Direct preparation of alpha-calcium sulfate hemihydrate out of sulfuric acid solution by reaction with lime under atmospheric pressure conditions is systematically investigated. Two preparation methods are investigated: one involving addition of dry lime to sulfuric acid solution (standard procedure) and the other involving addition of sulfuric acid solution to slaked lime (reverse procedure). On the basis of the standard procedure, the operating window is first determined, followed by thorough examination of preparation conditions such as acidity, retention time and CaO/H2SO4 molar ratio. Dihydrate was found to form as intermediate phase that converts to hemihydrate under all tested conditions. The typical morphology of alpha-hemihydrate crystals produced by the standard procedure is needle-shaped with 50-100 mum length and 1-3 mum thickness. However, when the reverse procedure was used, a different growth mechanism apparently applied yielding shorter (similar to60 mum) but thicker (5-10 mum) column-shaped crystals within otherwise the same operating window.