Succesive Formations of Portlandite and Hydroxyapatite from Tricalcium Phosphate in NaOH Thick Solutions Near the Singular Point Concentration

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Phase changes of tricalcium phosphate (TCP, 2.5g) were investigated in 25-35 mass% NaOH solutions (35ml) at 90 degree in C. In concentrations below ca. 30mass%, the successive reaction TCP → Ca(OH)₂ → Hydroxyapatite (HAp) was observed. On the other hand, in concentrations above ca. 30mass%, the reaction TCP → Ca(OH)₂ was observed. Consequently, the singular point between solubility isotherms of HAp and Ca(OH)₂ as a function of NaOH concentration was estimated to be 29.7mass% (9.7mol/dm³). The following successive reaction could be given, Ca₃(PO₄)₂ + 6NaOH → 3Ca(OH)₂ + 2Na₃PO₄•nH₂O → (3/10)Ca₁₀(PO₄₎₆(OH)₂ + 0.2Na₃PO4•nH₂O + 5.4NaOH, where the first step proceeded fast, and the second step very slow.