Effect of mineral composition on soil-water characteristics of red soil

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.