Hydration of tetracalcium phosphate

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.