All-solid-waste modified phosphogypsum-based backfill: Synergistic hydration, contaminant immobilization, and environmental assessment

Large-scale use of solid-waste-modified phosphogypsum (PG) backfill is vital for sustainable mining, but its hydration and contaminant immobilization mechanisms remain unclear. This study systematically reveals the multi-scale synergy, links mix design, microstructure, and macro-performance, and provides a scientific basis for sustainable mining engineering. This study elucidates the multi-scale synergistic mechanisms of a novel backfill formulated from PG, furnace ash, furnace slag, and gasification slag designated as the PCGFA matrix. Macroscopic evaluations demonstrate that the optimal mixture offers excellent pipeline workability, with a slurry fluidity of 284 mm and a bleeding rate of 12.2%. Its 28-d unconfined compressive strength reaches 1.41 MPa, which is functionally adequate for typical mine backfill requirements. Microstructural analyses reveal a highly coupled hydration mechanism where the alkaline environment co constructed by furnace and gasification slags depolymerizes robust aluminosilicate networks. This is suggested to trigger an intense pozzolanic reaction generating massive amorphous calcium aluminosilicate hydrate (C-A-S-H) gels and crystalline ettringite. Concurrently unreacted furnace ash microspheres provide a critical micro filler effect. This synergistic packing reduces the total cumulative pore volume to approximately 0.38 mL per gram and subdivides detrimental macropores from 70% to 56% transforming previously interconnected voids into an impermeable micro skeleton. Driven by the experimentally verified physical encapsulation and the proposed chemical entrapment, the leaching concentrations of heavy metals, fluoride and soluble phosphate are successfully suppressed. Finally comprehensive assessments reveal the optimized PCGFA achieves a low global warming potential of 67.13 kgCO 2 eq/t. This study provides crucial mechanistic guidance for the sustainable utilization of all solid waste modified PG backfill.