Abstract: A process for preparing calcined gypsum (stucco) which comprises treating a mass of calcined gypsum by adding, with thorough blending, small portions of water (about 1-10% by weight) to the calcined gypsum, allowing it to heal, and grinding the healed stucco to recapture the rate of strength development and the ultimate strength which are adversely affected by the water addition. The principal advantage provided by the addition of small portions of water is a reduction in water demand which is retained despite the grinding and optional drying of the healed stucco. If the treated calcined gypsum is not used shortly after the healing procedure, it should be dried to provide storage stability. The reduced water demand is particularly useful in gypsum board manufacture.

Abstract: An apparatus and process for preparing calcined gypsum which comprises continuously treating a mass of calcined gypsum by adding, with thorough blending, metered portions of water to the calcined gypsum. The water is incorporated into the calcined gypsum as a continuous, free-falling stream immediately after the gypsum enters the blender container near the top of its sidewall. The water must be added carefully so as not to contact either the side wall of the blender container or the propeller blade mounted in the center of the apparatus on a shaft projecting upwardly from the bottom and with the propeller spaced from the bottom of the container. The water is added in amounts ranging from about 1 to about 16% of free water by weight of the calcined gypsum fed to the blender apparatus.