Influence of Dry–Wet Cycles and Chemical Pollution on Red Soil Improved with Building Gypsum Powder

Red soil exhibits a high susceptibility to geological disasters and engineering instability owing to its significant dispersibility and substantial strength attenuation upon exposure to water. Consequently, there is an urgent necessity to enhance its engineering properties. The red soil region in the southwest of China has long been exposed to a complex environmental setting characterized by alternating dry and wet soil conditions and chemical erosion, influenced by the monsoon climate and industrial activities. In this research, building gypsum powder was employed as a modifying agent. By conducting direct shear and consolidation tests in conjunction with microscopic techniques, including SEM, XRD, and XRF, an investigation was carried out to explore the effects and underlying mechanisms of acetic acid, sodium hydroxide, and sodium sulfate on the mechanical properties of red soil modified by building gypsum powder under dry-wet cycles. The findings reveal that acetic acid leads to the dissolution of gypsum components, the initiation of pore development, and a substantial reduction in soil strength. Sodium sulfate, when present at low concentrations, crystallizes to fill pores and augment strength, whereas high-concentration crystallization results in expansion and structural damage. Sodium hydroxide triggers the thickening of the double electric layer and colloid precipitation, thereby cementing the soil skeleton and mitigating the adverse effects of dry-wet cycles. As the number of dry-wet cycles increases, the soil pores expand, the soil structure loosens, and the shear and compressive resistance diminish. By utilizing gray correlation analysis and fractal theory, a quantitative relationship between microstructural parameters and disintegration is established, which elucidates the predominant role of pore characteristics in determining macroscopic mechanical properties and the underlying mechanism of mineral component alterations. This research offers a theoretical foundation and strategic guidance for the prevention and control of red soil disasters as well as the optimization of geotechnical reinforcement materials.