Mechanical properties of cemented coal gangue–flyash backfill under coupled effects of water pressure and chloride salt erosion

In paste backfill mining, cemented coal gangue-flyash backfills (CGFB) can effectively control surface subsidence. CGFBs are subjected to water pressure and chloride ion erosion in the gob. Therefore, an improved understanding of the influence of pressurized water and chloride salt erosion on the performance of CGFB is crucial for realizing effective green mining. In this study, CGFB samples were soaked in a NaCl solution at 0, 0.5, 1.5, or 3.0 MPa for 15 d. The mechanical properties of the samples and deterioration mechanisms were investigated using uniaxial compression tests, acoustic emission tests, digital speckle strain measurements, scanning electron microscopy, and X-ray diffraction. The results show that the uniaxial compressive strength (UCS) increased and then decreased with the increase of soaking pressure. When the soaking pressure increased from 0 to 1.5 MPa, the average UCS increased by 43.5%. Then, when the soaking pressure increased from 1.5 to 3.0 MPa, the average UCS decreased by 18.9%. Moreover, water pressure promotes chloride ions into the interior of CGFB and the production of Friedel’s salt. Higher water pressures-chloride salt erosion coupling increases the porosity of CGFB, with the 3.0 MPa sample showing an 8.2% increase in porosity compared to the 0 MPa sample. Thus, internal pore cracks developed and penetrated the samples, which degraded their mechanical properties and reduced their strength and compactness.