A High-Phosphogypsum Multi-Solid-Waste Cementitious Binder for Backfill: Cross-Scale Insight into Pore Structure and Strength Development

Phosphogypsum (PG) is an industrial solid waste whose use in cementitious materials is limited by strength reduction at high dosages. This study evaluated a clinker-free multi-solid-waste binder containing 40 wt.% PG for cemented paste backfill using steel slag powder (SSP) and granulated blast-furnace slag (GBFS) as co-binders, with phosphate mine tailings and slime as aggregates. Uniaxial compressive strength (UCS), X-ray diffraction, scanning electron microscopy, and nuclear magnetic resonance were combined with image-based pore-structure sensitivity analysis to examine the relationships among hydration products, pore evolution, and strength development. The results showed that AFt and C–S–H-like gels were associated with pore refinement and strength gain. All mixtures reached UCS values above 0.5 MPa at 7 days and 1.0 MPa at 28 days. The A2 mixture achieved the highest 7-day UCS of 0.8 MPa, whereas A1 showed the highest 28-day UCS of 1.6 MPa. Porosity, pore probability entropy, and fractal dimension were negatively correlated with UCS, with pore probability entropy showing the highest sensitivity to 7-day strength. These findings support the use of high-PG clinker-free binders for targeted phosphate-mine backfill.