Hydrothermal Synthesis of Hydroxyapatite from Calcium Sulfate Hemihydrate

Abstract

Commercial grade calcium sulfate hemihydrate (CaSO4· 0.5H2O, or plaster of paris, PoP) an economically feasible and abundant precursor has been used in (Ca10(PO4)6(OH)2, HAp) synthesis. The synthesis was realized by reacting solid PoP precursor with 1M of (NH4)2HPO4 aqueous solution, at ambient and mild hydrothermal conditions (2 ± 0.2 atm and 120 ° C). The effect of reaction temperature and pressure on PoP to HAp conversion efficiencies and reaction kinetics has been reported. The formation of HAp at a reaction temperature lower than 50 C occurs at very limited extend; at 25 C HAp formation starts after 7 days. Extending the reaction time for low temperature reactions improves the HAp formation efficiency, meanwhile this also promotes precipitation of another calcium phosphate- CaHPO4 · 2H2O together with HAp. At a reaction temperature of 90 C HAp formation completes in 6 hours. Hydrothermal reaction conditions on the other hand significantly enhances reaction kinetics, where the transformation to HAp starts in 15 min and finalizes only in 90 min. Hydrothermal reaction also hampers the formation of secondary phases, most likely originating from the impurities of low grade PoP precursor. The scanning electron microscopy analyses showed that HAp crystals are preferentially growing on the reactant PoP powders in a reticulated need-like morphology, suggesting a dissolution and reprecipitation mechanism for HAp formation. The findings showed that HAp synthesis can be accomplished by using a single and relatively cheap solid precursor under technological relevant hydrothermal conditions. Such synthesis routes can be practically expanded for making phase pure HAp powders in industrial scale.