STUDY OF THE SETTING OF PLASTER

¹ Dental Research Section, National Bureau of Standards, Washington, D. C.

This study was performed to obtain a better understanding of the setting properties of plaster of Paris, CaSO₄ · H₂O. Dilatometer studies have been completed, showing the effect of stirring, accelerators, and retarders on the kinetics of the setting of plaster. Crystal morphology studies show that when plaster is reacted with different salt solutions the crystal habit of the resultant gypsum changes. This change in shape of the final crystals may explain the reduction in setting expansion effected by various additives to the plaster slurry.

The dilatometer experiments exaggerate the contribution of the final water solution to the setting expansion of plaster of Paris. In a set plaster there is no doubt that the bulk volume will depend principally upon the packing arrangement of the crystals rather than upon the solution effect.