Valorization of industrial by-product gypsum into high-strength α-hemihydrate: Phase transformation, impurity control, and continuous process scale-up

Industrial by-product gypsum, an abundant calcium sulfate-rich solid waste, faces significant challenges in its conversion into high-strength α-calcium sulfate hemihydrate (α-HH), primarily due to feedstock variability, soluble salts and metal impurities, organic contaminants, and scale-up limitations. This review presents a process-oriented framework. It integrates feedstock characteristics, impurity behavior, phase transformation mechanisms, crystal habit regulation, and continuous process scale-up. The framework systematically examines the production of α-HH from multiple perspectives. Unlike previous studies, this review moves beyond merely comparing preparation routes and offers a process-oriented perspective for the high-value utilization of industrial by-product gypsum. Various preparation routes, including autoclaving, salt-, acid-, and alcohol-mediated methods, as well as hydrothermal processes, are comparatively evaluated in terms of reaction media, driving forces, impurity migration behavior, product stability, and industrial feasibility. Subcritical hydrothermal conversion is further identified as a liquid-water-based subset of pressurized hydrothermal processes, in which gypsum slurry is maintained under high-pressure liquid-water conditions rather than under a water–steam coexistence environment. This distinction is important because compressed liquid water can simultaneously serve as the reaction, crystallization, and impurity-extraction medium. Future research should emphasize the standardized reporting of key process parameters, including solid loading, residence time distribution, water and heat balances, impurity accumulation, continuous operation duration, and product performance metrics, thereby facilitating process optimization and industrial implementation.