Influence of temperature and solution composition on the formation of calcium sulfates

Calcium sulfates (anhydrite and hydrates) were synthesized by mixing CaCl2 and Na2SO4 solutions at room temperature followed by aging the resulting slurries at elevated temperatures. The variation of the morphology and structure of the calcium sulfates with aging temperature was investigated. Experimental results indicated that CaSO4·2H2O plates, CaSO4·0.5H2O whiskers and CaSO4 spindles were formed at ≤100 °C, 130–160 °C and ≥170 °C, respectively. The formation and conversion of the calcium sulfates were discussed on the basis of characterization of the products and chemical analysis of the solutions. Compared to NaCl solution, pure water favors one-dimensional hydrothermal growth of CaSO4·0.5H2O whiskers owing to lower supersaturation.