Mechanism of the hardening process for a hydroxyapatite cement

The dynamics for the hydraulic process of calcium phosphate cement (CPC) were investigated by X-ray diffraction quantitative analysis. The results show that the hardening process of CPC is initially controlled by the dissolution of reactants in a 4-h period and subsequently by diffusion through the product layer of hydroxyapatite (HAP) around the grains. The compressive strength rises approximately linearly with the increase of the extent of conversion in a 4-h period, and a maximum compressive strength of about 51 MPa, which is superior to those reported by the references, is obtained in 4 h. Then the compressive strength drops a little with an increase in the extent of conversion. The final product of setting reaction is acicular HAP crystal. Crystal seed not only reduces the setting time but also drops the compressive strength. The variation of pH in CPC slurry from 7.5 to 10.5 reveals that the control step of the dissolution process in the hardening process is the dissolution of dicalcium phosphate anhydrous and the presence of crystal seed will reduce the supersaturation to produce HAP.