The Effect of Wetting-Drying Cycles on the Deterioration of the Physical and Mechanical Properties of Cemented Paste Backfill in Open-Pit Coal Mines

To promote the sustainable exploitation of open-pit coal resources, waste is used as backfill material to realize the comprehensive utilization of solid waste mine resources. We proposed a mining method that is a combination of the highwall mining and filling mining methods. Cemented paste backfill (CPB) samples were prepared with high-clay-mineral-content marl particles as aggregate and normal Portland cement, sulfoaluminate cement and gypsum as cementing materials. The physical and mechanical properties and microstructural evolution of CPB with different binder ratios under wetting-drying cycles were measured. The results showed that the CPB with 0-3 wetting-drying cycles underwent shear and tensile coalescence, and that with 4-10 cycles underwent shear coalescence. The unconfined compressive strength (UCS) and elastic modulus (EM) decreased exponentially with increasing number of wetting-drying cycles but decreased exponentially and cubically with increasing porosity, respectively. The EM is more sensitive to gypsum content than the UCS. CPB deterioration was divided into an initial deterioration stage and a secondary deterioration stage. The evolution curve of the total damage variable presents an 'S' shape, with an initial damage stage, an accelerated damage expansion stage, a decelerated damage expansion stage and an end damage stage. The research results provide a basis for improving the recovery rate of resources under highwall conditions, and the extensive utilization of stripping materials, and promote the coordinated development of coal resource exploitation and environmental protection.