Degradation Mechanisms of Concrete in Underground Coal Mine Water Tanks Subjected to Sulphate Attack

To simulate the partial immersion exposure typical of underground water storage, concrete specimens of different strength grades were partially submerged in Na2SO4 solution. Ultrasonic pulse velocity testing, microhardness measurements, and nanoindentation were employed to evaluate physical and micro-mechanical property degradation. In addition, the phase composition and content changes in different concrete regions using X-ray diffraction and thermogravimetric–differential scanning calorimetry are analysed to detect the time-varying phase compositions of concrete in various regions. The findings indicate that with the progress of corrosion, the overall infiltration of the wet area of C30 concrete tends to stabilize after attaining a certain height along the waterline. Subsequently, the corrosion solution gradually infiltrates and ascends along the concrete surface, leading to the overall breakdown and damage of the concrete within the climbing area from the water level line. In the corrosion and deterioration process of C40 concrete due to sulphate exposure, the corrosion solution gradually ascends from the concrete surface. The degree of surface degradation correlates with the height of the corrosion solution, eventually reaching the top of the concrete. As the sulphate surface climbing height increases, the ongoing alkali consumption reaction between the sulphate and the cement hydration product ultimately causes the fracture and decomposition of the cement stone structure.