Kinetics of calcium sulfate hemihydrate (HH) crystal growth plays an important role in mineralization of calcium sulfate phases in nature. HH crystal growth and the conversion of calcium sulfate phases form the basis for the production and application of gypsum based building material. alpha-HH crystals have been grown in 3.74 M CaCl2 solutions at a fixed initial ratio of calcium to sulfate under atmospheric pressure. The variations of sulfate ions were determined to obtain the alpha-HH crystal growth kinetics information. Effects of Mg2+ and K+ ions on alpha-HH growth were investigated to find an optimal composition of solution for alpha-HH preparation. The orders of alpha-HH growing in the CaCl2 solution were found, in most cases, to be near 2.0 in presence or in absence of Mg2+ and K+ ions. Mg2+ ions enhance the growth of alpha-HH in CaCl2 solution mainly due to initial supersaturation enhancing effects. K+ ions also improve the growth rate, which has been attributed to the reduction of interfacial energy. In a Ca (3.74 M)-Mg (0.20 M)-K (0.09 M) chlorides solution, the growth rate of alpha-HH increases with temperature from 80 to 100 degrees C, and the activation energy was calculated to be 40 kJ/mol. (C) 2009 Elsevier B.V. All rights reserved.