Comparative mechanistic effects of different non-ferrous metallurgical solid waste slags on beta-hemihydrate phosphogypsum: Insights into hydration kinetics, microstructural evolution and environmental compatibility

Phosphogypsum (PG) and non-ferrous metallurgical solid waste slags (NMSWS) are stockpiled, creating environmental risks and requiring safe utilization. Beta-hemihydrate phosphogypsum (β-HPG)-based composite binders were prepared by partially replacing β-HPG with four NMSWS: copper slag (CS), red mud (RM), lithium slag (LS) and lead-zinc smelting slag (LZSS). Workability, rheology, mechanical performance, water resistance, hydration heat and kinetics, phase assemblage, pore structure, microstructure, and immobilization of P, F and heavy metals were examined. NMSWS generally improved paste flowability and reduced the fitted yield stress from 161.87 Pa to 62.05–95.20 Pa, with all pastes exhibiting shear-thinning behavior. However, their effects on setting, hydration and hardened performance varied markedly. LS showed the most effective modification. The β-HPG + LS binder reached a 28 d -dry compressive strength of 22.47 MPa and a softening coefficient of 0.49, increasing by 27.74% and 4.26% relative to the control. This improvement was attributed to the higher later-stage hydration activity of LS, which promoted C-(A)-S-H gel formation and regulated AFt toward an appropriate content and favorable short-columnar morphology. Calorimetry and JMAK analysis confirmed that β-HPG + LS had the highest second exothermic peak, cumulative heat release and Avrami growth-rate constant among NMSWS-containing systems. LS refined the pore structure, reducing porosity, total intrusion volume and average pore diameter to 42.19%, 0.402 mL/g and 250.25 nm, respectively. Leaching concentrations of P, F, As, Cd, Cr, Cu, Mn, Pb and Zn were all below the corresponding Class II limit values specified in GB 3838–2002. These results provide a mechanistic basis for the synergistic utilization of PG and NMSWS in potentially low-carbon binders.