The influence of impurities in FGD gypsum (>90% CaSO4 center dot 2H(2)O) on the kinetics of its transformation to alpha-calcium sulfate hemihydrate (alpha-HH, CaSO4 center dot 0.5H(2)O) has been seldom investigated. This work demonstrates that CaCO3 and CaSO3 in FGD gypsum retard the transformation kinetics under hydrothermal conditions at atmospheric pressure, while SiO2 produces little effect. The induction time elapsed to initiate the transformation is significantly prolonged mainly because of CaSO3, which origins from its ability to decrease the driving force and simultaneously block a-HH nucleation sites on gypsum crystals. Transformation of FGD gypsum to a-HH can be significantly boosted by washing away the CaSO3. Enlarging the driving force via regulating temperature, salt solution concentration and acidity can also accelerate the transformation. The results indicate FGD process optimization, especially by a complete oxidation of CaSO3, would advance the utilization of FGD gypsum for a-HH production. (C) 2017 Elsevier Ltd. All rights reserved.