Sustainable synthesis of α-calcium sulfate hemihydrate from salt gypsum: Optimization of crystal morphology and mechanical properties for industrial applications

Salt gypsum (SG), a by-product derived from the salt manufacturing process, poses considerable challenges for efficient reuse owing to its elevated moisture and salt content. In this study, an innovative methodology was developed utilizing an oil-bath reaction unit to synthesize α-calcium sulfate hemihydrate (α-HH) from untreated SG in a sodium chloride (NaCl) solution, with the objective of achieving environmentally sustainable production. The influence of NaCl concentration, the type and content of transcrystallizing agents, reaction temperature, reaction duration, and stirring rate on the transformation process, crystal morphology, and mechanical strength of α-HH was systematically examined. The crystal morphology and chemical composition of the resultant α-HH products were meticulously characterized through XRD, SEM, EDS, FT-IR, and TG-DSC. The findings revealed that under optimized conditions (15 wt% NaCl solution, a reaction temperature of 115°C, a reaction duration of 2.0 h, a stirring rate of 200 rpm, and 0.20 wt% sodium succinate as a crystal modifier), the conversion efficiency of α-HH attained 80.90 %, accompanied by a compressive strength of approximately 31.93 MPa post-drying. NaCl induced a transformation in crystal morphology from flaky to long columnar, whereas sodium succinate further refined the morphology from long columnar to short columnar via physical adsorption mechanisms. This study proposes a high-value strategy for the valorization and recycling of industrial by-product gypsum, thereby broadening the application scope of SG and significantly enhancing its overall utilization efficiency. This approach is of paramount importance for advancing the green and sustainable development of the salt chemical industry.