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

Combinations of citrate (C6H5O73-), pyrophosphate (P2O74-) and sulfate (SO4 2-) ions were used to modify the physico-chemical properties of a calcium p hosphate cement (CPC) composed of beta-tricalcium phosphate (beta-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 pos itive effect of C6H5O73- ions on cement failure strain and their negative e ffect on cement pH. The position of the setting time maximum measured at an SO42- concentration of 0.09 M was not displaced by the addition of C6H5O73 - and P2O74- ions. However, the effect of SO42- ions on the setting time wa s depressed by C6H5O73- ions. Moreover, no increase in tensile strength was observed when increasing amounts of SO42- were added into a C6H5O73--conta ining cement. The latter results suggest a competitive effect of C6H5O73- a nd SO42- on setting time and tensile strength. Anhydrous dicalcium phosphat e (DCP; CaHPO4) appeared in cement samples dried just after setting, but no t 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 st ep, leaving a low internal pH in the sample, hence providing favorable cond itions for the transformation of dicalcium phosphate dihydrate (DCPD) into DCP. Interestingly, even though C6H5O73- ions dramatically lowered the equi librium pH of the cement with 5 ml of deionized water, they still prevented the occurrence of the transformation of DCPD into DCP. (C) 2000 Kluwer Aca demic Publishers.