Mechanical properties and macroscopic failure characteristics of magnesium slag-based backfill under unloading confining pressure

The processing of mine solid waste into backfill materials for filling mining areas is one of the key methods of green mining. The deformation and failure patterns of the solid waste cemented backfill during the extraction of mine pillars directly affect the stability of the mining area. Through triaxial unloading confining pressure tests, the mechanical properties and macroscopic failure characteristics of magnesium slag-based (MS-based) backfill were systematically investigated under different MS contents (0%, 20%, 30%, and 40%) and initial confining pressures (0.5 MPa and 2.0 MPa). The evolution laws of deformation parameters of the backfill as confining pressure during unloading confining pressure were explored, and the influences of MS content on deformation parameters of the backfill under unloading confining pressure were discussed. The results show that during unloading confining pressure, the deformation modulus gradually decreases with decreasing confining pressure and drops sharply at failure, exhibiting a non-linear relationship with volumetric strain. Conversely, the generalized Poisson’s ratio gradually decreases with decreasing confining pressure and increases sharply at failure. The peak deviatoric stress of the backfill first increased and then decreased with increasing MS content. Under an initial confining pressure of 2.0 MPa, when the MS content is 30%, the peak deviatoric stress of the MS-based backfill reaches a maximum value of 16.329 MPa, which is 129.82% higher than that at 0% MS content. When the MS content is 0%, 20%, and 30%, the peak deviatoric stress and elastic modulus of the backfill under 2.0 MPa confining pressure were both higher than those under 0.5 MPa confining pressure. Under unloading confining pressure conditions, the MS-based backfill did not exhibit large-scale fractures or crushing, with only cracks appearing on the surface. Higher initial confining pressure can effectively inhibit the propagation of cracks and failure within the backfill, thereby enhancing its structural integrity and stability. This study provides a reference for the mixing ratio optimization and stability analysis of the MS-based backfill materials in mining engineering.