Geopolymer-Stabilized Silty Soil: From Microscopic Mechanisms to Macroscopic Performance

Silty soil, prevalent across China’s coastal regions and inland river basins, significantly impedes sustainable infrastructure development due to its problematic engineering characteristics like high water content, low permeability, and weak structure. To overcome this challenge, soil stabilization technique for the beneficial reuse of silty soil is regarded as a potential solution. However, conventional inorganic binders like cement and lime come with substantial drawbacks, i.e., high carbon emissions, considerable energy consumption, and limited capacity for industrial solid waste assimilation. These increasingly conflict with China’s “dual carbon” goals for green transformation and sustainable growth. In this context, geopolymer as a novel type of green cementitious material formed through alkali activation using silicon-aluminum-rich industrial solid wastes (such as slag and fly ash) as precursors offers a highly promising sustainable solution. With near-zero carbon emissions, high reactivity, and remarkable waste valorization properties, it enables both the efficient utilization of silty soil and the synergistic disposal of industrial solid waste. This review systematically represents the research progress on the stabilization mechanisms, macroscopic properties, and key scientific issues of geopolymer-stabilized silty soil from a multi-scale perspective. This review aims to provide a theoretical basis for developing high-performance geopolymer-based silty soil solidifiers and proposing efficient silty soil disposal strategies, while offering practical references for promoting the resource utilization of silty soil and the synergistic treatment of industrial solid waste.

First, the physicochemical properties and engineering characteristics of silty soil in different districts of China are analyzed. It is indicated that although the properties of silty soil exhibit certain regional variations, they all share adverse engineering characteristics such as high water content, high void ratio, high thixotropy, low permeability, and low stability. These unfavorable properties severely limit their engineering applications. As a novel inorganic chemical stabilization method, alkali-activated geopolymer stabilization technology has demonstrated broad application prospects in the field of silty soil improvement due to its excellent engineering performance and low-carbon, environmentally friendly features.

In terms of microscopic mechanisms, this review introduces the formation principles of geopolymers, including their reaction processes, strength formation mechanisms, and differences among various types of geopolymers. The reaction mechanisms of different calcium-based geopolymer-solidified soils are revealed. The results show that the strength of low-calcium geopolymer-stabilized soil primarily stems from the N-A-S-H gel with a three-dimensional network structure, while the reaction process of high-calcium geopolymer-stabilized soil is more complex, resulting in diverse cementitious products. Subsequently, the microscopic mechanism of geopolymer-stabilized silty soil is analyzed in detail. Specifically, silicon-aluminum-rich geopolymer precursors such as slag and fly ash, as well as SiO₂ and Al₂O₃ in the silty soil, gradually dissolve and undergo a structural destruction under the action of alkali activators. This is followed by repolymerization, generating a large amount of flocculent gel substances, primarily C-(A)-S-H and N-A-S-H. These gel substances adhere on the surface of vitreous bodies, with some condensing and hardening to form a polymer skeleton, while others encapsulate soil particles and fill the pores between them, forming aggregates. The polymer skeleton supports and binds the soil particle aggregates, constituting the spatial structural system of geopolymer-stabilized soil, thereby enhancing its mechanical properties. The mechanisms behind the superior performance of geopolymer-stabilized silty soil are further explained by microanalytical methods.

In terms of macroscopic properties, the results of some strength tests on geopolymer-stabilized silty soil conducted globally in recent years are systematically summarized. Geopolymer-stabilized silty soil exhibits significant advantages over cement-stabilized soil in terms of unconfined compressive strength, shear strength, and foundation bearing capacity. Its mechanical properties are affected by multiple factors, among which alkali activator concentration, modulus of alkali solution, precursor type, and dosage are studied for their patterns. Optimizing reaction conditions can further enhance the mechanical properties of stabilized soil to some extent. Moreover, geopolymer-stabilized silty soil also demonstrates excellent durability, with significant improvements in water stability, freeze–thaw resistance, wet-dry cycle resistance, and chemical corrosion resistance, compared to cement-based materials. It exhibits a desirable environmental adaptability. Geopolymers can reconstruct the soil skeleton through their unique alkali activation and geopolymerization processes, significantly enhancing the mechanical properties and durability of silty soil, thereby providing a solution for the treatment and reinforcement of silty soil.

Utilizing geopolymers to stabilize silty soil and applying it in engineering construction represents a highly promising approach for resource utilization. This method enables efficient and sustainable use of silty soil and promotes large-scale consumption of industrial solid waste in China, demonstrating exceptional environmental, economic, and social benefits. Geopolymer-stabilized silty soil exhibits excellent mechanical properties, with improvements in compressive strength, shear strength, and bearing capacity, compared to conventional inorganic stabilized soil. In addition, it also shows significant advantages in key durability aspects such as water stability and freeze–thaw resistance. This performance enhancement is a macroscopic reflection of its unique microstructure and chemical bonding capabilities.

Although the existing experimental research on geopolymer-stabilized silty soil has made significant progress in macroscopic mechanical properties and microstructural characteristics, the in-depth analysis of its microscopic stabilization mechanisms and systematic studies on the influence of organic matter content on stabilization effects still require a further attention. Moreover, research on stabilized silty soil predominantly focuses on single-component geopolymer systems, while studies on binary or multicomponent geopolymer-stabilized soil remain insufficient. The application of geopolymers in soil stabilization still faces severe challenges, such as the lack of unified testing methods, performance evaluation standards, and engineering design specifications. Future research should focus on investigating the complex interfacial reactions and bonding mechanisms between geopolymers and silty soil particles, actively expanding exploration of silty soil stabilization under multicomponent geopolymer systems, and accelerating the establishment of design and construction specifications for geopolymer-stabilized soil in various engineering applications. Building a full-lifecycle standardized system can provide a technical assurance for the large-scale promotion and application of this technology.