Additive concentration effects on dicalcium phosphate dihydrate cements prepared using monocalcium phosphate monohydrate and hydroxyapatite.

In our previous study, we investigated the setting time, mechanical properties and microstructure of dicalcium phosphate dihydrate cements prepared using monocalcium phosphate monohydrate (MCPM) and hydroxyapatite (HA). Despite the use of sodium citrate as a setting regulator, setting occurs rapidly in the MCPM/HA system and further studies on other retardants are needed. In the present study, sodium pyrophosphate and sulfuric acid were tested to evaluate their effectiveness in maintaining workability of the cement paste. MCPM/HA cements at a powder to liquid ratio of 1.0 with sodium pyrophosphate and sulfuric acid at 10, 25, 50, 75 and 100 mM were manufactured and studied based on their setting time, mechanical and porosity properties, phase composition, and microstructure. These measurements were compared to our previous data using sodium citrate. The results showed that the additives have a dose-dependent effect on the setting time. Their order of efficiency is sodium pyrophosphate > sodium citrate > sulfuric acid. However, the sulfuric acid group exhibited the highest compressive strength (CS) compared to the other groups. A lack of correlation between the CS and the porosity of the cements suggested that a mechanism other than porosity reduction was responsible for the CS increase. Since x-ray diffraction analysis did not indicate an effect on composition, explanations based on calcium sulfate dihydrate formation and changes in microstructure were proposed based on scanning electron micrograph observations.