Thermodynamic equilibrium in the system H2O+P2O5+CaCO3 at 25 and 70 °C: Application for synthesis of calcium phosphate products based on calcium carbonate decomposition

The solid-liquid equilibrium of the ternary system of water-phosphorus pentoxide-calcium carbonate was studied using a synthesis method based on the measurement of the solution conductivity under atmospheric air pressure. Two isotherms were established at 25 °C and 70 °C with control of calcium carbonate decomposition in contact with aqueous solutions of P2O5. A phase sequences study was carried out at 25 °C in the isoplethic section of CaCO3-H2O-P2O5 system. The solid phases observed at these temperatures are: dehydrated calcium hydrogen phosphate CaHPO4·2H2O, Hydroxyapatite Ca10(PO4)6(OH)2, anhydrous calcium hydrogen phosphate CaHPO4 and calcium dihydrogen phosphate monohydrate Ca(H2PO4)2·H2O. All stable solids are characterized by X-ray diffraction and the Schreinemaker method.