Alpha calcium sulfate hemihydrate (α-HH) is an important class of cementitious material and exhibits considerable morphology-dependent properties. In the reverse microemulsions of water/n-hexanol/cetyltrimethylammonium bromide (CTAB)/sodium dodecyl sulfonate (SDS), the morphology and aspect ratio of α-HH are successfully controlled by adjusting the mass ratio of CTAB/H(2)O and the concentration of SDS. As the ratio of CTAB/H(2)O is increased from 1.3 to 4.5, the crystal length decreases from 120 to 150 μm to 0.5-1.2 μm with the corresponding aspect ratio reduced sharply from 180 to 250 to 2-7. With increasing SDS concentration, the crystal morphology gradually changes from submicrometer-sized long column to rod, hexagonal plate, and even nanogranule. The preferential adsorption of CTAB on the side facets and SDS on the top facets contributes to the morphology control. This work presents a simple, versatile, highly efficient approach to controlling the morphology of α-HH on a large scale and will offer more opportunities for α-HH multiple applications.

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For the first time, a-calcium sulfate hemihydrate (a-HH) was successfully prepared from calcium sulfate dihydrate (DH) in methanol-water solution under atmospheric pressure. The effect of methanol concentrations (40-73 mol%) on the transition from DH to a-HH were investigated at temperatures (601-751C) within a reaction time of 36 h. The results showed that an increase in methanol concentration could lower the transition temperature. The transition was speeded up by increasing methanol concentration from 47 to 57 mol% at 751C. When the temperature dropped to 701C, the transition rate firstly increased with the methanol concentration until it reached 69 mol% and then exhibited a reverse trend with increasing methanol concentration from 71 to 73 mol%. The accelerating effect, involved during the nucleation of a-HH, attributes to the elevated supersaturation due to the fall in the water activity resulting from the increasing methanol concentration. The retarding effect, involved during the crystal growth of a-HH, was likely derived from the specific adsorption of methanol molecules onto the surfaces of a-HH crystals. Overall, the aqueous solution of methanol could act as a novel category of medium suitable for the a-HH preparation from DH. D. Viehland-contributing editor

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