Preparation of Medium–Low Strength Hemihydrate Gypsum from Phosphogypsum and Grey Correlation Evaluation of Autoclaving Energy Consumption

This study explored the production of a medium–low strength α-hemihydrate gypsum (α-HH) from phosphogypsum using an autoclaved method. The influence of slurry concentration, spreading thickness, crystallizing agents, slurry pH, autoclaving temperature, and autoclaving time on the properties of α-HH was investigated. The energy consumption of various autoclaving processes was assessed using grey correlation theory. The research findings revealed that a 50% slurry concentration, 40 mm slurry spreading thickness, and 0.15% succinic acid as the crystallizing agent, with a slurry pH of 6, can yield α-HH crystals with an average aspect ratio of 1.42 and a high hemihydrate gypsum content of 80.06%. Increasing the autoclaving temperature and extending the autoclaving time resulted in a higher strength of α-HH. Autoclaving the slurry at 105 °C for 2.5 h yielded α-HH with a compressive strength of only 12.1 MPa. In contrast, autoclaving at 120 °C for 4 h produced α-HH with a higher compressive strength of 37.1 MPa. Grey correlation analysis revealed that the strength and energy consumption indices of the α-HH preparation had the highest correlation with the ideal sequence when autoclaved at 105 °C for 4 h, resulting in a compressive strength of 20.5 MPa.