Mechanical degradation induced by the alkaline water effects of weakly cemented fine-grained sandstone

Rock mechanical properties degradation induced by water-rock reactions may trigger various underground space disasters such as rib spalling or roof caving in underground spaces. To understand the alkalinity degradation effect on weakly cemented fine-grained sandstone (WCFS), a series of lab mechanics tests were conducted on the Jurassic fine-grained sandstone specimens which were collected from Da’nanhu No. 7 Coal Mine in China. By incorporating the results of X-ray diffraction, electron microscopy scanning and ion concentration detection, this paper investigations of the influence of different environments (Dry, deionized water, pH = 7, pH = 10, pH = 12) on the mechanical behavior and deterioration mechanisms of WCFS. Subsequently, we constructed a mechanical damage model to predict the behavior under varying dissolution environments. The results indicate that the mechanical parameters were negatively correlated to the pH level of solutions and alkaline water-rock interactive time duration. The UCS of corroded WCFS presents a nearly negative exponential relationship with the soaking duration; The hydro-chemical softening effect gradually decelerated with time duration; the higher the pH level, the more drastic the softening in the initial period; The decrease of elastic modulus experience a sharp decrease at initial immersion stage, a gradual decrease at the medium stage, and final stage approximation. Both cohesion and tensile strength were of a steep-gradual variation profile, while the impact of alkalinity was somewhat limited. Feldspar and kaolinite mainly participated in hydrolysis, hydration reactions, and alkaline corrosion, loosening the mineral particle structures with a resultant WCFS degradation. The weakening of rock strength is largely attributed to the transformation in the micro-fracture form, specifically the shift from transgranular to intergranular fracture. By introducing the chemical damage factor characterizing the molar mass variation of OH− in solutions, a constitutive model describing the stress and chemical damage of WCFS subjected to alkaline circumstances. The research findings contribute to a deeper understanding of the degradation behavior of WCFS in alkaline environments, providing new theoretical insights and practical guidance for roadway support design in weakly cemented strata.