recovery of sealing capacity, as demonstrated by direct permeability measurements [34].

Several autogenic mechanisms were observed by Zhao et al. [27]. They investigated geopolymer composites produced from fly ash and ground granulated blast-furnace slag and reinforced with polyvinyl alcohol (PVA) fibers. Self-healing was mainly driven by the continued geopolymerization of unreacted fly ash and slag particles, resulting in the formation of additional C(N)-A-S-H gel, while calcium carbonate precipitation contributed to crack filling. Microstructural analyses confirmed that C(N)-A-S-H gel and CaCO₃ were the principal healing products formed within the cracks. The results showed that moderate alkaline environments, particularly saturated Ca(OH)₂ solution, substantially enhanced crack closure and mechanical property recovery, whereas excessively alkaline conditions negatively affected long-term healing performance [27].

Ozel and Yildirim [19] described a fly ash/metakaolin-based geopolymer activated with a combination of sodium silicate and sodium hydroxide reinforced with PVA fibers, which helped control crack width and promoted strain-hardening behavior. The self-healing mechanism was mainly based on the formation of amorphous aluminosilicate gel products inside the cracks. When exposed to suitable curing conditions, these geopolymerization products precipitated and filled microcracks, leading to crack closure and restoration of the matrix continuity. In some geopolymer systems reported in the chapter, healing was additionally associated with the formation of calcium carbonate and aluminosilicate-based phases [19].

Also, PVA fibres were applied by Guo et al. [35] to support autogenous self-healing. The geopolymers developed in this study consisted of Class F fly ash as the sole aluminosilicate precursor, PVA fibres as crack-bridging reinforcement, and a sodium silicate/sodium hydroxide-based alkaline activator. Self-healing was governed by an autogenous mechanism involving the continued formation of N(C)-A-S-H geopolymer gel and the precipitation of calcite (CaCO₃) within microcracks. The PVA fibers played a crucial role by controlling crack widths and promoting the formation of multiple fine cracks that could be effectively healed. The results demonstrated that the geopolymer exhibited a remarkable recovery of mechanical properties, with the optimal damage level (2.0% tensile strain) producing the highest healing rate and nearly complete restoration of tensile performance [35].

Liu et al. [36] observed that an autogenic self-healing mechanism also takes place during 3D printing. They investigated a 3D-printed fiber-reinforced geopolymer (3DP-FRG) composed mainly of fly ash and slag, reinforced with polypropylene (PP) fibers, with a focus on printability and interlayer bonding performance. The authors found that the material exhibits interlayer self-healing, driven by continued geopolymerization reactions involving unreacted fly ash particles and dissolved silicate and aluminate species that migrate into the interlayer gaps. The main self-healing products identified in the gaps were N(C)-A-S-H gel, calcite, and sodium zeolite, which progressively filled and sealed the interlayer voids [36].

Overall, intrinsic self-healing in geopolymers relies not only on ongoing chemical reactions but also on effective crack-width control. The frequent use of PVA fibers in the reviewed studies suggests that the high healing performance reported for many geopolymer systems is often strongly dependent on fiber-induced microcracking behavior, rather than solely on the intrinsic healing capacity of the geopolymer matrix itself.

4.1.3. High Temperature Self-Healing

Some authors also describe a thermally induced self-healing mechanism in a geopolymer, where exposure to a very high temperature (even up to 1200 °C) leads to viscous sintering, porosity reduction, microcrack closure, and partial recovery of mechanical strength through the formation of ceramic phases, predominantly anorthite. However, this is not a classical autogenous self-healing process occurring under normal service conditions. Instead, the healing effect results from high-temperature microstructural reorganization, densification, and ceramic phase formation within the geopolymer matrix [37].

This kind of autogenic mechanism of self-healing was investigated by Mokhtari et al. [38]. They investigated metakaolin-based geopolymers activated with sodium and potassium waterglass in different proportions and subsequently transformed into ceramics through heat treatment at high temperature. In this case, no external healing agent was added; instead, the self-healing effect was achieved by optimizing the geopolymer composition, particularly a mixture containing 75 wt.% sodium waterglass and 25 wt.% potassium waterglass, which generated a sodium-rich amorphous glassy phase during firing [38]. The self-healing mechanism involved the formation and flow of this molten glassy phase at elevated temperatures, which filled and sealed thermally induced microcracks, while crystallization of nepheline and leucite transformed the original amorphous geopolymer into a polycrystalline ceramic. The best performance was obtained after heating to 1000 °C for 6 h, resulting in irreversible crack sealing, improved structural integrity, and a self-glazing effect, whereby excess glassy phase migrated to the surface and formed a smooth, glossy protective coating [38].

Glass frit was also used by Keane et al. [39]. A potassium-based metakaolin geopolymer with the stoichiometric composition K₂O·Al₂O₃·4SiO₂·11H₂O was reinforced with chopped basalt fibers and modified with a low-melting glass frit (Tm ≈ 815 °C), which served as the self-healing agent. The self-healing mechanism was thermally activated: upon heating above the glass melting temperature, the amorphous glass phase flowed into dehydration- and crystallization-induced microcracks, sealing the damaged regions and restoring microstructural integrity in both the geopolymer and the resulting ceramic material. As a result, the glass-containing composites exhibited markedly improved flexural performance, with approximately a 200% increase in flexural strength after high-temperature exposure and optimum properties achieved at about 1150 °C due to efficient crack sealing by the molten glass phase [39].

The same material was the subject of work conducted by Chadha and Kriven [40,41]. A potassium-based metakaolin geopolymer with the stoichiometric composition K₂O·Al₂O₃·4SiO₂·11H₂O was reinforced with 20 wt.% chopped basalt fibers and modified with 7.5 wt.% low-melting glass frit (Tm ≈ 815 °C), which acted as the self-healing agent. The self-healing mechanism was thermally activated: during heating, dehydration and geopolymer-to-leucite ceramic transformation generated microcracks, while the molten glass frit flowed into these cracks and pores, sealing them and producing both self-healing within the bulk and self-glazing on the surface. As a result, the glass-containing composites exhibited significantly improved flexural performance after high-temperature exposure, with the optimum behavior achieved at 1150 °C, where crack sealing by the glass phase and reinforcement by intact basalt fibers were maximized before fiber melting occurred [40,41].

A similar mechanism connected with high temperature self-healind was described by Bhuiya et al. [42]. Potassium-based metakaolin geopolymers were produced using either high-purity Metamax^® metakaolin or metakaolin derived from Mymensingh clay, and were reinforced with hydroxyapatite (HA, bone ash) or dicalcium phosphate particles added at 5–15 wt.%. The self-healing effect was mainly associated with the dicalcium phosphate -containing composites, where high-temperature treatment (1100–1165 °C) promoted the formation of an amorphous phosphate glass phase that flowed into microcracks and pores, sealing damage generated during dehydration and thermal exposure. As a result, dicalcium phosphate -reinforced geopolymers exhibited improved microstructural integrity, significantly reduced water absorption, and enhanced flexural strength, with the best-performing composition reaching approximately 32 MPa after heat treatment due to the crack-healing action of the phosphate glass phase [42].

In turn, Gomes et al. [43] modified a potassium-activated metakaolin geopolymer by partially replacing metakaolin with calcined Callovo–Oxfordian argillite, which acted not only as a filler but also as a reactive precursor capable of further alkali activation under heat and humidity. The self-healing mechanism was thermally induced and relied on viscous creep and viscous sintering of the amorphous geopolymer phase, which enabled the closure of microcracks generated during dehydration and thermal shrinkage. The addition of argillite significantly reduced thermal shrinkage, while the optimum composition containing approximately 67 wt.% calcined argillite and 33 wt.% metakaolin promoted crack closure during heating and preserved the integrity of the material at elevated temperatures. As a result, the composite exhibited improved dimensional stability, retained useful mesoporosity, and showed enhanced high-temperature self-healing behavior, reducing shrinkage from about 30% for the reference geopolymer to roughly 10% at 1000 °C [43].

In turn, Rashid et al. [44] investigated a fly ash-based geopolymer mortar activated with a combination of NaOH and Na₂SiO₃, and evaluated its behavior after exposure to temperatures of 400 °C, 600 °C, and 800 °C. A temperature-induced self-healing effect was observed particularly between 400 °C and 600 °C, where continued geopolymerization, densification of the matrix, and thermally activated reactions limited the development of cracks. The healing mechanism was associated with the activation of Na₂SiO₃, ongoing geopolymeric reactions, and the formation of hematite through the dehydroxylation of amorphous iron-containing phases, which contributed to partial filling and sealing of cracks and improved thermal damage resistance [44].

High-temperature self-healing in geopolymers represents a distinct class of healing mechanisms, where crack closure is achieved through thermally induced microstructural reorganization, viscous sintering, and the formation of glassy or ceramic phases rather than through conventional ambient-temperature geopolymerization. Although these systems have demonstrated remarkable crack-sealing efficiency and mechanical property recovery at elevated temperatures, their applicability remains largely restricted to refractory, ceramic, and other high-temperature engineering applications.

4.2. Extrinsic Self-Healing Strategies

4.2.1. Main Mechanism of Extrinsic Self-Healing

Extrinsic self-healing, also known as autonomous self-healing, relies on externally introduced healing agents that are released or activated after crack formation. In geopolymer systems, this approach commonly involves encapsulated healing agents, bacteria-based systems, or vascular networks that deliver repair compounds into damaged regions. Once activated, these agents promote crack sealing through precipitation reactions or the formation of new binding phases, thereby restoring the integrity of the material.

Figure 4 presents a schematic illustration of a representative (simplified) extrinsic self-healing mechanism in geopolymer systems, highlighting the most commonly observed processes. It should be noted that multiple alternative healing mechanisms may occur depending on material composition and environmental conditions.

Autonomous (engineered) self-healing technologies are designed to overcome the limitations of natural healing mechanisms by introducing additional agents or systems that actively respond to crack formation. In geopolymer materials, these approaches include microcapsules, bacteria-based systems, superabsorbent polymers, crystalline admixtures, and vascular networks, each operating through different activation and healing mechanisms [7,8,29,45].

Table 5. Selected, extrinsic (autonomous) technologies—comparison between geopolymers and cement-based systems.

Technology	Geopolymers	Cement-Based Systems
Capsule-based systems	Experimental	Widely studied (polymer or mineral capsules)
Bacteria-based	Limited applications	Mature concept: CaCO3 via microbial activity
Superabsorbent polymers (SAPs)	Emerging, affected by high alkalinity	Well-established (water retention + crack sealing)
Hydrogels/smart polymers	Early-stage research	Advanced responsive systems (pH, moisture, temperature)
Crystalline admixtures	Limited efficiency unless Ca added	Common and effective
Vascular systems	Rare	Developed but still experimental

summarizes the main autonomous self-healing technologies reported for geopolymers and compares their level of development with corresponding solutions used in OPC-based materials.

The comparison demonstrates that, although autonomous self-healing technologies in geopolymers are developing rapidly, most of them remain at an experimental stage and are less mature than their counterparts in OPC-based materials. Nevertheless, approaches such as microcapsules, bacterial systems, and smart admixtures show significant potential to extend the healing capacity beyond the limits of conventional autogenic mechanisms and enable the repair of larger and more complex damage [7,46].

While the previous comparison focuses on the technological approaches used to trigger autonomous healing, the effectiveness of any self-healing process ultimately depends on the nature and properties of the products formed within the damaged region. Different healing mechanisms generate distinct phases, which vary in their ability to seal cracks, restore mechanical properties, and improve the long-term durability of geopolymer materials.

Table 6. Types of healing products in geopolymers.

Phase	Self-Healing Process Type	Conditions of Formation	Role in Healing	Microstructural Characteristics
N-A-S-H gel	Re-polymerization	Low-Ca systems, high pH	Primary crack filling phase	Amorphous, porous gel forming continuous network
C-A-S-H gel	Ca-assisted precipitation	Presence of Ca (slag, additives)	Enhances crack sealing and densification	Semi-crystalline, denser structure than N-A-S-H
Hybrid N-A-S-H/C-A-S-H	Co-precipitation	Mixed Ca–Si–Al systems	Improved mechanical recovery	Interpenetrated gel network, reduced pore size
Zeolite-like phases	Long-term transformation	Extended curing, high alkalinity	Secondary crack filling	Crystalline, localized growth in pores
Carbonates (CaCO3)	Secondary (carbonation-assisted healing)	CO2 exposure, presence of Ca	Surface sealing, minor contribution	Crystalline deposits, often near surface
Efflorescence products	Side effect (ion leaching)	High alkali mobility, water transport	Negative or neutral	Crystalline, surface accumulation

summarizes the principal healing products associated with self-healing processes in geopolymers and their microstructural functions.

The diversity of healing products reflects the complexity of self-healing mechanisms in geopolymer systems. Among these products, N-A-S-H and C-A-S-H-based gels play the most important role in restoring matrix continuity and reducing permeability, while carbonate and zeolite-like phases generally provide supplementary crack-filling and densification effects.

4.2.2. Capsule-Based Systems

Capsule-based self-healing systems represent one of the most promising autonomous healing approaches in geopolymers, as they enable the on-demand release of healing agents after crack formation, thereby promoting crack sealing and partial recovery of the material’s functional properties. For example, Xue et al. [47] show that microcapsule-based self-healing in geopolymers not only repairs cracks but also actively slows microstructural degradation, enabling the material to maintain very low permeability even under high-concentration sulfate attack. This effect is achieved through crack-triggered rupture of microcapsules (≈17.8–21.6 μm in diameter) that release sodium silicate (Na₂SiO₃), supplying OH⁻, Na⁺, and Si(OH)₄, which promote secondary geopolymerization and reconstruction of the N-(C)-A-S-H gel network. As a result, the system is particularly promising for long-term applications in contaminant containment barriers such as vertical cutoff walls in sulfate-polluted groundwater environments, where durability and hydraulic sealing are critical [47].

In other research work, Xue et al. [25] used fly ash-based geopolymer backfill activated with sodium silicate and modified with microcapsules containing sodium silicate as a healing agent. Two types of microcapsules were developed, namely single-walled polyurethane capsules and double-walled polyurethane/melamine-formaldehyde capsules, designed to survive mixing and curing while remaining available for later activation. When drying–wetting cycles generate microcracks, the capsules rupture and release sodium silicate, which reacts with unreacted fly ash particles to form additional geopolymer gels that fill pores and seal cracks. The most important result was that the self-healing system significantly improved durability, keeping the hydraulic conductivity close to or below the design threshold of 1 × 10⁻⁸ m/s for several drying–wetting cycles, while reducing permeability by up to 99.38% compared with the reference material after the first cycle [25].

The microcapsules were also investigated by Han et al. [20]. They used a fly ash–GGBS geopolymer mortar incorporating hybrid self-healing microcapsules composed of a polyethylene wax (PE wax)/PbSO₄ shell and a hexamethylene diisocyanate (HDI) core. The microcapsules were designed to be activated either by crack-induced mechanical rupture or by chloride ions, as PbSO₄ embedded in the shell reacts with chlorides and promotes the release of the healing agent. After release, HDI reacts with water and hydroxyl groups present in the highly alkaline geopolymer matrix, forming polyurethane and polyurea products that fill cracks and chemically bond with the surrounding material. The most significant result was that chloride-assisted healing enabled nearly complete closure of cracks up to 0.31 mm wide, while simultaneously improving impermeability and mechanical performance recovery [20].

In turn, Ozen et al. [48] investigated fly ash–metakaolin geopolymer pastes incorporating encapsulated self-healing agents designed to autonomously repair cracks. The capsules consisted of sodium silicate carried in polyethylene glycol (PEG) and protected by an epoxy resin and fly ash coating, enabling the healing agent to remain inactive until cracking occurred. After preloading, the capsules ruptured and released sodium silicate into the cracks, triggering further geopolymerization reactions and the formation of new aluminosilicate products that filled and sealed the damaged regions. The most effective system contained 9 wt.% capsules and achieved a crack-sealing efficiency of 91.6 ± 18.5%, demonstrating the high potential of encapsulated sodium silicate for autonomous self-healing of geopolymers [48].

A similar system to encapsulation was applied by Zhao et al. [22]. They investigated a sodium aluminosilicate-based geopolymer refractory insulation produced primarily from metakaolin, sodium silicate, sodium hydroxide, silica fume, lightweight refractory aggregates, and aluminosilicate cenospheres. The material contained aluminosilicate cenospheres that created closed-cell porosity and enabled a controlled pore structure designed to support self-sealing behavior during molten salt exposure. The self-healing mechanism relied on the partial penetration of molten nitrate or chloride salts into small open pores, followed by salt freezing within the insulation layer, which formed a self-contained barrier; if cracks developed, newly infiltrating molten salt could re-freeze and autonomously seal the defect [22]. The main result was that the geopolymer insulation maintained structural integrity and thermal performance after exposure to molten nitrate salts for up to 53 days and molten chloride salts for up to 27 days, demonstrating the feasibility of a self-healing salt-barrier concept for high-temperature thermal energy storage systems.

Capsule-based systems were also investigated by Han et al. [49]. They investigated alkali-activated geopolymer mortars produced from a blend of fly ash (20%) and ground granulated blast furnace slag (80%), activated with a sodium silicate–NaOH solution. Self-healing was achieved by incorporating HDI@PE-wax microcapsules, consisting of a hexamethylene diisocyanate (HDI) core and a polyethylene wax shell, with dosages ranging from 0.5% to 2% of the geopolymer precursor mass. When cracking occurred, the microcapsules ruptured and released HDI, which reacted with water and hydroxyl groups present in the geopolymer matrix to form polyurethane–polyurea healing products capable of filling and bridging cracks. The optimum dosage was 1% microcapsules, which increased compressive strength by 6.2%, improved the compressive strength retention rate by up to 36.6%, and enabled effective crack repair in the geopolymer matrix [49].

Overall, capsule-based systems have demonstrated considerable potential for autonomous crack repair in geopolymer materials by enabling the controlled release of healing agents that promote secondary geopolymerization, polymer formation, or crack sealing. However, despite promising laboratory results, further research is needed to improve capsule durability, long-term stability, and compatibility with geopolymer matrices before large-scale practical applications can be realized.

4.2.3. Bacteria-Based Systems

Bacterial-based self-healing is currently considered one of the most widely studied and applied self-healing approaches in cementitious and geopolymer materials [50]. Many authors also point out that bacteria-based self-healing systems currently exhibit the highest healing efficiency among all investigated geopolymer healing agents, mainly due to their ability to induce calcium carbonate (CaCO₃) precipitation within cracks. The resulting CaCO₃ crystals effectively seal cracks, reduce permeability, and improve the durability and service life of geopolymer materials [51].

The healing mechanism is based on the activation of microorganisms within cracks, where they induce the precipitation of calcium carbonate (CaCO₃) as part of their metabolic processes. The newly formed CaCO₃ crystals accumulate within cracks and pores, physically sealing the voids and reducing permeability while restoring structural continuity. Additionally, this mineral precipitation densifies the microstructure around damaged regions, leading to measurable recovery of mechanical properties and improved resistance to further freeze–thaw deterioration [52,53].

One of the first works in this area was conducted by Jadhav et al. [54]. The geopolymer investigated was a metakaolin-based geopolymer activated with sodium silicate, into which Sporosarcina pasteurii bacterial spores were directly incorporated without encapsulation or immobilization, demonstrating that the geopolymer matrix could act as a suitable host for microbial self-healing agents. The self-healing mechanism was based on MICP: after activation in a nutrient medium containing urea and calcium ions, the bacterial spores germinated, hydrolyzed urea, and induced the precipitation of calcium carbonate (CaCO₃), which filled and sealed cracks in the geopolymer. The main finding was that the bacterial spores remained viable even in aged geopolymers and successfully sealed cracks through calcite formation, proving that direct addition of bacterial spores into metakaolin-based geopolymers is a feasible and effective strategy for autonomous self-healing without the need for encapsulation [54].

Tanyildizi et al. [52] demonstrated that biological self-healing in geopolymers via microbially induced calcium carbonate precipitation using Sporosarcina pasteurii is a highly effective mechanism for repairing cracks and pores formed during freeze–thaw cycles. The bacteria precipitate CaCO₃ within cracks (100–150 μm), leading to measurable improvements such as up to 10.9% higher tensile strength, reduced water absorption (≈8% decrease), and healing of approximately 91.3% of total porosity. As a result, the process not only seals cracks but also restores mechanical integrity and significantly enhances durability and impermeability of geopolymer concrete in cold environments [52].

An interesting comparison was made by Koseoglu et al. [55]. They demonstrated that several marine actinomycete isolates obtained from Marmara Sea sediments exhibited superior self-healing performance in geopolymer mortars compared with the reference bacterium Sporosarcina pasteurii. Among the tested strains, the marine isolate S13 (the authors do not give specific taxonomy) showed the most balanced performance, increasing compressive strength from 27.2 MPa to 37.2 MPa and reducing water absorption from 8.17% to 7.44%, while promoting extensive crack closure through microbially induced calcium carbonate precipitation. Other marine isolates also demonstrated effective crack-sealing capabilities, with microscopic observations confirming substantial healing of microcracks. In contrast, Sporosarcina pasteurii, which is widely used in self-healing concrete research, showed poor compatibility with the ceramic powder–slag geopolymer matrix and did not produce significant crack-healing effects [55]. These findings suggest that marine actinomycetes are better adapted to the highly alkaline conditions of geopolymers and may therefore represent a more promising microbial agent for self-healing geopolymeric materials.

In turn, Wulandari et al. [56] conducted an experiment with Sporosarcina pasteurii and yeast. The geopolymer studied was a fly ash-based geopolymer paste produced from Class F fly ash and activated with a mixture of sodium silicate and 4 M sodium hydroxide, and it was compared with a fly ash-containing Portland cement paste [56]. To introduce self-healing potential, the researchers added microbial agents consisting of Sporosarcina pasteurii bacteria and yeast, which promoted biomineralization within the matrix. The proposed healing mechanism was based on microbially induced calcium carbonate (calcite) precipitation, where microbial activity generated calcite that filled pores and densified the microstructure, thereby reducing porosity and potentially sealing microdefects. The main result was that samples containing microbial agents exhibited higher calcite content, greater closed porosity, and higher compressive strength than the control mixtures, demonstrating that microbial addition can significantly improve the performance of fly ash-based geopolymers and provide a promising basis for self-healing geopolymer materials [56].

Ziada et al. [57] demonstrated that 3D-printed fly ash-based geopolymer mortars reinforced with PVA fibers can be effectively self-healed using the bacterium Sporosarcina pasteurii. The healing process was based on MICP, in which bacterial activity promoted the formation of CaCO₃ deposits inside the cracks. The precipitated calcium carbonate gradually filled and sealed cracks with widths of approximately 100 μm, resulting in improved mechanical performance and reduced water permeability. In addition to enhancing flexural strength, PVA fibers supported the self-healing process by limiting crack width and providing bridging and nucleation sites for CaCO₃ precipitation, leading to more efficient crack closure [57].

To improve the self-healing process, Polat et al. [58] introduced bacteria to expanded perlite. They researched a metakaolin-based alkali-activated geopolymer mortar containing blast furnace slag, sodium silicate, sodium hydroxide, synthetic fibers, and expanded perlite aggregate. The self-healing system was based on the incorporation of Sporosarcina pasteurii bacterial spores, with expanded perlite serving as a low-cost and simple carrier for the bacteria, eliminating the need for complex and expensive encapsulation techniques. The self-healing mechanism relied on MICP, in which bacterial activity promoted CaCO₃ formation that gradually filled and sealed microcracks within the geopolymer matrix. The results demonstrated highly effective crack repair, substantial reductions in water absorption and permeability, and successful survival of bacterial spores in the alkaline geopolymer environment, although the healing products did not fully restore mechanical strength [58].

An attractive concept was also presented by Wulandari et al. [59], who joined bacteria with a fungal component [59]. A fly ash-based geopolymer activated with a mixture of 4 M NaOH and sodium silicate solution was modified with the microorganisms Sporosarcina pasteurii and Rhizopus oligosporus, which were introduced as potential self-healing agents. The proposed self-healing mechanism was based on MICP, where S. pasteurii generated CaCO₃ deposits while R. oligosporus formed fungal hyphae that supported bacterial growth and bio-mineralization within the geopolymer matrix. The precipitated calcite filled micropores and increased the proportion of closed porosity, leading to a denser microstructure and providing preliminary evidence of self-healing behavior. As a result, the compressive strength increased by 43.75%, from 16.44 MPa for the reference geopolymer to 23.53 MPa for the microbe-containing geopolymer, while other analyses confirmed hyphal growth and enhanced calcite formation [59].

However, Al Hayo et al. [60] show that these bacteria, that this is very effective in the case of cements, can be replaced by other counterparts that give even better results. They studied a GGBFS–ceramic waste powder geopolymer mortar activated with NaOH and potassium silicate (K₂SiO₃), in which self-healing was induced by daily injection of bacterial suspensions into pre-cracked specimens (crack width 0.20–0.26 mm) for 56 days. Three bacterial strains were evaluated: the ureolytic bacterium Sporosarcina pasteurii (SP) and the non-ureolytic strains Viridibacillus arenosi (A6) and Bacillus zhangzhouensis (D25); remarkably, D25 and A6 achieved higher healing efficiencies (96.9% and 91.9%, respectively) than SP (77.8%). A key finding was that non-ureolytic bacteria outperformed the conventional ureolytic strain, while avoiding ammonia production associated with urea hydrolysis; this makes them a more environmentally friendly option and potentially better suited to the highly alkaline environment of geopolymers, without compromising CaCO₃ precipitation efficiency [60].

Frahat et al. [61] investigated two other bacterial strains. They used a fly ash-based self-healing geopolymer concrete activated with NaOH and sodium silicate, incorporating eggshell powder as a supplementary calcium-rich additive and nutrient source for bacteria. Bacillus subtilis and Bacillus sphaericus were introduced into the geopolymer matrix to induce MICP, while eggshell was added at 5%, 10%, and 15% of the fly ash mass to support bacterial activity and biomineralization. The best overall mechanical performance was achieved by the mixture containing Bacillus sphaericus and 5% eggshell, which reached a compressive strength of 60.9 MPa after 28 days, clearly outperforming the control geopolymer concrete, 47.1 MPa [61].

Both of this strain was also investigated by Ganesh et al. [62]. A GGBS-based geopolymer concrete activated with 13 M NaOH and sodium silicate solution was used as the matrix, and it was reinforced with glass fibers, polypropylene fibers, and a bacterial consortium consisting of Bacillus subtilis and Bacillus sphaericus. The self-healing functionality was provided by the bacteria, which remained dormant in the geopolymer matrix and, upon water exposure, induced the precipitation of calcium carbonate (CaCO₃) inside cracks through microbial activity. The self-healing mechanism involved the gradual deposition of calcite within artificially induced cracks (up to 1 mm wide), leading to progressive crack closure. As a result, the bacterial geopolymer composites achieved 92–95% crack closure and recovered approximately 85–87% of their original compressive strength, while also exhibiting improved mechanical performance due to the combined action of fibers and biomineralization [62].

The same strain, Bacillus sphaericus, was investigated by Ahmed et al. [63]. In this case, the developed material was a fly ash-based geopolymer concrete incorporating recycled crushed brick aggregates and recycled PET particles. The bacteria become active when cracks and moisture are present and induce MICP, producing CaCO₃ that fills and seals microcracks within the geopolymer matrix. As a result, the self-healing mechanism promotes crack closure, densifies the microstructure, improves mechanical properties, and enhances the long-term durability of the geopolymer concrete [63].

Bacillus subtilis was the topic of research conducted by Choudhary et al. [64]. The geopolymers were composed of fly ash (FA), ground granulated blast-furnace slag (GGBS), and metakaolin (MK), activated with sodium hydroxide and sodium silicate solutions. To induce self-healing, the researchers incorporated 5% Bacillus subtilis spores, which remained dormant within the geopolymer matrix and became active when moisture entered cracks, promoting MICP. The main result was that bacterial incorporation significantly enhanced both self-healing and mechanical performance, leading to visible crack closure, pore filling by CaCO₃ deposits, and approximately 22% higher 28-day compressive strength compared with equivalent mixtures without bacteria [64].

A similar mechanism was observed by Yahya et al. [65]. They confirm that in the case of the use of the bacteria Shewanella oneidensis, the main self-healing mechanism is connected with biomineralization. The used bacteria promote the precipitation of calcium carbonate (CaCO₃) and other mineral products within pores and microcracks of the geopolymer matrix. These precipitates densify the structure, reduce pore connectivity, and improve the durability-related properties of geopolymer concrete [65].

Another investigated strain was Lysinibacillus sp. WH. Techo et al. [66] investigated a lime/pozzolan biogeopolymer produced from rice husk ash and incinerated sugarcane press mud, with Lysinibacillus sp. WH incorporated as a calcifying bacterium capable of MICP. The authors evaluated the ability of the bacterial geopolymer to heal artificially induced cracks and compared its performance with that of control geopolymers without bacteria. Self-healing was monitored through crack closure observations and microstructural analyses of the healing products. The results showed that bacterial specimens completely healed cracks of approximately 0.12–0.20 mm within 20 days, whereas no visible healing occurred in the control geopolymers [66].

The research made by Danilyan et al. [67] selected Lysinibacillus fusiformis JH2 as the bio-agent and incorporated it into the fly ash/fly ash–bottom ash geopolymer as dormant bacterial endospores, allowing the microorganisms to withstand the highly alkaline geopolymer environment and remain viable during curing. The incorporation of JH2 endospores produced an improvement in mechanical performance, increasing the 7-day compressive strength by up to 166%, while bacterial viability tests confirmed that the spores survived inside the geopolymer matrix. The proposed self-healing mechanism is based on the germination of endospores upon exposure to water entering through cracks, which reactivates bacterial metabolism and triggers biomineralization processes. Unlike conventional cement-based systems that mainly rely on CaCO₃ precipitation, the low-calcium geopolymer promoted the formation of aragonite (CaCO₃), natrite (Na₂CO₃), and brucite (Mg(OH)₂), which refined the pore structure, filled voids and cracks, and enhanced the durability and strength of the material [67].

In turn, Yahya et al. [65] evaluated the direct addition of Shewanella oneidensis to class F fly ash geopolymer concrete without encapsulation, while curing the specimens at ambient temperature (15–17 °C). The bacterial incorporation promoted self-healing-related benefits, including a 26.5% reduction in porosity (from 5.99% to 4.40%) and a 13.1% reduction in water absorption (from 3.06 to 2.66 kg/m²), which were attributed to bacterial biomineralization and pore-sealing by calcium carbonate precipitation. A key advantage of this approach is that the bacterial suspension was added directly to the geopolymer mixture without encapsulation or carrier materials. The results demonstrate that a simplified direct-inclusion method can still enhance durability and self-healing potential, while avoiding the cost and complexity associated with encapsulation techniques. This conclusion is also important because previous research shows that geopolymers provide a challenging environment for bacteria due to their extremely high pH (>13), and therefore require effective encapsulation strategies [68].

The other strategy of incorporating the bacteria was used by Mahmood et al. [69]. They investigated a fly ash Class F–silica fume geopolymer mortar activated with 10 M NaOH and Na₂SiO₃, incorporating several strains of Bacillus subtilis and Bacillus cereus. To improve bacterial survival in the highly alkaline geopolymer environment, the bacteria were immobilized within porous zeolite particles, which acted as a protective carrier and reservoir for water and nutrients. The main result was that B. subtilis demonstrated the highest self-healing performance, with almost complete crack closure after 14 days, complete healing of a 175.7 μm crack, and substantial reductions in water absorption (up to 60.4%) and porosity (up to 53.65%), indicating effective CaCO₃-mediated pore filling and durability enhancement [69].

However, it is worth noting that Ekinci et al. [70,71] show that in the case of strains of Bacillus subtilis this kind of protection is not necessary. They study a ground granulated blast furnace slag (GGBS)-based geopolymer mortar activated with sodium silicate (Na₂SiO₃), in which Bacillus subtilis endospore-forming bacteria were directly incorporated into the mixture without encapsulation or immobilization techniques. The self-healing system relied on MICP, where bacterial activity in the presence of urea and a calcium source promoted the formation of CaCO₃ that filled pores and microcracks within the geopolymer matrix. Microstructural analyses confirmed calcite precipitation and the development of a denser microstructure in healed specimens. The best performance was achieved with a bacterial concentration of 10⁷ CFU/mL and a dosage of 1% [71] or 3% [70], resulting in significant improvements in compressive strength, tensile strength, impermeability, and overall durability [70,71].

It is also worth noticing that not every bacterial species successfully used in ordinary Portland cement concrete will be effective in geopolymer systems. Nathania et al. [72] show that the incorporation of Bacillus megaterium and calcium lactate did not result in observable self-healing, as the authors explicitly concluded that the geopolymer concrete did not exhibit crack-repair capability. The authors attributed this failure primarily to the highly alkaline geopolymer environment produced by the 10 M NaOH activator (pH > 14), which likely reduced bacterial viability, inhibited biomineralization, limited CaCO₃ precipitation, and ultimately prevented the self-healing mechanism from occurring [72].

Comparison between the different bacteria strain was made by Doctolero et al. [73]. The geopolymer used in this study was a fly ash-based aluminosilicate binder, designed as a biogeopolymer by incorporating bacterial spores capable of inducing biomineralization. To enhance bacterial survival in the highly alkaline geopolymer matrix, the spores were immobilized on biochar, and both pure bacterial cultures (Bacillus subtilis, B. sphaericus) and a co-culture system (B. sphaericus + B. thuringiensis) were evaluated as self-healing agents. The self-healing mechanism was based on MICP, where bacterial activity promoted the formation of calcium carbonate (CaCO₃) crystals inside cracks. These biominerals gradually filled and sealed the damaged zones, reducing internal porosity and restoring the continuity of the geopolymer matrix. The main result of the study was that the use of co-cultured bacteria significantly improved healing efficiency, while biochar immobilization provided an optimal response at approximately 0.3–0.4 g/mL. The developed biogeopolymers were able to seal cracks up to 0.65 mm wide, with SEM-EDS, FTIR, XCT (X-ray computed tomography), and ultrasonic measurements confirming extensive CaCO₃ deposition within the crack network.

The interesting aspect is also the genetic modification of bacteria strain used in the self-healing process. Chatterjee et al. [74] studied class F fly ash-based geopolymer mortar activated with alkaline solutions and cured at ambient temperature, without any Portland cement addition. To provide self-healing functionality, the researchers incorporated genetically modified Bacillus subtilis cells (10⁵ cells/mL), transformed with a biosilicification (bioremediase-like protein) gene originating from a thermophilic hot-spring bacterium. This genetic modification enabled the bacteria to produce bioremediase protein capable of promoting mineral formation inside cracks and pores. The self-healing mechanism was based on microbially induced biosilicification and biomineralization, where bacterial activity generated crystalline healing products (including silica-rich phases and mineral precipitates such as CaCO₃/Gehlenite) that filled microcracks and reduced permeability. The spore-forming nature of B. subtilis also enhanced long-term survival within the highly alkaline geopolymer matrix. The main result was that the bacteria-amended geopolymer exhibited higher compressive strength, improved ultrasonic pulse velocity, lower water absorption and chloride permeability, and enhanced sulfate and acid resistance, demonstrating effective autonomous self-healing and durability improvement compared with the control geopolymer without bacteria [74].

Tanyildizi et al. [53] compared autonomous healing, where a suspension of Sporosarcina pasteurii (9 × 10⁸ cells/mL) was injected into pre-cracked specimens, with autogenous healing, where bacteria were incorporated into the geopolymer mortar during mixing. Autonomous healing consistently outperformed autogenous healing, resulting in higher compressive strength, flexural strength, and ultrasonic pulse velocity (UPV) after exposure to sulfate attack. In addition, autonomously healed specimens exhibited lower capillary water absorption, while XCT analysis demonstrated a healing efficiency of 98.51% pore filling, confirming the superior crack-sealing performance of the bacterial injection approach [53].

In general, bacteria-based systems represent one of the most effective autonomous self-healing approaches for geopolymer materials, owing to their ability to induce biomineralization and generate crack-filling products such as CaCO₃. Although numerous studies have demonstrated excellent crack closure, durability enhancement, and mechanical property recovery, challenges related to bacterial viability, long-term performance, and large-scale implementation in highly alkaline geopolymer environments still require further investigation.

4.2.4. Mineral Admixtures

Mineral admixtures constitute an alternative self-healing strategy in geopolymers, where reactive inorganic additives promote the formation of healing products capable of filling cracks, refining the pore structure, and enhancing long-term durability.

Chen et al. [75] investigated a fly ash-based geopolymer cutoff wall backfill composed of sand, fly ash, and sodium silicate activator, modified with reactive MgO particles (15 μm and 50 μm) as self-healing agents. The self-healing mechanism occurred in two stages. First, MgO hydrated to form brucite (Mg(OH)₂). Subsequently, brucite reacted with water and atmospheric CO₂, producing hydrated magnesium carbonates (HMCs), mainly nesquehonite (MgCO₃·3H₂O), hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O), and dypingite (Mg₅(CO₃)₄(OH)₂·5H₂O). These expansive crystalline products filled pores and cracks, forming interconnected networks within the geopolymer matrix. Although MgO slightly hindered geopolymerization, resulting in lower compressive strength and higher initial hydraulic conductivity, it substantially enhanced crack healing and long-term durability. Crack closure efficiency increased by 135–308% compared with the reference geopolymer, while crack width reductions of up to 60% were achieved after 28 days [75].

Hossain et al. [76] also confirmed the usefulness of MgO in the self-healing process. Geopolymer was produced using slag and class C fly ash (binary mix) or slag, class C fly ash and class F fly ash (ternary mix), activated with a dry blend of sodium silicate and calcium hydroxide, and reinforced with 2 vol.% PVA fibers to achieve strain-hardening behavior. To impart self-healing functionality, the researchers incorporated lightly burned MgO as an expansive self-healing agent, expecting its delayed hydration to form Mg(OH)₂ and potentially M–S–H phases capable of sealing microcracks and compensating for shrinkage. The main finding was that MgO-containing EGCs maintained good mechanical performance and strain-hardening characteristics, with the binary mix achieving the highest strengths (41.5 MPa compressive and 3.8 MPa tensile strength at 28 days), demonstrating the feasibility of developing self-healing geopolymer composites based on MgO activation [76].

Ozen and Stephan [77] investigated geopolymer mortars based on fly ash and metakaolin, activated with potassium silicate and modified with expanded perlite as a lightweight aggregate. For autonomous self-healing, the perlite was impregnated with sodium silicate (SS-Perlite) and coated with PVA and a thin fly ash layer to prevent premature release of the healing agent. After crack formation, water penetrating the crack dissolved the PVA coating, releasing sodium silicate, which reacted with unreacted aluminosilicate phases in the geopolymer matrix and generated secondary geopolymer products that partially filled the cracks. The main result was that SS-Perlite significantly improved compressive strength recovery compared with the control mixtures, although complete crack closure and full mechanical restoration were not achieved [77].

Borçato and Medeirosa-Junior [9] demonstrated that crystalline admixtures, which are widely used in Portland cement concretes, can also promote self-healing in geopolymer materials. The mixture containing crystalline admixture and hydrated lime showed the greatest amount of healing products, improved crack sealing, and a denser microstructure compared with the reference geopolymer [9].

A different approach was represented by Sun et al. [78]. They investigated DIW-printed aluminosilicate geopolymers and introduced diamond particles as a functional additive to enhance the self-healing performance and overall material properties. The diamond-reinforced geopolymer exhibited improved rheological behavior, better printability, and a more homogeneous microstructure compared with the reference material. The self-healing mechanism was mainly associated with the formation and growth of secondary mullite whiskers within the geopolymer matrix. These mullite crystals progressively filled micro- and nanopores as well as small cracks, while liquid-phase infiltration further densified the structure and restored its integrity. The main result was that diamond addition promoted microstructural self-repair, improved mechanical properties (hardness, Young’s modulus, and compressive strength), and enhanced thermal stability, demonstrating the feasibility of producing durable self-healing geopolymers through additive manufacturing [78].

A similar experiment was conducted by Tang and Tang [79] for extrusion-based 3D printing. A metakaolin-based geopolymer composed mainly of SiO₂ (55.06 wt.%) and Al₂O₃ (44.12 wt.%) was used as the matrix, while titanium-coated diamond particles (up to 15 wt.%) were incorporated as a functional additive. The diamond particles acted not only as a rheology modifier for extrusion-based 3D printing but also as the key component responsible for the self-healing behavior. The self-healing mechanism was based on the exceptionally high thermal conductivity of diamond. During geopolymerization, diamond particles stored and redistributed heat released by the reaction, promoting the formation and growth of secondary mullite crystals. These needle-like mullite structures filled pores and microcracks within the geopolymer matrix, leading to autonomous healing. As a result, the diamond/geopolymer composites exhibited a denser microstructure with significantly fewer pores and cracks, improved mechanical properties, and enhanced thermal stability up to 1200 °C. The study demonstrated that diamond addition can induce a novel self-healing mechanism in geopolymer composites through secondary mullite formation and pore filling [79].

Also, an atypical approach is represented by Keane et al. [80,81]. They investigated a potassium metakaolin-based geopolymer composite reinforced with alumina platelets and glass frit particles, which were uniformly dispersed throughout the geopolymer matrix. The glass particles served as the self-healing agent, while the alumina platelets were introduced to reduce shrinkage and improve dimensional stability during high-temperature exposure. The self-healing mechanism was thermally activated: upon heating to approximately 900 °C, the glass phase softened and flowed into microcracks generated during matrix shrinkage, thereby sealing the cracks and significantly reducing porosity [80,81]. As a result, a dense amorphous self-healing geopolymer was produced, which remained chemically stable and resistant to molten NaCl and KCl for more than 200 h without noticeable degradation [80].

Majdoubi et al. [82] suggest that a mechanism similar to self-healing can be responsible for the improved mechanical performance of phosphogypsum-modified geopolymers. It is associated with a crack-arresting mechanism, in which the rod-like phosphogypsum particles effectively bridge microcracks, hinder crack propagation, and enhance the toughness of the geopolymer matrix through strong interfacial adhesion [82]. Rather than representing true autonomous self-healing, the observed behavior is better described as a self-healing prevention mechanism, where waste phosphogypsum acts similarly to microfibers, reducing microcrack development and promoting the formation of a denser and more mechanically stable microstructure.

Overall, mineral admixtures enhance self-healing performance mainly by stimulating the formation of additional reaction products, improving matrix densification, and supporting crack-filling processes. Although their healing efficiency is often lower than that of bacterial or capsule-based systems, mineral additives offer a simple, cost-effective, and highly compatible approach for improving the durability and self-repair capacity of geopolymer composites.

4.2.5. Other Systems

In addition to capsule-based, bacterial, and mineral-admixture approaches, several alternative self-healing strategies have been explored in geopolymer materials, including polymers, superabsorbent materials, vascular systems, and nanomaterial-assisted concepts, aiming to enhance crack sealing and durability through diverse physical and chemical mechanisms.

One of the first works in the area of using organic polymers for self-healing was presented by Kusbiantoro et al. [83]. The study investigated a fly ash-based geopolymer mortar activated with alkaline solutions, in which 1 wt.% poly(ethylene-co-vinyl acetate) (EVA) was incorporated as a self-healing additive and the specimens were cured for 24 h at 70, 80, and 90 °C. The proposed self-healing mechanism was related to the polymeric EVA phase reducing drying-shrinkage damage and helping to maintain microstructural integrity during elevated-temperature curing, thereby limiting pore formation and microcracking associated with moisture loss from the geopolymer matrix. The main result was that geopolymer mortars containing EVA showed improved hardened properties, and curing at 90 °C produced the highest compressive strength, indicating that EVA can enhance the performance of fly ash geopolymers exposed to heat curing conditions [83].

From the current point of view, the designation of EVA as a self-healing agent should be interpreted with caution. No direct assessment of crack closure, damage recovery, or restoration of properties after mechanical loading was performed. Instead, the observed improvements appear to be associated with polymer modification of the geopolymer matrix and mitigation of drying-shrinkage effects during thermal curing, which is more characteristic of an organic–inorganic composite material than of a genuinely self-healing system.

Nowadays, stimuli-responsive polymers represent a key direction in the development of self-healing geopolymer systems, as they enable active crack sealing through mechanisms such as swelling, shape recovery, or controlled release of healing agents [7]. These systems have demonstrated the ability to improve durability and restore mechanical properties, particularly for cracks exceeding the limits of autogenous healing; however, their effectiveness is highly dependent on the chemical environment of the geopolymer matrix. Despite their potential, applications in geopolymers remain less explored, and further research is required to address issues related to polymer stability and long-term performance [7].

Moreover, the behaviour of superabsorbent polymers (SAP) is strongly dependent on environmental conditions. The water absorption capacity of SAP is not a constant value but strongly depends on the surrounding environment and the binder composition. Therefore, the design of SAP-modified materials should take into account the actual service conditions and the specific chemistry of the cementitious or geopolymeric system [10].

Yang et al. [10] investigated a fly ash-based geopolymer activated with sodium silicate (Na₂SiO₃) and compared its interaction with an acrylic acid—acrylamide SAP used as a potential self-healing and internal curing additive. The SAP was incorporated because of its ability to absorb and subsequently release water, thereby supporting ongoing geopolymerization reactions within the matrix. The self-healing mechanism was related to the controlled release of water stored in the SAP particles, which can sustain continued reaction of unreacted aluminosilicate species and facilitate crack sealing. Material characterization showed that the highly alkaline and ion-rich geopolymer pore solution significantly affected SAP swelling behavior. The main result was that SAP exhibited the lowest water absorption capacity in the geopolymer system compared with blended cement systems due to the high concentration of dissolved ions and multivalent cations. Nevertheless, SAP reached its maximum swelling much faster in the geopolymer environment, highlighting the need for dedicated SAP design strategies for self-healing geopolymer materials [10].

Other types of polymers were investigated by Han et al. [84]. They researched the self-healing mechanism based on the release of hexamethylene diisocyanate from microcapsules, which undergoes chemical reactions with water to form polyurea (PUA) and with hydroxyl groups in the geopolymer matrix to form polyurethane (PU). These reactions generate fibrous polymeric products that effectively fill and bridge cracks, contributing to structural restoration. The detailed analysis of the polymer–N-A-S-H gel interface demonstrates that PU exhibits 9.53% higher interfacial binding energy than PUA and forms a dense hydrogen-bonding network with a 398% increase in O–H interactions [84]. This results in significantly stronger adhesion between healing products and the geopolymer matrix, enhancing durability and long-term healing efficiency [84].

Rahman et al. [85] investigated a fly ash-based geopolymer cement activated with sodium hydroxide and sodium silicate, with the addition of slag cement as a strength enhancer. To introduce self-healing capability, the authors incorporated an elastomeric expandable additive (R-additive) based on styrene–butadiene rubber at concentrations between 10 and 25 wt.% [85]. In this case, the self-healing mechanism was primarily physical, relying on the swelling and expansion of the elastomeric particles to close cracks and seal leakage pathways within the cement sheath. As the elastomer expanded, it mechanically blocked flow channels and restored the sealing performance of the material. The main result was that cracked geopolymer specimens were able to autonomously stop fluid flow after exposure to liquids, demonstrating effective self-healing and significant reductions in permeability and porosity [85].

Inspired by the vascular systems of living organisms, hollow fibers are embedded in the geopolymer matrix and filled with a healing agent. When a crack intersects the fibers, they rupture and release the healing agent into the damaged zone, leading to crack sealing and significant recovery of load-bearing capacity [8].

Alshaaer [86] revealed that some natural fibres can be used as a self-healing agent. The geopolymer matrix was produced from metakaolin activated with sodium silicate and sodium hydroxide solutions, while the key self-healing component was a reinforcement of natural vascular Luffa cylindrica fibres, which was compared with carbon and jute fibre reinforcements. The self-healing mechanism relied on the unique vascular structure of the Luffa fibres, which stored moisture and released ions such as Ca²⁺, Na⁺ and Mg²⁺ into the crack region. In the presence of water and atmospheric CO₂, these ions reacted to form insoluble healing products, mainly calcium carbonate (CaCO₃) and sodium carbonate (Na₂CO₃), which gradually filled and sealed the cracks. The fibres also controlled crack propagation by promoting multiple fine cracks instead of wide fractures, facilitating autogenous healing. As a result, the Luffa-reinforced geopolymer was the only composite that exhibited effective crack closure and partial recovery of mechanical performance, with flexural strength increasing from 1.9 MPa after cracking to 8.5 MPa after 30 days of healing [86].

Chen et al. [87] show that the incorporation of nanosilica enhanced the degree of geopolymerization, promoted the formation of additional binding gels, reduced porosity, and refined the pore structure, resulting in a denser and more compact geopolymer microstructure [87]. This mechanism limited the number of cracks and thanks to it are valuable additives that create a supportive environment for self-healing processes.

Beyond the nanoparticles, such as SiO₂, Al₂O₃, other nanomaterials also show the potential to improve the self-healing properties of geopolymers. Wang et al. [88] demonstrated that optimizing the slag–metakaolin–alkaline activator system and incorporating 0.1% multi-walled carbon nanotubes (MWCNTs) significantly improved the strength, pore structure, and erosion resistance of high-performance geopolymer concrete exposed to MgSO₄–NaCl environments. The optimized geopolymer retained approximately 92% of its compressive strength after 90 days of exposure, while also exhibiting lower carbon emissions and lower overall costs than conventional Portland cement concrete [88].

Overall, these alternative self-healing approaches demonstrate that crack repair in geopolymers can be achieved through a wide range of mechanisms, including swelling, controlled release of healing agents, moisture storage, and microstructural densification. Although most of these solutions are still at an early stage of development, they offer promising opportunities for designing multifunctional geopolymer composites with enhanced durability, adaptability, and long-term self-repair capability.

4.3. Hybrid Approaches

Most current research shows that self-healing in geopolymer composites is governed by multiple concurrent mechanisms, including crack filling, interfacial repair, and continued geopolymerization. These processes involve the formation of calcium-based precipitates as well as amorphous aluminosilicate gels, depending on the environment [89]. Also, Gan et al. [23] confirm that self-healing in geopolymers is governed by a synergistic, multicomponent mechanism involving chemical, physical, and microstructural processes. These include CaCO₃ precipitation from Ca²⁺ migration and carbonation, continued geopolymerization forming N-A-S-H/C-A-S-H gels, and water-induced swelling that reduces crack width. The interaction of these mechanisms enables efficient crack filling and restoration of matrix continuity [23].

Different self-healing mechanisms in geopolymer composites act in a complementary way, each targeting a specific type and scale of damage. Microbial systems are particularly effective in sealing fine microcracks through CaCO₃ precipitation, while fibers help control crack propagation and maintain small crack widths, enabling repeated healing. In contrast, smart materials such as shape memory alloys can actively close larger cracks, making the overall system more efficient than relying on a single healing approach [26,90].

Consequently, hybrid systems combining multiple healing strategies (e.g., bacteria and fibers) are considered the most promising, as they significantly enhance crack closure (even up to hundreds of micrometers), improve mechanical properties, and extend the durability of geopolymer concrete [26,90].

Hybrid approaches were investigated by Yuan et al. [21]. The geopolymer reinforced with PVA fibers was produced using high-calcium fly ash and ground granulated blast-furnace slag as geopolymer precursors. The self-healing strategy was based on the external application of a nano-silica solution to pre-cracked specimens with crack widths of approximately 80 μm, followed by nano-silica solution/air cycling to stimulate healing at both the crack plane and the fiber–matrix interface. The best performance was achieved under nano-silica solution/air cycles, where surface cracks were completely sealed, the healing degree of the fiber–matrix interface reached 86.39%, and the recovery of fiber-bridging stress reached 91.56%. Nano-silica can act as additional nucleation sites and as a source of reactive silica for the formation of new geopolymerization products, thereby enhancing the self-healing process [21].

A similar approach was characteristic of research provided by Wu et al. [91]. They investigated metakaolin-based geopolymer mortars activated with sodium silicate and sodium hydroxide solutions, in which expanded perlite loaded with Bacillus pseudofirmus spores and basalt fibers were incorporated to enhance self-healing performance. The self-healing system consisted of bacteria immobilized in expanded perlite, which protected the microorganisms in the highly alkaline geopolymer environment and enabled their activation when water and oxygen entered through cracks. The healing mechanism was based on MICP, where bacterial metabolism produced CaCO₃ deposits that accumulated inside cracks, while basalt fibers acted as crack-bridging elements and adsorption sites for healing products. The main result was that the synergistic combination of bacteria and basalt fibers significantly improved both mechanical properties and crack repair, achieving 100% crack-area healing and enhanced compressive strength when the fiber and healing-agent contents were properly optimized [91].

The synergy effect was observed by Griño et al. [92]. They investigated an ambient-cured fly ash-based geopolymer mortar produced from low-calcium Class F fly ash and activated with a combination of sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium silicate solution (Na₂SiO₃). To enhance self-healing performance, the researchers incorporated polypropylene fibers (0–0.75%) and a bacterial co-culture consisting of Bacillus subtilis and Bacillus megaterium spores. The self-healing mechanism was based on MICP. The bacterial activity promoted the formation of carbonate ions and the subsequent precipitation of calcite (CaCO₃) and other carbonate minerals, which filled cracks and pores within the geopolymer matrix. Meanwhile, polypropylene fibers limited crack opening and acted as crack-bridging elements, creating more favorable conditions for bacterial healing products to seal the damage. The main result was that only the bacterial mixtures exhibited significant self-healing, with healing percentages ranging from 16.77% to 147.18%. A clear synergistic effect was observed between bacteria and polypropylene fibers: bacteria provided crack-filling mineralization, while fibers controlled crack width. The combination of bacterial co-culture and 0.75% polypropylene fibers produced the highest strength recovery, reaching nearly 200% strength regain, demonstrating a substantial enhancement of both healing efficiency and mechanical performance [92].

Also, the study made by Zhou et al. [89] demonstrates a synergistic effect and shows that mechanical properties can recover or even exceed their original values without complete crack closure, particularly in composites reinforced with polyethylene (PE) fibres [89]. This is due to enhanced fibre–matrix interaction, where healing products increase interfacial friction and bridging stress [89].

Alvee et al. [93] studied a fly ash-based geopolymer paste modified with nano-silica synthesized from rice husk ash and crystalline admixtures added at 3–5 wt.% of fly ash [93]. The crystalline admixtures, rich in calcium compounds, were introduced as self-healing agents, while nano-silica provided additional reactive silica and promoted the formation of geopolymer and C–S–H gels. The proposed self-healing mechanism was based on the formation of CaCO₃ crystals and additional C–S–H gel, which filled pores and microcracks within the geopolymer matrix. The best performance was achieved with 5% nano-silica and 5% crystalline admixture, which increased compressive strength by 33.6% and improved flexural strength and fracture toughness by more than 230% compared with the reference geopolymer. The authors concluded that nano-silica and crystalline admixtures act synergistically, enhancing geopolymerization and self-healing product formation, thereby significantly improving the mechanical performance of the geopolymer [93].

Some authors observed that self-healing phenomena are governed by a dual mechanism, rather than a single one. This mechanism combines ongoing geopolymerization with microbially induced calcium carbonate precipitation, which simultaneously strengthens the geopolymer network and seals cracks with biomineralized CaCO₃ [55].

Other authors point out even more complex processes [26,90]. For example, Ozel et al. [24] found that the autogenous self-healing of CDW-based engineered geopolymer composites was mainly governed by ongoing geopolymerization and carbonation reactions occurring inside the cracks. These processes produced new aluminosilicate phases, sodium carbonate (Na₂CO₃), and calcium carbonate (CaCO₃), which progressively filled and sealed the damaged regions [24].

In summary, hybrid self-healing approaches combine the advantages of different healing mechanisms, creating synergistic effects that enhance crack closure, mechanical recovery, and durability performance beyond what can be achieved by individual strategies alone. The available studies indicate that combinations of fibers, bacteria, nano-additives, and reactive healing agents represent one of the most promising directions for the development of next-generation geopolymer composites with improved and more reliable self-healing capability.

5. Characterization of Healing Processes

The basic method for monitoring crack closure is observation using an optical microscope. This method was applied, for example, by Frahat et al. [61]. The self-healing capability was evaluated by introducing controlled cracks into geopolymer specimens and monitoring crack closure after 1, 3, and 7 days through visual observation and photographic documentation. Additionally, the healing mechanism was further confirmed using SEM and EDS analyses, which revealed progressive deposition of CaCO₃ crystals within cracks and pores; partial crack filling was observed after 3 days, while substantial or nearly complete crack closure occurred after 7 days, particularly in mixtures containing Bacillus sphaericus [61].

The monitoring of crack closure was also used by Liu et al. [36]. They investigated the self-healing process for geopolymers manufactured by the 3D printing method. A significant reduction in interlayer gap width was observed during curing, demonstrating the autonomous self-healing capability of the geopolymer. The average gap width decreased from 280–370 μm after 1 day to 68–84 μm after 14 days, and finally to only 27–42 μm after 28 days, indicating progressive filling of the gaps by self-healing products [36].

SEM observations are useful for monitoring the process and products; for example, they revealed abundant calcite crystals and calcium-rich mineral deposits within the healed cracks and pore structure of the geopolymer mortar, confirming that bacterial biomineralization was responsible for crack filling and matrix densification [60].

Even better results can be achieved by using SEM/EDS. Thanks to these analyses, Yuan et al. [21] confirmed that the main healing products were amorphous C-(N)-A-S-H gels, which filled cracks, restored the fiber–matrix bond, and improved the ability of fibers to bridge and transfer stresses across damaged regions. They proved that nano-silica particles act as nucleation centers that attract dissolved alkali activators and release Si, Al, and Ca species, promoting the formation of additional healing products within cracks and along the fiber–matrix interface [21].

Danilyan et al. [67] also show the potential of using XRD research for self-healing investigations. XRD analysis played a key role in identifying the mineral phases formed by the bacterial activity within the geopolymer matrix, confirming the presence of aragonite (CaCO₃), natrite (Na₂CO₃), and brucite (Mg(OH)₂) as products of microbial-induced carbonation. The proposed self-healing mechanism relies on the germination of Lysinibacillus fusiformis JH2 endospores when water penetrates cracks, reactivating bacterial metabolism and promoting biomineralization. XRD results demonstrated that, unlike conventional cementitious systems dominated by CaCO₃ precipitation, the low-calcium geopolymer generated multiple carbonation products, including natrite and brucite, which contributed to pore filling and microstructural densification. Particularly, brucite (Mg(OH)₂) was considered beneficial because it refines the pore structure, enhances compressive strength, mitigates autogenous and drying shrinkage, and may further transform into M–S–H (magnesium silicate hydrate) phases that improve the long-term performance of the geopolymer matrix [67].

A helpful tool, especially if the self-healing is achieved by encapsulation, is FTIR. In the investigations made by Han et al. [49], FTIR spectroscopy was used to characterize the chemical composition of the microcapsules, pure HDI, HDI reaction products, and the self-healing products formed inside repaired cracks. The FTIR results confirmed the consumption of isocyanate groups and the formation of polyurethane and polyurea compounds, demonstrating that HDI reacted with both water and geopolymer hydroxyl groups to create chemically bonded healing products within the cracks [49].

Tanyildizi et al. [53] show XCT analysis as a useful tool for analysis of self-healing results. Using this method, they confirm that the healed metakaolin-based geopolymer mortar revealed that 98.51% of the total porosity was filled by biomineralization products, primarily CaCO₃ precipitated by Sporosarcina pasteurii. The analysis was performed on specimens containing microcracks with widths of approximately 100–200 μm, and showed that the bacterial precipitates effectively occupied pores and crack voids, resulting in nearly complete densification of the damaged matrix [53].

Also, other research confirms the usefulness of this tool. XCT was used by Mahmood et al. [69] as a non-destructive tool to visualize the internal distribution of zeolite particles containing immobilized bacteria within the geopolymer matrix, providing evidence that the bacterial carriers were dispersed homogeneously throughout the material. The VCT images demonstrated that this uniform distribution increases the likelihood that viable bacteria are located near newly formed cracks, thereby improving the effectiveness and reliability of the self-healing process and supporting the observed reductions in porosity and water absorption [69].

This technology was also useful for the investigation made by Rahman et al. [85]. XCT analyses revealed substantial microstructural changes after self-healing. Following the flow tests, porosity decreased from approximately 15% to 4.2%, while permeability was reduced from 2.83 mD to 0.4 mD. These results confirm that the healing process effectively sealed cracks and flow channels, leading to a denser and significantly less permeable geopolymer matrix [85].

Alternatively, in case of a lack of possibilities to provide XCT analysis, some authors use Mercury Intrusion Porosimetry (MIP). This test does not give as complex information as XCT; however, it can be a source of some important data. For example, MIP was employed by Chen et al. [75] to investigate the evolution of pore structure in the geopolymer matrices before and after exposure to dry–wet cycles. The technique enabled the authors to quantify total porosity, pore-size distribution, and the relative proportions of micropores and macropores, which are directly related to hydraulic conductivity and durability. The MIP results demonstrated that self-healing induced by MgO reduced the proportion of harmful macropores through the formation of hydrated magnesium carbonate products, confirming that pore refinement was a key mechanism responsible for the improved durability and reduced permeability of the geopolymer cutoff wall material [75].

An optional tool in self-healing investigation can be low-field nuclear magnetic resonance (NMR), which can be employed to evaluate the pore size distribution of geopolymer mortars before damage, immediately after pre-damage, and after three days of self-healing. In the case of research made by Han et al. [49], the NMR results showed that microcapsules refined the pore structure, reduced the proportion of harmful large pores, and promoted pore recovery after cracking, with higher microcapsule contents generally leading to more pronounced pore healing effects.

Ultrasonic Pulse Velocity (UPV) measurements are used as a non-destructive technique to assess internal densification, crack healing efficiency, and the overall integrity of the geopolymer matrix after the healing process, for example, bacterial treatment [55]. The highest UPV values indicate substantial crack filling, reduced internal porosity, and improved microstructural continuity compared with untreated cracked samples [55]. In turn, Griño et al. [92] used UPV to evaluate internal damage and healing by measuring the travel velocity of ultrasonic waves through the geopolymer specimens before cracking, after pre-cracking, and after a 14-day healing period. A reduction in UPV indicated crack formation and internal defects, whereas an increase after healing reflected crack closure and restoration of material continuity [92].

In turn, Guo et al. [94] confirmed that during the self-healing process, a continuous increase in UPV indicated that healing products progressively filled microcracks, reduced air voids, and improved the integrity of the material. A very strong correlation (R² ≈ 1) was found between UPV and tensile strength, demonstrating that ultrasonic measurements can be effectively used to predict the mechanical performance and self-healing efficiency of geopolymer composites [94].

UPV is particularly useful for self-healing studies because it is a non-destructive technique that enables continuous monitoring of the recovery process. Improvements in UPV values provide indirect evidence of successful self-healing through the filling of cracks and densification of the geopolymer matrix by healing products.

XCT was used by Zhao et al. [22] to characterize the internal pore structure of the geopolymer, distinguish between open-cell and closed-cell porosity, and evaluate whether molten salts penetrated and damaged the cenosphere-based closed pores during long-term immersion testing. The XCT results showed that most closed-cell cenospheres remained intact, salt penetration was largely restricted to small open pores, and the pore structure remained stable after prolonged exposure to both nitrate and chloride molten salts, confirming the durability of the proposed self-healing insulation concept [22].

The characterization of self-healing processes requires the application of different analytical techniques depending on the healing strategy and the nature of the healing products involved. While microscopy and spectroscopy are commonly used to investigate autogenous geopolymerization and gel formation, techniques such as FTIR, XRD, µCT, and non-destructive testing methods are particularly valuable for evaluating capsule-based, bacterial, and hybrid systems.

Since self-healing processes operate across multiple length scales, from atomic-scale bond reconstruction to macroscopic crack closure, no single characterization technique can provide a complete understanding of healing phenomena [8,26,95]. The relationship between the scale of observation, the dominant healing mechanism, and the corresponding experimental methods is summarized in

Table 7. Self-healing mechanisms across microstructural scales in geopolymers.

Scale	Process	Dominant Mechanism	Direct Link to Self-Healing	Experimental Evidence
Nano (≤100 nm)	Bond reorganization	Re-polymerization of aluminosilicate network (N-A-S-H restructuring)	Reformation of broken bonds after cracking → restores cohesion at the atomic level	FTIR, NMR
Sub-micron (100 nm–1 µm)	Gel nucleation and growth	Formation of N-A-S-H/C-A-S-H clusters	Initial precipitation sites for healing products → start of crack filling	SEM, TEM
Micro (1–100 µm)	Crack filling	Gel precipitation inside crack	Physical sealing of cracks → reduction in permeability and transport pathways	SEM, EDS
Meso (100 µm–1 mm)	Crack bridging/partial closure	Limited diffusion-driven gel formation	Partial sealing of larger cracks → depends on ion mobility and connectivity	XCT, permeability tests
Macro (>1 mm)	Surface deposition (limited)	Insufficient transport	Healing often ineffective → only superficial closure	Visual, mechanical tests

, which presents self-healing mechanisms across microstructural scales in geopolymer materials.

The presented classification demonstrates that autogenic self-healing in geopolymers is inherently a multiscale phenomenon, involving interconnected processes from nanoscale bond reorganization to macroscopic crack sealing. Consequently, a comprehensive assessment of healing efficiency requires the integration of characterization techniques operating at different spatial scales, as each scale provides unique information on the underlying healing mechanisms and their contribution to material recovery.

6. Performance Metrics for Self-Healing Evaluation

6.1. Mechanical Recovery

Mechanical recovery is one of the most important indicators of self-healing performance, reflecting the ability of geopolymer materials to restore their load-bearing capacity, stiffness, and toughness after damage.

Gan et al. [23] demonstrate that geopolymers are characterized by significant recovery of mechanical performance after healing, including restoration of strain-hardening behavior and tensile ductility. Even specimens preloaded to 3% tensile strain retained a ductility exceeding 3.5% after healing, confirming that self-healing extends beyond crack closure to mechanical functionality recovery [23].

Also, the studies conducted by Ozel et al. [24] confirm that self-healing led to substantial recovery and improvement of mechanical performance, particularly in flexural and splitting tensile strength. The most effective mixture showed a 59.6% increase in flexural strength after healing, while all healed specimens exhibited higher splitting tensile strengths than their unhealed counterparts. These improvements demonstrate that self-healing not only seals cracks visually but also restores the structural load-bearing capacity of the material [24].

The confirmation of these results can be found in the research provided by Ahmed et al. [63]. The incorporation of 4% self-healing microorganisms increased the 28-day compressive strength, tensile strength, flexural strength, and ductility index by approximately 27%, 40.5%, 81%, and 61.6%, respectively [63]. These significant improvements were attributed to bacterial biomineralization and CaCO₃ deposition, which refined the internal structure and strengthened the bond between the geopolymer matrix and aggregates.

Autogenous self-healing significantly increased the toughness of the composites by restoring crack-bridging mechanisms and strengthening the fiber–matrix interface. Crack sealing and the formation of additional reaction products produced a denser microstructure capable of absorbing more energy before failure. In the best-performing mixtures, toughness after healing was up to nearly five times higher than that of the corresponding unhealed specimens [24].

However, not all investigations show a full recovery of mechanical properties. The studies made by Yahya et al. [65] with adding as a healing agent bacteria Shewanella oneidensis suspension, point out a slight decreasing of mechanical properties. The bio-modified geopolymer exhibited a reduction in compressive strength from 20.44 MPa to 18.43 MPa (approximately 9.9%), which the authors attributed to the partial replacement of the alkaline activator with the bacterial suspension, resulting in slower geopolymerization kinetics [65].

Mechanical recovery is frequently considered one of the key indicators of self-healing efficiency; however, increases in strength or stiffness do not always directly reflect the actual crack-healing process. In fiber-reinforced geopolymer composites, particularly those containing PVA or PE fibers, apparent strengthening after healing may result not only from crack filling but also from enhanced fiber–matrix interfacial bonding and increased frictional resistance developed during subsequent loading. Consequently, mechanical property recovery should be interpreted with caution and complemented by direct observations of crack closure and microstructural analyses, as mechanical enhancement alone may overestimate the true self-healing efficiency of the material.

It should be noted that improvements in mechanical properties may also arise from the reinforcing effect of fibers themselves, which can alter crack propagation paths, increase energy absorption, and promote more ductile failure modes. Therefore, enhanced strength or toughness should not always be interpreted as direct evidence of self-healing, particularly in fiber-reinforced geopolymer composites where crack-bridging mechanisms may contribute significantly to the observed performance recovery [96].

6.2. Durability-Related Indicators

In addition to mechanical performance, self-healing efficiency must be evaluated through durability-related parameters that quantify the material’s resistance to fluid ingress, aggressive agents, and long-term environmental degradation.

Ozen and Stephan [77] investigated the self-healing results performer capillary water absorption tests on loaded and unloaded specimens by exposing only the cracked surface to water and monitoring mass gain over time to determine the water absorption coefficient and assess healing efficiency. The results showed that healing generally reduced water ingress only partially, indicating incomplete crack sealing, although mixtures with higher perlite contents exhibited improved recovery of water resistance, especially under water-healing conditions [77].

Also, Han et al. [20] noted that the sequential activation of hybrid microcapsules significantly improved impermeability recovery, increasing the impermeability repair rate by up to 22.7% through crack sealing and harmful pore refinement [20]. Permeability was also the main topic of the study made by Zheng et al. [34]. The investigated geopolymers exhibited very low permeability values, comparable to or lower than those of ordinary Portland cement. After damage, permeability increased significantly, but the geopolymer specimens were able to substantially reduce permeability during the healing period, demonstrating effective recovery of their barrier performance [34].

The aggressive chloride environment can also be supportive for self-healing mechanisms based on encapsulated components. This is because the self-healing system operates through a dual-trigger mechanism, where crack-induced stress causes microcapsule rupture and chloride ions promote shell dissolution, resulting in a more reliable and efficient healing process than single-trigger systems [84].

Han et al. [84] investigated the durability of geopolymer composites with self-healing components. The study developed a dual-triggered microcapsule-based self-healing system for geopolymers, where capsules release a healing agent in response to both mechanical cracking and chloride exposure under aggressive salt wet–dry cycling conditions. The results showed that the system significantly improved durability, reducing harmful pore content by 52.7%, enhancing impermeability by 8.32%, and limiting compressive strength loss, while maintaining structural integrity even after 150 cycles [84]. These improvements were attributed to the formation of polymeric healing products (mainly polyurethane) that effectively sealed cracks, strengthened the matrix interface, and inhibited the transport of aggressive ions.

In turn, Ahmed et al. [63] revealed that the self-healing system improved durability by reducing water absorption from 3% to 2% and decreasing freeze–thaw weight loss from 4.5% to 2.5%. Furthermore, the relative dynamic modulus of elasticity increased from 45% to 50%, indicating enhanced resistance to environmental deterioration and freeze–thaw damage [63].

Freeze–thaw performance was evaluated by subjecting geopolymer specimens to 56 cycles of freezing at −20 °C and thawing at +20 °C, followed by controlled cracking and subsequent healing treatment with or without bacterial solutions. The effectiveness of self-healing was then assessed by comparing properties before and after healing, including ultrasonic pulse velocity, splitting tensile strength, mass loss, and capillary water absorption, which reflect internal damage and recovery of the material. The results showed that bacterial self-healing significantly mitigated freeze–thaw damage, leading to higher strength, reduced permeability, and almost complete crack closure compared to non-healed samples. These results are quite important, taking into consideration the works that show a lack of self-healing mechanisms in freeze- thaw conditions for geopolymers compared to concrete [97].

In summary, durability-related indicators provide a more comprehensive assessment of self-healing performance than visual crack closure alone, as they directly reflect the recovery of the material’s protective and transport-barrier functions. The reviewed studies demonstrate that successful self-healing can significantly reduce permeability, water ingress, and the penetration of aggressive agents, thereby enhancing the long-term durability and service life of geopolymer composites.

6.3. Crack Healing Efficiency

Crack healing efficiency provides a direct measure of the self-healing process by assessing the extent of crack closure and the effectiveness of damage repair within the geopolymer matrix.

The study provided by Ozel et al. [24] investigated geopolymers produced primarily from construction and demolition waste, including recycled brick, roof tile, concrete, glass, recycled concrete aggregate, and hybrid PE/nylon fibers. The self-healing capability was mainly driven by ongoing geopolymerization and carbonation reactions, which generated aluminosilicate phases together with Na₂CO₃ and CaCO₃ inside the cracks, enabling complete closure of cracks up to 460 μm wide without capsules, bacteria, or additional healing agents [24].

The indicator of crack closure was also used by Han et al. [20]. They noticed that after 7 days of healing in water, specimens containing hybrid microcapsules achieved almost complete repair of cracks up to approximately 0.22 mm wide, whereas conventional microcapsules effectively healed cracks up to about 0.20 mm. When healing was performed in a 3.5% NaCl solution, the hybrid microcapsules exhibited substantially enhanced performance, enabling nearly complete closure of cracks as wide as 0.31 mm and increasing the crack-healing rate by 10.2% compared with water curing [20].

The self-healing performance was also evaluated by Ozen et al. [48]. They monitored the closure of preformed cracks with widths of up to approximately 200 μm, together with compressive strength recovery and water sorptivity after 28 days of healing. The best results were obtained under 100% relative humidity and 60 °C, where geopolymer samples containing 9 wt.% self-healing capsules reached a crack-sealing efficiency of 91.6 ± 18.5%, particularly for cracks smaller than about 50 μm [48].

Microscopic observations showed that cracks with widths of up to approximately 200 μm were completely sealed after the bacterial healing process, confirming the high crack-healing capacity of S. pasteurii in geopolymer mortars. Furthermore, XCT analysis revealed that 98.51% of the total porosity was filled by biomineralization products, resulting in reduced water absorption and improved impermeability; among the investigated approaches, autonomous healing (bacterial injection after cracking) was found to be more effective than autogenous healing (bacteria incorporated during mixing) [53].

The reviewed studies reveal that crack closure alone can overestimate the effectiveness of self-healing systems, as surface sealing does not always correspond to complete healing throughout the crack depth. Consequently, relying exclusively on optical observations may lead to misleading conclusions regarding the true healing efficiency and long-term durability of geopolymer composites.

6.4. Other Indicators

In addition to the commonly used indicators such as mechanical recovery, durability improvement, and crack closure, several supplementary parameters have been proposed to provide a more comprehensive evaluation of self-healing performance. These indicators often capture indirect effects of healing processes, including changes in pore structure, setting behavior, efflorescence formation, and microstructural evolution, which may not be fully reflected by conventional healing metrics.

Techo et al. [66] reported that the incorporation of bacterial spores can reduce both the initial and final setting times of geopolymer pastes. This finding suggests that calcifying bacteria may serve not only as self-healing agents but also as environmentally friendly admixtures for tailoring the fresh-state properties of geopolymer materials [66].

Another interesting note made by this team is that bacterial incorporation reduced the content of gaylussite (Na₂Ca(CO₃)₂·5H₂O), a mineral commonly associated with geopolymer efflorescence, providing the first microstructural evidence that MICP bacteria can mitigate efflorescence formation in geopolymers [66]. Future research should investigate the mechanisms by which bacterial activity limits alkali migration and evaluate the long-term effectiveness of this approach under practical environmental exposure conditions [66].

It is worth noting that some studies show that different healing indicators—such as crack closure, stiffness recovery, and tensile strength—are not directly correlated [89]. Mechanical recovery is mainly governed by interfacial healing, while crack filling primarily affects permeability and visual closure rather than load-bearing performance [89].

7. Factors Influencing Self-Healing Efficiency

7.1. Type of Geopolymer

It is worth noticing that recent studies highlight that the effectiveness of self-healing mechanisms strongly depends on the chemical composition of the geopolymer matrix (especially the availability of calcium) and the optimal dosage of healing agents, as not all techniques developed for OPC systems perform equally well in geopolymers [26,90].

The type of feedstock used for geopolymer production also has significance on self-healing processes. For example, Ozel et al. [24] show that mixtures with 100% construction and demolition waste (CDW)-based matrices exhibited superior self-healing performance compared to slag-containing systems. They explain this by more extensive ongoing geopolymerization and maintained higher ionic mobility within the matrix. The enhanced transport of healing-related ions and reaction products facilitated their migration into cracks, promoting the formation and accumulation of healing products and resulting in more effective crack sealing [24].

In turn, Yaswanth et al. [98] claim that fly ash–GGBS systems provide one of the best combinations of mechanical performance and self-healing potential. These blended matrices exhibit high compressive strength, enhanced ductility, and a dense microstructure that supports crack control and the development of healing products within microcracks [98].

The efficiency of self-healing in geopolymer materials is strongly controlled by the chemical composition of the binder and the characteristics of the alkaline activation system. Parameters such as the Si/Al ratio, alkalinity, calcium content, activator type, and liquid-to-solid ratio directly influence reaction kinetics, microstructural evolution, and the formation of healing products, thereby determining the overall healing potential of the material [7,99,100].

Table 8. Influence of geopolymer chemistry on self-healing.

Parameter	Range/Type	Effect on Self-Healing Mechanism	Microstructural Outcome
Si/Al ratio	~1.0–3.5	Controls degree of polymerization and gel reactivity	Low Si/Al (~1.0–1.5): loosely cross-linked N-A-S-H gel, higher nanoporosity, more reactive → facilitates dissolution and re-precipitation in cracks Optimal (~1.7–2.0): highly polymerized 3D network with lowest nanoporosity and highest packing density → efficient crack sealing High (>2.5): overly polymerized network, reduced mobility of species → limited healing potential
Alkali cation	Na+ vs. K+	Affects dissolution kinetics and ion mobility	Na+: smaller ionic radius → higher mobility, faster precursor dissolution → formation of more homogeneous but relatively less compact gel K+: larger ion → lower mobility but promotes denser and more rigid gel network with finer microstructure (smaller gel particles and pores) Structural effect: difference in pore size distribution and gel stiffness
Calcium content	low vs. high Ca	Enables coexistence of N-A-S-H and C-A-S-H phases	Low Ca: purely amorphous N-A-S-H gel → higher porosity, slower densification Moderate/high Ca: formation of hybrid gel (C-A-S-H + N-A-S-H) → reduced pore size, higher density matrix, improved crack filling Ca addition leads to refined pore structure and stronger interparticle bonding
Liquid/Solid ratio (L/S)	~0.25–0.8	Controls transport of ions and reaction extent	Low L/S: dense matrix with low total porosity and smaller gel pores → limited ion mobility but high mechanical integrity High L/S: increased pore volume and connectivity → improved ion transport but weaker final structure Trade-off: transport vs. compactness → directly affects healing efficiency
Activator type	NaOH vs. Na2SiO3 (silicate-rich)	Controls dissolution and gel formation kinetics	NaOH: promotes dissolution but slower gel formation → heterogeneous microstructure with localized gel clusters Na2SiO3 (silicate): enhances gel nucleation → more uniform gel distribution and denser matrix Silicate-rich systems show higher fraction of amorphous binding gel and fewer unreacted particles

summarizes the main chemical factors affecting self-healing mechanisms and their corresponding microstructural effects in geopolymer systems.

Another important element is the aggregate used. Finer precursors and finer aggregates enhance self-healing mechanisms in geopolymer composites. Their higher reactivity and denser microstructure improve the formation of healing products and reduce crack propagation, resulting in better crack closure efficiency [98].

The important issue in the self-healing process is also the type of activator used in geopolymerization. Ozel et al. [24] show that the combination of NaOH and Na₂SiO₃ proved to be the most effective activator system for autogenous self-healing, as NaOH promoted the dissolution of aluminosilicate phases while Na₂SiO₃ supplied soluble silicates that sustained further geopolymerization within the cracks. Mixtures activated with NaOH + Na₂SiO₃ exhibited significantly faster and more complete crack closure than those activated with NaOH + Ca(OH)₂, indicating that the presence of sodium silicate was essential for efficient healing [24]. The self-healing mechanism was governed by a combination of ongoing geopolymerization and carbonation reactions, leading to the formation of aluminosilicate gels together with Na₂CO₃ and CaCO₃ deposits that progressively filled and sealed the cracks [24].

Another important factor can be the self-healing additive used and the way in which it is applied. Koseoglu et al. [55] show that liquid bacterial formulations generally outperformed gel-based formulations in promoting self-healing in geopolymer mortars. Liquid treatments resulted in greater CaCO₃ precipitation, higher carbonate crystallinity, and superior recovery of mechanical properties, indicating more efficient crack sealing and matrix densification. The authors attribute this enhanced performance to improved nutrient diffusion, higher bacterial mobility, and more effective transport of calcium ions within the crack network, which collectively promoted more extensive biomineralization [55].

Beyond the intrinsic characteristics of the geopolymer matrix, various additives can significantly influence self-healing performance by modifying crack development, enhancing healing-product formation, or providing stimuli-responsive functionalities that support damage repair. The influence of these additives depends not only on the additive itself, but also on the parameters of the self-healing agent. For example, Chen et al. [75] researched the dependence of the efficiency of the self-healing process on the size of the introduced particles. They confirmed the influence of MgO particle size on the durability of the self-healing effect. Smaller MgO particles (15 μm) hydrated more rapidly and therefore provided faster initial healing, whereas larger particles (50 μm) reacted more slowly but remained active for a much longer period. As a result, the larger MgO particles generated a more persistent self-healing effect under prolonged dry–wet cycling and provided superior long-term durability, making them more suitable for long-service-life engineering applications [75].

Also, other additives can bring benefits for the self-healing process. One of them is eggshell powder, which acted as a sustainable calcium-rich nutrient source that enhanced bacterial viability and activity, promoting greater CaCO₃ precipitation, improved crack healing efficiency, and prolonged self-healing potential in the geopolymer matrix [61].

Table 9. The most popular additives and their role in geopolymer self-healing.

Additive	Self-Healing Process Type	Mechanism of Action	Effect on Healing	Microstructural Impact
Calcium sources (slag, Ca(OH)2)	Autogenic enhancement	Provides Ca2+ → C-A-S-H formation	Improves crack sealing efficiency	Reduced porosity, denser gel
Fibers	Indirect (crack control)	Limits crack width	Enables more effective autogenic healing	Smaller, distributed cracks
Silica fume	Autogenic enhancement	Additional Si → gel formation	Increased gel precipitation	More homogeneous microstructure
Nano-particles (SiO2, Al2O3)	Autogenic enhancement (nucleation)	Provide nucleation sites	Accelerates healing reactions	Finer gel distribution
Superabsorbent polymers (SAP)	Semi-autonomous (water supply)	Internal curing, water release	Improves long-term healing	Increased pore connectivity but sustained reaction
Bacteria (experimental)	Autonomous (MICP-like)	Biomineralization	Limited but potential healing	Localized mineral deposition

summarizes the main additives used in self-healing geopolymer systems and their mechanisms of action.

The reviewed studies indicate that the self-healing efficiency of geopolymers is strongly governed by the chemical composition of the matrix and the selection of additives. The most effective systems are typically those that combine favorable geopolymer chemistry with additives capable of controlling crack width, promoting the formation of healing products, or maintaining healing activity over prolonged periods. However, the effectiveness of a given additive remains highly dependent on its compatibility with the specific geopolymer formulation and exposure conditions.

7.2. Environmental Conditions

Also important are environmental conditions. Most authors agree that the self-healing process was found to be more effective in the presence of water, which supports bacterial activity and facilitates mineral precipitation [65]. In this case, water promotes the formation of CaCO₃ within pores and voids, leading to lower porosity and reduced water absorption in the bio-modified geopolymer concrete, for example, with bacterial usage [65]. Similar results were obtained by Ozen and Stephan [77]. According to them, autonomous healing is most effective under 100% relative humidity at 60 °C, where the released sodium silicate promoted further geopolymerization and resulted in the highest compressive strength recovery.

The type of healing products and mechanisms are strongly controlled by environmental conditions. Dry–wet cycles promote calcium carbonate crystallization through carbonation, whereas NaCl environments favor the formation of amorphous N-A-S-H gels via secondary geopolymerization reactions [89]. Other conclusions were made by Gan et al. [23]. Their research shows that the efficiency of self-healing is strongly dependent on environmental conditions, with water exposure providing the most favorable environment, followed by air and then wet–dry cycles. Water enhances healing by enabling ion transport, sustained geopolymerization, and swelling effects, whereas wet–dry cycles can degrade healing products and induce secondary cracking [23].

In turn, Ozen et al. [48] revealed that the primary self-healing products were amorphous aluminosilicate gels, formed as a result of additional geopolymerization induced by the released sodium silicate [48]. Smaller amounts of carbonation products were also detected, together with (Na/K)-Al-Si-H compounds and traces of (Na/C)-Al-Si-H phases, which contributed to crack filling and sealing [48].

Some other research claims that the most effective is the application of wet-dry cycles that promote the crystallization process [89]. This was partly confirmed by Yual et al. [21]. Among the investigated healing conditions, the nano-silica solution/air cycles provided the most effective healing performance, resulting in complete surface crack closure and the highest recovery of both fiber–matrix interfacial properties and fiber-bridging capacity. However, water/air cycles produced the weakest healing response, while simple air exposure yielded intermediate performance; furthermore, incorporating nano-silica directly into the matrix improved the initial mechanical properties but did not significantly enhance the later-stage healing efficiency because much of the nano-silica was consumed during early geopolymerization [21].

Temperature was found to significantly accelerate the self-healing process. Complete sealing of the cracked specimens occurred after approximately 410–450 min at 27 °C, whereas at 60 °C the healing time was reduced to about 310 min. These results suggest that self-healing elastomer-modified geopolymers may perform particularly well in deep subsurface environments, where elevated temperatures can promote faster crack closure and restoration of sealing properties [85].

In some cases, the self-healing process can be reinforced by the existence of particular compounds in the environment. For example, Han et al. [20] found that for designed capsules by this team, chloride ions were supportive for the self-healing process. They interact with PbSO₄ particles embedded in the microcapsule shell, creating pores and weakening the shell structure, which promotes the release of additional HDI healing agent. As a result, chlorides activate microcapsules that were not ruptured by mechanical loading alone, producing more healing products within cracks and leading to greater crack closure, strength recovery, and impermeability restoration [20].

A similar investigation was performed by Wu et al. [28]. They proved that the combination of sodium sulphate (Na₂SO₄) solution and wet–dry cycles effectively stimulated self-healing in geopolymers by promoting the formation of healing products and reducing crack widths [28]. Although sulphate-rich environments are generally considered aggressive for conventional cementitious materials, this study demonstrated that they can enhance the self-healing performance of geopolymer composites through activation of unreacted particles and continued geopolymerization. Similarly, Zhou et al. [89] show that under dry–wet cycles, higher slag content (lower fly ash) enhances healing, whereas in NaCl environments, higher fly ash content leads to superior self-healing efficiency.

Zhao et al. [27] evaluated the self-healing performance of geopolymers under air, water, NaOH solutions (7%, 14%, and 21%), and saturated Ca(OH)₂ solution. Water improved healing compared to air, while 7% NaOH and especially Ca(OH)₂ significantly enhanced crack closure, tensile property recovery, and damage reduction by promoting further geopolymerization. In contrast, high NaOH concentrations (14–21%) initially stimulated healing but later caused degradation due to depolymerization of the C(N)-A-S-H gel and the formation of new microcracks [27].

The different environments were also the subject of a study made by Zheng et al. [34]. They compared three fly ash-based geopolymer systems activated with liquid sodium hydroxide, liquid sodium silicate, and solid sodium silicate, using OPC as a reference material. Among the investigated formulations, the liquid sodium silicate -activated geopolymer exhibited the highest degree of geopolymerization, the lowest porosity, and the lowest permeability. In contrast to OPC, which showed no effective self-healing, all geopolymer systems demonstrated a clear ability to restore permeability after damage [34].

Rahman et al. [85] also performed healing tests using hydrocarbon fluids and found that crack sealing occurred even faster than in brine. The time required to stop fluid flow was nearly 50% shorter than that observed during water-based experiments. This behavior is especially relevant for oil and gas applications, as it indicates that contact with hydrocarbons may enhance the efficiency of the self-healing process under field conditions [85].

Ekinci et al. [70] demonstrated that effective self-healing could only be achieved when bacteria, calcium, and urea were simultaneously present in the curing environment, enabling the metabolic processes required for CaCO₃ precipitation. Specimens containing bacteria but cured without an additional calcium source and urea showed markedly lower healing efficiency, confirming that all three components are essential for successful microbial self-healing of geopolymer mortars [70].

Environmental conditions play a crucial role in governing autogenic self-healing processes in geopolymers by influencing reaction kinetics, ion transport, and the formation of healing products. However, the parameters presented in

Table 10. Representative environmental factors affecting self-healing in geopolymers; the reported ranges and effects may vary depending on geopolymer composition, crack characteristics, and exposure conditions.

Condition	Range/Type	Self-Healing Process Affected	Effect on Healing Mechanism	Microstructural Consequence
Temperature	20–60 °C	Dissolution + precipitation	Accelerates reaction kinetics	Faster gel formation, reduced induction time
Humidity	low–high	Ion transport	Required for mobility of species	Enables crack filling
Water exposure	immersion vs. curing	Diffusion-controlled healing	Enhances ion migration into crack	Increased gel deposition
CO2 presence	low–high	Carbonation-assisted healing	Promotes carbonate formation (if Ca present)	Surface crystallization
pH (alkaline)	12–14	All autogenic processes	Maintains dissolution and gel stability	Stability of N-A-S-H structure

should be regarded as representative rather than universal, since their effects and optimal ranges may vary considerably depending on factors such as geopolymer composition, precursor type, activator chemistry, crack characteristics, and exposure conditions.

In summary, environmental conditions represent one of the most critical factors governing autogenic self-healing in geopolymers, as they directly influence dissolution processes, ion transport, and the precipitation of healing products. However, the reviewed studies indicate that no universal environmental regime exists, and the effectiveness of healing depends on complex interactions between exposure conditions and material-specific parameters such as precursor chemistry, activator composition, and crack characteristics.

7.3. Other Factors

The level of pre-damage significantly affects self-healing efficiency, with higher pre-tensile strain leading to reduced healing performance. Specimens were preloaded to 1%, 2%, and 3% strain, and healing efficiency decreased with increasing strain due to the formation of wider cracks (>10 μm) that are more difficult to fully seal [23].

In turn, Yaswanth et al. [98] claim that crack width control below approximately 60 μm promotes effective self-healing [98]. Narrow microcracks facilitate the precipitation of healing products and continued geopolymer reactions within the crack, enabling autonomous crack closure and recovery of durability-related properties.

Overall, crack width is considered one of the most important parameters governing the effectiveness of self-healing in geopolymer materials. Previous studies summarized by the authors indicate that complete crack closure is typically achievable for crack widths up to 100–200 μm, whereas successful healing of wider cracks in the range of 400–600 μm has only been reported occasionally under favorable conditions [18,51].

Elmesalami and Celik [101] reported that the self-healing performance of geopolymer composites was strongly influenced by the curing age before cracking. The most effective self-healing was observed in specimens cured for approximately 7 days, as they still contained a sufficient amount of unreacted precursor particles that could participate in further geopolymerization after damage. At longer curing ages, the quantity of unreacted material decreased, reducing the potential for additional geopolymerization and consequently lowering the self-healing efficiency. As a result, older specimens generally exhibited a lower recovery of mechanical properties and crack-closing capability compared to younger composites [101].

Another factor that influences the efficiency of self-healing is the way of delivery of the self-healing agent into the material. Tanyildizi [102] investigated metakaolin-based geopolymer mortars incorporating the bacterium Sporosarcina pasteurii as a self-healing agent. Self-healing was achieved through MICP, in which bacterial activity led to the formation of CaCO₃ that filled and sealed cracks within the geopolymer matrix. Three self-healing methods were compared: injection, spray, and submersion. Cracks with widths of approximately 100–200 μm were created in the specimens and subsequently treated using these methods for 60 days. The results showed that the injection method was the most effective, providing the highest crack-closing efficiency and the greatest recovery of flexural strength [102].

The way of delivery of the bacteria was also investigated by Polat and Uysal [103]. The metakaolin-based geopolymer was modified with Sporosarcina pasteurii bacteria introduced either by adsorption onto metakaolin followed by drying (M-series) or by collecting bacteria from a solid agar medium and adding them in water suspension (S-series). Both methods enabled bacterial self-healing through MICP, where bacterial activity generated CaCO₃ crystals that filled cracks and pores; however, the metakaolin-adsorption method provided better bacterial survival, a denser microstructure, and more effective healing. As a result, the M-series geopolymer mortars achieved superior crack closure (frequently reaching 100% healing for cracks up to 200 μm) and a greater reduction in water absorption and porosity than the S-series, demonstrating that direct incorporation of bacterial spores into the geopolymer precursor was the more efficient self-healing strategy [103].

In summary, self-healing efficiency is influenced by a number of additional factors beyond material composition and environmental conditions, with crack width, pre-damage level, curing age, and healing-agent delivery method playing particularly important roles. The reviewed studies consistently indicate that effective crack-width control and appropriate activation of healing agents are essential for achieving reliable crack closure and long-term performance recovery in geopolymer composites.

8. Applications of Self-Healing Geopolymer Composites

Owing to their ability to autonomously repair cracks and restore functional performance, self-healing geopolymers have attracted increasing attention across a wide range of engineering applications. Their combination of enhanced durability, low environmental impact, and reduced maintenance requirements makes them particularly attractive for infrastructure, energy-related systems, additive manufacturing, and structures operating under aggressive environmental conditions, as illustrated in Figure 5.

Lekshmi et al. [50] identified several promising applications for biomineralized self-healing geopolymer materials, including pipelines and water infrastructure, tunnels, marine structures, basement walls, precast elements, and paving blocks exposed to heavy traffic conditions. In these applications, the autonomous healing of cracks through bacterial CaCO₃ precipitation can significantly reduce maintenance and repair requirements, thereby enhancing durability and extending the service life of structures [50,52].

Self-healing in geopolymer concrete significantly enhances structural durability by sealing cracks and limiting the ingress of harmful substances such as water, chlorides, and chemicals that accelerate degradation [26]. As a result, it reduces maintenance and repair requirements over the service life, contributing to more sustainable and cost-effective infrastructure systems [26,104].

In turn, Ahmed et al. [63] proposed the self-healing material for durable, lightweight, and thermally insulated structural elements in sustainable and cost-effective construction projects, including modern buildings and other applications where reduced weight and improved energy efficiency are desirable.

Xue et al. [25] analyzed the application of self-healing geopolymer on cutoff wall backfill. The material was developed as a low-permeability barrier for groundwater and soil contamination containment systems. The incorporation of self-healing microcapsules is particularly important for cutoff walls because it enables autonomous sealing of cracks induced by repeated drying–wetting cycles, thereby maintaining long-term hydraulic performance and durability [25].

Self-healing is considered one of the most important advantages of geopolymers for geothermal well applications, where microcracks can lead to fluid migration, loss of zonal isolation, cement degradation, and ultimately well failure. Geopolymers can mitigate these issues through their ability to autonomously seal cracks, thereby improving long-term wellbore integrity and operational reliability [11].

According to Madirisha and Ikotun [105], self-healing geopolymer cements can be used for hydrogen storage wells in depleted gas reservoirs, CO₂ storage and sealing operations, and permanent well plugging and abandonment applications. Their excellent durability under high-temperature and high-pressure conditions also makes them suitable for demanding subsurface energy and carbon storage infrastructures. Taking into consideration current economic trends, the application in hydrogen storage seems to be a very promising direction. Self-healing geopolymers are highly promising for underground hydrogen storage because they can autonomously repair microcracks, helping to maintain long-term wellbore integrity and hydrogen containment. The self-healing capability enhances zonal isolation, limits crack propagation, and reduces the risk of hydrogen leakage during long-term storage operations. As a result, geopolymer-based wellbore materials can improve the safety, durability, and reliability of depleted oil and gas reservoirs used for hydrogen storage [105].

The self-healing geopolymer cement is particularly attractive for oil and gas wells, where maintaining long-term zonal isolation and preventing fluid migration through microannuli and cracks are critical requirements. The ability to autonomously seal leakage pathways could significantly improve well integrity and operational safety. Similar benefits may be expected in geothermal wells, CO₂ geological storage (CCS), and underground energy storage systems, where cement barriers are continuously exposed to elevated temperatures, aggressive fluids, and long service periods. In such environments, self-healing geopolymers could enhance durability and reduce the risk of leakage from subsurface infrastructure [85].

Another potential area of application for self-healing geopolymers is 3D printing. Liu et al. [36] highlighted that the self-healing capability of 3D-printed geopolymer composites enhances interlayer integrity and long-term durability, demonstrating their strong potential for sustainable additive manufacturing applications in construction [36]. Also, Ziada et al. [57] confirm that the development of self-healing has strong potential for durable and sustainable 3D-printed construction materials, where autonomous crack healing can extend service life and reduce maintenance requirements.

For 3D-printed fiber-reinforced geopolymers, weak interlayer bonding is a critical challenge that reduces mechanical performance. The use of mineral- and polymer-based interlayer healing agents significantly enhances interlayer tensile and shear strength, reduces shrinkage, and promotes self-healing through the formation of geopolymer gel and mineral phases, thereby improving the durability of printed geopolymer structures [106].

The self-healing strategy is particularly relevant for marine structures, tidal-zone infrastructure, hydraulic engineering structures, and other systems exposed to sulphate-rich environments and repetitive wet–dry cycles. In such applications, the ability of geopolymers to autonomously seal cracks and recover stiffness could improve durability, reduce maintenance requirements, and extend service life. Moreover, some research findings suggest that sulphate exposure, which is traditionally regarded as a deterioration agent, can be exploited as a beneficial trigger for self-healing in geopolymer-based materials [28].

Zhao et al. [22] also show the potential of self-healing systems for application in high temperature. The developed material is intended for high-temperature molten-salt thermal energy storage systems, particularly in concentrating solar power plants where reliable insulation and containment of molten salts are critical. Its self-healing/self-sealing capability could improve the safety, durability, and economic viability of large-scale thermal storage tanks by reducing the risk of salt leakage and enabling the use of lower-cost structural materials [22].

Also, Mokhtari et al. [38] worked on high-temperature applications. They proposed the use of these self-healing and self-glazing geopolymer-derived ceramics in high-temperature construction and infrastructure applications, where autonomous crack sealing could improve durability and reduce maintenance requirements. Potential applications also include protective coatings, refractory and advanced ceramic components, aerospace structures, corrosion-resistant systems, and waterproofing layers, benefiting from both crack-healing capability and the formation of a protective glazed surface [38].

Research for self-healing materials for high temperatures was also conducted by Keane et al. [80]. The developed self-healing geopolymer was designed as a containment material for molten salts used in high-temperature thermal energy storage systems. Owing to its thermal stability, low porosity, and resistance to molten chlorides, the material has the potential to provide a low-cost solution for concentrating solar power plants and other large-scale energy storage technologies [80].

The other application that has a potential environmental dimension is demonstrated by Xue et al. [47,107]. They show that self-healing geopolymer backfills are particularly suitable for vertical cutoff walls used in contaminated groundwater containment, where maintaining low hydraulic conductivity is critical under long-term sulfate exposure [47]. By releasing sodium silicate from microcapsules upon cracking, the material can autonomously restore sealing performance and extend service life in aggressive hydro-chemical environments typical of industrial and mining sites [47]. Moreover, another study demonstrates that combining microencapsulated healing agents with geopolymer matrices offers a promising route toward durable, low-permeability self-healing barrier materials for long-term environmental protection applications [107].

Another research direction is presented by Su et al. [2]. In this case, geopolymer-based components incorporated in the capsule core act as a reactive mineral precursor, promoting additional C–S–H-type gel formation and long-term microstructural densification after capsule rupture. This suggests a promising future research direction in designing hybrid geopolymer–cement self-healing systems, where geopolymer chemistry is intentionally engineered to enhance healing kinetics and durability in aggressive environments [2].

A potential area of application in extreme environments is also seismic metamaterials. Self-healing materials in seismic metamaterials can be applied to maintain long-term performance by autonomously repairing microcracks generated during cyclic seismic loading, thereby preventing the degradation of wave attenuation capabilities. Such chemically adaptive systems are particularly promising for infrastructure applications (e.g., foundations or barriers), where they can extend service life and reduce maintenance by preserving the functional integrity of metamaterial structures over time [108]. A similar approach is represented by Mostofizadeh and Tee [109]. They noticed that self-healing technologies in geopolymer concrete have significant potential for seismic applications, as they can autonomously repair microcracks formed during earthquake loading, thereby extending the service life of structures and reducing the accumulation of damage over repeated seismic events. From an engineering perspective, self-healing geopolymer systems may reduce post-earthquake maintenance and repair costs while enhancing the long-term reliability and resilience of critical infrastructure such as bridges, hospitals, emergency shelters, and high-rise buildings located in seismic regions [109].

9. Challenges and Limitations

Despite the significant progress achieved in recent years, the practical implementation of self-healing geopolymers still faces several scientific, technical, and economic challenges. Understanding the current limitations of healing mechanisms, material design, long-term durability, and large-scale application is essential for assessing the technological readiness of self-healing geopolymer systems and identifying priorities for future research.

Geopolymer self-healing behavior is strongly influenced by the type of precursor, activator dosage, and curing conditions used during production. This material variability makes it difficult to establish universal design guidelines and predict healing performance consistently [8]. The healing process also strongly depends on external factors such as moisture, temperature, and nutrient availability. Insufficient humidity or unfavorable environmental conditions may substantially reduce the self-healing efficiency [18,51]. The healing process requires careful control of environmental conditions (e.g., moisture, temperature), as bacterial activity is sensitive to external factors [52]. All these factors influence the challenges in process standardization and regulation.

One of the major challenges in self-healing geopolymer research is the absence of standardized testing methods and evaluation criteria, making direct comparison between different studies difficult. The performance of self-healing systems is often assessed using different crack widths, curing conditions, healing periods, and recovery indicators, which leads to inconsistent results across the literature. Therefore, the development of unified testing protocols and performance benchmarks is essential for the reliable assessment and future commercialization of self-healing geopolymer materials [8].

Moreover, most research relies on short-term laboratory tests with varying conditions, making it difficult to compare results or assess real performance [7]. Additionally, most available studies focus on short-term laboratory performance, while evidence regarding long-term durability and healing efficiency is still scarce. Consequently, the long-term reliability of self-healing geopolymers under real service conditions remains insufficiently understood [18,51].

In the case of the usage of microcapsulation as a self-healing mechanism, some limitations are connected with long-term usage. Hue et al. [47] noticed that the long-term stability of microcapsule shells in highly alkaline sulfate environments remains uncertain, as gradual degradation could lead to premature leakage instead of crack-triggered release. A similar problem is observed in the case of bacteria-based systems. In this situation, the long-term durability and reliability of the healing mechanism under varying real field conditions remain uncertain and require further investigation [52]. Overall, for the self-healing studies of geopolymers, there is a significant lack of long-term field validation, meaning that the durability and reliability of self-healing systems under realistic service conditions remain largely uncertain [7].

Although microbial self-healing shows promising results, the long-term behavior of microorganisms in highly alkaline geopolymer matrices is still not fully understood. Moreover, the long-term effectiveness of bacterial self-healing may be limited due to the high cost of bacterial solutions, which can hinder large-scale practical implementation [52]. Further studies are needed to identify the most suitable microbial species and optimize their survival and healing efficiency [8]. In parallel, one of the major challenges of self-healing geopolymer systems is the relatively long time required for crack recovery. Biological healing mechanisms, especially those based on microbial precipitation of calcium carbonate, often require several weeks (usually 2–3 weeks) before significant crack closure is achieved [18,51].

The limitation of this technology application is also limited effectiveness for small crack widths. Current self-healing technologies are generally effective only for microcracks and narrow cracks. As crack width increases, the ability of healing products to completely bridge and seal the damaged zone decreases significantly [18]. The healing efficiency generally decreases as crack width increases, making most systems effective primarily for small cracks and microcracks [51].

There are also some challenges connected with scaling up this technology. Some of them are caused by not sufficient number of investigations in this area. For example, Xue et al. [47] pointed out that the durability and healing efficiency under multi-contaminant conditions are not fully understood, since complex chemical environments may accelerate matrix degradation and reduce the effectiveness of the released healing agents [47]. Although excellent results have been obtained in laboratory experiments, the implementation of self-healing geopolymer systems in full-scale structures remains challenging. Most self-healing geopolymer technologies have been validated only at laboratory scale, and their implementation in large-scale structural applications remains challenging [51]. Issues related to uniform healing performance, durability, and practical application methods still require further investigation [18].

One of the significant limitations is a performance gap between laboratory and field conditions, where microbial self-healing systems typically achieve 80–90% crack closure and 90–95% strength recovery in the laboratory, but only 50–60% crack closure and 70–85% strength recovery in real structures [68]. In geopolymer materials, this challenge is even more pronounced because their highly alkaline pore solution (pH > 13) reduces bacterial viability, with survival rates decreasing from about 90% after 180 days under controlled conditions to only 50–70% in field environments [68]. The reduced healing efficiency observed in practice is primarily attributed to nutrient leaching, temperature fluctuations, wet–dry cycles, UV exposure, and the complex geometry of real cracks, all of which limit bacterial activity and CaCO₃ precipitation within the geopolymer matrix [68].

Limitations of self-healing technologies are also connected with costs. Some of the most promising self-healing approaches rely on advanced materials such as nanomaterials, microcapsules, or encapsulated biological agents. These technologies can considerably increase production costs, which currently limits their widespread adoption in the construction industry [18,51].

Despite the remarkable progress in self-healing geopolymer research, significant challenges remain regarding crack-size limitations, long-term durability, standardization of testing methods, and large-scale implementation. Addressing these limitations will be essential for transforming self-healing geopolymers from promising laboratory concepts into reliable and commercially viable construction materials capable of delivering long-term structural and environmental benefits.

10. Future Research Directions

Based on the challenges identified in the current literature, future research on self-healing geopolymer composites is expected to focus on the integration of advanced healing strategies, smart monitoring technologies, and sustainability-oriented design concepts. As illustrated in Figure 6, the development of next-generation self-healing geopolymers will likely rely on the synergy between hybrid healing systems, novel functional materials, digital monitoring tools, multiscale modelling approaches, and comprehensive life-cycle assessment, ultimately facilitating their transition from laboratory-scale studies to large-scale industrial applications.

Future research should focus on combining multiple self-healing mechanisms (e.g., microbial agents, fibers, and microcapsules) to achieve synergistic and more reliable crack repair [90]. Combining multiple self-healing techniques (e.g., bacteria and fibers) is expected to significantly improve crack closure efficiency and overall performance [26]. Nodehi et al. [15] identify hybrid systems combining geopolymers, polymer additives, and self-healing bacteria as one of the most promising future research directions in advanced construction materials. Such self-healing polymer-modified geopolymer concretes could simultaneously provide low permeability, high chemical and fire resistance, and autonomous crack-healing capability [15].

Future research should also focus on developing polymers with enhanced resistance to highly alkaline environments (pH 12–14) to prevent premature degradation and ensure long-term healing performance [7]. More research is needed to identify and optimize bacterial strains that can survive and remain active in highly alkaline geopolymer environments [51].

Another promising direction is bacteria-induced self-healing, particularly through Microbial Carbonate Precipitation (MCP), is considered one of the most promising approaches for future geopolymer composites. This technology has the potential to provide autonomous crack sealing while improving the durability and service life of geopolymer structures [8]. Future studies should investigate not only bacteria but also fungi and other microorganisms that may better withstand the harsh alkaline environment of geopolymer materials. Such organisms could offer enhanced healing efficiency and longer-term viability within the geopolymer matrix [8]. It should also take into consideration developing advanced healing agents, such as enzyme-based systems, specifically tailored to geopolymer chemistry. Healing strategies must be better adapted to the chemical composition of geopolymer matrices, particularly considering the availability of calcium for effective crack sealing [26].

Further studies should investigate the stability of healing performance under realistic conditions such as wet–dry cycles, freeze–thaw, and chemical attack [90]. The long-term stability of healing agents remains a key concern for autonomous self-healing systems. Future research should focus on improving the durability of microcapsules and hollow fibers while ensuring reliable release of healing agents when cracking occurs [8].

In the area of advanced materials, Wang et al. [88] demonstrate a highly promising concept of nanomaterial-assisted self-healing [88]. Also, Al-Fakih et al. [51] point out that future studies should investigate nanomaterials such as nanosilica, carbon nanotubes, graphene, and nanofibers as potential enhancers of geopolymer self-healing performance. Other direction can the development of smart fibers capable of controlling crack propagation and promoting autonomous healing represents a promising research direction [51].

Future research should investigate whether TiO₂ nanoparticles can actively enhance the self-healing efficiency of geopolymers by accelerating geopolymerization reactions, acting as nucleation sites for healing products, and promoting a denser microstructure that facilitates crack closure [110]. Hegyi et al. [110] highlight that TiO₂-modified self-healing geopolymers remain largely unexplored, and suggest that the most promising research direction is the development of multifunctional materials combining self-healing, self-cleaning, antimicrobial performance, and improved durability within a single geopolymer system.

Another pointed direction of the studies is advanced microcapsule and hollow-fiber systems should be developed to improve healing-agent storage, release efficiency, and compatibility with geopolymer matrices. Optimized systems could lead to higher crack-healing efficiency and better recovery of mechanical properties after damage [8]. The integration of stimuli-responsive materials and self-healing coatings is expected to enhance autonomous crack closure and structural resilience [90].

Advanced monitoring techniques should be developed to accurately track crack closure and quantify self-healing efficiency in real time [51]. The development of multi-stimuli-responsive systems capable of reacting to combined triggers such as moisture, pH, temperature, and ionic concentration is essential to improve reliability under real service conditions. There is a critical need to establish standardized testing methods for evaluating crack closure, mechanical recovery, and durability to enable consistent comparison across studies [7].

Advanced multiscale modeling approaches, including machine learning and digital twin technologies, should be developed to predict self-healing performance and structural durability more accurately [7]. Work provided by Ulloa et al. [111] focused on modeling the compressive strength of metakaolin-based self-healing geopolymer concrete incorporating Bacillus bacteria, using a database of 147 experimental mixtures containing fly ash, silica fume, metakaolin, and bacterial additives. Several machine-learning techniques were evaluated and compared, including Group Method of Data Handling Neural Networks (GMDH-NN), Generalized Support Vector Regression (GSVR), k-Nearest Neighbors (KNN), Decision Trees, Random Forest, and XGBoost. The results demonstrated that all models provided high predictive accuracy; however, the GMDH-NN model achieved the best performance, with an accuracy of approximately 99%, an RMSE of 0.4 MPa, and an R² value of 1.00. Sensitivity analysis further revealed that metakaolin (30%), silica fume (29%), and fly ash (27%) were the main factors controlling compressive strength, whereas Bacillus bacteria had a smaller direct influence (14%) and primarily contributed to self-healing and long-term durability [111].

Future developments should aim to reduce the environmental footprint of healing agents while maintaining high performance through improved material design and life cycle assessment [90]. Ozel et al. [24] demonstrate that self-healing capability can be achieved in materials produced almost entirely from construction and demolition waste, without the need for costly biological approaches or encapsulation-based technologies.

Tamoor and Zhang [112] stress that future research should integrate self-healing geopolymer technologies with life-cycle assessment (LCA) to quantify whether the enhanced durability, crack-sealing capacity, and reduced maintenance requirements can offset the environmental burdens associated with geopolymer activators and self-healing agents over the entire service life of a structure. They highlight that sustainability evaluations should move beyond cradle-to-gate impacts and consider long-term performance, as extended service life and fewer repair interventions may significantly improve the overall environmental and economic viability of self-healing geopolymer systems [112].

The previous analyses made by Garces et al. [113,114] did not show clear results. They evaluated the environmental performance of self-healing geopolymer concrete containing polyurethane-urea microcapsules filled with alkali activators (NaOH and sodium silicate), using a cradle-to-gate LCA and comparing it with conventional geopolymer concrete and OPC concrete. The results showed that although geopolymer concrete had a 37% lower global warming potential (CO₂ footprint) than OPC concrete, the addition of self-healing microcapsules substantially increased environmental impacts in most other categories because microcapsule production and alkali activators were highly resource- and energy-intensive [113]. The authors concluded that the environmental justification for self-healing geopolymer concrete cannot yet be fully confirmed, because the additional environmental burden can only be offset if self-healing significantly extends service life and reduces maintenance; therefore, further studies including long-term durability and healing performance are required [113,114].

Although self-healing systems may increase the initial environmental footprint because of the added healing agents, their ability to autonomously repair cracks and reduce maintenance requirements can result in lower life-cycle environmental impacts compared with conventional repair approaches [115].

Future studies should include long-term and field-scale validation under realistic environmental conditions, such as freeze–thaw cycles, chloride exposure, and carbonation, to assess durability over the service life. Research must also address scalability challenges, particularly ensuring uniform dispersion of polymers and preventing agglomeration or damage to microcapsules during large-scale production [7]. It should focus on scalable and cost-effective manufacturing technologies that enable large-scale production of self-healing geopolymer materials [51]. For this purpose is necessary to establishing standardized testing methods and developing cost-effective solutions will be essential for broader industrial adoption [90].

Future work should focus on applying self-healing geopolymer concrete in real-world infrastructure, such as bridges and pavements, to reduce maintenance and extend service life [26]. There is a strong need to validate self-healing geopolymer systems through full-scale field applications rather than only laboratory experiments [90].

11. Conclusions

Self-healing geopolymer composites represent a rapidly developing class of sustainable construction materials capable of autonomously repairing damage and extending the service life of infrastructure. Based on the reviewed literature, the following conclusions can be drawn:

• Self-healing in geopolymers is governed by both intrinsic and extrinsic mechanisms. Intrinsic healing is primarily associated with continued geopolymerization, dissolution–precipitation reactions, and the formation of N-A-S-H and C-(N)-A-S-H gels, whereas extrinsic approaches involve microcapsules, bacteria, mineral admixtures, polymers, and vascular systems.
• Crack-width control is one of the key factors determining healing success. Fiber reinforcement plays a crucial role by promoting multiple microcracking and maintaining crack widths within a healable range.
• Bacteria-based and capsule-based systems currently show the highest autonomous healing potential. These systems can achieve nearly complete crack closure, substantial permeability reduction, and significant recovery of mechanical performance through CaCO₃ precipitation or the release of reactive healing agents.
• The evaluation of self-healing performance requires a multiscale and multidisciplinary approach. Mechanical recovery, crack closure, permeability reduction, and durability-related indicators should be combined with microstructural characterization techniques such as SEM/EDS, XRD, FTIR, and XCT to obtain a comprehensive assessment of healing processes.
• Self-healing geopolymers offer significant potential for a wide range of applications, including transportation infrastructure, marine and hydraulic structures, environmental containment barriers, underground energy storage systems, geothermal wells, hydrogen and CO₂ storage facilities, and additive-manufactured construction elements.
• Despite substantial progress, several barriers still limit large-scale implementation. The lack of standardized testing procedures, limited long-term field validation, uncertainty regarding multiple healing cycles, reduced effectiveness for large cracks, and economic challenges associated with advanced healing agents remain key issues requiring further investigation.
• Future research should prioritize hybrid self-healing systems, smart monitoring technologies, field-scale validation, and life-cycle assessment. The integration of advanced healing concepts with sustainable geopolymer formulations may enable the development of next-generation low-carbon, durable, and resilient construction materials.

Overall, self-healing geopolymer composites demonstrate considerable potential to support circular-economy principles by reducing maintenance interventions, extending service life, and improving the durability of low-carbon infrastructure. However, achieving reliable and commercially viable applications will require further advances in material design, performance assessment, and long-term validation under realistic service conditions.