Carbonation-Primed Hydration Strategy (CPHS) for stabilizing high-phosphogypsum OPC cement

Phosphogypsum (PG) presents significant challenges in ordinary Portland cement systems due to its tendency to cause delayed setting, strength reduction, and expansion-related instability, limiting its dosage to below 5 %. To address these limitations, this study proposes a Carbonation-Primed Hydration Strategy (CPHS), wherein phosphogypsum composite cement undergo accelerated CO₂ curing followed by water curing. This two-step approach enables stable incorporation of high PG contents while maintaining mechanical and volumetric performance. Under CPHS, a system with 20 % PG achieved 35 MPa after 24 h and over 60 MPa at 28 days. Even at 40 % PG, compressive strength exceeded 27 MPa. The Le Chatelier expansion remained below 1.5 mm across all groups, and softening coefficients above 0.9 confirmed excellent water resistance. Microstructural results showed that carbonation transformed C₃S/C₂S into CaCO₃ and silica gel, forming a dense, stable matrix that encapsulated PG and suppressed sulfate leaching. Subsequent hydration refined the pore structure and restored reactivity via C–S–H growth. This study demonstrates that CPHS effectively mitigates delayed ettringite formation and performance deterioration at high PG contents, offering a practical solution for the safe, durable, and carbon-efficient use of PG in cementitious materials.