Calcium sulfate-phosphate composites with enhanced water resistance

Abstract

A novel method of processing gypsum, CaSO₄·2H₂O, is described, forming composites of calcium sulfate anhydrite crystallites bound by a water-resistant phosphate matrix. The materials have been characterised using various analytical techniques, including powder X-ray diffraction (PXRD), infrared spectroscopy, scanning electron microscopy and ³¹P solid-state nuclear magnetic resonance (NMR). While PXRD shows the major crystalline phase present in the composites is calcium sulfate anhydrite, the NMR shows the presence of a polyphosphate matrix, consistent with IR analysis. The NMR also allows the identification of an interfacial layer between the anhydrite crystallites and phosphate coating by comparison with the mixed sulfate-phosphate mineral ardealite, whose NMR spectra we also report. Comparison of conventional and proton-decoupled cross polarisation NMR spectra allows the presence and location of included protons to be probed. Physical measurements of mechanical strength show that addition of aluminium to the composite allows the formation of high density materials that have dry strength comparable to gypsum and no loss of strength on immersion in water.