Effects of the granularity of raw materials on the hydration and hardening process of calcium phosphate cement.

Effects of the granularity of the raw materials on the hydration and hardening process of calcium phosphate cement (CPC) composed of equimolar tetracalcium phosphate (TECP) and dicalcium phosphate anhydrous (DCPA) were investigated systematically. The variation of pH value in CPC slurry indicated that the control step of CPC hydration was the dissolution of DCPA under these experimental conditions. Reducing the particle size of DCPA could accelerate the hydration rate, and decreasing the particle size of TECP would expedite the dissolution of DCPA, which would obviously result in a faster hydration rate. The results of isothermal conduction calorimetry showed that reducing the particle size of TECP could increase the conversion ratio of starting materials to hydration products, which would lead to an increase in the compressive strength of the hardened body of CPC. The sample composed of the smallest particle size of DCPA and TECP obtained the compressive strength of 41 MPa, which would not attain the highest compressive strength, 49 MPa. The smaller the particle size of either DCPA or TECP, the shorter the setting time was. During the setting process of CPC, the microstructure progresses from a gel structure to an agglomeration-crystallization structure. The calculated values of setting time from the rheological model coincided with the experimental data very well. The parameters of AC impedance spectroscopy were closely correlated with the mean pore diameter and porosity of the CPC hardened body. The results of AC impedance spectroscopy further verified that a small particle size of raw materials could result in high hydration rate and the compressive strength of 49.1 MPa.