Microstructural Evolution of Coal Immersed in High-Mineralization Mine Water

To address the issue of microstructure damage and stability deterioration of coal pillar dams in underground coal mine reservoirs caused by long-term exposure to highly mineralized mine water, this study conducts indoor simulation experiments to investigate the evolution of coal microstructures under immersion in salt solutions with varying mineralization degrees (1000–2000 mg/L). Changes in solution chemical parameters are monitored through dynamic water quality analysis, while the pore-fracture structure of coal is quantitatively characterized using scanning electron microscopy (SEM) and a pore-crack analysis system (PCAS). Results indicate that with increasing mineralization, solution pH initially decreases due to H+ release from pyrite and siderite oxidation, followed by a slight recovery in later stages owing to the buffering capacity of HCO3−. The trends of TDS and EC are consistent, initially slightly decreasing due to ion adsorption, and then rising with the dissolution of minerals in the later stage. The continuous decline in ORP indicates a progressive enhancement in the solution’s reducing potential. Microstructural observations reveal that after immersion in highly mineralized solutions, the coal matrix undergoes more severe fragmentation, with increased pore quantity and irregular pore morphology, and the fracture network becomes more developed and interconnected. Quantitative analysis further demonstrates that, with increasing mineralization, the proportion of large pores (>24 μm) significantly rises; the fractal dimension first decreases and then increases; the rise in probability entropy reflects enhanced spatial disorder in pore arrangement; and the pore area ratio increases. The research results provide a theoretical basis for the long-term stability evaluation and safety control of coal pillars in highly mineralized mine water environments.