Synthesis of calcium sulfate hemihydrate whiskers from high-purity salt gypsum in a glycerol–water solution at atmospheric pressure

Calcium sulfate hemihydrate whiskers (CSHWs) were prepared from high-purity salt gypsum in a glycerol–water system. The effects of the glycerol/water ratio, reaction temperature, reaction time, and stirring speed on the crystal morphology of the CSHWs were investigated. Results showed that the average length and average length/diameter (L/D) ratio of the CSHWs first increased and then decreased as the glycerol/water ratio, reaction time, and stirring speed were increased, but they continued to gradually increase with increasing reaction temperature. The CSHWs had the highest L/D ratio and smooth surfaces—with an average length of 42.91 μm, an average diameter of 0.72 μm, and an average L/D ratio of 55—when the preparation conditions were a glycerol/water ratio of 2.0, a reaction temperature of 100 °C, a reaction time of 2.0 h, and a stirring speed of 300 rpm. This study provides a feasible idea for the treatment of salt gypsum, which achieves high value-added utilization of salt gypsum as well as the elimination of solid waste. As a new reaction medium for the preparation of calcium sulfate hemihydrate from calcium sulfate dihydrate, aqueous solutions of glycerol provide a crystallization environment different from that provided by conventional salt solutions. The CSHWs had the optimal length/diameter (L/D) ratio and smooth surfaces—with an average length of 42.91 μm, an average diameter of 0.72 μm, and an average L/D ratio of 55—when the preparation conditions were a glycerol/water ratio of 2.0, a reaction temperature of 100 °C, a reaction time of 2.0 h, and a stirring speed of 300 rpm. [Display omitted] •High value-added use of solid waste salt gypsum is reported for the first time.•High quality calcium sulfate hemihydrate whiskers form in glycerol–water solutions.•The best conditions are a glycerol/water ratio of 2.0, 100 °C for 2.0 h, 300 rpm.•A right glycerol/water ratio promotes the dissolution of calcium sulfate dihydrate.•Glycerol selectively adsorbs on the gypsum surfaces to extend the c-axis growth.