CARBONATION OF CALCIUM-PHOSPHATE CEMENTS AFTER LONG-TERM EXPOSURE TO NA2CO3-LADEN WATER AT 250-DEGREES-C

The rates of Na2CO3-induced carbonation of calcium phosphate cements ( CPC) at 250-degrees-C derived from various calcium aluminate cement (C AC) reactants, depended primarily on the amount of non-reactive CAC re actants remaining in the CPC material after it was autoclaved. Secar 4 1 CAC, consisting of monocalcium aluminate (CA) and gehlenite (C2AS), had a high reactivity with the NH4H2PO4-based fertilizer (Poly-N) form ing a CPC matrix, thereby resulting in rapid setting. Such behavior re flected the formation of hydroxyapatite (HOAp) and boehmite which play an important role in minimizing the carbonation rate at a hydrotherma l temperature of 250-degrees-G. In contrast, Secar 80, having monocalc ium dialuminate (CA2) as its major component, retarded the reaction be tween CAC and Poly-N, so that a large amount of non-reactive CAC remai ned in the 250-degrees-C-autoclaved CPC specimens. Thus, the high susc eptibility of the remaining CAC reactant to carbonation resulted in a yield of almost-equal-to 1.7 % CaCO3 after exposure for 120,days. The loss of strength for all CPC specimens depended on the degree of cryst allinity of the HOAp and gamma-AlOOH phases growing in the amorphous p hases, but was independent of its rate of carbonation.