Polycarboxylic superplasticiser (PS) and naphthalene-based superplasticiser (NBS) were used to achieve a denser structure of α-hemihydrate calcium sulfate and better mechanical properties. The hydration process was investigated by thermogravimetric analysis (TGA), which showed that the PS decelerated the whole hydration process of α-hemihydrate calcium sulfate, whereas the NBS accelerated the process in the early stage and decelerated it in the late stage. It was noteworthy that they both could increase the highest hydration heat. The PS delayed the hydration heat peak whereas the NBS advanced it. Both scanning electron microscopy and pore property analysis confirmed the formation of a more compact structure of the hydration product. Optimal mechanical properties of α-hemihydrate calcium sulfate were obtained with 0·5% NBS or 0·4% PS by mass. Under the same conditions, the PS was more efficient at improving the properties of α-hemihydrate calcium sulfate than the NBS