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.