To prepare alpha-calcium sulfate hemihydrate (alpha-HH) from flue gas desulfurization (FGD) byproduct under atmospheric pressure, the dehydration of FGD gypsum in a concentrated Ca and Mg chlorides solution medium at 95 degrees C was investigated, and the effects of K(+) ions were focused on. The crystal water content variation, material phases, and morphological characteristics of the dehydration products were examined and compared. The solubilities of alpha-HH in the mixed salt solutions with different K(+) ion concentrations were also determined. It was found that with low concentration of K(+) ions in the mixed salt solution (i.e. 0.001-0.035 M), the dehydration rate was enhanced, while with higher K(+) ion concentration (i.e. 0.087-0.263 M), this process was retarded. alpha-HH was the dehydration product when K(+) ion concentration was at appropriate level, while anhydrite (AH) occurred when K(+) ion concentration was above 0.173 M. K(+) ions were assumed to behave as a capping agent to control the morphology of the alpha-HH, resulting in crystals of lower aspect ratio. FGD gypsum can be successfully transformed into alpha-HH in concentrated Ca and Mg chloride solution under mild conditions, and K(+) ions play an important role in the process. (C) 2009 Elsevier Ltd. All rights reserved.