Effects of Different Confining Pressures and Curing Temperatures on the Mechanical Properties and Microscopic Mechanisms of Cemented Backfill Materials

Alkali-activated slag–loess cemented backfill materials are subjected to coupled confining pressure and curing temperature in open-pit end-slope backfill mining. This study investigates their mechanical properties and microscopic mechanisms under four confining pressures (0, 2, 4, and 6 MPa) and four curing temperatures (5, 20, 35, and 50 °C) at a curing age of 7 days. Uniaxial and triaxial compression tests were conducted to obtain stress–strain curves, peak strength, elastic modulus, cohesion, and internal friction angle. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and an improved simulated annealing algorithm for three-dimensional pore reconstruction were used to characterize pore diameter, porosity, connectivity, and hydration products (calcium-silicate-hydrate (C-S-H), calcium-aluminosilicate-hydrate (C-A-S-H), and sodium-aluminosilicate-hydrate (N-A-S-H)). The results show that increasing confining pressure flattens the post-peak softening curve and transitions failure from brittle to ductile, while rising curing temperature shortens the compaction stage and increases elastic modulus. Both factors increase peak strength synergistically. Cohesion increases nonlinearly with temperature (2.64 MPa at 5 °C to 6.27 MPa at 50 °C), whereas the internal friction angle (13°) is temperature-insensitive. Microscopically, confining pressure reduces pore diameter, porosity, and connectivity via physical compaction; curing temperature promotes gel production, decreasing porosity from 26.23% to 13.95% and connectivity from 64.87% to 34.89%. This study provides a theoretical basis for backfill design and ground pressure management in open-pit end-slope mining.