In gypsum board production, the amount of water required for a stucco slurry with a suitable fluidity is considerably greater than the amount necessary for hydration of the stucco. Since excess water must be removed by drying, it is advantageous, from an energy point of view, to reduce the consistency or water demand of stucco. Constant board density is maintained thereby through increased addition of foam.

The high water demand of stucco produced by conventional calcination methods is due largely to the ability of the stucco to disintegrate, a process during which larger stucco particles break down into smaller particles upon contact and subsequent mixing with water. The water demand of stucco may be reduced by aging, that is, the exposure of stucco to water vapor over a period of time, which reduces the ability of the stucco to disintegrate. However, aging also has a negative effect on both setting characteristics of the stucco and, as a result, on the strength of the gypsum formed.

The objective of the work described in this paper was to study the relationships between water demand and particle size distribution as affected by changes in disintegration during aging. Particle size analysis and several methods of consistency measurement were used for this purpose. It was found that a direct relationship exists between water demand of stucco and particle size distribution as defined by percent disintegration, with minor deviations at initial stages of aging. In contrast, nonlinear relationships were found, with preliminary experiments, for stuccos containing aridizing agents or surfactants, which indicates that the water demand of stucco is also affected by surface properties. The water demand of stucco is not a function of the surface area of the stucco, that is, other properties, such as particle shape and surface characteristics, play a significant role. Similarly, the magnitude of the differences in water demand of various types of stucco indicates that particle size distribution, although the major factor, is not the only one influencing this property.