Influence of solid solutions on the properties of some apatite-type calcium salts relevant to the production of phosphate and silicate cements

Abstract: The effects of non-stoichiometry and isomorphic substitutions on the properties of some apatite-type compounds were investigated. These investigations were performed both in the fields of Portland cement chemistry and apatitic calcium phosphate-based biocements. Apatite compounds were synthesized by solid state methods and by hydrolysis of the reactant α-tricalcium phosphate (α-TCP). The materials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Apatite-type chlorellestadite is a compound found in cement kiln deposits. Its formation in the inlet part of the kiln obstructs the feed flow and was found to be an indication of the presence of high HCl(g) concentrations, principal cause of corrosion in the kiln lining. In order to quantify, by XRD analysis, the chlorellestadite phase in the coating samples, a good starting model of the crystal structure had to be chosen. For that purpose, many synthetic chlorellestadites were prepared varying the experimental conditions, and their crystal structure refined using the Rietveld method. At the same time, the range of thermal stability of nonstoichiometric chlorellestadite was determined providing a better understanding of the chemistry of silicate sulphate chloride-apatites. Biological hydroxyapatite is the main inorganic compound present in bones and teeth in all vertebrates. Synthetic hydroxyapatite with chemical composition and crystal structure similar to biological apatite represent therefore a suitable biocompatible material for orthopaedic applications. Calcium deficient hydroxyapatite (CDHA) was prepared by mixing, at room and body temperature, an α-TCP powder with a 2.5 wt% Na2HPO4 accelerating solution. The resulting paste hardens forming CDHA as hydration product. Incorporation in the cement of Sr and Si, elements present as traces in bones, was achieved by hydrolysis of Sr-, Si-substituted α-TCP. The effect of substitutions on the hydraulic and mechanical properties of the cement pastes was monitored by isothermal calorimetry, XRD, SEM, and compressive strength testing. The bioactivity of the set cements was investigated by examination with SEM of the new apatite layer formed on the surface of the cement samples, after 4 weeks soaking in simulated body fluid.