title: Dissolution kinetics of polycrystalline calcium sulfate-based materials: influence of chemical modification.
abstract: 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, CaSO(4).2H(2)O, 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 (CaSO(4)) crystallites bound by a polyphosphate matrix were also studied. For some of these samples, Ca(2+) 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.
authors.affiliation: Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom.
authors.forename: Robin D
authors.initials: RD
authors.lastname: Fisher
authors.forename: Michael M
authors.initials: MM
authors.lastname: Mbogoro
authors.forename: Michael E
authors.initials: ME
authors.lastname: Snowden
authors.forename: Maxim B
authors.initials: MB
authors.lastname: Joseph
authors.forename: James A
authors.initials: JA
authors.lastname: Covington
authors.forename: Patrick R
authors.initials: PR
authors.lastname: Unwin
authors.forename: Richard I
authors.initials: RI
authors.lastname: Walton
mesh_terms: D001888:Boric Acids / Q000737:chemistry; D002133:Calcium Sulfate / Q000737:chemistry*; D003460:Crystallization; D005707:Gallic Acid / Q000737:chemistry; D007700:Kinetics; D011122:Polyphosphates / Q000737:chemistry; D013499:Surface Properties; D013644:Tartrates / Q000737:chemistry; D014867:Water / Q000737:chemistry
chemical_list: D001888:Boric Acids; D011122:Polyphosphates; D013644:Tartrates; D014867:Water; D005707:Gallic Acid; C032688:boric acid; C029768:tartaric acid; D002133:Calcium Sulfate