Optimizing hydration and performance of phosphogypsum based cementitious system through multiphase composites

Thermal treatment of phosphogypsum (PG) to produce construction-grade gypsum is a promising approach for large-scale utilization. However, the single-phase composition of calcined gypsum necessitates the addition of retarders to control hydration speed, often compromising material performance. To address this, we propose a multiphase gypsum system that leverages synergistic interactions among various gypsum phases to regulate hydration kinetics. This study examines the workability, mechanical properties, water resistance, hydration heat, and microstructure of multiphasic PG. We systematically analyze the interaction mechanisms between different gypsum phases, including II-anhydrite (AII), III-anhydrite (AIII), β-hemihydrate (HH), and dihydrate (DH), within the multiphasic PG system. Results indicate that incorporating optimal amounts of AIII and AII effectively adjusts PG hydration process, enhancing workability and water resistance. Specifically, a composite of 30 % AIII and 20 % AII yields significant improvements in mechanical strength and water resistance (with a softening coefficient reaching 0.81), extends setting time, and reduces water demand. Interactions among AII, AIII, HH, and DH effectively regulate hydration rates in phosphorus-based gypsum cementitious materials. Early-stage hydration of AIII releases substantial heat, promoting the hydration of HH and AII. In turn, AII modulates HH’s hydration rate, providing a retarding effect that enhances early strength. At later stages, hydration of AIII and HH increases the exothermic rate of AII’s hydration, while DH serves as a nucleation site for AII crystallization, producing a dense structure. Additionally, unhydrated AII absorbs infiltrated water molecules, further improving water resistance and enhancing long-term strength.