Mineral reconstruction of high-volume phosphogypsum driven by a slag-based composite stabilizer: mechanisms of subgrade performance enhancement and phosphorus/fluoride immobilization.

To enable large-scale utilization of phosphogypsum (PG) in road subgrades, a slag-based composite stabilizer (CSL) comprising ground granulated blast-furnace slag (GGBFS), clinker, and quicklime was developed and optimized using constrained Scheffé mixture models and multi-objective optimization. With ordinary Portland cement-stabilized PG (OPC-PG) as a reference, engineering performance, P and F immobilization, and global warming potential (GWP) were evaluated, while multiscale characterization clarified the links among mineral reconstruction, structural strengthening, and contaminant immobilization. The recommended clinker:GGBFS:quicklime mass ratio was 15:75:10. Compared with OPC-PG, CSL-PG exhibited superior strength development and durability. Both systems rapidly immobilized P, whereas F immobilization showed pronounced age dependence and increased with curing age. CSL-PG achieved 98.9% P immobilization at 1 d, while F immobilization increased from 43.6% at 1 d to 86.9% at 180 d, representing relative improvements of 30.15% and 13.59% over OPC-PG, respectively. CSL promoted PG-derived sulfate participation in ettringite formation, while reactive Si/Al species sustained C-(A)-S-H formation, jointly driving mineral reconstruction and cementitious network densification. P immobilization was mainly associated with Ca-P precipitation, whereas F immobilization involved Ca-F association, adsorption, local coordination, and encapsulation by C-(A)-S-H. Compared with OPC, CSL reduced production-stage GWP by 57.53%, demonstrating potential for safe, low-carbon, large-scale PG utilization.