Aging deterioration of mechanical properties on coal-rock combinations considering hydro-chemical corrosion

Coal-rock masses located in water-rich environments, such as heavy water mines, and water-isolated coal-rock pillars installed underground, are consistently subjected to erosion from acidic groundwater. In this study, we employed various techniques including acoustic emission (AE), digital image correlation (DIC), scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), and X-ray diffraction (XRD) to investigate the evolution of mechanical properties and AE damage characteristics of coal-rock combinations under water-chemical corrosion. Moreover, we analyzed the microscopic deterioration mechanism. Our results reveal that as the acidity of the solution increases, the cumulative energy and AE count of the specimen decrease to varying degrees. The AE ringing counting process can be categorized into stable, active, and sharp development stages. Moreover, as the pH value of the solution decreases, the AE b-value follows a "V" type variation. The dynamic b-values demonstrate localized sudden drops, indicating the occurrence of large-scale cracks. The internal cracks within the coal-rock combination transform from shear cracks to tensile cracks, corresponding to a transition in failure mode from shear failure to cleavage failure. Furthermore, a detailed analysis of the deterioration mechanism of the coal-rock combination under water-rock interaction was conducted. Under on-site groundwater conditions (pH = 6.24), the overall degradation of uniaxial compressive strength (UCS) and elastic modulus reached 13.37% and 28.73%, respectively. The physical, chemical, and mechanical effects of the water-chemical solution induce microstructural changes within the coal-rock combination. Consequently, the total percentage of macropores (>1 mu m) in the sandstone section increased by 8.85%, reaching 35.3%, thereby further deteriorating its mechanical properties. In contrast, the proportion of macropores in the coal section only increased by 0.95%. Therefore, water-rock action has a more severe damaging impact on the sandstone section compared to the coal section of the combination.