(180o) Transformation of Calcium Sulphate Dihydrate to Anhydrous Calcium Sulphate in Aqueous Solutions

The mechanism of transformation of calcium sulphate dihydrate (CaSO₄?2H₂O_(s)) to anhydrous calcium sulphate (CaSO_4(s)) in aqueous solutions was investigated. The kinetics of the transformation was followed by monitoring the changes both in the concentration of dissolved calcium in the solution measured by ICP-OES and in the mole fraction of dihydrate/anhydrite in the solid phase obtained from X-ray diffraction and thermo gravimetric analysis (TGA). It was found that the transformation occurs in such a way that dihydrate, which is the metastable phase under the conditions studied, dissolves and anhydrite, the stable phase, precipitates from the solutions. During the dehydration of calcium sulphate dihydrate at temperatures above 90°C, metastable hemihydrate (CaSO₄?1/2H₂O_(s)) may form as an intermediate stage. Addition of an anhydrite seed to the solution diminishes the induction period and accelerates the transformation process.