Recycling of phosphogypsum to prepare gypsum plaster: Effect of calcination temperature

The calcination of phosphogypsum (PG) was employed to produce gypsum plaster to achieve its recovery and utilization. Although previous studies have indicated the possibility of producing hemihydrate and anhydrous gypsum, the influence mechanism of the impurities in calcined PG (CPG) on the properties of plaster is unclear. Herein, the paper aims to investigate the effect of the impurities on the properties of gypsum plaster at elevating calcination temperatures. The phase transformation of PG and impurities were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM), and the hydration properties of gypsum plaster were investigated, including water requirement for normal consistency (WR), setting time, compressive and flexural strength, hydration products and degree, and microstructure. The results show that the increase in calcination temperature resulted in higher anhydrite content, and CPG showed similar chemical and mineral compositions at high temperatures (500°C–800 °C). Additionally, harmful impurities barely changed at low calcination temperatures (150°C–400 °C), resulting in the reduced mechanical properties despite its higher hydration degree; however, the calcination at high temperatures converted harmful impurities into inert insoluble calcium pyrophosphate (Ca2P2O7), calcium metaphosphate (Ca(PO3)2), calcium fluoride (CaF2) and sodium sulfate (Na2SO4). With the best calcination temperature of 800 °C, gypsum plaster showed a relatively quick setting and maximum strength with a compact microstructure and low hydration degree due to the large formation of Na2SO4 and nucleation sites. Recycling PG to prepare anhydrite is recommended because of its excellent performance and lower energy consumption than traditional building materials.