Reactivity of α-tricalcium phosphate

The reactivity of α-tricalcium phosphate (α-TCP) in forming hydroxyapatite (HAp) at 37°C was investigated. The effects of synthesis route, HAp seeding and the presence of calcium salts on the mechanism and extent of HAp formation were examined by pH measurements and/or isothermal calorimetric analyses. A synthesis temperature at the lower end in the temperature range of 1100–1300°C and the reaction of α-TCP with a high specific surface area greatly improved rate and extent of HAp formation. The time for complete reaction decreased from 18 h to 14 h, when the reaction was carried out in the presence of 1 wt% of HAp seeds; the hydrolysis mechanism did not change. At HAp seeds proportion of 5 wt% and 10 wt%, transformation occurred without a nucleation period. The calcium salt additives studied were anhydrous and dihydrate form of dicalcium phosphate (CaHPO4 and CaHPO4 · 2H2O), calcium carbonate (CaCO3), and calcium sulfate hemihydrate (CaSO4 · 1/2H2O). All the additives delayed HAp formation as determined by the isothermal calorimetric analyses. Their retarding effects in decreasing order are CaCO3, CaSO4 · 1/2H2O, DCPD, DCP. CaCO3 almost completely retarded HAp formation. After 24 h, hydrolysis was complete only for pure α-TCP and for the α-TCP-DCP blend. Reaction was complete in other formulations before 48 h except for the CaCO3-containing blend. In all mixtures conversion to HAp occurred without forming any intermediates. However gypsum formed in the mixture containing CaSO4 · 1/2H2O. All the α-TCP-additive mixtures, excluding α-TCP-CaCO3, reached nominally the same strength value after 24 h of reaction as governed by the transformation of α-TCP to HAp. For phase-pure α-TCP, the average tensile strength changed from 0.36 ± 0.03 MPa to 7.26 ± 0.6 MPa. Upon hydrolysis only the CaSO4 · 1/2H2O-containing mixture exhibited slightly higher strength averaging 8.36 ± 0.9 MPa.