The effect of additives on the hydration of CaSO4·0.5H2O: A synchrotron X-ray diffraction study

To modulate the rate of hydration of calcium sulfate hemihydrate (HH) to calcium sulfate dihydrate for industrial applications, additives (both accelerants and retardants) are added to the HH. Here we explore in detail the kinetics and mechanism of HH hydration in the presence of the accelerators K2SO4 or ZnSO4, and also when a retardant (citric acid or tartaric acid) is added. Retardants tend to lower the maximum temperature of the reaction and increase the time needed to reach 50% hydration, yet the total conversion percentage is much higher for citric acid/tartaric acid then when an accelerant is added. The reaction process is found to involve heterogenous nucleation regardless of the nature of the additive. More soluble accelerants act mainly though increasing the rate of crystal growth, while less soluble species have a more significant effect on the rate of nucleation