Performance improvement of high-concentration phosphogypsum backfill materials: the effect of silica fume

This study investigated the effects of silica fume (SF) content on the rheological properties, stability and mechanical homogeneity of high-concentration phosphogypsum (PG) backfill materials to address the technical challenges including poor fluidity, high bleeding rate and non-uniform strength distribution. Five mixes with 0, 3%, 6%, 9% and 12% SF (10% cement, 0.46 water-to-solid ratio) were tested for fluidity, bleeding, rheology, packing density, water film thickness (WFT) and unconfined compressive strength (UCS) (7/28 d). The results demonstrated that SF significantly enhanced the stability of the backfill slurry, reducing the bleeding rate by 60.23% (from 9.90% to 2.77%) at 6% SF. SF transformed the slurry from shear-thickening to Bingham fluid behaviour with a 174.55% increase in dynamic yield stress. These changes amplified shear resistance and viscous resistance on particles, thereby inhibiting particle sedimentation and improving stability. WFT emerged as the critical parameter controlling fluidity and rheological properties. SF regulated WFT by modifying packing density and specific surface area. At 12% SF content, WFT was reduced by 83.74%, resulting in 16.67% decreased fluidity and 174.55% increased dynamic yield stress. Mechanical tests revealed enhanced homogeneity in backfill with SF incorporation, as evidenced by the non-uniformity coefficient decreasing from 1.37 (0% SF) to 1.05 (12% SF).