The Structural Performance of Fiber-Reinforced Geopolymers: A Review

Abstract

Geopolymers (GPs), as promising alternatives to ordinary Portland cement (OPC)-based concrete, have gained interest in the last 20 years due to their enhanced mechanical properties, durability, and lower environmental impact. Synthesized from industrial by-products such as slag and fly ash, geopolymers offer a sustainable solution to waste management, resource utilization, and carbon dioxide reduction. However, similarly to OPC, geopolymers exhibit brittle behavior, and this characteristic defines a limit for structural applications. To tackle this issue, researchers have focused on the characterization, development, and implementation of fiber-reinforced geopolymers (FRGs), which incorporate various fibers to enhance toughness, ductility, and crack resistance, allowing their use in a wide range of structural applications. Following a general overview of sustainability considerations, this review critically analyzes the structural performance and capability of geopolymers in structural repair applications. Geopolymers demonstrate notable potential in new construction and repair applications. However, challenges such as complex mix designs, the availability of alkaline activators, curing temperatures, fiber matrix compatibility issues, and limited standards are restricting its large-scale adoption. The analysis and consolidation of an extensive dataset would support the viability of geopolymer as a durable and sustainable alternative to what is currently used in the construction industry, especially when fiber reinforcement is effectively integrated.

1. Introduction

The cement industry is a major contributor to global carbon dioxide (CO₂) emissions, accounting for approximately 7.4% of total anthropogenic emissions [1], due to the calcination and processing of limestone (clinker) at higher temperatures [2]. For these reasons, OPC-based concrete will dominate in the future due to economics, established infrastructure, market confidence, and the increase in production as per statistical trends—estimated from 4.3 billion metric tons (2015) to 6.1 billion metric tons in 2050 [3]. To mitigate environmental impacts, the adoption of supplementary cementitious materials (SCMs), such as ground granulated blast furnace slag (GGBFS), fly ash (FA), rice husk ash (RHA), and metakaolin (MK), is strongly suggested for reducing OPC clinker content. Moreover, the variations in manufacturing processes and raw materials, the use of alternative fuels with low CO₂ emissions, and the installation of waste heat recovery systems can also contribute to a lower carbon footprint due to cement production [4,5].

In this regard, geopolymers can be the best sustainable alternative to OPC-based matrices, and they are developed from waste materials (aluminosilicate source material) such as fly ash, silica fume, slag, quarry rock dust, and metakaolin. A comparative oxide analysis of various source materials utilized in geopolymers is presented in Figure 1. Alkaline activators, such as sodium silicate and hydroxide and potassium silicate and hydroxide, are used for the extraction and activation of alumina and silica in geo-polymerization processes [6], which develop binding gels, therefore exhibiting exceptional properties in fresh and hardened stages [7], durability in aggressive environments [8], low environmental impact [9], and the structural retrofitting of existing structures [10,11]. The properties of geopolymer are highly dependent on the source material, alkaline activator, molarity of activator, and curing conditions [12,13,14,15,16].

The potential of numerous source materials as sustainable alternatives to ordinary Portland cement concrete—highlighting their environmental benefits, replacement possibilities (%), cost reductions, and improved performance characteristics—is presented in Table 1.

The field applications of geopolymers cover various fields in terms of material applications, such as in the precast industry, pavements, bridges, sustainable housing (due to their excellent thermal and acoustic insulation), and as structural repair materials for retrofitting reinforced and masonry structures. However, geopolymers show poor tensile behavior, similarly to normal concrete, with other issues reported by the authors of [11] and [25]. To enhance ductility and tensile behavior, numerous studies have focused on the characterization, development, and implementation of fiber-reinforced geopolymers for a broader range of applications [26,27]. The compatibility of fibers with materials needs to be thoroughly assessed, and it must be ensured for reinforcement application because the performance of a fiber-reinforced geopolymer relies more on fiber materials rather than binders. Moreover, for structural applications, to achieve enhanced stress transmission and post-cracking behavior, effective fiber–matrix interactions and an optimum aspect ratio need to be realized [28,29,30]. Similarly, for assessing the fiber’s performance, its surface area in composites and its geometry—such as length, cross-section of fibers, and cross-sectional area of fibers in a matrix—are the basic aspects that require consideration [31,32,33,34].

Table 1. Source materials of geopolymers and environmental sustainability potential in comparison to normal concrete.

Precursor	Source	Possible Cement Replacement %	Optimum Substitute % in the Literature	Lower Global Warming Potential	Per m3 Cost Reduction	Remarks	References
Fly ash (FA)	Coal power plant	7% due to limited alkaline activator	80–100% GPC (Geopolymer concret) 10–30% OPC	Environmental benefits 49.5%; human health 35%	10.87–17.77%	Its alumina content and cement-like appearance make it a viable alternative material for geopolymers	[35,36,37,38,39]
GGBFS	Steel and iron industry	6.5% replacement as per current slag production	35–50% GPC 30–60% OPC	Energy consumption 49%; CO2 emission 39%	10–15%	Enhanced strength and durability properties with more amorphous XRD patterns than the crystalline phases of fly ash	[40,41]
Metakaolin	Kaolinite rock	1.4% replacement as per current production	5–40% GPC 15–20% OPC	Energy demand 2.3%; CO2 emission 1.4%	4.2%	Enhanced cohesive matrix and C-S-H gels; improved durability, thermal resistance, and mechanical properties	[35,42,43,44,45,46,47]
Silica fume (SF)	Silicon metal Ferrosilicon alloy	2% replacement as per current production	5–15% GPC 0–25% OPC	47.61% for geopolymer with silica fume and without Na2SiO3	10.87–17.77%	SF improved mechanical durability due to fine spherical particles and amorphous silica causing binder—aggregate interfacial bonding and the pore size refinement of composites	[38,48,49,50,51]

Table 1. Source materials of geopolymers and environmental sustainability potential in comparison to normal concrete.

Precursor	Source	Possible Cement Replacement %	Optimum Substitute % in the Literature	Lower Global Warming Potential	Per m3 Cost Reduction	Remarks	References
Fly ash (FA)	Coal power plant	7% due to limited alkaline activator	80–100% GPC (Geopolymer concret) 10–30% OPC	Environmental benefits 49.5%; human health 35%	10.87–17.77%	Its alumina content and cement-like appearance make it a viable alternative material for geopolymers	[35,36,37,38,39]
GGBFS	Steel and iron industry	6.5% replacement as per current slag production	35–50% GPC 30–60% OPC	Energy consumption 49%; CO2 emission 39%	10–15%	Enhanced strength and durability properties with more amorphous XRD patterns than the crystalline phases of fly ash	[40,41]
Metakaolin	Kaolinite rock	1.4% replacement as per current production	5–40% GPC 15–20% OPC	Energy demand 2.3%; CO2 emission 1.4%	4.2%	Enhanced cohesive matrix and C-S-H gels; improved durability, thermal resistance, and mechanical properties	[35,42,43,44,45,46,47]
Silica fume (SF)	Silicon metal Ferrosilicon alloy	2% replacement as per current production	5–15% GPC 0–25% OPC	47.61% for geopolymer with silica fume and without Na2SiO3	10.87–17.77%	SF improved mechanical durability due to fine spherical particles and amorphous silica causing binder—aggregate interfacial bonding and the pore size refinement of composites	[38,48,49,50,51]

The compressive strength of geopolymers is an important characteristic that influences other mechanical properties such as tensile strength and the modulus of elasticity, and the available code for normal concrete can be adopted for the assessment of geopolymer composites [28]. An optimal addition of fibers and ultra-fine silica material—for instance, nano silica—to geopolymer concrete could enhance its residual compressive strength and energy absorption capacity [52]. The deformation of geopolymer composites depends on the breaking or rupture mechanism, while toughness depends on fiber–matrix interactions [33,34,52,53,54]. These also depend on the fiber’s nature; for example, basalt fibers possess more mechanical strength than polyvinyl alcohol fibers, and due to basalt fibers’ brittle nature, the failure behavior of composites cannot be improved [55]. The energy absorption ability of fiber-reinforced geopolymers depends on crack bridging, fiber-to-binder interaction, friction, surface area contacts, and aspect ratios.

Fiber-reinforced geopolymers as a structural and repair material for retrofitting exhibit enhanced properties [17]. Steel fiber (0.5-1%) improves flexural and fatigue resistance [56] and enhances bond strength [57], and hooked steel fibers show better results compared to other steel fiber shapes [58]. Polyethylene terephthalate (PET) fibers in geopolymer composites—up to 1% wt. content—improve mechanical properties, with superior interfacial bonds observed [59]. Cotton fiber (up to 0.5 wt. %) enhances strength, the flexural modulus, and toughness [60], while flake-shaped PET particles contribute to cracking reduction and an increase in ductile modes [61].

The comparative analysis reported in

Table 2. Comparative analysis of mechanical and durability properties of OPC and GPC.

Properties	OPC	GPC	Remarks	References
Compressive strength	Lower	Higher	Higher compressive strength initially; however, this depends on the aluminosilicate source, activator type, reactivity of precursors curing time, duration, and temperature.	[62]
Tensile strength	Lower	Higher	Higher tensile strength than OPC-based concrete, depending on aluminosilicate sources, activator type, and curing condition.	[63]
Setting time	Slower	Faster	Setting time depends on the activator type and aluminosilicate source used in GPC; GPC setting time is faster than OPC.	[64]
Water absorption	Lower	Moderate	Water absorption in GPC is moderate due to internal matrix pores readily affecting its water absorption.	[64]
Durability	Lower	Higher	Silica and alumina in aluminosilicate source forms CASH (Calcium Alumina Silicate Hydrate) gel, and the dense packing of GPC matrix shows superior durability.	[65]
Chloride attack	Lower	Higher	Depends on the alumina silicate source, activator type, and curing condition. However, the chloride ingress rate in fly-ash-slag-based GPC and slag-based GPC is low; age factor also higher than OPC; and higher protection against corrosion than OPC.	[66]
Acid attack	Lower	Higher	Silica and alumina in pozzolanic sources; reactivity with alkaline activators of GPC shows better acid resistance.	[67]
Fire resistance	Lower	Higher	GPC maintains its microstructure at elevated temperatures up to 800 °C and performs better than OPC. GPC shows better insulating properties depending on various curing conditions, precursors, additives, and activators.	[68]
CO2 emissions	Higher	Lower	During the life cycle, GPC exhibits lower CO2 emissions and global warming potential than OPC.	[69]
Freeze–thaw effect	More affected	Less affected	GPC shows excellent chemical and physical properties in freeze and thaw cycles compared to OPC.	[70]
Shrinkage	Lower	Moderate	Shrinkage in GPC depends on curing conditions.	[71]
Porosity	Lower	Moderate	Due to dense packing of GPC matrix, GPC shows superior internal geopolymer structures than OPC.	[71]

shows that geopolymer concrete exhibits higher strength, acid and fire resistance, and durability, and imposes lower environmental impacts than normal concrete, although moderate shrinkage, water absorption, and variations in porosity are also observed. Many reviews are available in the literature regarding the history, background, characterization, and factors affecting the mechanical properties of geopolymer composites [12,13,14,15,16,17,18,19,20,21]. However, the literature still lacks a detailed review of geopolymers as structural materials, and the performance of geopolymers in the rehabilitation of structures still requires research.

This review aims to advance the understanding of geopolymers in structural appli-cations and provides a comprehensive foundation for geopolymer adoption in engineering practices.

2. Structural Performance of Geopolymer

Geopolymer concrete exhibits similar behaviors to conventional concrete. Some researchers have claimed improved load-bearing capacities for geopolymer beams, although failure modes and crack patterns have been found, similarly to conventional concrete. The performance of geopolymers is influenced by many factors, such as variations in source materials, curing methods, testing conditions, and fiber adoption and reinforcement types, therefore highlighting the need for standardized protocols to ensure consistent comparisons and to widen applicability in structural engineering applications [72]. Metakaolin-based binders have more aluminosilicate content, resulting in a dense matrix; however, they have better bonding, resulting in enhanced tensile strength and ductility [73]. GGBS (ground granulated blast furnace slag)-based binders develop calcium–silicate–hydrate (C–S–H)-type gels in addition to geo-polymeric gels due to their higher calcium content, and this dual gel phenomenon results in improved strength, stiffness, and load-bearing capacities [74,75]. Fly-ash-based binders have lower reactivity due to low calcium levels, resulting in lower initial strength gains. However, once properly cured, they provide long-term strength and better durability [76].

2.1. Role of Fibers in Performance Enhancement

Geopolymers, similarly to normal concrete, are brittle, encouraging research into fiber reinforcement to improve ductility and tensile performance. Effective fiber reinforcement requires improved fiber–matrix bonding, stress transfer, bridging cracks, and post-cracking behavior. Each fiber type has advantages and disadvantages; therefore, the selection of fibers depends on requirements, applications, and environmental conditions. Synthetic fibers (PET, PVA (polyvinyl alcohol), and PP (polypropylene fiber) ) may degrade in alkaline environments and have low heat resistance. Glass fibers are alkaline-sensitive and brittle. Steel fibers may corrode, while stainless-steel fibers are expensive. Carbon fibers are brittle, prone to leaching, and expensive. Natural fibers, such as jute, sisal, hemp, and flax, offer economic and environmental advantages, although their performance is mainly dependent on pre-treatment methods for improving durability and bonding with the geopolymer matrix.

The performance of fiber-reinforced geopolymers is more dependent on the material properties of fibers than on binders. Similarly, fiber geometry, such as the length and cross-section of fibers, the fiber’s surface area in composites, and the cross-sectional area of fibers in a matrix, are key aspects in the assessment of fiber performance [31,32,33,34]. Enhanced mechanical properties are achieved with fiber contents below 2% by volume, as higher fiber contents increase viscosity [77,78]. Basalt fibers (0.025-0.15 vol%) can enhance flexural and compressive strength, toughness, and fracture energy due to microcracks and their superior stability in alkaline environments [79]. The incorporation of an ultra-low content (0.2 vol%) of polyethylene fibers reduces crack widths and enhances mechanical properties while offering lower costs and fewer environmental impacts than conventional concrete. Moreover, they perform competitively with polyvinyl alcohol (PVA) fiber-reinforced cementitious composites in terms of cracking responses and strength [80]. Flax-fiber-reinforced geopolymers improved toughness and flexural strength due to crack bridging and the failure resistance of frictional debonding in composite matrices [81].

Variations in the shear span-to-depth (a/d) ratio and beam depth affect the structural response of geopolymer beams. Visintin et al. [82] stated that the ultimate load-carrying capacity and shear strength decrease with an increase in the a/d ratio. Wu et al. [83] also reported an increase in shear strength with a decrease in the a/d ratio in slag-based geopolymer beams. Similar observations were also found by the authors of [84,85] for fiber-reinforced geopolymers (glass fiber and steel fiber), which confirms that a/d ratio changes are independent of the variations in fiber addition with or without shear reinforcements. However, the inclusion of fibers, such as steel fibers (SFs), basalt-fiber-reinforced (BFRP) bars, and polypropylene fibers (PFs), in geopolymers enhanced the shear capacity [86,87,88]. For enhancing the capacity of GPC beams, fibers such as steel, polypropylene, basalt, glass fibers, etc., are used. Reinforced geopolymer concrete in structural elements possesses all required mechanical properties, which are needed to replace the OPC in conventional concrete [89,90]. Nikmehr et al. [91] investigated the mechanical and structural performance of fly-ash-based and slag-based geopolymers with recycled aggregates and basalt fibers. The improved tensile strength, load-bearing capacity, and crack resistance were observed compared to a plain mix. Zuaiter et al. [92] studied fly-ash-based and slag-based geopolymer concrete with glass fibers. The results showed that hybrid combinations of long and short fibers (1:1 and 1:3) at 1 vol% content enhanced flexural and shear performances more effectively than using a single type of glass fiber, even with higher contents used. Ozbayrak et al. [93] studied fly-ash-based geopolymers and OPC beams with similar strength and tensile reinforcement. The results showed that reinforcement ratios and curing methods significantly affect GPC beam behavior. The experimental and numerical values of OPC and GPC samples differ by approximately 5–6% with respect to the yield and ultimate points. At yielding, GPC samples showed 7.4% higher stress than OPC, and 10.4% was observed upon failure. Li et al. [94] assessed and compared autoclaved aerated concrete (AAC) panels and light-weight fly-ash-based and slag-based geopolymer composite (LGC) panels under four-point bending, and the results show that LGC exhibits better structural performance, with an ultimate bending capacity that is 57–110% higher than that of the AAC panel. Wen et al. [95] assessed the fly-ash-based and slag-based geopolymer concrete reinforced with silica fume and steel fibers. The results showed that the increase in calcium, alkali, and steel fiber content enhanced mechanical properties. At peak stress, the stress of specimens without fibers decreases abruptly, as shown in Figure 2. With the increase in fiber content, specifically at 1.5 vol%, the decrease in stress becomes slower, confirming the ductility enhancement and prolonging the life of a structure.

Natural fibers, such as sisal, hemp, and jute fibers, can also be utilized as reinforcements in geopolymers due to their improved properties compared to synthetic and artificial fibers: environmentally sustainable, light in weight, and cost-effective. However, high water absorption capacities limit their application, but pretreatment via water run-off and alkali treatment can improve their bonding performance with geopolymers [96,97,98]. Sisal fibers treated in a 10 wt% NaHCO₃ solution in metakaolin-based geopolymers show better mechanical properties, enhancing strain capacity in comparison with non-treated fibers [99]. Benfratello et al. assessed the mechanical behavior of metakaolin-based geopolymers reinforced with sisal (0–2 wt%) fibers via numerical modeling. The result showed enhanced mechanical properties, and the coupled damaged plastic microplane model (ANSYS) adopted for numerical modeling produces satisfactory results, specifically in terms of strength and elastic stiffness [100]. Coconut fibers, when treated with sodium hydroxide, reduce tensile strength. However, when treated with sodium alginate and calcium chloride, they improve the fiber–matrix bond strength [101]. Moreover, hemp fiber surfaces that are treated in NaOH solutions experience a removal of impurities, making the fiber surface rough and improving bonding strength. Wet preserved hemp fibers improve bond properties and reduce treatment costs because of their 4% higher cellulose content [102]. The durability of sisal fibers can be improved using chemical treatment (NaOH and acetylation), also enhancing bonding in fiber–matrix interactions. However, beyond NaOH concentrations that are higher than 20%, surface damage can be inflicted on the fiber [103].

From the above discussion, it can be concluded that fiber-reinforced geopolymers have the potential to serve as a sustainable and high-performance alternative to conventional materials. For improved performances and wider applications in the construction industry, continued research on fiber selection, treatment, and interaction with geopolymer composites will be required.

From our literature review, the optimum natural fiber content recommended for improved properties is below 2%, as shown in Figure 3.

2.2. Seismic Resistance of Fiber-Reinforced Geopolymers

Huang et al. [108] analyzed the impact performance of fly-ash-based and slag-based GPC and OPC columns (alkaline sol/binder ratio of 0.6) reinforced with basalt-fiber-reinforced bars (BFRPs) under impact loading, with the addition of polymer coatings and steel bars. The results showed that the higher compressive strength and elastic modulus of the reinforcement materials resulted in the superior impact resistance of the columns; moreover, the geopolymer specimens showed comparable or even better impact resistance performance in failure modes and dynamic responses than normal concrete columns (Figure 4).

Akduman et al. [109] assessed the seismic performance of monothetic and demountable construction with demolition waste (CDW)-based geopolymer concrete column connections under various axial (10-30%) compression ratios. The demountable samples exhibited enhanced load capacities, curvature characteristics, and energy dissipation, but ductility was decreased at higher axial loads. Huang et al. [110] assessed the six columns with various reinforcement types as a substitute for steel bars in strengthening geopolymer columns under impact loads. Columns with steel basalt carbon fiber (SBCB) exhibited comparable mid-height deflections with respect to the column reinforced with steel bars under impact load velocities of 0.45-0.49 m/s. The incorporation of carbon fiber (CF) and hybrid basalt microfibers (BMFs) improved the rigidity of columns, resulting in a reduction in mid-point deflection (Figure 5). These findings confirm that SBCB reinforcement bars have the capacity to reduce the reliance on steel bars, as they have more sustainable and enhanced properties in reducing crack damage and improving the stiffness of the column.

Chen et al. [111] assessed the dynamic tensile properties of geopolymers reinforced with steel and polypropylene fibers. Fiber stretching was observed after cracking, which contributed to a reduction in crack expansion and improvements in energy dissipation; in contrast, in terms of split tensile behavior, steel fibers performed better than polypropylene fibers. The damage analysis and residual behavior are also essential for structural assessment, as they help evaluate the impact of repeated loading and the resulting vibrations on the structure. Xu et al. [112] assessed and compared the nonlinear behavior of fly-ash-based and slag-based geopolymer concrete under cyclic tension; they found that geopolymers showed enhanced properties with an increase in slag content, but a lower elastic modulus was observed in comparison to conventional concrete. The stress–strain curves for GPC and OPC under cyclic tension (Figure 6) exhibit comparable nonlinear features, such as residual deformation, stiffness degradation, and reloading strain, confirming the geopolymer’s suitability as a sustainable alternative to OPC. Sarnaya et al. assessed the beam column joint of steel-fiber-reinforced geopolymer concrete under negative cyclic loading. GPC exhibited enhanced ductile behavior, specifically at 0.75 vol%. Steel fibers increased the ductility factor (73%) and absorption energy. Moreover, the crack width was more prominent in OPC-based and GPC-based samples in comparison with steel-fiber-reinforced geopolymer samples, as shown in Figure 7 [113].

Li et al. [114] assessed a 1:2 scale, two-story, one-bay structural frame comprising recycled-fireclay-brick-based geopolymer concrete (RFBC-GPC) with GGBS and FA under seismic loading. The shaking table test results showed that the RFBC-GPC frame exhibits similar damage patterns with respect to the plastic hinges installed on the first floor’s beam ends, but enhanced stiffness and seismic performance were observed compared to conventional RC frames, confirming the potential of geopolymers in seismic-resilient construction. Moreover, the concrete damage plasticity model (CDP) adopted in Abaqus for the RFBC-GPC frame accurately predicted the experimental results within a marginal error. Buyuktapu et al. [115] studied the physical and mechanical performance of slag-based fiber-reinforced geopolymer mortar (Na₂ SiO₃/NaOH (10 M) = 2) with a hybrid combination of carbon, glass, and polypropylene fibers for masonry walls. The results showed superior bonding, shear, and axial strength, with enhanced failure behavior in comparison to cement mortar.

The punching shear response under impact loading is primarily governed by the peak impact load, whereas damage progression and flexural response are influenced by impact duration and impulse. Furthermore, dynamic punching shear resistance is dictated by internal structural integrity and the material strength of the slab, as presented in

Table 3. Structural performance of geopolymer.

Materials	Structural Member	Investigated Variables	Key Findings	References
FA, GGBS with steel wire mesh and poly vinyl alcohol (PVA) fibers	Slab	Blast loading, steel wire mesh, and fiber-reinforced geopolymer	Fiber-reinforced slabs improved blast resistance compared to steel-wire-mesh-reinforced composites. Moreover, one-part geopolymer concrete having PVA fibers on the top layer of the slab significantly improved energy dissipation capacity.	[116]
FA, GGBS, and steel wire mesh with poly vinyl alcohol (PVA) fibers	Slab	Static and dynamic loading; experimental and numerical modeling	ACI-318 and Eurocode 2 can predict the static slab -punching resistance of GPC slab, but not for dynamic punching shear capacity. The punching shear response is reliant on the peak impact load, while damage and flexural response are correlated on the duration of impact loading and impulse.	[117]
FA, GGBS, and basalt fiber reinforcement (BFRP)	Slab	Shear performance and FEM analysis; reinforcement ratio and comp strength	Diagonal tension or shear compression failure depends on the reinforcement ratio. Existing guidelines of RCC (reinforced cement concrete) can be adopted for predicting shear resistance; moreover, numerical models show compatibility with experimental results.	[118]
Textile-reinforced (TRGM) and polymer-modified (TRPM)	Slab	Static and numerical analysis; reinforcement ratio, layers, slab thickness	TRGM enhanced post-cracking stiffness and delayed progression and development of cracks, while flexural capacity increases with the number of layers; the numerical results found were in good agreement with experimental analysis.	[119]
Fly-ash-based and slag-based GPC	Beams	Compressive and flexural strength	Slag-based GP enhanced strength compared to normal concrete beams. Moreover, the cracking behavior, crack width, flexural cracks spacing, and number of cracks were observed to be similar to normal concrete beams.	[87,88]
FA- and slag-based GPC beams	Beams	Flexural behavior	Load carrying capacity and deflection in peak and service load stages were higher than conventional concrete. However, ductility factor was similar to conventional concrete.	[84]
CFRP and GFRP FA slag-based GP beams	Beams	Flexural behavior along with deflection	GFRP-reinforced GPC exhibits 2.5 times higher ultimate strength than CFRP sample. Both samples demonstrate a stiffness reduction of 40% in comparison with conventional RC beams.	[120]
FA-based GPC with GFRP bars	T-Beams	Reinforcement ratio and compressive strength	Increasing the bottom reinforcement ratio with GFRP bars improved load carrying capacity and reduction in deflection.	[121]
FA-based GPC with waste lime and glass powder as sand	Deep beams	Compressive and tensile strength with varying ratios	Incorporation of waste glass and lime caused degradation in compressive and tensile strength, and numerical analysis models based on loading capacity matched with experimental results.	[122]
FA with steel and polypropylene fibers	Beam	Flexural behavior with varying fibers	Strength capacity increased up to 30% in hybrid combinations of polypropylene and steel fibers.	[123]
FA-based GPC	Beams	Shear span ratio	The GPC properties of shear friction occurred within the range of shear friction properties of normal OPC-based concrete.	[82]
FAGLLSS-based GPC	Short and slender column	Loading position, ultimate, deflection, failure mode	The short column capacity decreased when the eccentricity of the load was decreased, displaying similar failure mode by crushing in the compressed face near the mid-height of column.	[124]
GGBS-based GPC	Column	Molarity activator stress–strain	Increasing the molarity of NaOH enhanced the load carrying capacity of columns, with deflection of columns decreased.	[125]
FA, GGBS, and recycled fireclay brick as aggregate	Column	Cyclic and seismic analysis, stirrup spacing	On cyclic loading, all samples showed similar flexural failure modes, failure drift ratio was 4.11%, and peak load observed was 288.87KN. Increasing RFBAC (recycled fireclay brick aggregates concrete) content decreased bearing capacity and lateral stiffness but enhanced seismic performance.	[126]
FA/slag with carbon and basalt micro-fibers and steel fiber composite bars	Column	Fiber content SFCBs (Steel, carbon and basalt fiber); impact resistance	Similar failure mode observed independent of fiber content and reinforcement type. However, CFs and BMFs decreased mid-height deflection (7-42%) and cracking damage. Steel basalt composite bars are recommended for the replacement of steel bars for durable and sustainable structures.	[110]
FASLAG-based GPC	Column	Slenderness eccentricity	At ambient curing, FA-based/slag-based GPC columns show scaling issues in structural level in testing.	[90]
FA and GGBS with recycled brick and recycled aggregate	Column	Axial load-bearing capacity, ductility, reinforcement	Damage progression and failure mode were observed to be similar to OPC-based RCA (Recycled coarse aggregates ) concrete; ultimate bearing capacity decreased with an increase in RCA and enhanced with the increase in reinforcement ratio.	[127]

.

3. Retrofitting/Rehabilitation of Structures Using Geopolymers

Existing structures are experiencing serious damage due to aging, environmental issues, and increased loads, making retrofitting mandatory. External reinforcements such as steel and concrete jacketing not only strengthen structures but also cause an increase in weight, steel corrosion, and construction difficulties. For improving flexural and shear strength, externally bonded reinforcement (EBR) and near-surface-mounted (NSM) techniques are commonly used in strengthening structures. Engineered cementitious composites (ECCs) are widely used for the retrofitting of structures. Due to the environmental impact of OPC, it can be replaced with a geopolymer-based alternative, offering a more sustainable solution with enhanced strength and durability [128]. FA and GGBS-based geopolymer concrete and fiber-reinforced geopolymers in column jacketing can enhance the ultimate load-carrying capacity (1.1 times)—with better compatibility than normal concrete—due to the improved bond interface between the RC column and the jacketed material [129,130].

The selection of a repair product is mainly dependent on bond strength and compatibility with the substrate. Various materials, including cementitious mortars, geopolymer mortars, high-performance fiber-reinforced cementitious composites and polymer-modified compounds, are currently used in practice for repair applications [131]. Advanced composite materials, such as laminated composites and fiber-reinforced composites, provide improved structural reinforcement and resilience. Carbon-fiber-reinforced CFRP and glass-fiber-reinforced polymers (GFRPs) are widely used due to their lightweight properties and high strength-to-weight ratio. The addition of nanoparticles or fiber-reinforced materials enhanced matrix performance, significantly improving structural resilience and durability [132]. The interfacial bond between the composite matrix and fiber surface acts as a branching effect in crack deflection and energy dissipation, preventing localized failure [36] . Hybrid fiber-reinforced geopolymer (HFRG) composites exhibited superior performance in durability, bond strength with existing structures, fire resistance, and environmental sustainability compared to cement-based strain-hardening cementitious (SHC) materials [133].

Epoxy resin (4 wt%)-based geopolymers (ERGs) improved bonding performance and mechanical strength, and they are recommended as efficient repair materials in structural applications [134]. Tashan et al. [135] studied the flexural behavior of fiber-reinforced (polypropylene PP) GGBS-based geopolymer concrete beams. Their results showed that epoxy resin injections in damaged beams restore strength, with improved flexural behavior and ductility (Figure 8). This is a result of the presence of polypropylene fibers in geopolymer concrete, as they have smaller diameters and increased fiber volume, and they are equally dispersed throughout the matrix, assisting crack bridging.

Figure 8. Load-deflection behavior of high-strength geopolymer concrete (HSGC) and repaired high-strength geopolymer concrete (RHSGC). Adopted from Ref. [135].

Figure 8. Load-deflection behavior of high-strength geopolymer concrete (HSGC) and repaired high-strength geopolymer concrete (RHSGC). Adopted from Ref. [135].

Qu et al. [136] repaired damaged recycled fireclay-brick-based geopolymer concrete (RFBC-GPC) frames using epoxy resin, and they tested the frame under seismic conditions on a shaking table. The results showed that stiffness was not fully restored; however, damage resistance and functionality recovery (34%) were improved. This is due to the adoption of epoxy resin with RFBC-GPC, which effectively covered the damage-induced cracks and improved the initial defects, such as pores and microcracks, thereby enhancing structural repair performance.

Fly-ash-based carbon-textile-reinforced geopolymer mortars (CTRGMs) improved ductility, flexural strength, and energy absorption in damaged beams, especially with the increased textile layers (Ls) and U-strip anchorage (A) (Figure 9), which is a result of the improved bonding and stress distribution between the textile layer and FAGGBS-based geopolymer mortar [137].

Figure 9. Load-deflection diagram of (a) control and pre-damaged beams; (b) beams retrofitted with 1 layer (L) of CTRGM; and (c) beams retrofitted with 4 layers (Ls) and 4 anchorage (A) points of CTRGM [137].

Figure 9. Load-deflection diagram of (a) control and pre-damaged beams; (b) beams retrofitted with 1 layer (L) of CTRGM; and (c) beams retrofitted with 4 layers (Ls) and 4 anchorage (A) points of CTRGM [137].

Palizzolo et al. [138] adopted metakaolin-based geopolymers with sisal fibers as the plaster layer (0.03 m thickness with 2% wt. sisal fibers) to analyze masonry panels. The results showed that the geopolymer plaster increased the ultimate load-bearing capacity of the masonry and lowered the damage level with respect to both compression and tension when compared with standard plaster—this is due to the stitching effect of fibers (Figure 10 and Figure 11).

Stochino et al. [139] studied the retrofitting of masonry walls by adopting jute-fiber-based textile-reinforced mortar. The results showed enhanced structural performance accompanied by an increase in load-carrying capacity (520%), and thermal insulation was observed with a 36% reduction in thermal transmittance. These findings emphasize that natural fibers provide dual benefits when adopting them in sustainable masonry. The near-surface-mounted (NSM) technique comprises placing FRP strips or bars into grooves on concrete surfaces and applying bonding with adhesives such as cement grout or epoxy. In comparison to externally bonded reinforcements, the NSM technique has more benefits: ease of installation, simple anchorage, and superior bond performance. However, its adoption in geopolymers is still limited. Geopolymer mortars showed enhanced structural performance in NSM techniques; however, they still require further investigation [140,141].

Table 4. Performance of geopolymers as repair materials.

Source Materials	Investigated Variables	Key Findings	References
Naturally hydrated lime with FA (20-100%) and 12M NaOH	FA/NHL (Natural hydrated lime) ratios; mechanical and Rheological	FA-based grouts at 100% replacement of NHL exhibited higher compressive strength (8-23MPa), low water absorption, and chloride resistance, making them suitable for structural rehabilitation. Yield stress and consistency im-proved up to 50% NHL replacement.	[142]
Fly ash with steel, polypropylene, and glass fibers	Mechanical properties, ductility coefficient, and Flexural toughness	Flowability of hybrid fiber-based GP decreases with increasing fiber content and remains workable up to 2% fiber volume. Hybrid fiber (80% steel and 20% glass fibers or 70% steel fibers and 30% PP fibers) enhanced flexural toughness (10-181%) and increased flexural strength (7-18%) and ductility by 263% (PP + steel fibers) compared to monofiber geopolymer composites. It improved flexural toughness when compared with mechanical strength, as the repair material’s toughness is more important for structures subjected to dynamic loading.	[133]
Low-calcium FA, polypropylene fibers using styrene butadiene rubber, and epoxy resin	Slant shear, rebar pull-out, pull-off flexural corrosion	GPC mortar exhibits higher bond strength than a cementitious mix for both saturated and dry surface conditions. Epoxy resin performs better as an adhesive between geopolymer and cementitious mixes. PP fiber geopolymer mortar enhanced flexural performance in toughness, ultimate capacity, and crack control. A 2mm geopolymer repair coating is recommend for better corrosion resistance.	[143]
Ultra-fine-ground granulated blast furnace slag with FA (20-50%)	FA and slag molarities, plasticizer variation, and mechanical properties	Slag-based geopolymer with 30% fly ash ensured a homogenous matrix and improved bonding. UGGBFS as a repair material in structural retrofitting achieved 60% of its strength (28 days) in 24hrs. Improved bond strength with PCC (Plain cement concrete) surface and rebar, but workability decreases without fly ash, increasing the molarity (14M) of the alkaline activator.	[144]
Metakaolin (50%) and fly ash (50%) with and without textile-reinforced (TR) fibers	Comparison of TRG and TR-reinforced beams, and adhesive-type variation	Textile-reinforced geopolymer mortar (TRGM) enhanced shear capacity up to 47% and 106% with one and two TRG mortar layers in comparison with unstrengthened beams. Similarly, their effective strain matches with textile-reinforced mortar beams with similar wrappings. With enhanced properties in durability, TRGM is an optimal RC repair material.	[145]
Slag-based GP plaster with light-weight glass aggregates, air-entraining agents, and GFRP mesh	Seismic performance and energy efficiency	Alkali-activated geopolymer plaster with air-entraining agents and expanded glass aggregates exhibits 8 MPa compressive strength (28 days) and thermal conductivity of 0.35 W/mK at 700 kg/ m3. Moreover, using modified starch (MS), methylcellulose (MC), a shrinkage-reducing admixture (SRA), polypropylene fibers, and silane-based surface treatment enhanced adhesion, microcrack resistance, and detachment resistance, and low water absorption was observed.	[146]
Textile-reinforced geopolymer (TRM) mortar with jute and basalt fibers	Fiber variation %; textile layers	TRM with natural fibers are selected as a sustainable alternative to epoxy-based FRP systems. Enhanced load capacity and durability in various environmental conditions. Validation with finite element modeling emphasized the potential of TRM with natural fibers for structural retrofitting.	[147]

shows the structural performance of geopolymers as repair materials, focusing on their bond strength, mechanical properties, durability, and suitability in different conditions.

4. Geopolymer Applications

The adoption of geopolymer concrete in the construction industry is increasing due to its superior strength, durability, and structural performance. From a materials science point of view, the applications cover a wide range of fields, and they depend on the fiber composition and type of fiber in the composite, as described in Figure 12. Therefore, it is important to highlight the efficiency of each composite independently for each application. The applications include pavements and bridges due to their high strength, enhanced freeze–thaw resistance, and durability; moreover, these materials have improved resistance to sulfate and chloride attacks, making them ideal for marine structures.

Due to their rapid strength improvement and lower carbon footprint, geopolymers are widely adopted in the precast industry. Moreover, low permeability and the chemical stability of geopolymers render them ideal for mobilizing toxic and radioactive waste. In building construction, geopolymers can be utilized for their excellent thermal and acoustic insulation, resulting in the construction of sustainable housing. Their ability to impact loads and their higher strength and resistance to deicing chemicals render them ideal for airport runways. Similarly to structural retrofitting, due to their excellent bond behavior with old structures, they are also ideal as repair materials. Figure 13, Figure 14, Figure 15 and Figure 16 show some applications and projects that have been accomplished by implementing geopolymer concrete.

Figure 12. Prospective applications of fiber-reinforced geopolymer composites [148].

Figure 12. Prospective applications of fiber-reinforced geopolymer composites [148].

Figure 13. Commercial production of geopolymer sewer tubes in Belgium [149].

Figure 13. Commercial production of geopolymer sewer tubes in Belgium [149].

Figure 14. Clay-based geopolymer application in 3D printing [150].

Figure 14. Clay-based geopolymer application in 3D printing [150].

Figure 15. Strengthening an RC girder using CFRP bars and an external geopolymer coating in Zhanjiang, China [151].

Figure 15. Strengthening an RC girder using CFRP bars and an external geopolymer coating in Zhanjiang, China [151].

Figure 16. Rehabilitation of damaged culvert using geopolymers (Ottawa, Canada) [152].

Figure 16. Rehabilitation of damaged culvert using geopolymers (Ottawa, Canada) [152].

5. Conclusions and Recommendations

The adoption of geopolymer concrete in the construction industry remains limited, despite extensive research on its fresh, mechanical, and durability properties. The main constraint in this regard is the limited number of studies available on the structural assessment of geopolymers and the lack of reviews on various factors affecting their structural performance. To cover this paucity of research, this critical review examines the available literature on the structural applications of geopolymers and their use as repair materials in structural retrofitting. From our extensive review, the following conclusion can be drawn.

Geopolymer concrete in structural elements, such as beams, columns, slabs, and structural frames, exhibits comparable/superior properties to normal concrete, with improved strength, impact resistance, ductility, and structural performance. These improvements are mainly influenced by the geopolymer mix composition, geometric characteristics of the fiber type, bond behavior, and reinforcement ratio.

The shear span-to-depth ratio is a critical factor that can affect the structural performance of geopolymers. Fiber content (%) and the compressive strength of geopolymers are also key factors in structural applications.

Regarding variations in source materials, mix designs, fiber type, geometry, and curing conditions, extensive studies have been conducted on geopolymer concrete, resulting in diverse findings. These findings challenge the development of standardized performance characteristics. However, this variation does not imply that geopolymers are weaker than normal concrete. Geopolymer concrete exhibits superior structural performance in terms of load-bearing capacity and flexural, shear, and seismic resilience, making it a sustainable alternative for infrastructure development.

Fiber-reinforced geopolymers enhance structural performance under static, impact, and dynamic loading conditions. However, their high costs and dependency on specific source materials limit their use in commercial applications. These challenges can be solved by adopting natural fibers, such as sisal, jute, and hemp fibers, and extensive experimentation should be carried out for alternative mix designs, source materials, and alkaline activators, which would be more cost-effective.

Fiber reinforcements—specifically steel fibers, synthetic and hybrid combinations, and natural fibers—enhance structural performance, although each fiber type has unique advantages and limitations. Steel fibers enhance strength and toughness, but higher costs and corrosion limit their application. Synthetic fibers, such as polypropylene fibers, have durability issues, while natural fibers, such as jute, hemp, or sisal fibers, are economical and sustainable, but pretreatment is required for enhanced properties.

Apart from structural applications, geopolymers used as external reinforcement/repair materials in the form of external plaster layers and grout materials have shown superior properties than conventional repair materials. Textile-reinforced mortar and epoxy-modified geopolymers used as grouting can be an effective solution for seismic retrofitting and for strengthening masonry. Natural fibers such as sisal fiber (2%)—when used in geopolymer plaster as repair materials—can improve load-bearing capacity, energy absorption capacity, and thermal insulation, making it a promising alternative for durable and eco-friendly structural retrofitting in vulnerable infrastructures,.