A method of forming a gypsum based product is disclosed. The method comprises the steps of: calcining a mixture of water and gypsum under conditions of raised temperature and pressure within a vessel to produce an alpha-hemihydrate slurry therein; passing the alpha-hemihydrate slurry from the vessel to a mixer for mixing with additional water to produce a settable slurry, which is arranged to set to form the gypsum based product.