Mineral composition is one of the critical factors regulating the soil-water characteristic curve (SWCC) and micro-pore structure in red soil. The purpose of this study is to analyze the mechanism of mineral composition on the soil-water characteristics of red soil from the microstructural level. The response curve method was employed to systematically evaluate the effects of different mineral compositions on matrix suction, with three key ratios selected for investigation. Using the contact filter paper method, SWCCs were obtained for humidified soil samples with three distinct mineral proportions. Additionally, scanning electron microscopy and mercury intrusion porosimetry were utilized to reveal pore microstructure and distribution characteristics. The results indicate that the regression model constructed using Box-Behnken design demonstrates significant effects of kaolinite and illite on matric suction during variance fitting analysis, whereas montmorillonite exhibits no significant influence. The interaction between kaolinite and illite has a much greater impact on matric suction than the interaction between kaolinite and montmorillonite when a specific variable is fixed. Furthermore, the influence of mineral composition on the SWCC is minimal in the near-saturation and residual regions, where the curve remains relatively flat. In the transition region, the slope of the SWCC increases significantly for samples with high kaolinite content, while the slope is relatively small for samples rich in the other two mineral components. Regarding microstructure, samples with high kaolinite and illite content exhibit uneven sand particle boundaries, large flocculated structures with cross-distribution, and a rich macroporous structure. In contrast, the other two groups of samples display a relatively loose dispersed structure. Moreover, there is a strong correspondence between the dominant pore size range in the pore size distribution curve and the transition region of the SWCC, further validating the profound influence of mineral composition on the soil-water characteristics of red soil.

Peaty soil, a distinct category of soft foundation soil, exhibits unique physical and mechanical properties that are strongly influenced by its microstructure. Its high water content, organic matter content, low strength and permeability often result in significant engineering challenges. Enhancing the mechanical strength of peaty soil has thus become a central focus in geotechnical engineering. Using slag-based geopolymer to synergize with cement for solidification, the mechanical properties of peaty soil before and after stabilization were examined through unconfined compressive strength and direct shear tests. The mechanisms of improvement were further analyzed through microscopic techniques, including scanning electron microscope (SEM), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and Fourier transform infrared spectroscopy (FTIR). The results demonstrate that all three alkali activators contribute to the enhancement of the mechanical strength of the peaty soil, with NaOH showing the highest activation efficiency. Cement stabilization of peaty soil improves shear strength by reducing pore space and strengthening interparticle bonding via ion exchange, hydration product crystallization, and the formation of CaCO₃ and calcium silicate hydrate (C-S-H). Four stages i.e., dissolution activation, ion exchange, gel formation, and structural reorganization are identified in the reaction process of activated slag improving peat soil. The alkali activator facilitates the dissolution of the slag’s vitreous phase, promoting ionic polymerization that leads to the formation of calcium-alumino-silicate-hydrate (C-A-S-H) gel. Simultaneously, organic functional groups in the peaty soil engage in ion exchange, forming CaSiO₃ precipitates and establishing a “calcium bridge” structure. These reactions collectively contribute to the formation of a dense composite matrix, thus enhancing compressive strength. Grey relational analysis reveals that compressive strength is most strongly correlated with pore area, while shear strength shows the highest correlation with the shape factor. Modified soil specimens undergo five dry-wet cycles, with a minimum strength loss rate of 27%. These findings provide a theoretical foundation for the partial replacement of cement with alkali-activated slag in peaty soil stabilization, contributing both to soft soil improvement and the valorization of industrial byproducts. Furthermore, these results offer valuable insights for ground improvement in peat-rich regions, such as Yunnan, China.

The cement has important influence on the mechanical properties of granite residual soil. Kaolinite powder, montmorillonite powder, illite powder and red clay powder are used to replace the fine particles of granite residual soil to remold soil samples. Compression, shear, tensile and disintegration tests are carried out respectively to study the effects of different cements on the mechanical strength of soil. Results show that the soil particle aggregates after kaolin treatment are increased, and a skeleton structure with high strength is formed between the aggregates. However, the existence of large number of pores between the skeletons provides a channel for water intrusion, resulting in enhanced water absorption of the soil. Benefiting from the swelling property of montmorillonite in water, the treated soil surface has fewer pores and dense structure, and the expansive body provides high strength cementation capacity, making the mechanical characteristics of soil significantly improved. The mechanical strength of the soil sample treated with illite is low, but the soil surface appears relatively dense. From perspective of particles dispersed on the soil surface, illite powder is more used as fine material to fill the pores between large particles than as a cement with high strength between soil particles. After physical compaction, the occlusion between particles is enhanced due to compression; At the same time, after a long time of hydration reaction, the cement formed bridge cementation on the particle surface. Under the combined actions above, the mechanical strength of soil has been greatly improved. The type of cement would have a great impact on the mechanical properties. On the one hand, it depends on the characteristics of the cement itself, such as the water swelling characteristics of montmorillonite. On the other hand, it depends on the binding effect of the cement on the soil. The compressive, shear, tensile and collapse resistance of the soil samples are controlled by the binding mode of the cement and the soil sample.