Control of α‑Calcium Sulfate Hemihydrate Morphology Using Reverse Microemulsions

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/H2O and the concentration of SDS. As the ratio of CTAB/H2O 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.