Dissolution of CaSO 4·1/2H 2O and precipitation of CaSO 4·2H 2O : A kinetic study by synchrotron X-ray powder diffraction

Abstract

Time-resolved X-ray powder diffraction has been performed on hydrating samples of calcium sulfate hemihydrate with 0.50 wt.% dihydrate seeds added. Data were recorded in transmission mode using a position sensitive detector and synchrotron X-ray radiation of optimised intensity (wavelength 1.4 Å). The dissolution of the hemihydrate and the formation of the dihydrate were both monitored. Varying the water/solid weight ratio (w/s) from 0.50 to 1.50 did not affect the reaction rate significantly. Addition of 0.50 wt.% potassium sulfate to the water accelerates the reaction and 0.25 wt.% citric acid monohydrate causes retardation; times for complete reaction: 17 and 42 min, respectively, compared to 28–32 min without additive (w/s=1.00). A comparison of the reaction curves shows (i) that hemihydrate dissolves and dihydrate precipitates at the same rate, and (ii) that preferred orientation effects are absent in the diffraction experiments.