COMPOSITION EFFECTS ON THE PH OF A HYDRAULIC CALCIUM-PHOSPHATE CEMENT

The pH of a hydraulic calcium phosphate cement (HCPC) made of monocalc ium phosphate monohydrate (Ca(H2PO4)(2) . H2O; MCPM), beta-tricalcium phosphate (beta-(Ca-3(PO4)(2); beta-TCP) and water was measured as a f unction of reaction time and composition at room temperature. During s etting, the cement pH varies from very acidic pH values, i.e., 2.5, to almost neutral pH values, i.e., 6. The cement pH profile significantl y depends on the initial cement com position. However, all profiles ar e characterized by a sharp initial decrease of the pH due to the disso lution of MCPM crystals and the precipitation of dicalcium phosphate d ihydrate (CaHPO4 . 2H(2)O; DCPD) crystals. With an excess of MCPM, the final pH stays low, and its value can be predicted from the initial c omposition of the cement and solubility data. With an excess of beta-T CP, the end pH is close to 5, which is much lower than 5.9, the value predicted by calculation. Results suggest that the difference may be d ue to the presence of impurities in the cement. Replacing MCPM by phos phoric acid renders the cement paste very acidic for the initial 30 s, but then the pH profile follows that obtained with MCPM. Adding pyrop hosphate ions into the cement paste postpones the position of the pH m inimum. The delay, which is proportional to the concentration of pyrop hosphate ions, is thought to be due to the inhibiting action of pyroph osphate ions on the precipitation of DCPD crystals.