Effect of several additives and their admixtures on the physico-chemical properties of a calcium phosphate cement

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Abstract
Combinations of citrate (C6H5O 7 3-− ), pyrophosphate (P2O 7 4− ) and sulfate (SO 4 2− ) ions were used to modify the physico-chemical properties of a calcium phosphate cement (CPC) composed of β-tricalcium phosphate (β-TCP) and phosphoric acid (PA) solution. The results obtained with only one additive at a time are similar to those previously published. New facts are: the positive effect of C6H5O 7 3− ions on cement failure strain and their negative effect on cement pH. The position of the setting time maximum measured at an SO 4 2− concentration of 0.09 M was not displaced by the addition of C6H5O 7 3− and P2O 7 4− ions. However, the effect of SO 4 2− ions on the setting time was depressed by C6H5O 7 3− ions. Moreover, no increase in tensile strength was observed when increasing amounts of SO 4 2− were added into a C6H5O 7 3− -containing cement. The latter results suggest a competitive effect of C6H5O 7 3− and SO 4 2− on setting time and tensile strength. Anhydrous dicalcium phosphate (DCP; CaHPO4) appeared in cement samples dried just after setting, but not in cement samples incubated for 24 h in deionized water before the drying step. It is believed that the setting reaction is stopped by the drying step, leaving a low internal pH in the sample, hence providing favorable conditions for the transformation of dicalcium phosphate dihydrate (DCPD) into DCP. Interestingly, even though C6H5O 7 3− ions dramatically lowered the equilibrium pH of the cement with 5 ml of deionized water, they still prevented the occurrence of the transformation of DCPD into DCP.

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