Alpha-tricalcium phosphate (α-TCP): Solid state synthesis from different calcium precursors and the hydraulic reactivity

The effects of solid state synthesis process parameters and primary calcium precursor on the cementtype hydration efficiency (at 37°C) of α-tricalcium phosphate (Ca₃(PO₄)₂ or α-TCP) into hydroxyapatite (Ca_10-x HPO₄(PO ₄)_6-x(OH)_2-x x = 0-1, or HAp) have been investigated. α-TCP was synthesized by firing of stoichiometric amount of calcium carbonate (CaCO₃) and monetite (CaHPO₄) at 1150-1350°C for 2 h. Three commercial grade CaCO₃ powders of different purity were used as the starting material and the resultant α-TCP products for all synthesis routes were compared in terms of the material properties and the reactivity. The reactant CaHPO₄ was also custom synthesized from the respective CaCO₃ source. A low firing temperature in the range of 1150-1350°C promoted formation of β-polymorph as a second phase in the resultant TCP. Meanwhile, higher firing temperatures resulted in phase pure α-TCP with poor hydraulic reactivity. The extension of firing operation also led to a decrease in the reactivity. It was found that identical synthesis history, morphology, particle size and crystallinity match between the α-TCPs produced from different CaCO₃ sources do not essentially culminate in products exhibiting similar hydraulic reactivity. The changes in reactivity are arising from differences in the trace amount of impurities found in the CaCO₃ precursors. In this regard, a correlation between the observed hydraulic reactivities and the impurity content of the CaCO₃ powders-as determined by inductively coupled plasma mass spectrometry-has been established. A high level of magnesium impurity in the CaCO₃almost completely hampers the hydration of α-TCP. This impurity also favors formation of β-instead of α-polymorph in the product of TCP upon firing.