Effect of Freeze–Thaw Cycles on the Microstructure Characteristics of Unsaturated Expansive Soil

The term “engineering cancer” refers to expansive soil, whose properties threaten the stability and safety of structures. As a result, appropriate steps must be taken to guarantee the sustainable development of buildings. To explore the impact of freeze–thaw cycles (FTCs) on the microscopic characteristics of unsaturated expansive soil in the cold region, the mineralogical composition and microstructure were analyzed using X-ray diffraction (XRD), thermogravimetric analysis, and scanning electron microscopy (SEM). The influence of repeated FTCs on the characteristics of particle morphology and pore structure in expansive soil was quantitatively examined. The findings indicate that, in comparison to other expansive soil samples, the Yanji expansive soil is particularly susceptible to failures due to its high sand content and low liquid limit. The FTCs significantly alter the microstructure, leading to increased complexity in the particle edge shapes, a transition in particle distribution from dispersed to more concentrated, a reduction in larger particles, and a more intricate spatial arrangement of particles. As moisture content rises, the impact of FTCs becomes increasingly pronounced. The particle distribution’s area probability index and fractal dimension are identified as medium-variability parameters, with a high-variation coefficient before the 3rd FTC, which then gradually decreases. The repeated FTCs result in particle breakage and agglomeration, causing the particle size to become more uniform and the soil’s microstructure to stabilize after 3–5 FTCs. These findings contribute to understanding the FTC behavior of expansive soils, provide theoretical support and scientific guidance for disaster prevention and control measures, as well as for the sustainable development of engineering projects involving expansive soil sites.