Thermodynamic study of formation of amorphous β-tricalcium phosphate for calcium phosphate cements

The reactivity and setting properties of calcium phosphates in water are considerably determined by the particle size / specific surface area and the crystallinity of the compounds. Wet or dry grinding is a common method for adjusting the particle size of calcium phosphate compounds, but this process can also lead to a partial amorphous transformation of the materials. In this work we could show that this mechanically induced phase transformation of beta-tricalcium phosphate is strongly increasing the formation enthalpy DeltaH of the compound up to 420 KJ/mol, which is a result of the formation of intrinsic defects within the crystal lattice. The increase of DeltaH is linearly correlated to the loss of crystallinity as well as the weight loss during heating. X-ray diffraction analysis showed, that the amorphous fraction is responsible for a setting reaction of mechanically activated betaTCP to calcium deficient hydroxyapatite.