Hydration of tetracalcium phosphate

Abstract

Hydrating tetracalcium phosphate evolves measurable heat for only a few hours when reacted isothermally in aqueous solution at temperatures between 25 and 70°C. The hydration reaction is incomplete, and tetra calcium phosphate is the only crystalline solid detected by X-ray diffraction analysis. The apparent activation energy of tetracalcium phosphate hydration is 36 kJ/mol. An activation energy of this magnitude is indicative of an interfacially controlled reaction. As with many hydration reactions, which are initially interfacially controlled, a layer of hydration product accumulates on the hydrating surfaces, and subsequent hydration becomes diffusionally controlled. Consistent with such a mechanistic path, seeding with calcium hydroxide reduces both the maximum rate of heat evolution and the total heat of reaction , while seeding with hydroxyapatite increases the total heat evolved. Complete hydration may be achieved when a tetracalcium phosphate slurry is continuously agitated for an extended period. After storage for one year large Ca(OH)2 crystals have precipitated and an equilibrium involving hydroxyapatite and Ca(OH)2 has been achieved. Carbonate readily enters the hydroxyapatite lattice and carbonated hydroxyapatite is formed in a CO2 atmosphere. Although its composition is uncertain, the carbonated hydroxyapatite formed has a Ca/PO4 ratio of 2, and it is the only phase observed at the complete reaction of tetra calcium phosphate under CO2.