Hydrothermal conversion of industrial by-product gypsum to α-calcium sulfate hemihydrate: A review of mechanisms, process regulation, and sustainability.

Industrial by-product gypsum (IBPG) is an abundant secondary resource generated from various industrial processes, but its large-scale accumulation and low-value utilization remain significant challenges for sustainable waste management. The conversion of IBPG into high-value α-calcium sulfate hemihydrate (α-HH) represents a promising pathway for resource recovery and circular utilization. This review evaluates hydrothermal strategies for α-HH production from IBPG, focusing on formation mechanisms, process regulation, crystal engineering, impurity effects, and sustainability considerations. A Process-Structure-Property-Environment (PSPE) framework is established to elucidate the relationships among process parameters, crystal structure, product performance, and environmental implications. The review clarifies the mechanisms underlying hydrothermal α-HH formation, particularly the dissolution-recrystallization pathway, and how process parameters, crystal modifiers, and inherent impurities influence crystal evolution and product quality. Compared with conventional utilization routes, hydrothermal processing provides an integrated aqueous platform that enables phase transformation, impurity migration, and morphology control. This approach reduces additional purification requirements, improves resource efficiency, and facilitates the upgrading of impurity-containing gypsum residues. However, industrial implementation remains constrained by reactor scale-up, corrosion management, process optimization, and insufficient quantitative sustainability assessments. Future research should integrate predictive process modeling, life cycle assessment, techno-economic analysis, and continuous reactor development to promote the sustainable valorization of IBPG into high-value gypsum materials.