New hydraulic cements based on α-tricalcium phosphate–calcium sulfate dihydrate mixtures

Calcium sulfate dihydrate (CSD) powder was added to a cement consisting of α-tricalcium phosphate (α-TCP) and water. The changes of the physico-chemical properties of the cement were investigated as a function of the CSD amount, the phosphate concentration in the mixing solution, and the solution volume. An increase of the phosphate concentration in the mixing liquid and small additions of CSD powder strongly reduced the cement setting time. Simultaneously, the fraction of unreacted α-TCP powder present after 1 day of incubation increased, indicating that α-TCP hydrolysis was inhibited. The effects of the CSD amount and the phosphate concentration were synergetic, i.e. the effect of CSD powder was increased with an increase of the phosphate concentration and vice versa. Interestingly, none of the factors affected the cement diametral tensile strength. The present results were explained based on solubility calculations. The present study shows that the use of CSD crystals in combination with phosphate ions is an easy and interesting way to control the setting time of α-TCP–water mixtures, in particular, because the mechanical properties of the cement are not modified.