The ground gypsum dihydrate (the ground) has powerful accelerating effect to the setting of gypsum hemihydrate (hemihydrate), and it is increased by cooperation with usual accelerators, and the setting can be shortened to 20-90%.<BR>The ground deducts the retarding effect of usual retarders, and shortenes the setting time to 20-70%.<BR>Set gypsum mixed with the ground has higher compressive strength than normal set gypsum.<BR>By scanning electron micrograph of the ground crystal which was taken from the slurry under setting, the activated nucleation on the crystal surface was observed.<BR>The ground loses water of about 0. 7% by atomospheric drying or by coevaporation with ethanol. Quantity of this loss was calculated from the increase of crystal surface, and was nearly equal to experimental value.<BR>The dehydration of the ground begins at about 60°C, and the activation energy of this dehydration was estimated as 250 J/mol. Uuder atomosphere of RH 81% and room temperature the ground loses water of about 0. 7%, and then absorbs moisture to 0.4% increase of the weight. Under this condition mixture of hemihydrate and the ground absorbs moisture quickly and converts to dihydrate.<BR>Under RH 9% and room temperature the ground acts as under RH 81%, but the mixture of hemihydrate loses water of about 0. 7% and then absorbs moisture to 0. 4% loss of the weight regardless of the coexistence of the ground.<BR>The mechanism of this moisture absorption can be explained that the ground carry the water to hemihydrate from moisture by repeating dehydration and absorption.