Research Progress on the Preparation and Performance of Recycled Mortars Using Solid Waste-Based Cementitious Materials

Abstract

Solid waste-based cementitious materials (SWCMs) represent an innovative class of binders derived mainly from construction and demolition waste as well as industrial byproducts. Their application in recycled mortar offers a promising pathway to partially replace conventional cement, thereby advancing resource recycling and facilitating a low-carbon transition in the cement industry. This review systematically examines the properties, activation techniques, strength development, and corrosion resistance of recycled mortar prepared with SWCMs. Recycled powder (RP) and industrial solid waste have gelation potential, but their low reactivity requires activation treatment to enhance utilization efficiency. Activation methods, including thermal activation, carbonation, and alkali activation, effectively enhance reactivity and promote the formation of dense gel structures (e.g., C-(A)-S-H, N-A-S-H). While low replacement ratios optimize pore structure via the microfiller effect, higher ratios introduce excessive inert components, impairing mechanical properties. SWCMs demonstrate superior resistance to sulfate and chloride attacks, but their acid resistance is relatively limited. They also have excellent freeze–thaw resistance. SWCMs represent a viable and sustainable alternative to conventional cement, exhibiting commendable mechanical and durability properties when properly activated and formulated, thereby contributing to resource recycling and environmental sustainability in the cement industry.

1. Introduction

The advancement of economic globalization has accelerated the rapid development of industries worldwide, yet it has also resulted in substantial resource consumption and increased greenhouse gas emissions. Statistics indicate that the construction sector accounts for approximately 40% of the natural resources used in industrial production. In addition to emitting greenhouse gases during production and construction, this sector generates enormous amounts of construction and demolition waste [1]. Cement, as one of the most essential building materials, is responsible for approximately 40% of CO₂ emissions during its production process while also consuming significant amounts of natural resources. Moreover, the total volume of solid waste generated from construction and demolition activities has reached 3.5 billion tons [2], constituting 30% to 50% of the global solid waste stream [3,4]. Simultaneously, industrial by-products such as fly ash, slag, and desulfurization gypsum, which are produced by metallurgical, chemical, and other industries, have also accumulated on a massive scale. Collectively, these materials exert considerable environmental pressure [5]. Currently, the resource utilization rate of such solid wastes remains relatively low, with a large proportion ultimately disposed of in landfills or stockpiles. This not only occupies valuable land resources but also causes complex pollution of soil, water, and the atmosphere, emerging as a critical bottleneck to sustainable urban development.

Among construction waste materials, waste concrete and waste clay bricks constitute the two predominant components. In recent years, beyond merely crushing and sieving these materials for use as recycled aggregates, further grinding them into recycled powder for application as cementitious materials has emerged as a cutting-edge research direction aimed at achieving high-value recycling of construction waste [6]. This approach seeks to transform waste from a simple physical filler into a key component that participates in cementitious reactions and is regarded as one of the core pathways to advance construction waste recycling from “downcycling” to “upcycling.” However, recycled powder often contains stabilized hydration products (such as C-S-H gel and Ca(OH)₂) or high-temperature sintered inert phases (e.g., quartz and mullite), which generally lead to inherent drawbacks such as a low content of active components, high chemical inertness, irregular particle morphology, and high water demand [7]. As a result, composite cementitious systems incorporating recycled powder as a partial replacement for cement often suffer from slow hydration rates, insufficient early-age strength development, and poor microstructural densification. When used as a source of aluminosilicates in geopolymer synthesis, recycled powder presents challenges such as low reactivity, rapid consumption of alkaline species in solution, and the formation of loosely structured polymerization products [8,9]. These fundamental limitations in material activity severely restrict the high-volume application of recycled powder in structural materials, posing significant challenges to its widespread adoption in practical engineering.

Moreover, the disposal of industrial solid waste has become an equally pressing issue. Byproducts such as fly ash from coal-fired power plants, blast furnace slag from iron and steel smelting, and desulfurization gypsum and red mud have accumulated in massive quantities, posing serious environmental threats. In recent years, the advanced utilization of industrial solid waste has become a leading research direction for achieving high-value recycling. This approach moves beyond conventional applications, such as road base materials or cement blends, instead of using these wastes as essential raw materials for green construction materials, including geopolymers and alkali-activated binders [10,11,12]. This approach aims to transform industrial solid waste from an “environmental burden” into a strategic resource that supports the development of “zero-waste cities” and low-carbon building solutions, representing a shift from low-end disposal toward value-added creation [13,14,15]. However, owing to differences in sources and production processes, these wastes often present inherent drawbacks, such as significant variability in chemical composition, complex active phase constituents, and diverse physical characteristics [16]. These limitations make it challenging to precisely control reaction pathways, workability, and long-term volume stability when such wastes are used as sole or primary precursors in alkali-activated systems [17]. Furthermore, the design of blended systems and processing strategies for performance optimization is complicated by unclear structure-activity relationships among reaction mechanisms, resultant phases, and macroscopic properties, largely due to the heterogeneity and complexity of solid wastes [18,19]. These fundamental bottlenecks at both the material utilization and engineering application levels severely restrict the reliable, large-scale use of industrial solid waste-based cementitious materials in structural engineering, resulting in considerable challenges in industrialization.

Promoting the systematic integration and synergistic utilization of construction waste and industrial solid waste in the field of cementitious materials is crucial for achieving both high-performance and low-carbon objectives. By combining the use of recycled powder with alkali activation technology for industrial solid waste, it is possible not only to leverage their complementary advantages and enhance the overall performance and stability of the cementitious system but also to significantly reduce reliance on conventional cement [20,21], thereby substantially decreasing energy consumption and carbon emissions at the source.

Given the above considerations, there is a pressing need to systematically review, consolidate, and critically assess research progress in the field of solid waste-based cementitious materials for recycled mortar. This review specifically addresses a critical gap by establishing a comprehensive framework for solid waste-based cementitious materials, which synergistically utilize recycled powder derived from construction and demolition waste (CDW) and industrial solid wastes. It systematically elucidates how various activation methods, such as thermal activation, carbonation, and alkali activation treatments, relate to material properties. The review systematically links these activation mechanisms to the resulting material performance, particularly mechanical strength and resistance to diverse aggressive environments, including sulfate attack, acid corrosion, chloride penetration, and freeze–thaw cycles. By providing a comparative analysis of degradation mechanisms under different aggressive conditions and correlating them with gel chemistry and microstructure, this work offers novel insights for designing high-performance, low-carbon binders with enhanced durability. Thus, it bridges the knowledge gap between isolated activation studies and the practical, durable application of fully waste-based mortars.

2. Methodology Adopted for the Review

To ensure systematicity, representativeness, and relevance in selecting references for this review, a dynamic, evolutionary, and layered in-depth search and screening strategy was employed. The retrieval databases included the Web of Science (WoS) Core Collection (encompassing sub-databases such as SCI, SSCI and AHCI) and the China National Knowledge Infrastructure (CNKI) (including core databases of Peking University Chinese and CSCD), ensuring comprehensive coverage of internationally authoritative literature and important domestic research achievements. The determination of keywords was a gradual process, refined and expanded as research and understanding deepened. Initially, “recycled mortar” served as the entry point, which was then extended to material system keywords such as “polymer mortar” and “alkali-activated cementitious materials”. After clarifying the basic reaction mechanisms, the scope was further broadened to preparation methods such as “thermally activated mortar” and “carbonized mortar”. The focus shifted to material properties, introducing evaluation terms such as “corrosion resistance”, “freeze-thaw resistance”, “sulfate resistance”, and “acid corrosion resistance”. Based on this principle, the study constructed a multi-level keyword system comprising core concepts and their synonym clusters. These elements were strategically combined with Boolean operators including AND and OR, which enabled an optimal balance between recall and precision throughout the retrieval process and laid a reliable foundation for the review. After the initial literature acquisition, clear inclusion and exclusion criteria were defined: priority was given to original research articles and authoritative peer-reviewed reviews, while unreviewed conference abstracts, news reports, and literature for which the full text could not be obtained were excluded. Through this structured process, the study aimed to build a comprehensive and precise literature base, thereby supporting the reliability and relevance of the review’s arguments.

3. Feasibility of Solid Waste-Based Cementitious Materials for Recycled Mortars

Solid waste-based recycled mortar is an environmentally sustainable construction material made from solid wastes, including construction and demolition debris and industrial by-products. It is produced through the partial or complete replacement of conventional natural aggregates and cementitious materials via physical, chemical, or hybrid processing techniques according to specific mixture formulations. This approach seeks to valorize solid wastes as valuable resources, thereby reducing the consumption of natural resources, mitigating environmental pollution, and fostering the development of a circular economy.

3.1. Material Characteristics

3.1.1. Construction Solid Waste Materials

Recycled powder (RP) is an ultrafine powder (particle size < 0.16 mm) primarily produced from construction and demolition waste, such as waste concrete, bricks, and masonry, through a series of processing steps, including screening, crushing, and grinding. The raw materials are diverse in origin and are derived mainly from waste concrete and clay bricks generated during demolition projects, as well as from construction residues and laboratory waste. The physicochemical properties of RP, including its reactivity index and particle size distribution, exhibit considerable heterogeneity. This stems from significant variations in the source materials, such as building type, service life, and original composition. During production, the raw materials undergo meticulous processing involving crushing, sorting, drying, and high-precision grinding to meet standardized specifications [22]. Consequently, RP particles are characterized by rough surfaces, irregular and angular morphologies, and porous structures, as shown in Figure 1 [23,24]. The irregular particle shape increases interparticle friction, which adversely affects mortar fluidity, whereas the porous structure elevates water demand, thereby influencing rheological performance. However, the angular and rough surfaces of the particles provide a larger reactive area that promotes hydration, and the porous texture facilitates internal curing by absorbing and retaining water [25].

Owing to variations in raw material sources, the composition of RP can vary significantly. Waste concrete powder (WCP) primarily consists of both crystalline and amorphous phases, including quartz (SiO₂), calcite (CaCO₃), dolomite (CaMg(CO₃)₂), C-S-H gel, and ettringite, and unhydrated clinker phases, such as C₂S, C₃S, and C₄AF, as illustrated in Figure 2a [23]. Quartz and dolomite originate from fine and coarse aggregates, respectively, introduced during the concrete crushing process. Calcite results from the carbonation of both the aggregate and the concrete. As shown in Figure 2b, the major chemical constituents of WCP are SiO₂, CaO, Fe₂O₃, Al₂O₃, and MgO. Among these, SiO₂ is the most abundant component, followed by CaO. In contrast, waste brick powder (WBP) has a distinctly different composition, characterized by high contents of SiO₂ and Al₂O₃, as indicated in Figure 3b. The X-ray diffraction (XRD) pattern of WBP confirms that quartz is the predominant crystalline phase, as shown in Figure 3a [24]. The high concentrations of key oxides, such as CaO, SiO₂, and Al₂O₃, in RP indicate its potential pozzolanic reactivity [26]. The presence of amorphous SiO₂ and Al₂O₃ particles may promote the reaction of volcanic ash. However, CaO exists in the form of calcium carbonate, which prevents it from participating in new hydration reactions.

3.1.2. Industrial Solid Waste

Industrial solid wastes, such as slag and fly ash, present inherent advantages for developing alkali-activated materials because of their high contents of silicon and aluminum components, which are readily soluble in alkaline environments. By directly activating the silicoaluminate component in the precursor with an alkaline activator, depolymerization and reconstruction of the active components in solid waste can be achieved. This process overcomes the traditional cement’s reliance on C₃S and C₂S minerals, providing a more efficient reaction pathway. Owing to their low-carbon characteristics, these systems have attracted widespread attention.

Compared with ordinary Portland cement, alkali-activated materials exhibit lower energy consumption, higher mechanical strength, and superior durability, making them promising alternatives to traditional cement [29,40]. The chemical compositions of two commonly used precursors, fly ash and slag, are presented in Figure 4. Although variations exist depending on the raw material sources, the main components of both are concentrated in three oxides, namely, SiO₂, CaO, and Al₂O₃. Fly ash and slag represent two typical systems. As shown in Figure 4a, fly ash is a low-calcium system dominated by silica and alumina (SiO₂ and Al₂O₃). Slag is a high-calcium system rich in silica and calcium oxides (SiO₂ and CaO), as indicated in Figure 4b. The proportions of Si, Ca, and Al significantly influence the performance of the resulting hardened mortar [41].

Fly ash (FA), one of the most prevalent industrial byproducts, is a fine powder collected from flue gas following the combustion of bituminous coal in coal-fired power plants. It typically exhibits gray to dark gray coloration, occasionally appearing light gray, depending on the unburned carbon content and mineral composition. Generally, higher carbon levels result in darker shades. Most FA particles range from 1–100 μm in size and have a high specific surface area [42,45]. Class F fly ash, also known as low-calcium FA, is typically derived from anthracite or bituminous coal, whereas Class C (high-calcium) FA is produced from lignite or subbituminous coal. Compared with high-calcium FA, low-calcium FA is richer in SiO₂ and Al₂O₃, with a CaO content generally less than 15%, whereas the CaO content of high-calcium FA is between 15% and 30% [43]. As shown in Figure 5a, the FA particles are predominantly spherical and consist largely of an amorphous glass phase, along with minor crystalline phases such as quartz, mullite, and magnetite, as evidenced by the XRD pattern in Figure 5b [67]. Although fly ash is widely used as an alkali-activated precursor, the large-scale closure of coal-fired power plants and the transition of the power industry towards a more environmentally sustainable model are expected to significantly reduce its production in the future. Moreover, the extensive application of fly ash as a supplementary cementitious material in conventional building materials has generated significant competitive demand. This demand may hinder its large-scale development and utilization as an alkali-activated precursor for other applications.

Ground granulated blast furnace slag (GGBS) is a significant by-product generated during the iron-making process in blast furnaces and is widely utilized as an industrial solid waste material. As illustrated in Figure 5c, GGBS particles exhibit an angular and irregular morphology, with colors ranging from white to light gray. Its chemical composition predominantly consists of CaO, SiO₂, Al₂O₃, and MgO. Upon rapid cooling from the molten state, GGBS solidifies into a primarily amorphous glassy phase. This metastable glassy structure imparts latent hydraulic activity, which is essential for its application as a cementitious material [28,46,47,48].

Silica fume (SF), also known as microsilica, is an ultrafine gray powder collected from flue gases during the production of silicon or ferrosilicon alloys. As illustrated in Figure 6, the SF particles are predominantly spherical and exhibit an amorphous structure. With an average particle size of approximately 0.1 μm, silica fume is approximately two orders of magnitude finer than cement particles are, classifying it as a nanoscale material. It is characterized by an exceptionally high SiO₂ content, generally no less than 85%, along with minor oxides such as Al₂O₃, Fe₂O₃, CaO, and MgO. The silica in SF is almost entirely amorphous, which endows it with high chemical reactivity and significant pozzolanic activity [69].

Red mud (RM) is a highly alkaline solid residue generated during the extraction of alumina (Al₂O₃) from bauxite ore. It is a characteristic reddish-brown color due to its high iron oxide content. As shown in Figure 7a, the microstructure of the RM predominantly consists of irregular platelets, along with some spherical beads and porous agglomerates. The pH value of red mud ranges from 10.5–13.5, indicating strong alkalinity. Its chemical composition includes Fe₂O₃, Al₂O₃, SiO₂, TiO₂, CaO, Na₂O, and other oxides, which contribute to its potential suitability for use in alkali-activated systems. The main crystalline phases identified by XRD (Figure 7b) include hematite, goethite, calcite, katoite, and cancrinite. However, the presence of heavy metals such as Cr, V, and As, as well as naturally occurring radionuclides such as uranium-238 (²³⁸U), thorium-232 (²³²Th), and radium-226 (²²⁶Ra), may limit their widespread application [45,47].

Desulfurization gypsum (DG) is an industrial byproduct generated during the flue gas purification process via limestone–gypsum wet desulfurization technology in coal-fired power plants and steel mills. Compared with natural gypsum, desulfurization gypsum appears grayish-white or light yellow in color, with a microscopic morphology characterized by short columnar or plate-like crystals, as shown in Figure 8a. Its main component is calcium sulfate dihydrate (CaSO₄·2H₂O), although it may contain small amounts of soluble salts such as chloride ions and magnesium oxide [46]. After drying treatment, this material exhibited excellent cementitious properties.

3.2. Activation Methods for Solid Waste-Based Cementitious Materials

Owing to the generally low reactivity of solid waste materials, their use as cementitious components is limited. However, certain potentially active constituents in solid waste, including amorphous silicoaluminates and substances within the glassy silicon-aluminum phase, can be activated. Through specific methods, these materials are transformed into highly reactive cementitious components, which enables efficient solid waste utilization.

3.2.1. Thermal Activation

Subjecting WCP to elevated temperatures significantly alters its physical and chemical properties, thereby affecting the performance of the mortar prepared with it. At high temperatures (>500 °C), dehydration and decomposition occur in hydration products, such as calcium silicate hydrate (C-S-H) and calcium carbonate (CaCO₃). At high temperatures, CaCO₃ undergoes thermal decomposition reactions, directly releasing CO₂ gas and generating CaO. As the temperature increases, the C-S-H gel initially loses adsorbed water and some bound water, and its structure will gradually be damaged. With further temperature rises, its amorphous structure undergoes reconstruction, and calcium silicate minerals such as β-C₂S and C₃S crystallize through atomic rearrangement, as shown in reaction Equations (1) and (2). The common characteristic of these decomposition products is their relatively high reactivity [69].

As the temperature rises, the phase composition undergoes significant changes. The XRD pattern of the thermally activated WCP is shown in Figure 9, where mineral phases such as quartz and dolomite (CaMg(CO₃)₂) can still be identified within the temperature range of room temperature to 600 °C. When the temperature rises from 600 °C to 800 °C, a diffraction peak of CaO appears at 2θ ≈ 37.5°, indicating the decomposition of calcite (CaCO₃) at high temperatures. In addition, diffraction peaks of C₂S and C₃S are detected, which result from the decomposition of C-S-H at high temperatures. Since C-S-H gel is an amorphous mineral phase, it cannot be detected by XRD. All these decomposition products have potential pozzolanic activity. When the temperature continues to rise to 1000 °C, the diffraction peaks of CaO, C₂S and C₃S become significantly more intense, indicating ongoing decomposition [40]. At 1200 °C, diffraction peaks of gypsum are detected [31]. Furthermore, existing studies have shown that ettringite decomposes between 350 °C and 400 °C, while calcium hydroxide (CH) remains stable [72]. AFt gradually dehydrates within the range of 75 °C to 300 °C. Around 150 °C, AFt is almost completely dehydrated and can form Al₂O₃ and CaO, which possess certain gelling abilities [30].

$${\mathrm{CaCO}}_3\to \mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{CO}}_2$$

(1)

$$\mathrm{\text{C-S-H}}\to \mathrm{\beta }{\mathrm{\text{-C}}}_2\mathrm{S}+{\mathrm{C}}_3\mathrm{S}$$

(2)

Furthermore, thermal activation significantly altered the microstructural characteristics of WCP. Unlike untreated WCP, the matrix has a denser microstructure within the temperature range of 300–400 °C because of the decomposition of hydration products (CH and C-S-H) and partial calcite [73]. At temperatures between 400 °C and 1000 °C, the particles exhibit increased density, which is attributed to the interlinking of newly formed hydration products on their surfaces [28]. However, when the temperature reaches 1200 °C, the WCP particles soften, and nearly all the hydration products decompose [74]. While this enhances particle flowability, it simultaneously weakens their nucleation effect. The microstructural evolution of WCP under different thermal activation temperatures is illustrated in Figure 10 [28]. As the temperature increases, the particles become finer and more rounded. However, above 750 °C, the finer particles gradually coalesce, leading to a reduction in the reactive surface area of the WCP particles [72]. Therefore, when conducting thermal activation treatment on raw materials, precise temperature control must be maintained to achieve the required performance.

Thermal activation treatment involves drying and calcining raw materials at specific temperatures and durations. This process requires external energy input, resulting in additional energy consumption.

Table 1. Energy consumption of cement, untreated RP and heat-activated RP.

RP		OPC	Ref.
Before thermal activation	18 kwh/t	105 kwh/t	[8]
After thermal activation	70.5 kwh/t
Before thermal activation	0.1–1.2 MJ/Kg	5.5 MJ/Kg	[76]
After thermal activation	2.85–3.95 MJ/Kg

presents the energy consumption of cement, untreated RP, and thermally activated RP, with the energy consumption of thermally activated waste powder calculated based on previous studies [75]. The results indicate that although the energy consumption for thermal activation of RP is higher than that of untreated RP, it remains relatively low compared to that of cement. Thermal activation enhances the activity and microstructure of RP, enabling it to replace cement to a greater extent. Future research should focus on expanding its applications to promote the recycling and utilization of solid waste more broadly. Therefore, thermally activating RP and applying it is both feasible and promising. Additionally, it is advisable to utilize clean energy sources such as wind, solar, and nuclear energy during the thermal activation process. This approach will further reduce energy consumption and make the process more low-carbon and environmentally friendly.

Research shows that the thermal activation of WCP enhances its reactivity and microstructure. By improving these properties, this method enables a higher cement replacement ratio and unlocks the material’s full utilization potential. However, thermal activation involves increased energy consumption, as achieving the required calcination temperature still depends on fossil fuels and electricity. Additionally, it requires precise control over the calcination parameters, including the temperature, duration, and cooling methods, which makes it relatively complex. Consequently, achieving efficient and low-carbon applications of thermal activation remains a critical challenge for future research.

3.2.2. Carbonation

Previous studies have confirmed that key components in solid waste materials, such as CH, C₂S, and C₃S, undergo carbonation when exposed to CO₂. Existing research indicates that upon introducing CO₂ gas, the components in solid waste materials (such as CH, C₂S and C₃S) undergo carbonation reactions with CO₂. The essence of this reaction is “decalcification followed by re-precipitation”. For example, CH is directly carbonized to form CaCO₃. Under the synergistic effect of CO₂ and H₂O, calcium ions from C₂S and C₃S precipitate as CaCO₃, while the silicon-oxygen framework in the original structure reorganizes, primarily generating amorphous C-S-H gel. Therefore, the main products of the carbonation reaction are CaCO₃ and C-S-H, which improve the material composition and optimize the microstructure, thereby enhancing the performance of the mortar. The carbonation reactions are represented by the following equations:

$$\mathrm{Ca}{(\mathrm{O}\mathrm{H})}_2+{\mathrm{CO}}_2\to {\mathrm{CaCO}}_3+{\mathrm{H}}_2\mathrm{O}$$

(3)

$$3\mathrm{C}\mathrm{a}\mathrm{O}\cdot 2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\cdot 3{\mathrm{H}}_2\mathrm{O}+3\mathrm{C}{\mathrm{O}}_2\to 3\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\cdot 2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\cdot 3{\mathrm{H}}_2\mathrm{O}$$

(4)

$$3(3\mathrm{C}\mathrm{a}\mathrm{O}\cdot 2\mathrm{S}\mathrm{i}{\mathrm{O}}_2)+(3-\mathrm{x})\mathrm{C}{\mathrm{O}}_2+\mathrm{y}{\mathrm{H}}_2\mathrm{O}\to \mathrm{x}\mathrm{C}\mathrm{a}\mathrm{O}\cdot 2\mathrm{S}\mathrm{i}{\mathrm{O}}_2\cdot \mathrm{y}{\mathrm{H}}_2\mathrm{O}+(3-\mathrm{x})\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3$$

(5)

$$\begin{array}{l}2(3\mathrm{CaO}\cdot {\mathrm{SiO}}_2)+(2-\mathrm{x}){\mathrm{CO}}_2+\mathrm{y}{\mathrm{H}}_2\mathrm{O}\to \\ \mathrm{x}\mathrm{CaO}\cdot 2{\mathrm{SiO}}_2\cdot \mathrm{y}{\mathrm{H}}_2\mathrm{O}+(2-\mathrm{x}){\mathrm{CaCO}}_3\end{array}$$

(6)

As noted in Section 3.1, untreated WCP, with quartz and calcite as the primary mineral phases, also contains a certain amount of CH, AFt, and partially hydrated C₂S, C₃S, and other compounds. XRD analysis (Figure 11) [29] revealed that after carbonation treatment, quartz and calcite remained the dominant crystalline phases, while the diffraction peak intensity of dolomite (CaO·MgO·2CO₂) was markedly enhanced. In contrast, the characteristic peaks of phases such as CH, AFt, C₂S, and C₃S are significantly diminished or have disappeared. These observations indicate that the reactive phases mentioned above participate in carbonation reactions with CO₂, leading to the formation of new products, predominantly CaCO₃.

The carbonation reaction significantly influences the microstructure and macroscopic properties of the material. The precipitated CaCO₃ and silica-based gels deposit and accumulate on particle surfaces and within pores, markedly refining the microstructural morphology. The microstructure of the carbonized WCP is shown in Figure 12 [30]. The carbonated WCP sample exhibited reduced porosity and a more regular pore morphology, with characteristic rhombohedral calcite crystals appearing on the surface. Previous studies have indicated that well-formed, fine calcite crystal precipitates facilitate nucleation and promote the growth of hydration products in cementitious composites [30], thereby contributing to the increased strength of the mortar. However, in large-scale processing, it is not guaranteed that all materials will achieve the desired carbonation effect. Uneven reactions can result in localized over-carbonation or under-carbonation, which may introduce new stress concentration points or weak areas within the material. Furthermore, carbonation improved the granulometric properties of WCP. The particle size analysis results before and after carbonation are presented in Figure 13. The median particle size of the samples after carbonation is generally larger than that before carbonation. This indicates that the carbonation reaction not only alters the phase composition but also increases particle size and enhances overall uniformity. This improvement is achieved through the deposition of newly formed fine CaCO₃ crystals on the particle surfaces, which effectively fill the pores [29]. However, excessive carbonation may cause the carbonation products to block the pores, which instead hinders the further diffusion of CO₂ and creating a self-inhibitory effect.

Carbonation treatment is a process that utilizes CO₂-rich gas to treat solid waste under controlled conditions, such as temperature and relative humidity, converting CO₂ into thermodynamically stable carbonates that can be permanently stored [80,81]. This process not only enhances material performance but also enables the storage of CO₂ gas. Compared to traditional improved technologies, carbonation treatment is a green and sustainable technology. The amount of CO₂ absorbed serves as a measure of the potential for carbon sequestration.

Table 2. Absorption of CO₂ after carbonation.

Carbonation Conditions					Absorption Capacity	Ref.
Materials	Relative Humidity	Carbonation Time	CO2 Concentration	Temperature
Cement paste powder	60 ± 5%	-	99%	20 ± 1 °C	47.5%	[82]
50% OPC + 50% FA	65%	-	0.04%	20 ± 1 °C	1.74	[83]
50% OPC + 50% GGBS					1.82
50% OPC + 50% FA	65%	28 d	20%		19.68
50% OPC + 50% GGBS					28.41
Recycled concrete fines	5–10%	30 min	>99%	20	2.76	[34]
				60	10.86
				100	21.65
				140	12.11
Recycled concrete fines	70 ± 5%	12 d	20 ± 3%	20 ± 2	44	[41]

presents the amount of CO₂ absorbed after carbonation. The data indicate that selecting appropriate carbonation conditions can maximize the carbon sequestration potential of the material. Therefore, future research should focus on optimizing carbonation conditions to improve the performance of carbonized materials, increase the carbon sequestration capacity, and reduce energy consumption and costs associated with CO₂ capture and transportation. This will help achieve an optimal balance among performance, environmental protection, and cost in carbonation treatment.

In summary, carbonation treatment of RP effectively modifies the composition, microstructure, and particle size distribution of solid waste materials. By increasing their reactivity, carbonation increases the utilization rate of RP, reduces the cement demand, and simultaneously sequesters CO₂, thereby lowering greenhouse gas emissions. Moreover, this process does not require high-temperature calcination, resulting in low energy consumption. However, variability in powder composition, including low CaO content, can lead to inconsistent carbonation efficiency. Additionally, the high costs associated with CO₂ capture and purification, along with potential long-term stability issues caused by volumetric changes in carbonation products, present significant challenges. Therefore, achieving large-scale application of carbonation treatment remains a critical research priority.

3.2.3. Alkali Activation

The latent reactive components in solid waste materials can be effectively activated by alkaline activators to form a dense three-dimensional network structure. This reaction process does not depend on the C₃S and C₂S minerals found in traditional Portland cement. The performance of the resulting material can be compared with or even better than that of traditional cement materials. Therefore, it has the potential to be used as an alternative to traditional cement.

In such systems, the type and modulus of the activator, along with the reactivity of the precursor, jointly regulate the kinetics of gel formation and the densification of the microstructure. This synergy not only facilitates rapid development of early-age strength but also enables continuous polymerization, which enhances long-term mechanical performance. This unique reaction characteristic offers a novel approach to overcoming the inherent limitations of recycled micropowder in traditional cement. It also underscores the flexibility and potential of alkali-activated systems in material design and performance regulation.

The alkali-activated elements Ca, Si and Al in cementitious materials work synergistically to regulate the reaction process and the formation of final products, thereby influencing the macroscopic properties of the materials. When the Ca content is relatively high, it is a high-calcium aluminosilicate system, which typically reacts efficiently at ambient temperature, primarily forming a C-A-S-H gel as the reaction product. The second type is a low-calcium aluminosilicate system, which often requires elevated temperatures and longer curing periods, with N-A-S-H gel as the main resulting phase [84]. The activation mechanism of high-calcium alkali-activated systems is illustrated in Figure 14. Under the influence of alkaline activators such as NaOH or Na₂SiO₃, elements such as Si, Ca, and Al are released from calcium silicate or aluminosilicate sources, forming soluble ionic species or hydroxyl compounds such as Si(OH)₄⁻, Al(OH)₃⁻, and Ca(OH)₂. These dissolved species further react in an alkaline environment to produce amorphous calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) gels [85]. In an environment rich in calcium elements, the reaction rate accelerates, promoting the rapid formation of a denser C-A-S-H gel, thereby enhancing the material with higher early strength [86]. In contrast, the reaction of low-calcium alkali-activated materials involves three main stages: (1) dissolution of aluminosilicate raw materials under strong alkali, releasing reactive aluminate and silicate tetrahedral units; (2) polycondensation of these active units, where silicate and aluminate tetrahedra connect via shared oxygen atoms, forming Si-O-Si and Si-O-Al bonds, leading to a three-dimensional gel network; and (3) continuous growth and cross-linking of the gel phase, resulting in final hardening [87,88,89], as depicted in Figure 15. The three-dimensional N-A-S-H gel network, composed of Si and Al, provides excellent long-term strength development and structural stability [90], thereby enhancing the material with better durability.

Alkaline activators stimulate the amorphous aluminosilicate components in solid waste materials to form cementitious reaction products. The type of activator influences mortar performance by modulating reaction pathways, gel phase composition, and microstructural development. For example, sodium-based base activators are used. The main function of NaOH is to provide a high-pH environment. OH⁻ destroys the glassy structure of raw materials, which releases active SiO₂ and Al₂O₃ monomers and forms silicate ([SiO₄]⁴⁻) and aluminate ([AlO₄]⁵⁻) ions. Additionally, the Na⁺ ions supplied by NaOH not only maintain charge balance within the system but also catalyze polycondensation reactions. The dissolution rate is governed by both the NaOH concentration and the duration of the dissolution process. On the other hand, sodium silicate provides both OH⁻ and SiO₃²⁻ ions. While OH⁻ contributes to the alkaline environment and facilitates the release of Si and Al species, SiO₃²⁻ directly participates in the formation of the gel network. Owing to its higher silicon content, sodium silicate promotes the development of a more stable aluminosilicate network during the reaction, thereby increasing the mechanical strength [48].

The molar ratio of SiO₂ to Na₂O is known as the silicate modulus (denoted as M). It is a critical parameter influencing the performance of alkali-activated systems. By quantitatively regulating the alkalinity of the activator, the silicate modulus controls the composition and structure of the resulting gel phases, thereby ultimately determining the material properties. In current practice, composite activators comprising strong alkalis and silicate compounds are commonly used. Strong alkalis provide a high-pH environment that promotes the dissolution of Si and Al from the precursor, whereas silicate sources supply reactive silica that facilitates cross-linking within the gel network. By adjusting the proportions of alkali and silicate, the silicate modulus can be precisely controlled [49]. Research indicates that the optimal range of M varies with the type of precursor. For low-calcium systems, the modulus should generally not exceed 2 [91], as sufficient OH⁻ is required to break down the Al-O-Si networks. In contrast, for high-calcium systems, the modulus should not fall below 2, since although Ca²⁺ promotes the formation of C-S-H gel, adequate silicate ions are necessary to support the reorganization of aluminosilicate species. However, unreacted bases may remain in this system, and these alkaline substances can serve as potential internal alkali sources. This significantly increases the risk of alkali-aggregate reactions and severely compromises the long-term volumetric stability and durability of the material.

Furthermore, the type of alkali metal cation significantly influences the reaction process, gel structure, and final properties of the activated mortar. As shown in Figure 16a [52], the microstructure of the potassium-based activator (PS)-activated mortar exhibited a dense and homogeneous morphology with no obvious unreacted particles. The reaction products are uniformly distributed and compact, displaying minimal pores or cracks, along with a dense interfacial transition zone between the paste and fine aggregates. This indicates that the larger ionic radius and lower charge density of K⁺ result in slower migration rates, which moderate the dissolution kinetics of aluminosilicate precursors. The consequently more controlled reaction progression leads to homogeneous gel formation, reduced chemical shrinkage, and a strong bonding capacity with silicate tetrahedra (SiO₄⁴⁻) [50]. The resulting C-(K)-A-S-H gel possesses a stable structure with fewer microdefects, such as pores and cracks. Its chain-like configuration is more flexible than that of the C-(N)-A-S-H gel, contributing to a uniform pore structure [92]. As presented in Figure 16b, the microstructure of the sodium-based activator (SS)-activated mortar shows a more porous and cracked morphology. It has a microstructure with numerous pores and cracks, and many unreacted particles can be observed. Moreover, there are many cracks between the slurry and fine aggregates, and the interface transition zone is relatively weak [93]. This can be attributed to the smaller ionic radius and higher charge density of Na⁺, which accelerates the dissolution of aluminosilicate phases and leads to rapid early-age hydration. The resulting C-(N)-A-S-H gel tends to encapsulate unreacted particles, hindering further hydration, while the rapid reaction often induces localized shrinkage stress concentrations, resulting in microcracking. Moreover, the C-(N)-A-S-H gel is more rigid [50].

However, when solid waste undergoes alkaline activation treatment, self-shrinkage tends to occur, leading to the formation of structural cracks that compromise the performance of the mortar. Self-drying is the primary cause of mortar shrinkage [94]. The capillary pressure generated by water evaporation leads to drying shrinkage within the mortar. The drying shrinkage of AAM mortar is significantly greater than that of OPC mortar. This drying shrinkage in AAM mortar gradually develops after the final setting, with the shrinkage value increasing over time [95]. In the AAM system, this factor is not the sole dominant influence; the characteristics of the C-A-S-H gel also have a significant impact. The hydration product of AAM, C-A-S-H gel, should not be considered a continuous porous material. It is more like a gel-like or granular material containing micro-defects. Its chemical shrinkage during drying is associated with the rearrangement and redistribution of C-A-S-H nanoparticles [96]. Moreover, the contractions caused by both mechanisms are irreversible [92]. Therefore, mitigating mortar shrinkage requires addressing both of these aspects. Relative humidity (RH) significantly influences shrinkage by regulating capillary stress and surface free energy. At high RH levels (>50%), the rearrangement and reorganization of the C-A-S-H microstructure result in pronounced viscous behavior. This rearrangement causes the collapse of gel pores and refinement of the pore structure [97]. Conversely, at low RH levels (<40–50%), the direction, magnitude, and location of stresses induced by surface free energy differ, promoting the densification of the particles themselves [98,99]. Therefore, addressing shrinkage and cracking in alkali-activated material (AAM) mortar requires attention to both material design and curing conditions. Optimizing the mix ratio and activator parameters can produce a denser, more stable gel. Additionally, incorporating components that generate an appropriate amount of expansive hydration products (such as ettringite) during the early stages can induce compensatory expansion, offsetting some chemical and drying shrinkage. However, the dosage of these expansive components must be carefully controlled to prevent excessive expansion, which could cause greater damage. Finally, establishing a scientific curing regime and maintaining appropriate humidity levels are essential for effective shrinkage control.

Alkaline activation treatment of solid waste materials enables their efficient utilization by allowing the use of all solid waste cementitious materials, thereby addressing the high carbon emissions associated with traditional cement production. However, the dosage and modulus of alkali must be precisely controlled. Prolonged exposure can lead to alkali–aggregate reactions or carbonation shrinkage, which may compromise the service life of the structure. Additionally, the use of strong alkalis increases health and safety risks and complicates construction operations.

By replacing conventional cement, solid waste-based cementitious materials offer a viable route for significantly reducing resource consumption and carbon emissions. These materials incorporate construction solid wastes, such as discarded concrete and brick powder, as well as industrial solid wastes, such as slag and fly ash. The potential cementitious properties of these materials can be effectively activated through various treatment methods. These processes include thermal activation to break down the glassy structure, carbonation to capture CO₂ and form strength-enhancing calcium carbonate, and alkali activation to depolymerize silico aluminate phases and reconstruct a three-dimensional gel network. However, it faces challenges such as low activation efficiency, high energy consumption, and significant contraction. Future efforts should focus on the development and application of solid waste materials [100] to achieve diverse application scenarios [101,102]. Additionally, future efforts should prioritize the development of low-carbon activation technologies, such as efficient carbonation and low-temperature alkali activation. Additionally, the synergistic effects of multiple solid wastes should be optimized, and long-term durability studies should be conducted. These measures are critical for overcoming technical bottlenecks and advancing the large-scale application of solid waste recycled mortar, thereby contributing to a circular economy.

4. Mechanical Strength of Solid Waste-Based Cementitious Recycled Mortars

In the engineering application of materials, their mechanical behavior in practical use is crucial in addition to their basic properties. As a core component of building structures, the strength of mortar is a key indicator for measuring its mechanical properties, which directly determines the safety and stability of the structure. Therefore, in-depth research on how replacing cement with solid waste affects mortar strength is essential. This research not only further verifies the effectiveness of using solid waste materials but also provides a necessary foundation for promoting the engineering application of recycled solid waste mortar.

4.1. Effect of the Solid Waste Replacement Ratio on Mortar Strength

Compared with that of conventional cement mortar, the incorporation of recycled powder (RP) generally leads to a reduction in mortar performance. This decline is attributed primarily to the dilution of the main reactive phases in the cement, namely, C₂S and C₃S, combined with the inherently low reactivity of RP, which has a limited capacity to participate in hydration reactions. Consequently, the formation of hydration products decreases, and the microstructural compactness is reduced, thereby compromising the overall mechanical properties of the mortar.

Studies have shown that when waste concrete powder (WCP) is used to partially replace ordinary Portland cement, the development trend of compressive strength follows the pattern illustrated in Figure 17. The compressive strength of the mortar decreases as the WCP replacement ratio increases. Under conditions of a low substitution rate, WCP has a distinct “microaggregate effect”. Its particles optimize the graded composition of the cementitious system, fill internal pores, block water migration channels, and thereby inhibit the self-shrinkage behavior of the mortar. Furthermore, the porous structure of the WCP can adsorb part of the free water, helping to maintain internal humidity and promote continued cement hydration, which contributes to increased compactness, reduced drying shrinkage, and an improved interfacial transition zone (ITZ) structure. Although the incorporation of WCP dilutes the active components in cement, leading to a reduction in the total amount of hydration products [103], the surfaces of WCP particles can serve as nucleation sites for hydration products, accelerating the cement hydration process. Additionally, unhydrated cement particles and active components (such as SiO₂ and Al₂O₃) in WCP can undergo secondary hydration reactions, generating additional C-(A)-S-H gel, which partially compensates for the performance loss caused by the reduced cement content. Consequently, the impact on mortar strength remains relatively limited at low replacement ratios. At higher replacement levels, the dilution effect of WCP increases the proportion of inert components, which not only significantly reduces the amount of cementitious hydration products in the system but also retards the hydration process due to the adsorption of substantial free water [104]. This leads to pore coarsening, increased connectivity, and exacerbation of both autogenous and drying shrinkage, ultimately resulting in deterioration of the ITZ, reduced compactness, and decreased mortar strength.

The use of RP with higher reactivity can lead to certain improvements in mortar performance. Waste brick powder (WBP), which contains a greater proportion of amorphous aluminosilicates capable of undergoing pozzolanic reactions, is more reactive than WCP. Consequently, when WBP is used to partially replace cement, the resulting mortar demonstrates greater strength than WCP-modified mortar at the same replacement ratio, as shown in Figure 17 and Figure 18. When the dosage ranges from 15% to 20%, the mortar can achieve its highest strength. This improvement is not only attributed to the previously mentioned volcanic ash reaction [109], but also because WBP exhibits a “micro-aggregate effect” similar to that of WCP, effectively filling the pores of the slurry [110]. This dual action comprehensively enhances the strength of the mortar. However, when the dosage exceeds this optimal range, the dilution effect of WBP becomes dominant, reducing the relative content of effective cementitious components in the system and subsequently causing a decline in macroscopic strength. Regarding the XRD patterns of pastes, different replacement levels of WBP are presented in Figure 19 [111]. In WCP-blended pastes, the diffraction peaks of CH are noticeably reduced, whereas those of CaCO₃ and SiO₂ increase significantly [72]. This indicates that the incorporation of WCP not only reduces the formation of cement hydration products but also introduces inert phases, such as quartz (SiO₂) from aggregate minerals and calcite (CaCO₃). Consequently, the overall reactivity and degree of hydration decrease. These factors contribute to the formation of a loose and porous microstructure, as illustrated in Figure 20a [112]. In contrast, WBP-modified pastes clearly exhibit a decrease in the diffraction peaks of CH and CaCO₃, accompanied by a significant increase in the SiO₂ peak. This indicates that WBP reduces the content of CH and CaCO₃ in the binder system. Since WBP contains no hydrated phases, it initially decreases the total amount of hydration products. However, the SiO₂ in WBP is predominantly present as amorphous silica with pozzolanic activity, which can further react with CH to form additional C-S-H gel [25]. This reaction refines the pore structure, resulting in a matrix with fewer capillary pores and cracks and greater compactness than WCP blends do, as shown in Figure 20b [113]. The abundant presence of C-S-H in the microstructure of the WBP paste indicates that, unlike WCP, the incorporation of WBP enhances the overall degree of hydration in the cementitious system.

4.2. Effects of Various Activation Methods on the Mortar Strength

4.2.1. The Influence of Thermal Activation on the Mortar Strength

Compared with mortar containing untreated RP, the use of thermally activated RP significantly enhances the mechanical performance. This improvement is attributed primarily to changes in the phase composition and microstructure of the RPs induced by the thermal treatment. On the one hand, the original cement hydration products, such as CH and C-S-H gel, undergo dehydration and decomposition, transforming into more reactive phases, including C₂S, C₃S, Al₂O₃, and CaO. On the other hand, the internal porosity and specific surface area of RP particles increase [31,72,73,114]. These modifications not only provide abundant nucleation sites that accelerate early-age hydration kinetics but also enable the activated components to participate in secondary pozzolanic reactions, generating additional C-S-H gels. Consequently, the microstructure of the paste is refined, resulting in improved mechanical properties.

The thermal activation temperature is a critical parameter governing the pozzolanic activity of RP. As shown in Figure 21, the compressive and flexural strengths of the mortar initially increase and then decrease with increasing activation temperature, reaching a maximum at approximately 800 °C, which corresponds to the peak pozzolanic reactivity of RP under different thermal treatment conditions. Studies indicate that within the temperature range of 750–800 °C, RP attains an optimal reactive state. Under thermal activation, portlandite (Ca(OH)₂) fully decomposes, and the C-S-H gel is largely converted into an active amorphous phase, whereas the decomposition of calcite (CaCO₃) remains limited. This limitation mitigates the potential volume instability caused by excessive free CaO [115], thereby contributing to strength enhancement. However, when the temperature exceeds 800 °C, some amorphous phases undergo sintering and crystallization, forming low-reactivity crystalline phases such as wollastonite (CaSiO₃) and gehlenite (Ca₂Al₂SiO₇), as evidenced by the distinct diffraction peaks in the XRD patterns (Figure 22) [116]. Owing to their chemical stability and low reactivity, the formation of these crystalline phases significantly diminishes the pozzolanic effect, thereby restricting further development of mechanical properties. At temperatures above 1000 °C, the gypsum (CaSO₄·2H₂O) potentially present in RP decomposes into soluble anhydrite (CaSO₄) [117], which can rapidly rehydrate upon contact with water, disrupting normal hydration processes and adversely affecting the final mortar strength.

4.2.2. The Influence of Carbonation on the Mortar Strength

After carbonation treatment, the strength of the mortar significantly improves. As shown in Figure 23, the compressive strength of the carbonized mortar at both 7 d and 28 d is higher than that of the non-carbonized group. This improvement is attributed to the formation of dense calcite microcrystals on the surface of WCP following carbonation [118]. These microcrystals possess a high specific surface area and active surfaces, which serve as nucleation sites for the C-S-H gel, thereby promoting early-age cement hydration. Furthermore, CaCO₃ can guide the ordered growth of C-S-H gel, contributing to early strength development. The influence of the carbonation treatment on the mortar strength is illustrated in Figure 23. Additionally, CaCO₃ reacts with aluminate phases such as C₃A in cement to form needle-like monocarboaluminate (Ca₄Al₂(CO₃)(OH)₁₂·5H₂O). This reaction product not only enhances matrix densification but also consumes free water during its formation, reducing overall porosity [66]. Simultaneously, the precipitation of CaCO₃ fills the capillary pores within the WCP, helping mitigate internal defects. As mentioned in Section 3.2.2, carbonation treatment increases the WCP particle size and optimizes the particle size distribution, which improves the ITZ between the aggregates and the cement paste, thereby refining the microstructure and mechanical performance of the mortar. Moreover, carbonated WCP retains partially unhydrated reactive components such as Ca(OH)₂, C₂S, and C₃S [119]. With prolonged curing, these components continue to hydrate, generating additional C-S-H gel, which contributes to the favorable long-term mechanical performance of the mortar.

The carbonation temperature serves as a pivotal factor in regulating the polymorph of CaCO₃ crystals, thereby influencing the microstructure and resultant strength of mortar. The microstructural evolution of WCP carbonized at different temperatures is presented in Figure 24 [78]. At 20 °C, spherical vaterite constituted the primary carbonation product. As the temperature increased, the proportion of cubic calcite gradually increased. By 140 °C, vaterite is almost entirely replaced, and the calcite crystals exhibit larger dimensions than those formed at 100 °C. At 60 °C, carbonation predominantly yields vaterite, which has a high specific surface area and a fine particle size. Its high surface activity provides effective nucleation sites for cement hydration, accelerating the early hydration of C₃S and C₃A [120] and promoting the formation of hydration products, thereby increasing early strength. However, owing to its metastable nature, vaterite tends to dissolve readily [121], which limits the extent of early strength improvement. When the temperature exceeds 100 °C, vaterite transforms into more stable calcite through a dissolution–reprecipitation process [122]. Calcite crystals are dense and stable; although their nucleation effect is relatively weaker in the early stages, their stability helps retard the decomposition of hydration products at later ages. Through pore filling and reinforcement of the ITZ, calcite compensates for some of the strength loss resulting from reduced cement content. A temperature of 100 °C promotes CO₂ diffusion and ion migration, thereby improving the carbonation efficiency. However, beyond 140 °C, moisture evaporation reduces the relative humidity in the system, inhibiting the carbonation reaction [34]. Under such conditions, although the calcite content increases, its larger particle size may weaken the nucleation effect. In addition, insufficient encapsulation by CaCO₃ leaves unreacted siliceous gels, which can lead to a decline in long-term strength.

4.2.3. The Influence of Alkaline Activation on the Mortar Strength

The type of alkaline activator significantly influences the strength development and microstructure formation of mortar. Sodium-based activators create a highly alkaline environment that rapidly dissolves Si and Al from the raw materials, accelerating early gel formation and resulting in high early-age strength. However, this rapid reaction alters the internal moisture distribution through the fast consumption and migration of water, leading to structural stress imbalances and inducing shrinkage. In contrast, potassium-based activators promote a more moderate reaction process with different moisture retention and migration behaviors, avoiding significant structural stress changes at early stages and thus resulting in negligible shrinkage. Compared with that of sodium-based systems, the resulting gel structure is more homogeneous, contributing to a denser microstructure and consequently better strength development [105]. As shown in Figure 25 [51,52,53], the compressive strength of mortar activated with either sodium- or potassium-based activators at 28 days is significantly greater than that at 7 days, indicating a continuous reaction and stable microstructural development in both systems at later ages. Notably, the 28-day compressive strength of the K⁺-activated mortar is consistently greater than that of the Na⁺-activated samples. This confirms that the denser microstructure guided by K⁺ ultimately results in superior macroscopic mechanical performance.

The silicate modulus of the alkali activator is a critical parameter influencing the mechanical performance of mortar. As shown in Figure 26 [49,54,55,123,124], the effect of the modulus on the compressive strength generally tends to initially increase but then decreases. At lower moduli, the relatively high Na₂O content in the activator rapidly elevates the pH of the system, promoting the breakage of Si-O and Al-O bonds in the precursor and accelerating the release of reactive aluminosilicate monomers. This facilitates the early formation and cross-linking of the gel phase, thereby enhancing early strength. Within the optimal modulus range, a balanced ratio of SiO₂ to Na₂O in the activator maintains sufficient alkalinity while supplying adequate silicate species for polycondensations [125]. The resulting gel phase exhibited a high degree of polymerization, significantly reduced microporosity, and a dense matrix structure. The reduction in strength observed at a lower modulus is attributed to the high alkali concentration in the mixture. This high alkalinity systematically shortens the silicate chain length, hindering the polymerization of alkali-activated materials and consequently compromising the mortar’s durability and stability. An appropriately modulated activator can enhance the interfacial bonding between the cementitious matrix and aggregates, reducing microcracking at the interface and consequently improving the overall compressive strength. However, when the modulus is too high, the significantly increased SiO₂ proportion in the silicate activator increases the system’s viscosity and reduces the OH⁻ concentration [56]. This slows the dissolution rate of the precursor, limits the release of reactive aluminosilicate monomers, and leads to an incomplete reaction. Unreacted SiO₂ may remain in a free state, creating structural defects. Excessive SiO₂ can also interfere with the polycondensation process of the gel, resulting in a loose gel structure and increased porosity. Furthermore, an insufficient Na⁺ concentration in high-modulus systems may lead to dealkalization of the gel phase, reducing the matrix density and consequently diminishing the strength.

The influence of solid waste as a cement substitute on mortar strength is complex and variable. At low replacement ratios, the “microaggregate effect” contributes to structural refinement, resulting in only a marginal reduction in strength. However, at high replacement levels, the predominance of inert components and insufficient hydration lead to a significant decline in strength. Activation methods such as thermal treatment, carbonation, and alkali activation can enhance the mechanical performance. To clearly present the strength development patterns of solid waste-based recycled mortar under various research conditions, the key findings of this section are summarized in

Table 3. The influence of different factors on the strength of recycled mortar made from solid waste-based cementitious materials.

Categories of Influencing Factors	Specific Conditions		Trend or Range of Compressive Strength Variation	Ref.
Solid waste substitution rate	Waste concrete powder, low substitution rate (<20%)		The intensity is slightly reduced (approximately 85–95% of that in the control group)	[36,105,106,107,108]
	Waste concrete powder, high substitution rate (≥30%)		The intensity decreases significantly (approximately 50–80% of that in the control group)	[36,105,106,107,108]
	Waste brick powder, with the same substitution rate		The strength is generally higher than that of the WCP system	[37,38,39,108]
Activation method	Thermal activation	Temperature	The intensity has increased significantly and reached its peak.	[28,31,32,72,74,75]
		Excessively high temperature (>1000 °C)	The strength has decreased significantly.	[28,31,32,72,74,75]
	Carbonation	Carbonation	The strength of both 7 d and 28 d has been enhanced.	[27,29,30,33]
		Carbonation temperature	Regulate early and long-term strength development by influencing the crystal type.	[34,78,122]
	Alkali activation	Cationic type of alkaline activator	The strength of the K+ system at 28 d is usually higher.	[51,52,53]
		Modulus of alkali activator	The intensity initially increases with the modulus, then decreases, indicating the presence of an optimal range.	[49,54,55,123,124]

. Nevertheless, the large-scale application of these materials continues to face multiple constraints. These include the high energy consumption of thermal activation, the elevated cost and inconsistent efficiency of carbonation, as well as the stringent control requirements and long-term reaction risks related to alkali activation. Future research should prioritize the development of green activation technologies, such as low-temperature alkali activation and biocatalyzed carbonation, to reduce energy consumption and carbon emissions. Additionally, further investigations are needed to explore the synergistic “complementary Ca-Si-Al mechanism” through blending construction and industrial solid wastes, establish accelerated aging models to clarify long-term strength degradation patterns, and incorporate internal curing materials to balance strength development and shrinkage.

5. Durability of Solid Waste-Based Cementitious Recycled Mortars Against Chemical Attacks

In practical engineering applications, mortar is often subjected to complex and variable service environments, such as normal, marine, or industrial settings. When used in structures exposed to contaminants, the intrusion of corrosive media can significantly compromise structural durability. Therefore, investigating the corrosion resistance of solid waste-based mortar is not only essential for enhancing its overall performance but also a necessary prerequisite for promoting its application in harsh environments. This chapter focuses on typical corrosive scenarios, including sulfate attack, acid corrosion, and chloride ion ingress, and analyses the corrosion resistance mechanisms, performance variations, and influencing factors of solid waste-based mortar.

5.1. Corrosion Resistance

5.1.1. Sulfate Attack

Sulfate ions represent one of the most common anions in aqueous environments. During sulfate attack, sulfate ions (SO₄²⁻) first penetrate the cementitious matrix, where they react with portlandite (CH) to form gypsum (CaSO₄·2H₂O). The gypsum subsequently reacts with aluminum-containing phases (e.g., C-A-H) in the cement paste to generate ettringite (AFt). Under specific conditions such as low temperatures, SO₄²⁻ can also interact with C-S-H and carbonates to form thaumasite, as illustrated by the following reaction equations. The formation of these products can induce substantial internal stresses, leading to cracking and degradation, which explains the generally poor sulfate resistance of conventional cement-based materials. In contrast, alkali-activated cementitious materials prepared from industrial solid wastes, such as slag and fly ash, exhibit improved resistance to sulfate attack to a certain extent.

$$\mathrm{C}\mathrm{a}{(\mathrm{O}\mathrm{H})}_2+\mathrm{S}{\mathrm{O}}_4^{2-}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}+2\mathrm{O}{\mathrm{H}}^{-}$$

(7)

$$\begin{array}{l}3\mathrm{C}\mathrm{aO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot 6{\mathrm{H}}_2\mathrm{O}+3\mathrm{C}\mathrm{a}{(\mathrm{O}\mathrm{H})}_2+3\mathrm{S}{\mathrm{O}}_4^{2-}+25{\mathrm{H}}_2\mathrm{O}\to \\ 3\mathrm{C}\mathrm{aO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot 3\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4\cdot 31{\mathrm{H}}_2\mathrm{O}+6\mathrm{O}{\mathrm{H}}^{-}\end{array}$$

(8)

$$\begin{array}{l}\mathrm{C}{\mathrm{aSi}}_2{\mathrm{O}}_7\cdot 3{\mathrm{H}}_2\mathrm{O}+2\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4\cdot 2{\mathrm{H}}_2\mathrm{O}+2\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3+24{\mathrm{H}}_2\mathrm{O}\to \\ \mathrm{C}{\mathrm{a}}_6{\left[\mathrm{Si}\right(\mathrm{OH})}_6]{(\mathrm{C}{\mathrm{O}}_3)}_2{(\mathrm{S}{\mathrm{O}}_4)}_2\cdot 24{\mathrm{H}}_2\mathrm{O}+\mathrm{C}\mathrm{a}{(\mathrm{O}\mathrm{H})}_2\end{array}$$

(9)

Sulfate attack on cement-based materials involves a complex combination of physicochemical processes. When sulfate ions (SO₄²⁻) penetrate the paste, chemical attack is initiated as SO₄²⁻ reacts with the C-(A)-S-H gel, leading to its decalcification. The released calcium ions dissolve into the pore mixture, where they combine with water and SO₄²⁻ to form gypsum and ettringite (AFt) [57]. In the initial stage, an appropriate amount of gypsum can promote the formation of well-developed and stable AFt crystals. The precipitation of AFt helps fill pores and microcracks, thereby increasing the compactness of the mortar and temporarily mitigating further sulfate ingress. However, as the attack continues, excessive gypsum leads to a high sulfate ion concentration in the matrix. Under these conditions, AFt continues to grow and crystallize, generating a crystallization pressure that induces internal microcracks and porosity [126]. At this stage, physical damage begins to dominate the degradation process, establishing a cyclic “ingress–damage–accelerated ingress” feedback loop that ultimately results in structural failure [127]. When the attacking medium is magnesium sulfate (MgSO₄), the deterioration mechanism becomes even more severe. Because Mg²⁺ is more reactive than Ca²⁺, it displaces Ca²⁺ in cement hydration products such as CH and C-S-H, forming noncementitious Mg(OH)₂ and low-strength M-S-H [128], as shown in the following equations. This process not only consumes the key cementitious phase in the system but also leads to disintegration of the cementitious structure and a sudden drop in strength. Moreover, the released SO₄²⁻ continues to participate in the abovementioned sulfate erosion cycle, exacerbating the deterioration of the material.

$$\mathrm{C}\mathrm{a}{(\mathrm{O}\mathrm{H})}_2+\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4\to \mathrm{M}\mathrm{g}{(\mathrm{O}\mathrm{H})}_2+\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4$$

(10)

$$\mathrm{\text{C-S-H}}+\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4\to \mathrm{\text{M-S-H}}+\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4$$

(11)

The composition of raw materials significantly influences the mechanical properties and sulfate resistance of alkali-activated materials by determining the types of hydration products formed. In alkali-activated slag-fly ash systems, as the slag content gradually decreases, the dominant gel phase transitions from calcium-rich C-A-S-H to sodium-containing C-(N)-A-S-H. With further reduction in slag content, the system primarily forms N-A-S-H gel. This phase evolution is attributed to the decreasing Ca²⁺ concentration resulting from the reduced addition of high-CaO slag. The mechanical strength of the mortar varies with these changes in gel type. A decrease in slag content leads to a gradual reduction in strength, which can be explained by the dense layered structure of the C-A-S-H gel, which minimizes structural defects. However, the three-dimensional network of the N-A-S-H gel tends to contain more micropores [129], resulting in a comparatively lower strength. Thus, variations in the hydration product content substantially affect the mechanical performance of the matrix.

Under sulfate attack conditions, the degradation mechanisms and resistance of different gel structures exhibit distinct behaviors. As the immersion time in the Na₂SO₄ solution increased, all the mortar samples experienced progressive strength loss. In the case of the C-A-S-H gel, SO₄²⁻ ions induce decalcification through ion exchange reactions, leading to disintegration of the gel framework. This process is accompanied by the formation of expansive products such as gypsum and ettringite (AFt), which significantly increase internal porosity and microcracking, resulting in rapid strength deterioration. As shown in Figure 27 [130,131,132,133], mortar formulations with higher slag contents exhibit more pronounced strength loss in the later stages of sulfate exposure. In contrast, although the N-A-S-H gel undergoes dealumination [134], this reaction typically does not cause complete breakdown of the gel network. The three-dimensional aluminosilicate framework remains largely intact. Moreover, the XRD pattern of sulfate erosion (Figure 28) shows that the peak intensity of the corrosion product ettringite decreases or even disappears at low slag ratios. This phenomenon may be attributed to the reduced slag content, which lowers the Ca/Si ratio. Consequently, the hydration products gradually transform from C-A-S-H to N-A-S-H gel, with the Al used for ettringite formation being redirected to generate N-A-S-H gel. Therefore, the strength degradation rate of low-calcium materials is slower, demonstrating superior long-term resistance.

Studies on the durability of alkali-activated cementitious materials in sulfate environments have revealed significantly lower resistance to MgSO₄ attack than Na₂SO₄ exposure [44,136,137]. The erosion mechanism of sodium sulfate and magnesium sulfate on mortar is shown in Figure 29. This difference stems from the high charge density of Mg²⁺, which replaces Ca²⁺ in the C-A-S-H gel, leading to the formation of gypsum and AFt. It can further substitute for Al³⁺ in the gel network, resulting in depolymerization of the three-dimensional framework composed of silicon (aluminum) oxygen tetrahedra [138]. Concurrently, H⁺ ions generated by the hydrolysis of MgSO₄ decrease the pH of the system, accelerating gel decomposition [58]. The XRD patterns of the mortar samples under magnesium sulfate attack are shown in Figure 30. Under the exposure of magnesium sulfate, the diffraction peaks of C-A-S-H gradually diminished, whereas those of gypsum and brucite became more pronounced. Notably, AFt does not exhibit clear diffraction peaks in the XRD patterns, as it becomes unstable at pH values below 10.7 [139]. From a microstructural perspective [58,59], all mortar samples after sulfate attack exhibit noticeable pores and cracks caused by crystallization pressure from expansive products, ultimately forming interconnected microcracks and significantly reducing the density of the ITZ. Compared with the samples exposed to Na₂SO₄, those subjected to MgSO₄ presented a more porous and loose microstructure, indicating a greater degree of deterioration and a faster rate of performance degradation.

5.1.2. Acid Corrosion

When the surface of traditional Portland cement building structures comes into contact with acid rain or other acidic media, H⁺ ions initially undergo a neutralization reaction with Ca(OH)₂ in the structure. This process causes the dissolution of solid-phase hydrated silicates and hydrated aluminate salts, thereby reducing the alkalinity of the structure. The H⁺ ions further attack the Ca²⁺ in C-S-H, forming a noncementitious silica gel (SiO₂·nH₂O), as represented in reaction Equations (12) and (13). This process degrades the cementitious framework and leads to a decrease in mortar strength. Notably, H⁺ corrosion often occurs in conjunction with sulfate attack and carbonation, which collectively accelerate the deterioration process.

$$\mathrm{C}\mathrm{a}{(\mathrm{O}\mathrm{H})}_2+2{\mathrm{H}}^{+}\to \mathrm{C}{\mathrm{a}}^{2+}+2{\mathrm{H}}_2\mathrm{O}$$

(12)

$$\mathrm{\text{C-S-H}}+\mathrm{n}{\mathrm{H}}^{+}\to {\mathrm{Ca}}^{2+}+{\mathrm{SiO}}_2\cdot \mathrm{n}{\mathrm{H}}_2\mathrm{O}$$

(13)

In contrast to ordinary Portland cement systems, alkali-activated materials derived from industrial solid wastes primarily form a three-dimensional network-structured aluminosilicate gel (N-A-S-H). The superior acid resistance of these systems can be attributed to their distinct chemical compositions and microstructures. On the one hand, the three-dimensional framework composed of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedra exhibits better resistance to acid attack than the C-S-H gel in conventional cement [122]. The absence of Ca(OH)₂ as a vulnerable phase for acid reactions further enhances the overall chemical stability. On the other hand, since calcium is the primary target during acid corrosion and tends to form soluble salts such as CaCl₂ or CaSO₄, the C-(A)-S-H gel formed in industrial waste-based systems generally has a low Ca/Si ratio. Compared with the high Ca/Si C-S-H in OPC, the low-Ca C-(A)-S-H releases fewer Ca²⁺ ions under H⁺ attack, thereby reducing gypsum formation and avoiding the expansion caused by AFt and internal cracking due to gypsum precipitation [90]. Furthermore, the alkali activation process leads to the formation of a highly dense microstructure characterized by low porosity and poor pore connectivity, effectively hindering the penetration and diffusion of acidic media. Compared with traditional Portland cement systems, alkali-activated materials derived from industrial solid waste generally exhibit better acid resistance.

In alkali-activated systems, H⁺ ions initially undergo ion exchange with cations (e.g., Na⁺, K⁺, and Ca²⁺) in the gel network. This constitutes the primary degradation mechanism at the early stage, which elevates the pH of the pore solution [60] and induces weight gain due to water absorption after cation leaching. As the acid concentration increases or is prolonged, H⁺ begins to attack and breakdown the Si-O-Al and Si-O-Si network structures [61], triggering dealumination and desilication reactions that ultimately dissolve the gel framework. This process represents the fundamental cause of severe material deterioration.

The chemical composition of raw materials fundamentally governs the acid resistance of mortar, with the calcium content and silicon-to-aluminum (Si/Al) ratio being particularly influential in determining the acid degradation mechanism. From the perspective of phase composition, the calcium content of the system directly dictates the type of dominant gel phase and its stability under acidic conditions. High-calcium systems such as slag-based materials primarily form C-A-S-H gels. This gel is prone to calcium leaching during acid attack, which leads to structural disintegration and loss of cohesion. Consequently, these systems exhibit relatively poor acid resistance [141]. In contrast, low-calcium systems such as fly ash-based materials primarily form a three-dimensional N-A-S-H-dominated network. Owing to the lower content of exchangeable calcium ions that can be replaced by H⁺, the aluminosilicate framework remains more stable in acidic environments, resulting in superior durability [142]. Furthermore, from the perspective of chemical bond stability, the Si/Al ratio of the raw material directly affects the degree of polymerization and hydrolytic stability of the gel network. Since the bond energy of Si-O is significantly greater than that of Al-O, a higher Si/Al ratio implies a greater proportion of more hydrolysis-resistant Si-O-Si bonds and a lower proportion of more acid-vulnerable Si-O-Al and Al-O-Al bonds within the gel structure. Therefore, increasing the Si/Al ratio effectively enhances the resistance of a material to acid attack [138].

Furthermore, the deterioration mechanisms of alkali-activated cementitious materials in hydrochloric acid (HCl) and sulfuric acid (H₂SO₄) environments are fundamentally distinct. This is primarily manifested in the differences in attack mechanisms, reaction products, and failure modes. Figure 31 shows the change ratios of the compressive strength of mortar after corrosion by sulfuric acid and hydrochloric acid, respectively. It is evident that the reduction in compressive strength after sulfuric acid exposure is less pronounced than that after hydrochloric acid exposure. The microstructures of the mortar samples after exposure to sulfuric and hydrochloric acid are compared in Figure 32 [143]. The corrosion mechanism in HCl is predominantly governed by H⁺ attack, leading to dissolution-based degradation. Chloride ions do not participate directly in chemical reactions but accelerate H⁺ penetration through ion exchange. The resulting corrosion products (e.g., chlorides) are highly soluble and can be continuously leached away by the acid solution, constantly exposing fresh surfaces and allowing the corrosion front to progress inwards [144]. In contrast, sulfuric acid attack involves a dual degradation mechanism. In addition to the direct damage to the gel network caused by H⁺, it is accompanied by sulfate-related reactions [145], which constitute the key difference between the two acids. SO₄²⁻ reacts with Ca²⁺ and Al³⁺ in the system to form low-solubility precipitates such as gypsum and ettringite. These products may initially block capillary pores and slow acid ingress; however, as the reaction continues, their continued precipitation generates increasing crystallization pressure, leading to internal stress accumulation, microcrack propagation, and eventual surface spalling. Consequently, mortar exposed to HCl exposure exhibits a relatively linear strength loss due to progressive dissolution, whereas H₂SO₄ attack causes expansion and cracking, which often leads to sudden failure, reflected in a more abrupt, nonlinear decline in strength.

5.1.3. Chloride Ion Ingress

Chloride ion ingress is one of the primary causes of deterioration in the durability of reinforced concrete structures, posing a particularly severe threat to constructions in coastal or saline soil environments. Although chloride ions do not corrode the cement paste or coarse aggregates in concrete, they induce corrosion of the steel reinforcement. This corrosion generates expansion stress, leading to cracking along the reinforcement, a reduction in the bond strength between the steel and the concrete, and eventual spalling of the concrete cover. These processes further accelerate chloride penetration, ultimately resulting in structural deterioration. Therefore, the porosity and water absorption capacity of a material are critical indicators of its resistance to chloride ion attack.

After chloride ions (Cl⁻) penetrate cement-based materials via capillary action, they initiate a series of physicochemical processes. Upon entering the matrix through capillary pores, Cl⁻ reacts with C₃A to form Friedel’s salt, as shown in Equation (14). This low-solubility product can initially bind chlorides and slow the ingress process. However, in later stages, the expansion associated with Friedel’s salt generates internal stress [62], inducing microcracks that facilitate further chloride penetration. Simultaneously, Cl⁻ reacts with portlandite (CH) to form highly soluble CaCl₂ [63], as indicated in Equation (15). This process continuously consumes the CH phase, which is essential for maintaining high alkalinity, and significantly reduces the pore solution pH. It also diminishes the contribution of CH to the strength framework, thereby destabilizing the chemical environment required for C-S-H gel stability. In addition, Cl⁻ can adsorb onto the surface of C-S-H gel [150], where it interacts with Ca²⁺ to form soluble CaCl₂. This leads to depolymerization of the gel network, increased gel porosity, and enhanced permeability, collectively accelerating chloride ingress.

$$2{\mathrm{C}}_3\mathrm{A}+\mathrm{C}{\mathrm{a}}^{2+}+2\mathrm{C}{\mathrm{l}}^{-}+10{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{C}\mathrm{a}\mathrm{O}\cdot \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\cdot \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{l}}_2\cdot 10{\mathrm{H}}_2\mathrm{O}$$

(14)

$$\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2+2\mathrm{C}{\mathrm{l}}^{-}\to \mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{l}}_2+2\mathrm{O}{\mathrm{H}}^{-}$$

(15)

In contrast to ordinary Portland cement (OPC), the main hydration products of alkali-activated materials (AAMs) derived from solid wastes are aluminosilicate gels with low calcium-to-silicon ratios, such as N-A-S-H or C-A-S-H. These gels generally form a denser and more homogeneous microstructure than do the high Ca/Si and C-S-H gels in OPC systems [151]. At the microscale, this compact structure creates tortuous transport paths that significantly hinder the diffusion of chloride ions in the pore solution. Moreover, the refined pore system with reduced connectivity further restricted Cl⁻ migration. Unlike OPC, AAMs contain little or no free CH, which fundamentally minimizes the formation of expansive phases such as Friedel’s salt, thereby avoiding the associated crystallization pressure and microcrack propagation [152]. The low-calcium nature of AAM gels limits not only the total amount of Ca²⁺ available for reaction with Cl⁻ but also the tendency to form expansive products [153]. However, more importantly, under corrosive conditions, the limited leachable calcium helps preserve the structural integrity of the main silicate-aluminate framework, which is composed of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedral units. This considerably slows the gel depolymerization caused by calcium leaching [154]. Consequently, AAMs establish multiple protective mechanisms through their dense microstructure, optimized phase composition, and stable chemical bonding, collectively contributing to their superior resistance to chloride ion penetration compared with that of conventional OPC systems.

The composition of raw materials in solid wastes significantly influences the chloride resistance of mortar. In high-calcium systems, Ca²⁺ promotes the formation of the C-(A)-S-H gel [155], which develops a dense microstructure during early hydration. This not only enhances early-age strength but also establishes an effective physical barrier by reducing pore connectivity, thereby significantly hindering chloride transport and diffusion. Moreover, the resulting Friedel’s salt exhibited excellent chloride-binding capacity, contributing to the immobilization of chloride ions [156]. In addition, aluminum and silicon, as essential elements in the aluminosilicate gel network, play a vital role in forming a dense binding framework. A lower Si/Al ratio facilitates the formation of highly cross-linked N-A-S-H or C-N-A-S-H gels [157]. In such structures, the increased proportion of [AlO₄]⁵⁻ tetrahedra promotes a more compact three-dimensional network, which reduces the overall porosity and suppresses potential chloride penetration pathways, thereby achieving an efficient physical blocking effect against Cl⁻ ingress.

The corrosion behavior of different chloride salts in mortar is notably different. Owing to their smaller ionic radius, monovalent cationic chlorides (e.g., NaCl and KCl) exhibit greater mobility in the pore solution, which may facilitate faster diffusion of Cl⁻ into the mortar. In contrast, divalent cationic chlorides, characterized by larger ionic radii and higher charge densities, migrate more slowly, potentially slowing the overall Cl⁻ penetration rate [158]. More importantly, divalent cations such as Ca²⁺ and Mg²⁺ contribute to the formation of Friedel’s salt, brucite, and minor calcium chloride hydrate (CaCl₂·6H₂O) [64], which can partially fill pores and refine the pore structure, thereby mitigating the chloride ingress rate. However, compared with CaCl₂, MgCl₂ is more destructive to mortar. The underlying mechanism lies in the direct degradation of the gel network by Mg²⁺, leading to the formation of weakly cementitious M-S-H and crystalline Mg(OH)₂. Although these products may initially fill pores to some extent, the resulting M-S-H phase has very low strength. Furthermore, this cation exchange process directly disrupts the chemical structure and binding capacity of the primary gel phases, ultimately causing significant deterioration in mortar strength, as shown in Figure 33.

Solid waste-based cementitious materials have remarkable advantages in terms of corrosion resistance. Their low-calcium aluminosilicate gels (e.g., N-A-S-H) possess a dense microstructure and high chemical stability, providing effective resistance against sulfate, acid, and chloride ion attack. Compared with conventional cement, low-calcium systems exhibit superior resistance to Na₂SO₄ exposure, although their performance against MgSO₄ remains relatively limited. Under acid corrosion, systems with high Si/Al ratios exhibit increased durability due to the stability of Si-O-Si bonds. In the case of chloride ingress, the dense gel matrix acts as a physical barrier, whereas Friedel’s salt contributes to chemical binding, synergistically inhibiting chloride penetration. However, the current understanding of the degradation mechanisms under combined multi-ion corrosion conditions remains insufficient, and variability in solid waste composition may pose long-term environmental risks. Future efforts should prioritize the development of multifactor corrosion models, optimize the gel composition through blending diverse solid wastes, and strengthen engineering validation in aggressive environments such as marine and saline–alkali areas. These advances are essential for promoting the large-scale application of solid waste-based mortar in high-durability green construction.

5.2. Freeze–Thaw Resistance

Freeze–thaw damage is one of the most significant forms of deterioration affecting building structures in cold regions, including walls, floors, water conservancy projects, bridges, and culverts. Freeze–thaw failure is a complex physical fatigue process. It mainly occurs when water penetrates the internal pores of a structure and freezes at low temperatures. This freezing causes the pore volume to expand, resulting in the formation of microcracks. As the temperature rises, water gradually seeps deeper into the structure through these pores and cracks. Under repeated cycles of expansion pressure and water osmotic pressure, the surface of the structure shows a peeling process, progressing layer by layer from the outside inward. Consequently, the microcracks and pores within the structure increase and expand, leading to a loss of quality and deterioration in structural strength [162].

Compared to ordinary Portland cement, industrial solid wastes such as fly ash, mineral powder and silica fume have smaller particle sizes and a slower hydration rate. These characteristics enable them to effectively fill the voids within cement-based materials, thereby enhancing structural density [11,91]. Calcium hydroxide, a primary hydration product of cement, possesses a loose and fragile microstructure with large pore diameters. Through the pozzolanic reaction, solid wastes consume calcium hydroxide, producing additional C-S-H and C-A-S-H gels. The newly formed gels deposit within existing capillary pores and macropores, dividing them into smaller, more closed pores [163]. This significantly reduces the number of harmful pores (>50 nm) while increasing the number of harmless pores (<20 nm). This denser and more refined pore structure effectively hinders the intrusion of external moisture and significantly reduces the total amount of internal frozen water and its migration ability. Therefore, solid waste systems typically demonstrate superior freeze–thaw resistance compared to OPC, as shown in Figure 34 [164,165]. However, due to the relatively slow hydration reaction rate of solid waste materials, their early strength development is relatively sluggish [11,25]. Exposure to freeze–thaw cycles during the early stages can cause more severe structural damage. Additionally, solid waste particles are small and possess a large specific surface area, which increases water demand. Excessive water demand may lead to a higher water-to-binder ratio, thereby increasing the system’s porosity [166] and adversely affecting freeze–thaw resistance.

Pore structure is a key factor affecting the freeze–thaw resistance of mortar. High porosity indicates that, in a saturated state, the mortar contains more freezable water. When freezing occurs, the volume expansion potential is greater, resulting in stronger destructive forces. However, total porosity alone is not a decisive indicator; pore size distribution is more critical. Generally, pores with diameters greater than 50 μm are classified as harmful pores, while those exceeding 200 μm are considered highly harmful pores [165]. These pores are the primary sites of freeze–thaw damage because they facilitate rapid water penetration and migration and serve as main reservoirs for free water. Their abundance directly determines the quality of the freeze–thaw resistance of the mortar. With the increase of the number of freeze–thaw cycles, the number of pores within the mortar also rises, causing cumulative damage until the material’s performance deteriorates completely [164,167,168].

Based on the above, solid waste-based cementitious materials have demonstrated significant advantages in freeze–thaw resistance. These solid waste materials enhance the density of mortar by refining its pore structure, thereby optimizing its microstructure and further improving its freeze–thaw resistance. Although the use of solid waste materials may result in slower early strength development and an increased risk of early freeze–thaw damage, their ongoing secondary hydration reactions continuously optimize the pore structure over time. Consequently, the frost resistance of these materials improves with age, exhibiting a long-term durability advantage compared to traditional cement mortar.

6. Future Prospects

Future research should prioritize three key directions to advance the field:

(1) Develop a synergistic utilization strategy for multi-component solid waste systems to achieve complementary effects in chemistry and microstructure. In the future, this approach will lead to the creation of high-performance, multifunctional, and recyclable advanced building material systems. Furthermore, the application of solid waste materials should not be limited to building materials alone. For example, copper slag and zinc slag inherently provide antibacterial metal ions [169]. Future developments could include sustained-release antibacterial coatings for use in environments with stringent hygiene requirements, such as hospitals and schools. By leveraging the alkaline properties of steel slag, mine slag, red mud and other materials, along with the various metal oxides they contain [170], non-toxic anti-rust coating pigments can be produced to gradually replace traditional heavy metal-based anti-corrosion pigments, such as those containing chromium and lead. The application of solid waste materials offers new sustainable development pathways for the construction, industrial and household sectors.

(2) The focus is shifting from the activation of a single type of solid waste to the coordinated activation of multiple solid wastes. By scientifically designing the proportions of different solid wastes and leveraging the solid-phase reactions that occur during the heat treatment process, new highly active phases are generated, achieving a synergistic enhancement of the activation effect. Non-traditional thermal activation methods, such as microwave and infrared radiation activation, are being developed. These technologies utilize the selective coupling of electromagnetic waves with materials to achieve internal heating, significantly enhancing thermal efficiency and potentially generating more active phases. Future carbonation technologies will evolve from merely treating solid waste to manufacturing value-added products. By precisely controlling process parameters such as temperature, humidity and CO₂ concentration, the crystal form, morphology, and particle size of calcium carbonate can be directionally regulated to produce specialized calcium carbonate suitable for high-value industrial applications, including plastics, coatings, and papermaking, thereby maximizing value. Additionally, solid activators based on industrial by-products (such as complex alkali salts) are being developed to address issues related to the strong corrosiveness and inconvenient transportation of liquid activators. Through precise regulation of proportions and processes, alkali-activated materials with intelligent properties, such as self-repair, self-sensing, moisture regulation, and antibacterial are prepared, greatly enhancing their added value. Furthermore, innovative low-carbon activation approaches are being pursued to minimize energy consumption and environmental impact while maintaining performance.

(3) Establish a reliable long-term durability model based on on-site performance under actual usage conditions. These efforts aim to transform laboratory-scale achievements into structurally feasible and environmentally sustainable applications. In future developments, solid waste materials will become advanced engineering materials characterized by longer lifespans, enhanced safety, and reduced maintenance requirements in harsh environments. This will be achieved through mechanism-driven material design and performance improvement through the integration of multiple technologies. Develop pH-sensitive coatings based on solid waste that change color when an internal corrosive environment is initiated, providing a visual early warning of corrosion. Combine solid waste carriers with phase change materials. When the temperature drops, the phase change material solidifies and releases heat, mitigating sudden temperature declines within the mortar. This process delays the formation of ice crystals and reduces freeze–thaw damage.

7. Conclusions

This review provides a systematic analysis of solid waste-based cementitious materials for recycled mortar, focusing on raw material properties, activation methods, mechanical strength, and corrosion resistance. The main conclusions are as follows:

(1) By analyzing the composition, mineral composition and microscopic morphology of solid waste, it has been found that construction solid waste (such as discarded concrete powder and brick powder) and industrial solid waste (such as slag and fly ash) possess the technical feasibility to partially or fully replace cement in the preparation of recycled mortar due to their high content of active SiO₂, Al₂O₃ and CaO components. However, their inherent inertness limits direct utilization efficiency, and their active ingredients must be activated through specific methods.

(2) Activated solid waste materials exhibit a higher utilization rate. Thermal activation can significantly enhance material reactivity; however, it is limited by high energy consumption and process complexity. Carbonation technology, which fixes CO₂ and generates calcite, offers both environmental protection and structural enhancement benefits. However, its efficiency is significantly affected by variations in raw material composition. Alkali-activated systems are most effective in forming high-strength and high-density gels, but require precise control of the activator modulus and alkali content, as well as management of potential alkali-aggregate reactions and long-term shrinkage risks. Future efforts should focus on the coordinated optimization of multiple activation methods and the development of low-energy, environmentally friendly activation pathways.

(3) The solid waste substitution rate significantly impacts the strength of mortar. At low substitution levels, the “micro-aggregate effect” predominates, resulting in limited strength loss. However, a high substitution rate causes a significant decrease in strength due to the dilution of cementitious components and insufficient hydration. Activation treatments can improve strength to varying degrees, with alkaline activation showing particularly notable effects. Future research should further explore the synergistic effects of multi-source solid waste to promote the precision of mix proportion design.

(4) Solid waste-based mortar generally outperforms traditional cement in terms of corrosion resistance. Its dense low-calcium silicoaluminate gel structure significantly enhances resistance to acid attack and chloride ion penetration. Specifically, it exhibits superior resistance to Na₂SO₄ erosion but is more susceptible to damage from MgSO₄. The mortar remains stable against HCl erosion; however, exposure to H₂SO₄ can lead to gypsum-induced expansion failure. Chloride ion erosion is inhibited through the combined effects of the gel’s physical barrier and chemical curing. Additionally, solid waste particles refine the pore structure, reduce the amount of freeze–thaw water, and enhance frost resistance. However, caution is required regarding the potential risk of early freeze–thaw damage due to the slow development of early strength.

Through systematic analysis, this review demonstrates that solid waste-based recycled mortar is technically feasible. Its mechanical properties and durability can be effectively enhanced through activation methods.