Strategic Curing Period Optimization for Phosphogypsum-Based Cementitious Backfill: Mitigating Fluoride Leachate...

The resource utilization of phosphogypsum, which is a significant industrial solid waste, is crucial. In this study, a phosphogypsum-based cementitious backfill (PCB) was developed to promote its resource utilization using phosphogypsum, mine waste rock, and cement. The evolution of key performance parameters of PCB was monitored under simulated rainfall and groundwater leaching environments across an 80 days experimental time frame. Indoor leaching tests revealed a transition in leachate pH from strong to weak alkalinity, significant decreases in electrical conductivity and total dissolved solids after 20 days with stabilization at 80 days, and fluoride concentrations dropping below 10 mg/L during 30-60 days of leaching. The gypsum and calcite remained the dominant mineral phases; their relative contents and the chemical composition exhibited selective evolution under the coupled effect of curing period and leaching duration, with CaO content increasing significantly. Microstructural analysis revealed that the 28 day curing group exhibited the optimal structure, featuring well-developed ettringite crystals and a dense network of the C-S-H gel. Leaching increased the water content and decreased the density of PCB, reduced the proportion of fine particles, and led to granular coarsening. The porosity change depended on the curing period: it continuously increased for the 7 day curing group but initially increased and then decreased for the 14 day and 28 day curing groups. In terms of mechanical properties, short-term leaching reduced compressive strength, but strength gradually recovered under prolonged leaching. This research identified 28 days as the optimal curing period, providing theoretical support for the safe application of phosphogypsum in mine backfilling.