Effects of H_2O and carbonate ions on the hydrolysis of mechanochemically treated β-tricalcium bis(orthophosphate)

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β-tricalcium bis(orthophosphate) [Ca3(PO4)2] (β-TCP) was mechanochemically treated with a mortar grinder in three different atmospheres of air, wet N2, and wet CO2. H2O and CO32- were incorporated into ground β-TCP, the amount of which depended on both the grinding time and the atmosphere. As the grinding time of β-TCP increased, it hydrolyzed easily to Ca-deficient hydroxyapatite. The apatite-formation ratio for β-TCP ground in wet CO2 was remarkably lower than that for β-TCP ground in air or wet N2. The role of H2O and CO32- in the hydrolysis of ground β-TCP was investigated by calorimetry together with spectroscopy. In calorimetry for each ground β-TCP, a strong exothermic band was observed, which was followed by a broad exothermic band. It is considered that the first band corresponds to the process of adsorption of H2O in the solvent onto the fresh surface of ground β-TCP and the second broad band corresponds to the apatite-formation process. With increasing amount of H2O incorporated into ground β-TCP, both the heat evolution of the first exothermic band and the apatite-formation ratio increased linearly. The H2O incorporated into ground β-TCP promotes the dissolution of β-TCP and apatite formation. In terms of the effect of the incorporated CO32- on hydrolysis, the incorporated CO32- prevents not only apatite formation but also the dissolution of β-TCP.
2009-08-01