Degradation Behavior and Damage Evolution Model of Sandstone under Coupled Stress-Temperature-Chemical Action

Seepage from water-rich strata and persistent roof water inflow during coal mining can substantially weaken the mechanical properties of coal and rock masses. Elevated in situ stress and geothermal temperature in deep roadways further accelerate deterioration and increase the risk of engineering instability. In this study, sandstone, a common coal-seam roof rock, was selected as the test material. Immersion and uniaxial compression tests were conducted under coupled thermal-alkaline water conditions. Scanning electron microscopy and X-ray diffraction analyses were used to clarify the strength degradation mechanism, and a uniaxial compressive strength characterization model accounting for thermal-chemical-stress damage was developed. The results show that immersion extends the compaction stage and reduces the elastic modulus and peak stress; these effects become more pronounced with increasing temperature and solution alkalinity. The uniaxial compressive strength decreases linearly with temperature and exponentially with pH, and temperature, pH, and their interaction all have significant effects on the uniaxial compressive strength of sandstone (p < 0.001). The strength degradation rate gradually declines as alkalinity increases, whereas higher temperature accelerates strength deterioration. The most pronounced deterioration occurs at 60 °C and pH = 12, where the uniaxial compressive strength and elastic modulus decrease to 37.724 ± 1.094 MPa and 1.260 ± 0.046 GPa, respectively, corresponding to reductions of 58.50% and 58.25% relative to the dry specimens at the same temperature. Microscopically, sandstone damage evolves from local crack development and fragment spalling to lamellar delamination, edge curling, and propagation of internal defects with increasing alkalinity. The net increase in SiO₃ ^2- concentration rises from 5.694 to 14.235 mg/L, indicating that the dominant water-rock interaction progressively shifts from wetting-induced softening and hydrolysis to alkaline dissolution. The proposed segmented damage constitutive model can describe the compaction stage, pre-peak damage evolution, and mechanical response near the peak. Three-fold leave-one-out validation yields mean relative errors of 6.17-12.89%, with an overall mean error of 9.63%. These findings provide a theoretical basis for evaluating and controlling the stability of surrounding rock in deep mines under elevated geothermal and water-temperature conditions.