Natural curing of cold-bonded phosphogypsum-based aggregates: Physico-mechanical evolution and phosphorus/fluoride release behavior

Phosphogypsum (PG) valorization is constrained by poor physical stability and the potential release of residual phosphorus and fluoride. This study proposes a cement-free, function-oriented cold-bonding strategy to convert PG into engineered aggregates and investigates ambient natural curing (NC) as a coupled structural and chemical regulation route. Four PG-based aggregates were designed for mechanical reinforcement, pore-structure regulation, surface-chemistry modification, and pH buffering. Their physico-mechanical properties, aqueous stability, P/F release, and microstructural evolution were compared with those under standard curing (SC) over 28 days. Under NC, moisture loss and atmospheric CO₂ exposure occurred simultaneously, promoting drying-induced densification and carbonate formation. System A showed a cylinder compressive strength of 2.30 MPa and a post-leaching disintegration rate of 1.59% after 28 days of curing. The leachate pH under NC gradually decreased to 7.3–8.0. Phosphorus concentrations remained below 0.2 mg/L throughout the test period, while the influence of curing regime on fluoride release was more pronounced at early ages. Microstructural analyses further indicated formulation-dependent differences in matrix structure and pore characteristics. These results suggest the feasibility of using natural curing to regulate the structural and environmental behavior of cement-free PG-based aggregates.