Impact of Dry–Wet Cycles on the Strength Characteristics of Phosphogypsum-Based Composite Stabilized Laterite Soils

Laterite soil (LS) is widely distributed and commonly utilized in engineering construction. However, its natural bonding degrades under cyclic dry–wet (D-W) conditions, resulting in volume instability and strength deterioration. To mitigate this problem, phosphogypsum (PG), as a sulfate-bearing industrial by-product, is often used in combination with cement or other stabilizing agents to improve the engineering properties of LS. Nevertheless, the durability and degradation mechanisms of PG-cement (PGC) stabilized LS under D-W cycles remain insufficiently understood. This study systematically investigates the durability of PGC-stabilized LS subjected to D-W cycles through unconfined compressive strength (UCS), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) tests. The results reveal that the UCS of PGC stabilized LS initially decreased after the first three cycles owing to internal loosening, then increased between the third and fifth cycles. Beyond ten cycles, a significant decline in UCS was observed, resulting from the completion of PG-cement hydration and ongoing structural degradation. SEM and MIP tests demonstrated that the degradation mechanism of stabilized soil arises from complex interactions among water, materials, chemical processes, and mechanical forces. The hydrophilicity of clay minerals influenced pore shrinkage and strength loss, with microstructural damage ultimately leading to macroscopic strength reduction. The findings provide both theoretical insights and practical guidance for the long-term performance assessment and green utilization of PGC in geotechnical engineering.