Compensation-deterioration effect of phosphogypsum-red mud-based backfill under chloride salt conditions and wet-dry cycles.

The use of bulk industrial solid wastes such as phosphogypsum and red mud for mine backfilling promotes synergistic solid waste utilization and ecological restoration in mining areas. However, uncertainties remain in the long-term performance of backfill and heavy metal stability under chloride-rich groundwater erosion. In this study, a multisource industrial solid waste synergistic cementation system (PRFQ) was prepared from raw phosphogypsum (PG), red mud (RM), fly ash (FA), and quicklime (QL). Its performance evolution and degradation mechanisms were investigated after 12 wet-dry cycles in 0%-11% NaCl solutions. The results indicate that the chloride-salt corrosion resistance threshold of the PRFQ system was 4%, and the recommended PG content in the PRFQ system was 40-70 wt%. At a PG content of 50 wt% and an RM/FA/QL ratio of 5:1:2, after 12 wet-dry cycles in an 11% NaCl solution, the residual compressive strength was 1.82 MPa, the mass loss was 13.12%, and the heavy metal leaching concentrations met the Class III limits of the groundwater quality standards. Low-concentration Cl- promotes Fe/Al gel formation and particle bonding, compensating for microstructural defects. A higher Cl- concentration leads to Na+-induced decalcification, Friedel's salt expansion and NaCl crystallization stress, causing pore expansion and degradation. Wet-dry cycles accelerate Cl- migration and the dissolution of hydrates, but some decomposed products bond with Cl-, providing filling and passivation and slowing degradation. This study supports the safe, high-value utilization of PG, RM, and FA in chloride-rich environments.