Dissolution kinetics of polycrystalline calcium sulfate-based materials : influence of chemical modification

Using a channel flow cell (CFC) system, the dissolution kinetics of polycrystalline gypsum-based materials have been examined with the aim of understanding their interaction with water, a property that limits the applications of the material in many situations. ICP (inductively coupled plasma) analysis of elemental concentrations in solution as a function of time yields surface fluxes by using a finite element modeling approach to simulate the hydrodynamic behavior within the CFC. After correction for surface roughness, a value for the intrinsic dissolution flux into water of pure polycrystalline gypsum, CaSO4.2H2O, of 1.1 (±0.4) × 10–8 mol cm–2 s–1 has been obtained. The addition of known humid creep inhibitors to the gypsum samples, including boric acid, tartaric acid and 3,4,5-trihydroxybenzoic acid (gallic acid), was found to have little measurable effect on the dissolution kinetics of gypsum: all yielded dissolution fluxes of 1.4 (±0.6) × 10–8 mol cm–2 s–1. However, trisodium trimetaphosphate (STMP) was found to have a small detectable inhibitory effect relative to pure gypsum yielding a flux of 7.4 (±2.0) × 10–9 mol cm–2 s–1. The data strongly suggest that models for humid creep inhibition that involve dissolution–crystallization of gypsum crystallites are less likely than those that involve a hindered ingress of water into the gypsum matrix. For comparison, composite materials that comprised of calcium sulfate anhydrite (CaSO4) crystallites bound by a polyphosphate matrix were also studied. For some of these samples, Ca2+ surface fluxes were observed to be 1 order of magnitude lower than values for polycrystalline gypsum control substrates, suggesting a useful way to impart water resistance to gypsum-based materials