Preparation and Performance Optimization of an Expansive Backfill Material for Active Roof Contact in End-Wall Mining

Conventional cemented backfill used in narrow end-wall mining entries commonly suffers from shrinkage and insufficient roof contact, reducing its support effectiveness. In this study, a solid-waste-based expansive backfill was prepared from mine overburden, ordinary Portland cement, fly ash and hydrogen peroxide. Orthogonal tests were performed to evaluate the effects of fly ash dosage, aggregate-to-binder ratio and hydrogen peroxide dosage on slurry flowability, expansion ratio and compressive strength. The slurry exhibited a flowability of 17.9–24.7 cm and an expansion ratio of 11.23–46.52%, confirming the gas-generating expansion effect of hydrogen peroxide. The aggregate-to-binder ratio was the dominant factor influencing flowability, expansion and 28 d strength. Multi-index optimization identified an optimal mix of 40% fly ash, an aggregate-to-binder ratio of 4:1 and 10% hydrogen peroxide. The optimized backfill showed age-dependent increases in uniaxial compressive strength (UCS) and elastic modulus, with failure evolving from inclined shear to tensile and tensile–shear composite modes. Its shear behavior followed the Mohr–Coulomb criterion, with cohesion of 1.61 MPa and internal friction angle of 30.13°. SEM observations indicated that aggregate skeleton support, cementitious bonding and gas-generating expansion jointly controlled the material performance.