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 degrees C and 70 degrees C with control of calcium carbonate decomposition in contact with aqueous solutions of P2O5. A phase sequences study was carried out at 25 degrees C in the isoplethic section of CaCO3-H2O-P2O5 system. The solid phases observed at these temperatures are: dehydrated calcium hydrogen phosphate CaHPO4 center dot 2H(2)O, Hydroxyapatite Ca-10(PO4)(6)(OH)(2), anhydrous calcium hydrogen phosphate CaHPO4 and calcium dihydrogen phosphate monohydrate Ca(H2PO4)(2)center dot H2O. All stable solids are characterized by X-ray diffraction and the Schreinemaker method. (C) 2017 Elsevier B.V. All rights reserved.