The formation of hydroxyapatite by an acid-base reaction between solid calcium phosphates at temperatures from 5 to 60 °C was examined. The basic reactant used is Ca_4(PO_4)_2O, while the acidic reactants include CaHPO_4, CaHPO_4 · H_2O, and Ca(H_2PO_4)_2 · H_2O. Rates of heat evolution during reaction were determined by isothermal calorimetry. The variations in the proportions of reactants and hydroxyapatite and the formation of intermediate products were assessed by x-ray diffraction. Development of microstructure was observed. Generally, hydroxyapatite formation occurs by rapid initial reaction followed by a period during which reaction occurs slowly. Apparent activation energies calculated for the reaction when CaHPO_4 is the acidic reactant show differing values depending on its surface area. When the acidic reactant is Ca(H_2PO_4)_2 · H_2O, intermediate products are formed. At low temperatures the intermediate is CaHPO_4 · 2H_2O, while at higher temperatures it is CaHPO_4. Above 38 °C, the rate during the period of slow reaction decreases with increasing temperature. This appears to be related to the retrograde solubilities of the reactants, CaHPO_4 · 2H_2O and CaHPO_4, and of HAp.
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