Structural Evolution and Hydromechanical Properties of Magnesia Materials–Solidified Sludge under Wet–Dry Cycles

Under wet–dry cycling conditions, the structural evolution and hydromechanical behavior of magnesia-stabilized sludge exhibit significant complexity, which is critical for its engineering applications. This study conducted systematic experiments through macro- and microstructural analyses, soil–water characteristic measurements, and mechanical performance tests. The results reveal that, although magnesia materials effectively suppress crack development, the progressive dissolution of cementitious compounds (e.g., magnesium silicate hydrate, magnesium aluminate hydrate, Phase 5, and brucite) with increasing cycle numbers leads to a reduction in interparticle bonding strength. This induces a dual structural deterioration effect: at the microscale, porosity accumulates progressively, while at the macroscale, large interconnected cracks form, ultimately causing a systematic decline in the soil’s water retention capacity, manifested as reduced saturated water content, diminished matric suction, and enhanced hydraulic hysteresis. This coupled degradation mechanism between bonding strength and pore structure results in a nonlinear decay of the material’s initially enhanced mechanical properties (e.g., compressive strength and elastic modulus) with ongoing cycles and fluctuating moisture content. Additionally, a bimodal pore model based on fractal theory accurately predicts soil–water characteristic curves, with microstructural parameters showing strong correlations with mechanical properties. The resulting predictive model for mechanical strength, integrating microstructural parameters and soil–water characteristics, provides a quantitative basis for the durability design of sludge stabilization projects.