[Effects of addition of calcium phosphates on the properties of gypsum].

Abstract

Calcium hydrogen phosphate (CHP), calcium hydrogen phosphate dihydrate (CHPD) and calcium sulfate dihydrate (gypsum) crystals were added to calcium sulfate hemihydrate as gypsum model materials for improvement of the mechanical properties. The compressive and the bending strength of the hardened mixture were improved by the addition of CHPD, while the addition of CHP and gypsum crystals had no effect. The setting time was markedly reduced by the addition of CHPD, but remarkably increased in the case of CHP crystals. On the other hand, the setting expansion was decreased by the addition of CHP and CHPD crystals. Based on the observation of the surface profile of the calcium sulfate hemihydrate set in contact with glass, and scanning electron microscopy (SEM) of the fractured surface of the hardened mixture, the surface roughness was markedly decreased by the addition of CHPD. When the crystals were added to the metastable solution of calcium sulfate supersaturated with respect to gypsum, the presence of gypsum crystals deposited on the CHPD surface was confirmed by atomic absorption analysis, fourier transform infrared spectroscopy. SEM and electron X-ray microanalysis. Therefore, epitaxial growth of gypsum occurred on the CHPD surface and the addition of CHPD improved the mechanical properties of the gypsum materials.