Research Review of Reaction Mechanism and Mechanical Properties of Chemically Solidified Silt

Abstract

Dredged silt, characterized by high moisture content, low shear strength, and poor permeability, presents significant challenges for direct engineering application, leading to excessive land occupation and unsustainable resource management. To address these issues, solidification-lightweight composite technology has emerged as a promising approach to transform dredged silt into sustainable geo-materials. This review systematically evaluates international research progress on silt solidification, focusing on (1) the chemical reaction mechanisms of varied solidification agents, (2) the quantitative effects of key factors (e.g., agent dosage, curing time, and organic content) on the mechanical properties (unconfined compressive strength and shear strength) of treated silt, and (3) a critical discussion on technological limitations and future research directions. The findings provide insights for optimizing treatment protocols and advancing large-scale applications.

1. Introduction

With the rapid expansion of global infrastructure construction, coastal regions and inland waterways generate vast quantities of dredged silt and silty clay annually. Characterized by their microstructural morphology in Figure 1, these sediments possess unfavorable engineering properties—such as high compressibility, low shear strength, excessive water content, and high void ratios—rendering them unsuitable for direct construction applications. Current disposal practices exacerbate environmental contamination, impose significant urban land-use pressures, and incur substantial transportation and storage costs. Transforming these waste materials into viable geotechnical resources through advanced treatment technologies is therefore essential for sustainable infrastructure development and the transition toward a circular economy [1].

Multiple techniques have been developed for silt modification and stabilization, including well-established methods such as physical consolidation, chemical stabilization, and biological remediation [3,4,5,6]. Among these, chemical stabilization has emerged as the most widely adopted approach due to its ability to (1) facilitate soil resource recycling, (2) eliminate waste disposal requirements, and (3) provide cost-effective, high-performance binders [7]. The process involves homogenizing stabilizing agents (e.g., cement, lime, or industrial byproducts) with silt to induce physicochemical reactions that enhance mechanical properties, ultimately transforming the waste material into engineered geomaterials suitable for construction applications [8].

Soil curing agents refer to materials that can chemically bond soil particles or react with clay minerals at ambient temperature to form cementitious compounds, thereby enhancing the mechanical characteristics of soils. These curing agents are typically classified according to: (1) physical state (solid powders or liquid solutions), and (2) chemical composition, encompassing inorganic binders (e.g., cement, lime, fly ash), organic polymers (e.g., polyacrylamide, epoxy resins), bioenzymes, ionic curing agents, and advanced composite formulations (see

Table 1. Common solidified materials and main components.

Type	Name	Main Components
Inorganic curing materials	Cement	3CaO·SiO2, 2CaO·SiO2, 3CaO·Al2O3, 4CaO·Al2O3·Fe2O3
	Lime	CaO
	Fly ash	SiO2, Al2O3, Fe2O3, CaO, TiO2, MgO, K2O, Na2O, SO3, MnO2, etc.
	Waste gypsum	CaSO4
	Phosphogypsum	CaSO4·2H2O
	Slag	Ca, Mg, Fe, Si and their oxides
	Alkaline residue	CaSO3, CaCO3, CaCl2, CaO, etc.
	Silicon powder	SiO2
	Coal gangue	Al2O3, SiO2
Organic curing materials	Epoxy resin	Organic polymer compounds containing two or more epoxy groups in their molecules
	Polymer materials	Polymer-based materials, including rubber, fibers, adhesives, asphalt, etc.
	Sodium silicate	Na2SiO3
Composite curing materials	Composite curing agent	Formulated by compounding two or more types of inorganic and organic curing materials

for a comprehensive comparison).

Recent research has achieved significant breakthroughs in sludge stabilization technology, particularly in understanding the fundamental mechanisms and developing novel stabilization approaches. However, the extant literature reveals that current research efforts are often fragmented, focusing on a single type of curing agent or a specific mechanical property. A paucity of cross-comparative studies exists that systematically evaluate the advantages, limitations, and applicability ranges of different curing systems under unified criteria. Furthermore, while mechanistic studies have progressed, the translation of nanoscale findings to predictable macroscopic performance and long-term field durability remains a significant challenge, often hindered by the complex and variable nature of silt compositions.

This review critically examines three key aspects: (i) nanoscale interaction mechanisms between curing agents and soil components, (ii) engineering performance (including strength, compressibility, and permeability characteristics), and (iii) long-term durability under various environmental conditions. By establishing comprehensive structure-property-performance relationships, this paper aims to provide scientific guidance for developing sustainable and cost-effective silt stabilization strategies to address pressing geo-environmental challenges.

On this basis, this review systematically compiles and analyses a substantial body of research on silt solidification, with particular emphasis on the reaction mechanisms and mechanical behavior of chemically stabilized silt. The primary contributions of this work lie in its integrative analysis of conventional and emerging curing agents, which include inorganic, polymer-based, industrial waste-derived, and geopolymer materials. Furthermore, it provides a detailed examination of the key factors governing their effectiveness. By providing a critical evaluation of technological limitations, this review offers valuable insights for the selection, optimization and application of silt stabilization strategies. Moreover, it identifies significant research gaps and proposes future directions to improve the feasibility of large-scale silt reuse. This work thus serves as a comprehensive reference for researchers and engineers engaged in the development of eco-friendly and efficient geo-materials, thereby supporting advancements in sustainable infrastructure and waste valorisation.

2. Reaction Mechanisms of Silty Soil Stabilization Using Different Types of Curing Agents

The stabilization mechanism, which serves as the fundamental criterion for assessing treatment effectiveness, is primarily governed by the chemical nature of the curing agent. Based on their distinct chemical interactions, silty soil stabilization mechanisms can be systematically classified into four major categories.

2.1. Reaction Mechanism of Cement-Stabilized Silty Soil

The reaction mechanism of cement-stabilized silty soil involves five sequential physicochemical processes (Figure 2) [9]: (1) cement hydrolysis and hydration, (2) interfacial reactions between soil particles and cement hydrates, (3) ion exchange and particle agglomeration, (4) hydration hardening, and (5) carbonation. These processes generate characteristic cementitious products, including crystalline ettringite (AFt), calcium silicate hydrate (C-S-H) gels, and calcite (CaCO₃)—which contribute significantly to the improved engineering properties of the stabilized soil [10]. The resultant microstructure enhancement is achieved through three synergistic mechanisms: (i) pore space infilling by nano-scale hydration products, (ii) strong interparticle bonding via C-S-H formation, and (iii) structural densification through recrystallization, collectively responsible for the improved mechanical performance of the stabilized soil. The specific reaction process is detailed in Appendix A.

In summary, cement-stabilized soil fundamentally results from the dual mechanisms of Ca(OH)₂ physicochemical interactions and the skeletal framework provided by the cementitious matrix. The former facilitates the organization of soil particles and microaggregates into stabilized aggregated structures, while the cementitious matrix encapsulates and interconnects these aggregates, ultimately forming a rigid monolith [11].

2.2. Mechanism of Silty Soil Stabilization Using Industrial Waste-Based Curing Agents

Typically, we prepare this type of curing agent by blending industrial waste with a specific proportion of lime and cement. The elemental composition of industrial waste is similar to that of soil, primarily consisting of reactive silica oxides, alumina oxides, and other components [12]. The stabilization mechanism involves the reaction of reactive silica and alumina in the industrial waste under alkaline conditions, leading to the formation of cementitious materials through a hardening process [13]. These cementitious materials create effective bonding forces between the curing agent and soil particles. As a result, the stabilized soil exhibits a steady increase in strength over an extended period, along with enhanced water stability and frost resistance. Figure 3 shows the stabilization mechanism of soil stabilized with industrial waste-based curing agents.

Figure 3. The Stabilization mechanism of soil stabilized with industrial waste-based curing agents [14].

Figure 3. The Stabilization mechanism of soil stabilized with industrial waste-based curing agents [14].

2.3. Mechanism of Polymer-Based Stabilization of Silty Soils

The stabilization mechanism of polymer-based soil curing agents in silty soil can be categorized into three key aspects:

• Thinning of the double layer
  When polymer materials are mixed into the soil, they adsorb onto the surfaces of soil particles. This adsorption shields the binding of water molecules, reducing their adsorption on particle surfaces. Concurrently, the polymer increases the ion concentration and ionic valence in the pore water, thereby decreasing the degree of ion hydration. These combined effects reduce the thickness of the double electric layer. A thinner double layer weakens the soil’s hygroscopicity. Since water absorption typically induces significant soil expansion, reduced hygroscopicity minimizes volumetric swelling, leading to improved soil strength.
• Enhancing interparticle bonding
  The adsorption of abundant polymer materials onto soil particle surfaces lowers the surface free energy of the particles while amplifying intermolecular attractive forces. This promotes stronger aggregation of soil particles, resulting in enhanced soil strength and structural integrity.
• Optimization of pore distribution
  The incorporation of polymer curing agents reduces the proportion of large-diameter pores within the soil matrix. This modification enhances the structural stability of the soil under load and promotes a more uniform pore size distribution. These changes collectively improve the soil’s bearing capacity and resistance to deformation.

2.4. Mechanism of Silty Soil Stabilization Using Geopolymer-Based Curing Agents

The solidification of soil using geopolymers involves the alkaline activation of silica-alumina-rich precursors, which undergo a geopolymerization reaction to form a geopolymer gel. During this process, a portion of the gel condenses and hardens into a rigid polymer skeleton, while the remaining gel encapsulates soil particles and fills interparticle voids, creating densely packed aggregates [15]. The interconnected polymer skeleton and aggregates collectively establish a three-dimensional structural framework, enhancing the soil’s macroscopic mechanical properties, such as compressive strength and load-bearing capacity, through improved interparticle bonding and pore structure optimization [16]. This reaction mechanism is illustrated in Figure 4.

At present, research on geopolymer-stabilized silty soils predominantly focuses on macroscopic mechanical properties or microscopic structural analysis, with limited attention given to the underlying reaction mechanisms governing geopolymer-soil interactions. This gap can be attributed to two primary factors. Firstly, the composition of geopolymers is significantly influenced by precursors, alkali activators, water-binder ratios, and other variables. The presence of soil has been shown to alter these parameters, particularly in geopolymer-soil systems with low geopolymer content. However, excessive geopolymer content can lead to substantial cost increases, making the process economically unviable. Consequently, controlling geopolymer properties in geopolymer-soil systems is considerably more complex than in pure geopolymer systems. Secondly, the high alkalinity of geopolymers has been found to alter soil properties. For example, the dissolution of silica and alumina components in alkaline solutions has been shown to induce changes in soil characteristics. Moreover, the physical properties and structural components of silty soils vary across regions. Using a single type of geopolymer to solidify soils from different geographical areas may therefore result in significantly different outcomes.

To address these challenges, future research should comprehensively consider the diversity of soil types and the various factors influencing geopolymer performance. Further investigation into the reaction mechanisms of geopolymer-soil solidification is necessary to develop a more precise model of the reaction mechanisms within geopolymer-soil systems.

3. Mechanical Properties of Silty Soils Stabilized with Different Types of Curing Agents

3.1. Unconfined Compression Strength

The unconfined compressive strength (UCS) serves as the most direct mechanical parameter reflecting the macroscopic performance of solidified silt soil treated with curing agents [18]. Studies by Li et al. [19] and Chen et al. [20] demonstrate that the type of curing agents is a primary factor influencing the UCS of solidified silt soil. Significant variations in solidification effects occur when different categories of curing agents are employed for silt soil stabilization. The following sections elaborate on the solidification effects of four distinct types of curing agents: traditional inorganic curing agents, industrial waste-based curing agents, polymer-based curing agents, and geopolymer-based curing agents.

3.1.1. Traditional Inorganic-Based Silty Soil Curing Agent

Traditional inorganic curing materials generally consist of powdered cementitious substances, predominantly cement and lime. The dosage of curing agent constitutes the principal factor governing the strength enhancement of stabilized silt. As evidenced by studies [18,21,22], the compressive strength of cement-treated silt is predominantly controlled by cement content, with the initial water content of the native soil being a secondary determinant. Notably, each soil type exhibits a critical minimum cement dosage threshold; sub-threshold applications result in negligible solidification efficacy [23]. Experimental investigations by Tang et al. [21], Boutouil et al. [22], and Horpibulsuk et al. [24] establish a linear proportionality between the UCS of stabilized soils and cement dosage within defined ranges of additive content and curing duration (Figure 5). While this correlation aligns with findings from other researchers [25,26], discrepancies emerge in the quantification of minimum effective dosage. Through systematic analysis, Tang et al. [23] identified soil initial water content as the primary determinant of these variations, demonstrating an exponential increase in the critical cement dosage with elevated moisture levels. Moreover, Tang’s empirical studies revealed a quadratic inverse correlation between the compressive strength of cement-stabilized soils and their water content.

The engineering characteristics of lime-stabilized silt are predominantly governed by lime content. Zhu et al. [28] demonstrated that lime dosage exerts a pronounced influence on the performance of lime-treated silt. Within optimal lime content ranges, the UCS exhibits a near-linear enhancement proportional to lime addition [29]. Lin et al. [30] further identified specific surface area as a critical parameter influencing stabilization efficacy. Through innovative application of a slaked lime dehydration method, they engineered a high-performance lime-based curing agent using secondary quicklime. This advancement primarily stems from two mechanisms: (1) the secondary quicklime’s exceptionally high specific surface area and superior dispersibility facilitate intensified clay-lime interactions, yielding greater cementation products compared to conventional lime; (2) the accelerated carbonation kinetics at the interface of secondary lime-stabilized soil induces synergistic strength enhancement through rapid surface densification mediated by nanoscale pore refinement and interparticle bonding reinforcement.

Furthermore, Wang et al. [31] demonstrated through comparative testing that cement-stabilized silt exhibits significantly superior mechanical properties to lime-treated sediments. Jauberthie et al. [32] revealed enhanced stabilization efficacy when employing a dual-agent system combining lime and cement for dredged sediments from the Saint-Malo estuary in France, achieving higher UCS compared to single-agent treatments.

In summary, traditional inorganic curing agents represented by cement and lime exhibit certain solidification effects on silty soil, yet this approach still presents the following limitations and deficiencies: (1) the required dosage of curing agents is relatively high, while the early-stage strength of solidified soil generally remains low; (2) for specific moisture content, the UCS of cement-stabilized soil demonstrates distinct segmented zones with varying cement content—active zone, inert zone, and degradation zone. In the active zone, soil UCS shows significant improvement; in the inert zone, UCS growth slows markedly; whereas in the degradation zone, excessive cement content leads to insufficient water available for hydration, consequently resulting in reduced soil UCS with increasing cement content [33]; (3) the production process of traditional curing agents consumes substantial energy and emits large quantities of carbon dioxide, imposing a significant burden on the environment [34]; (4) cement/lime-stabilized soils commonly suffer from issues such as proneness to cracking, excessive shrinkage, poor water stability, and inadequate durability [35].

3.1.2. Industrial Waste-Based Silty Soil Curing Agent

Industrial waste refers to by-products generated during industrial production processes, including mining waste rocks, mineral processing tailings, fuel residues, and metallurgical or chemical process slags [36]. Currently, the most commonly used industrial waste additives include fly ash, gypsum, and metakaolin. Research indicates that in specific regions or individual projects, cement-based curing agents incorporating industrial waste materials have demonstrated promising results in soil improvement [37]. Domestic and international scholars have conducted valuable theoretical research and pilot projects in this field [38,39,40,41,42].

Huang et al. [38] investigated the use of waste gypsum combined with cement to stabilize soft soil. They found that the cement-waste gypsum curing agent proved to be an effective binder for loose and porous soft soils, demonstrating significantly enhanced stabilization effects compared to cement alone. This improvement stems from the dual action of hydration products: C-S-H generated from cement binds loose soil particles, while ettringite formed by the reaction between cement and waste gypsum expands to fill pores. However, studies revealed that the strengthening effect of ettringite depends critically on the concentrations of CaO and OH⁻ in the system. When these concentrations reach saturation, excessive ettringite growth can degrade structural strength. To optimize ettringite formation for strength enhancement, Xun Yong [37] proposed incorporating fly ash alongside waste gypsum. In the cement-waste gypsum system, the SiO₂ and Al₂O₃ in fly ash react with Ca(OH)₂ to form additional C-S-H and C-A-H. This reaction not only reduces Ca(OH)₂ concentration, promoting beneficial ettringite growth, but also increases the quantity of C-S-H, enhancing inter-crystalline cementation. Furthermore, the C-S-H can react with sulfates to generate additional ettringite, collectively improving the curing agent’s performance. Meng et al. [39] studied the stabilization effects of cement, fly ash, and gypsum on dredged sludge under varying ratios. Through orthogonal experiments, they identified an optimal mix ratio for East Lake dredged sludge at specific moisture contents: 20% cement (by mass relative to soil), with a cement: fly ash: gypsum ratio of 1:3:0.3. Their analysis clarified the roles of each component: cement dominated the stabilization process, exerting the most significant influence; fly ash accelerated early-stage strength development by effectively reducing the initial moisture content of the sludge; and gypsum contributed to both early and long-term strength formation. Additionally, the introduction of auxiliary curing agents altered the stress-strain curve morphology of stabilized sludge, shifting from strain-hardening behavior in pure cement-stabilized samples to strain-softening patterns. Similarly, Yu et al. [40] reported that a composite curing agent incorporating phosphogypsum and fly ash as partial replacements for lime exhibited superior soil stabilization performance.

Research indicates that metakaolin (MK) not only enhances the strength development of cement-stabilized soil but also improves its durability. Chu et al. [41] pioneered the study on incorporating metakaolin to mitigate the corrosion of cement-stabilized soil caused by soluble salts. Their findings revealed that metakaolin accelerates hydration and pozzolanic reactions in cement-stabilized soil, promoting the formation of C-S-H while inhibiting the generation of harmful phases such as calcium chloroaluminate hydrate and expansive compounds like ettringite and thaumasite. This mechanism effectively enhances both the mechanical properties and long-term durability of the stabilized soil. Zhang et al. [42] further investigated the reinforcement mechanisms of metakaolin. They demonstrated that metakaolin not only modifies pore volume distribution through physical filling but also triggers secondary hydration and pozzolanic reactions. These processes generate additional binding phases, including C-S-H, C-A-H and ettringite, which collectively strengthen interparticle bonding. Consequently, the incorporation of metakaolin significantly improves the compressive strength of cement-stabilized soil, particularly during the early stages of curing.

In summary, composite curing agents formed by partially substituting cement and lime with industrial waste materials demonstrate superior stabilization performance compared to single-use cement or lime. However, the actual consumption of industrial waste remains limited in such applications, and the associated cost improvements are relatively marginal.

3.1.3. Polymer-Based Silty Soil Curing Agent

Numerous scholars have investigated the application of polymer materials for stabilizing silty soils, with notable effectiveness observed in lignin-based solutions, urea-formaldehyde resin systems, and polyurethane grouts. Cai et al. [43] demonstrated that lignin additives improve the engineering properties of silty soils, where both lignin content and curing duration significantly influence the strength of stabilized soils. Microchemical analyses revealed that these improvements stem from reduced double-layer thickness and enhanced cementation through positively charged lignin polymers. Figure 6 illustrates the mechanism of silty soil stabilization using lignin combined with enzyme-induced carbonate precipitation (EICP). Studies by Zhang et al. [44,45] and Gao et al. [46] further confirmed that appropriate proportions of lignin effectively enhance the unconfined compressive strength of silty soils, though excessive proportions may cause strength degradation. Zou et al. [47] developed a modified urea-formaldehyde resin by blending it with other polymers. When applied to subgrade soil stabilization, this modified grout simultaneously improved soil strength and water resistance.

Li et al. [48] explored the strength characteristics and mechanisms of polyurethane-grouted silty soils. Their findings indicated that water-permeable polymer grouts markedly enhance soil properties, with strength improvements scaling proportionally to polymer dosage. Notably, polyurethane primarily enhances soil strength through physical mechanisms—such as adhesion, encapsulation, pore filling, and particle bridging—rather than chemical reactions with the soil matrix.

In summary, polymer-based curing agents exhibit advantages over other materials, including low dosage requirements, precise control over the strength of stabilized soils, simplified construction processes, and broad applicability. However, they demonstrate limitations such as poor water stability, limited long-term durability, and high susceptibility to environmental conditions. These constraints have consequently restricted their widespread practical application.

Figure 6. The mechanism of silty soil stabilization using lignin combined with EICP [49].

Figure 6. The mechanism of silty soil stabilization using lignin combined with EICP [49].

3.1.4. Geopolymer-Based Silty Soil Curing Agent

The term “geopolymer” was first proposed by French scientist Davidovits [50], who defined it as a class of cementitious materials formed through the dissolution of aluminosilicate precursors under alkaline activation, followed by polycondensation reactions that generate a three-dimensional network gel structure [51,52]. Studies [53,54,55,56,57] demonstrate that the reaction products and hardening characteristics of geopolymers are intrinsically linked to precursor composition. Louthenbach et al. [58] classified geopolymers into high-calcium, low-calcium, and calcium-free systems based on the Ca/Si ratio in precursor materials. The mechanistic influence of calcium on polymerization is illustrated in Figure 7. The calcium content in the reactants determines the types of final reaction products, thereby governing the macroscopic physical properties of geopolymers. Consequently, calcium-based geopolymer curing agents exhibit significant variations in their efficacy for stabilizing silty soft soils. Geopolymer-based soil curing agents typically consist of two components: a solid precursor and an alkaline activator. Experimental investigations [59,60,61,62,63,64] identify precursor type, precursor dosage, and alkaline activator formulation as the primary factors influencing the unconfined compressive strength of geopolymer-stabilized soils.

Figure 7. Effects of calcium on the mechanism of geopolymer reaction [65].

Figure 7. Effects of calcium on the mechanism of geopolymer reaction [65].

The type of precursor significantly influences the UCS of geopolymer-stabilized soils. Commonly used precursors include slag, fly ash, and metakaolin. Fly ash is categorized into high-calcium (HCFA) and low-calcium (LCFA) types based on its CaO content [60]. Both types have been successfully applied in stabilizing silty soils. As illustrated in the mechanistic model for fly ash-based geopolymer stabilization (Figure 8), Cristelo et al. [60] found that HCFA-based geopolymers exhibit higher early strength, while LCFA-based geopolymers achieve superior long-term strength. Calcium acts as a network modifier [66], playing a critical role in forming the aluminosilicate framework and influencing the polymerization degree of the glassy phases in fly ash. LCFA’s lower calcium content leads to a more polymerized glassy structure, reducing its reactivity [67,68] and resulting in lower early strength of stabilized soils [69,70,71]. Conversely, the higher CaO content in HCFA promotes the formation of calcium-sodium aluminosilicate hydrate (C-N-A-S-H) gels, enhancing early strength. To address the low early strength of LCFA-based geopolymers, Yang et al. [72] developed an alkali-activated, high-volume fly ash curing agent (60% LCFA, 30% inorganic binder, and 10% alkali activator by mass). The activator (sodium sulfate) accelerates silicate hydration, generating ettringite and Ca(OH)₂ crystals. The Ca(OH)₂ interacts with the alkaline activator to create a strongly alkaline environment, boosting pozzolanic reaction rates and improving early strength development. Additionally, studies have shown that slag-based geopolymers exhibit high early strength but slow long-term strength gain [73], prompting research into hybrid slag-fly ash systems. Yaghoubi et al. [74,75] demonstrated that slag-fly ash geopolymers can serve as sustainable binders for silty soils, though their UCS is highly dependent on the slag-to-fly ash ratio. Wu et al. [76] further optimized the slag-fly ash ratio in geopolymer curing agents, demonstrating that a 9:1 mass ratio achieves the highest UCS of 1.5 MPa after 14 days of curing. This enhancement stems from two synergistic mechanisms: (1) Alkali-activated slag releases Ca²⁺, which reacts with fly ash-derived SiO₂ and Al₂O₃ to form cohesive hydrates (C-S-H, C-A-S-H and C-A-H), and (2) the spherical fly ash particles improve slurry fluidity, ensuring uniform curing agent distribution. However, excessive fly ash triggers preferential formation of rigid sodium aluminosilicate hydrate (N-A-S-H) gels [77], whose stable tetrahedral networks resist secondary reactions with slag hydrates, weakening soil cohesion. These findings underscore the critical balance between fly ash’s dual roles as a chemical reactant and physical fluidizer in hybrid geopolymer systems.

Othman et al. [78] systematically evaluated the performance of metakaolin (MK)-based geopolymers in stabilizing silty soil and demonstrated a clear dose-dependent enhancement in UCS. Their experimental results indicated that the UCS of soft soil increased significantly with the addition of MK, particularly at concentrations of 10% and 12%, achieving values of approximately 9.9 MPa and 9.18 MPa, respectively (Figure 9). These findings confirm the efficacy of MK-based geopolymers as effective stabilizers for improving the mechanical strength of soft soils. This improvement confirms the efficacy of metakaolin-based geopolymers in enhancing soil strength. Guo et al. [59] studied the strength characteristics and micro-mechanisms of metakaolin-alkali cement blends in stabilizing high-plasticity clay. Their findings revealed that the UCS of treated clay increased with higher metakaolin substitution ratios for cement. When metakaolin replacement rose from 10% to 20%, the UCS improved by 21.8%, accompanied by a significant refinement in pore structure, characterized by a shift from larger to smaller pores.

Figure 8. Mico-mechanism model of alkali-activated fly ash solidified sludge [79].

Figure 8. Mico-mechanism model of alkali-activated fly ash solidified sludge [79].

Figure 9. Effect of MK content on the UCS of MK-based Geopolymer-stabilized soil at various curing age [78].

Figure 9. Effect of MK content on the UCS of MK-based Geopolymer-stabilized soil at various curing age [78].

The UCS of stabilized soils exhibits distinct development patterns depending on precursor dosage. Horpibulsuk [80] investigated the use of carbide slag and fly ash blends to improve silty clay strength, identifying three characteristic zones: active zone, inert zone, and degradation zone (as shown in Figure 10). In the active zone, strength increases significantly with carbide slag content until reaching a saturation threshold. Beyond this threshold (inert zone), strength gains slow with further dosage increases. In the degradation zone, strength declines as carbide slag content rises. Arulrajah et al. [81] determined an optimal geopolymer precursor dosage of 20%, with deviations (either lower or higher) adversely affecting UCS. At 10% precursor content, insufficient geopolymer network formation occurs, resulting in low strength. When dosage increases from 20% to 30%, strength growth decelerates due to non-uniform geopolymer gel distribution. Excess precursor promotes flocculent crystalline particles rather than cohesive networks, and crystal expansion over time induces internal repulsive forces, causing localized bond fractures and strength reduction. Yao et al. [16] also studied UCS variations with geopolymer dosage (geopolymer-to-soft soil mass ratio), concluding that 14% precursor content yields optimal strength.

Figure 10. Relationship between unconfined compressive strength and calcium residue content.

Figure 10. Relationship between unconfined compressive strength and calcium residue content.

The type of alkali activator significantly influences the UCS of geopolymer-stabilized soils. Commonly used alkali activators are categorized into four classes: caustic alkalis, alkaline silicates, alkaline carbonates, and alkaline sulfates [82,83]. Wang et al. [79] systematically compared the effects of NaOH, Na₂SiO₃·9H₂O, and Na₂CO₃ on the UCS of fly ash-based geopolymer-stabilized soils. Their results revealed that NaOH demonstrated the highest UCS enhancement, followed by Na₂CO₃, while Na₂SiO₃·9H₂O exhibited the least effectiveness. Yaghoubi et al. [74] further found that sodium-based activators achieved greater strength development in geopolymer-stabilized soils compared to potassium-based counterparts. Specifically, when K₂SiO₃, KOH + K₂SiO₃, or Na₂SiO₃ were used, the UCS showed no significant improvement. In contrast, KOH, NaOH, and NaOH + Na₂SiO₃ activators enabled substantial strength gains with extended curing periods.

It is evident from Figure 11 that the alkali activator dosage also significantly impacts the strength of stabilized soils. Chimoye [84] observed that the strength of soils stabilized with novel composite binders increases with higher alkali activator dosages. Wang et al. [85] in their study on metakaolin-based geopolymer-stabilized soils, identified an optimal alkali activator dosage. At low dosages, geopolymer-stabilized clay soils show marked strength improvements, whereas excessive dosages negatively affect strength development. Liu et al. [17] further demonstrated that the optimal alkali activator dosage varies substantially depending on geopolymer type. High-calcium geopolymers exhibit a distinct optimal dosage range, while low-calcium geopolymers benefit from progressively stronger activation efficacy under increasingly alkaline environments.

Figure 11. UCS of stabilized soils with different alkali activator contents [85].

Figure 11. UCS of stabilized soils with different alkali activator contents [85].

In summary, the widespread application of geopolymer-based soil curing agents can effectively enhance the resource utilization rate of bulk industrial solid waste, significantly reduce greenhouse gas emissions such as carbon dioxide, and decrease the consumption of non-renewable energy sources such as coal, thereby contributing to the achievement of the “Carbon Peak and Carbon Neutrality” strategic goals. However, due to the diversity of raw materials and alkaline activators, coupled with significant variations in soil properties across different regions, it is essential to develop tailored geopolymers with optimal stabilization effects for soils of distinct characteristics during implementation.

3.2. Shear Strength

Shear strength, one of the primary mechanical properties of soils, serves as a critical indicator for evaluating the mechanical performance of stabilized silty soils. Liu et al. [86] investigated the shear strength of dredged sludge stabilized with industrial solid wastes (slag, desulfurization gypsum, and fly ash) and construction residues. Comparative analysis of shear strength parameters across multiple samples revealed that specimens with higher slag content exhibited greater shear strength, while those with higher fly ash content showed lower values. This indicates that slag plays the most significant role in sludge stabilization, followed by desulfurization gypsum and fly ash. In a related study, Liu et al. [87] examined the effects of fly ash and slag content on the stabilization of river-dredged sludge. They observed that increasing fly ash and slag dosages initially decreased and then increased the water content of stabilized sludge, while shear strength followed an inverse trend—initially rising and subsequently declining. An optimal mix ratio (6% fly ash and 1.5% slag) was identified, yielding the most effective stabilization performance. Wang et al. [88] explored shear strength variations in geopolymer-stabilized soils, demonstrating that geopolymers substantially enhance the shear strength of soft soils. The shear strength of stabilized specimens exhibited a positive correlation with compressive strength, where both cohesion and internal friction angle increased with compressive strength within a defined range.

Li et al. [89] demonstrated that the shear strength of stabilized soils increase with the incorporation of fibers. Tang et al. [90] investigated the mechanical performance of non-cemented and cement-stabilized soils reinforced with randomly distributed short polypropylene fibers. Their findings revealed that fiber addition increases both the unconfined compressive strength and shear strength of stabilized soils. While interfacial bond strength and frictional resistance appear to govern the reinforcement efficacy, the dominant factors differ between fiber-reinforced non-cemented and cemented soils. In non-cemented systems, fiber-clay particle interactions primarily dictate interfacial strength, whereas in cemented soils, interactions between fibers and cement hydration products play the dominant role. Prabakar and Sridhar [91] incorporated sisal fibers into clay and analyzed the shear strength of fiber-reinforced soil through triaxial tests, observing significant improvements. Jiang et al. [92] further identified an optimal fiber dosage range through extensive laboratory tests. Within this range, both the internal friction angle and cohesion of the soil increase markedly with higher fiber dosage ratios. Beyond this threshold, additional fibers provide diminishing returns or even adverse effects.

In summary, the incorporation of fibers in moderate quantities effectively enhances the shear strength of stabilized soils through two principal mechanisms. First, reinforcement is achieved through the mismatch in elastic modulus between the fibers and the soil matrix, which induces relative displacement and differential deformation under external loading. This mismatch results interfacial friction and interlocking forces at fiber-soil contact surfaces (Figure 12). Additionally, fibers redistribute tensile stresses during shear deformation, mitigating localized failure and improving shear resistance [93]. Second, the three-dimensional interwoven network formed by fibers homogenizes stress distribution and imposes spatial confinement on the soil matrix, further strengthening shear capacity. However, exceeding a critical fiber dosage threshold leads to adverse effects: overcrowded fibers cause entanglement and disrupt soil continuity, while their intricate arrangement promotes micro-pore formation, degrading overall performance [94].

A comprehensive analysis of the published literature reveals that the field tends to priorities maximizing a single parameter—such as UCS or shear strength—under idealized conditions. Although this approach has proven effective for preliminary screening, it has consistently fallen short in predicting in-situ performance with sufficient accuracy. The existing body of work would be significantly strengthened through the following improvements: first, systematic studies that concurrently measure UCS, shear parameters, and stress-strain behavior are necessary to fully understand the material’s mechanical response. Second, greater emphasis should be placed on the durability of these mechanical improvements under repeated loading, wet-dry cycles, and freeze-thaw conditions. Third, a more critical evaluation of the economic and environmental costs associated with reported strength gains is essential—moving beyond technical feasibility to assess practical and sustainable viability. True progress in silt stabilisation technology lies not merely in achieving higher strength values, but in demonstrating balanced, durable, and sustainable mechanical performance.

4. Discussion

4.1. Analysis of Various Curing Agents

Table 2. Comparison of a typical geopolymer and OPC [96,97,98,99].

Parameter	Geopolymer	OPC
Energy consumption (calcination and crushing)	990 × 106 J/ton	3430 × 106 J/ton
Carbon emission	Low (169 kg CO2/m3)	High (306 kg CO2/m3)
Environmental impact	Alternative waste management solution	Release of cement kiln dust (CKD)
Major raw material	Industrial and agricultural wastes	Limestone, shale, rocks etc.
Thermal characteristics	Higher resistance to high temperatures	Lower resistance to high temperatures
Advantages	Strong, durable, sufficient, systematic case studies	Strong, durable, Low carbon footprint
Disadvantages	High carbon footprint, disposal issues, high alkalinity	limited application, high alkalinity, heat treatment, high dosages

summarizes the major types of polymers used in soil stabilization in comparison with the traditional curing agent—Portland cement. While cement has been extensively employed over the past several decades and has proven effective in numerous practical engineering scenarios, geopolymers also emerged as a promising alternative for soil stabilization [95]. Owing to the similarity in their cementitious reaction products, soil stabilized with geopolymers can achieve strength levels comparable to those stabilized with cement. Nevertheless, the mechanism of geopolymerization in soil systems is not yet fully understood, and further research is required to validate the performance of geopolymers in soil stabilization.

4.2. Challenges of Geopolymer Stabilization

Table 3 summarizes the unconfined compression test results for soils treated with different types of curing agents. Although geopolymers demonstrate considerable potential for soil stabilization based on existing literature, their practical implementation faces several critical challenges.

The predominant limitation hindering the widespread adoption of polymer soil curing agents lies in the scarcity of comprehensive, independently verified performance data [100,101,102]. Additionally, the absence of standardized protocols for predicting geopolymer curing agent performance presents a significant obstacle. This standardization gap is exacerbated by inconsistent dosage recommendations from manufacturers, which empirical studies have shown to be inadequate for achieving target performance enhancements in certain scenarios [101].

4.3. Future Research Priorities

4.3.1. Fundamental Mechanisms of Geopolymer Stabilization

Multiple mechanisms have been proposed to explain polymer stabilization in soils, including electrostatic attraction, hydrogen bonding, van der Waals forces, entropy increase, and physical binding. However, these mechanisms still lack conclusive experimental validation, particularly regarding relative contribution of each mechanism under different conditions, time-dependent evolution of interfacial interactions and quantitative structure-activity relationships, etc.

Common analytical techniques (XRD, SEM, FTIR, NMR) have been extensively employed to study soil mineralogy, polymer chemistry, and their potential interactions. While XRD is effective for characterizing crystalline soil minerals, FTIR and NMR are primarily employed to analyze polymer composition. Although fundamental, such material characterization represents only the preliminary stage of investigating polymer-soil interactions. Currently, SEM is the predominant technique for analyzing soil-polymer mixtures in published research. Its high-resolution imaging reveals polymers enveloping soil particles, binding grains, and filling voids—providing valuable visual evidence of interactions. Nevertheless, SEM cannot determine whether these interactions are purely physical or involve reaction products. Critical questions remain unresolved, particularly regarding how specific polymer functional groups interact with soil minerals, moisture, cations, and anions.

Solving these issues would eventually unveil the mechanisms of polymer stabilization, understand the limitations and applications of different polymers, select the correct polymers based on different soil conditions [96].

Furthermore, alkaline binder formulations require optimization to reduce their environmental footprint. Specifically, the production and transportation of NaOH and Na₂SiO₃ contribute significantly to CO₂ emissions. Minimizing this carbon impact is essential for climate change mitigation [103].

4.3.2. Field Performance Challenges

The establishment of reliable correlations between laboratory-measured properties and in-situ performance of stabilized soils remains a critical knowledge gap in geotechnical engineering [102]. For geopolymer-based curing agents to achieve widespread adoption as alternatives to conventional cementitious binders (e.g., Portland cement, lime, fly ash), it is essential to rigorously quantify their field-scale behavior based on laboratory characterization.

Whereas traditional stabilizers exhibit well-documented empirical relationships between unconfined compressive strength (UCS) and fundamental design parameters (resilient modulus, flexural strength), such correlations are often inadequate for polymer-stabilized systems. This limitation stems from intrinsic material differences: geopolymers demonstrate pronounced viscoelastic behavior with marked time-dependence, fundamentally altering their dynamic response under in-situ loading regimes. The resulting divergence in stress-strain characteristics between cementitious and polymeric systems necessitates development of specialized constitutive models, as conventional UCS-based predictions of field modulus fail to capture these complex rheological phenomena.

Table 3. Compressive characteristics of stabilized silty soils using different types of curing agents.

Raw Materials or Precursor	Alkali Activators	Alkali Binder Ratio	Curing Age	UCS(MPa)	References
OPC	-	-	28	1.27	[35]
MK	Na2SiO3 + NaOH	-	28	31.22	[104]
OPC	Na2SiO3 + NaOH + CaCl2	0.6	7	0.124	[105]
FA	Na2SiO3 + NaOH	0.8	28	4.08	[106]
FA and GGBS	Na2SiO3 + NaOH	0.35	28	1.53	[107]
FA and GBFS	NaOH	-	7	3.32	[108]
POFA	KOH	-	7	0.59	[109]
FA	OPC	0.5	120	6.84	[110]
CMK	Na2SiO3 + NaOH	1.5	90	0.317	[111]
MK	Na2SiO3 + NaOH	5.13	1	2	[78]
FA	Na2SiO3 + NaOH	1	14	0.649	[112]
SF	-	-	7	0.054633	[113]
FA and Cement	SR	2.14	7	0.41	[114]
FA(F) and GGBFS	Na2SiO3 + NaOH	0.5	28	0.574	[115]
Palm oil fuel ash	Na2SiO3 + NaOH	0.76	28	4.18	[116]
FA and Slag	Na2SiO3 + NaOH	1	28	1.034	[81]
FA and GGBS	Na2SiO3 + NaOH	0.67	7	2.27	[117]
MK	Na2SiO3 + NaOH	1.0	14	4.928	[118]
MK	Na2SiO3 + NaOH	3.1	28	5.6	[119]
FA and GGBS	Na2SiO3 + NaOH	0.32	28	0.86	[120]
FA and GGBS	Na2SiO3 + NaOH	-	28	8.5	[74]
FA and Slag	Na2SiO3 + NaOH	0.4	28	0.4	[121]
GGBS	Na2SiO3 + NaOH	17.4	28	-	[122]
MK	NaHCO3 + CaO	0.5	7	0.15	[85]
MK	NaOH	0.7	28	2.09	[123]
POFA	Na2SiO3 + NaOH	1.0	7	0.294	[124]
GP	CaO + MgO	-	7	1.2	[125]
GGBS	CS	0.25	28	0.22	[126]
WGP	OPC	1	28	0.18	[127]
POFA	KOH	-	28	1.48	[128]
FA(F) and PP	KOH	0.91	28	0.645	[129]

FA—fly ash; GGBS—ground granulated blast furnace slag; MK—metakaolin; OPC—cement; POFA—palm oil fuel ash; CMK—coal-bearing metakaolin; GP—glass powder; WGP—waste glass powder; PP—polypropylene fiber; SR—soda residue; CR—carbide sludge.

Table 3. Compressive characteristics of stabilized silty soils using different types of curing agents.

Raw Materials or Precursor	Alkali Activators	Alkali Binder Ratio	Curing Age	UCS(MPa)	References
OPC	-	-	28	1.27	[35]
MK	Na2SiO3 + NaOH	-	28	31.22	[104]
OPC	Na2SiO3 + NaOH + CaCl2	0.6	7	0.124	[105]
FA	Na2SiO3 + NaOH	0.8	28	4.08	[106]
FA and GGBS	Na2SiO3 + NaOH	0.35	28	1.53	[107]
FA and GBFS	NaOH	-	7	3.32	[108]
POFA	KOH	-	7	0.59	[109]
FA	OPC	0.5	120	6.84	[110]
CMK	Na2SiO3 + NaOH	1.5	90	0.317	[111]
MK	Na2SiO3 + NaOH	5.13	1	2	[78]
FA	Na2SiO3 + NaOH	1	14	0.649	[112]
SF	-	-	7	0.054633	[113]
FA and Cement	SR	2.14	7	0.41	[114]
FA(F) and GGBFS	Na2SiO3 + NaOH	0.5	28	0.574	[115]
Palm oil fuel ash	Na2SiO3 + NaOH	0.76	28	4.18	[116]
FA and Slag	Na2SiO3 + NaOH	1	28	1.034	[81]
FA and GGBS	Na2SiO3 + NaOH	0.67	7	2.27	[117]
MK	Na2SiO3 + NaOH	1.0	14	4.928	[118]
MK	Na2SiO3 + NaOH	3.1	28	5.6	[119]
FA and GGBS	Na2SiO3 + NaOH	0.32	28	0.86	[120]
FA and GGBS	Na2SiO3 + NaOH	-	28	8.5	[74]
FA and Slag	Na2SiO3 + NaOH	0.4	28	0.4	[121]
GGBS	Na2SiO3 + NaOH	17.4	28	-	[122]
MK	NaHCO3 + CaO	0.5	7	0.15	[85]
MK	NaOH	0.7	28	2.09	[123]
POFA	Na2SiO3 + NaOH	1.0	7	0.294	[124]
GP	CaO + MgO	-	7	1.2	[125]
GGBS	CS	0.25	28	0.22	[126]
WGP	OPC	1	28	0.18	[127]
POFA	KOH	-	28	1.48	[128]
FA(F) and PP	KOH	0.91	28	0.645	[129]

FA—fly ash; GGBS—ground granulated blast furnace slag; MK—metakaolin; OPC—cement; POFA—palm oil fuel ash; CMK—coal-bearing metakaolin; GP—glass powder; WGP—waste glass powder; PP—polypropylene fiber; SR—soda residue; CR—carbide sludge.

5. Conclusions and Future Perspectives

This comprehensive review synthesizes global advances in chemical stabilization of silty soils, elucidating the mechanisms of different curing agents, and key factors governing mechanical performance (e.g., unconfined compressive strength, shear strength) of chemically stabilized soils. Based on above analysis, the following conclusions and recommendations are derived:

• Sustainable Development Trends

The evolution of silt stabilization technology is increasingly aligned with eco-friendly and low-carbon objectives, driven by advancements in novel materials and policy mandates for energy conservation and environmental protection. Future research should prioritize lifecycle assessment (LCA) to quantify the environmental benefits of emerging stabilization approaches.

2.

Industrial Waste Valorization

The adoption of industrial byproduct-based curing agents (e.g., fly ash, slag) offers a dual advantage: (i) enhancing geotechnical performance while reducing material costs, and (ii) mitigating environmental challenges associated with waste disposal (e.g., land occupation, leaching risks). However, standardized protocols are needed to ensure consistent waste-to-resource conversion and address variability in byproduct composition.

3.

Geopolymer System Optimization

Low-calcium geopolymer-stabilized soils exhibit lower early-stage strength but stable long-term strength development, while high-calcium geopolymers achieve higher initial strength with less stable later-stage performance. Hybrid calcium-based geopolymer systems, combining the advantages of both while mitigating their limitations, represent a promising avenue for future research.

4.

Curing Agents’ Diversity Gap

Current research disproportionately focuses on conventional precursors (FA, POFA, GGBS, metakaolin), neglecting other industrially abundant byproducts. A systematic evaluation of alternative precursors is essential to establish comprehensive utilization frameworks and expand the sustainable material portfolio.

5.

Region-Specific Optimization

The efficacy of curing agents demonstrates strong regional dependency, primarily attributable to substantial variations in the physicochemical properties of dredged silt across different geological settings. Dredged silt exhibits significant spatial heterogeneity in particle composition, mineralogy, and chemical characteristics across different geological conditions, resulting in substantial performance fluctuations when employing identical curing agents. Therefore, a region-specific optimization selection of curing agents must be implemented, making curing agents tailored to the distinct properties of local silts.