Controlling the morphology of calcium sulfate hemihydrate using aluminum chloride as a habit modifier

The morphology and aspect ratio of calcium sulfate hemihydrate (HH) are successfully controlled using AlCl3 as a morphology modifier in the hydrothermal synthesis process. As the concentration of AlCl3 is increased from 0 to 7.5 x 10(-2) mol L-1, the crystal length decreases from 130 mu m to 0.1-0.3 mu m and the corresponding aspect ratio declined dramatically from 150-240 to 1-2, and the crystal morphology gradually changes from whiskers to rods, and even irregular nanogranules. The preferential adsorption of Al3+ on the side facets of HH would lower the surface energy and inhibit the elongation along these facets. The doping of Al3+ in the HH would destroy the main chain of HH and inhibit the elongation along the [001] direction. The two facts contribute to the morphology control. This work exhibits an efficient method to control the morphology of HH over a large range of size by simply adjusting the concentration of AlCl3.