Carbonation of calcium phosphate cements after long-term exposure to Na2CO3-laden water at 250°C

The rates of Na2CO3-induced carbonation of calcium phosphate cements (CPC) at 250°C derived from various calcium aluminate cement (CAC) reactants, depended primarily on the amount of non-reactive CAC reactants remaining in the CPC material after it was autoclaved. Secar 41 CAC, consisting of monocalcium aluminate (CA) and gehlenite (C2AS), had a high reactivity with the NH4H2PO4-based fertilizer (Poly-N) forming a CPC matrix, thereby resulting in rapid setting. Such behavior reflected the formation of hydroxyapatite (HOAp) and boehmite which play an important role in minimizing the carbonation rate at a hydrothermal temperature of 250°C. In contrast, Secar 80, having monocalcium dialuminate (CA2) as its major component, retarded the reaction between CAC and Poly-N, so that a large amount of non-reactive CAC remained in the 250°C-autoclaved CPC specimens. Thus, the high susceptibility of the remaining CAC reactant to carbonation resulted in a yield of ≈ 1.7 % CaCO3 after exposure for 120 days. The loss of strength for all CPC specimens depended on the degree of crystallinity of the HOAp and γ-A100H phases growing in the amorphous phases, but was independent of its rate of carbonation.