Dissolution of CaSO4 center dot 1/2H(2)O and precipitation of CaSO4 centerdot 2H(2)O - A kinetic study by synchrotron X-ray powder diffraction

Time-resolved X-ray powder diffraction has been performed on hydrating samp les of calcium sulfate hemihydrate with 0.50 wt.% dihydrate seeds added. Da ta were recorded in transmission mode using a position sensitive detector a nd synchrotron X-ray radiation of optimised intensity (wavelength 1.4 Angst rom). 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.% ci tric acid monohydrate causes retardation. times for complete reaction: 17 a nd 42 min, respectively, compared to 28 - 32 min without additive (w/s = 1. 00). A comparison of the reaction curves shows (i) that hemihydrate dissolv es and dihydrate precipitates at the same rate, and (ii) that preferred ori entation effects are absent in the diffraction experiments. (C) 2001 Elsevi er Science Ltd. Ail rights reserved.