Carbonate apatite is expected as a bone substitute with an excellent resorbability, because of its similarity in composition of the hard tissue. In this study, set gypsum containing calcite at various ratios were treated in trisodium phosphate solution at 100C° for Iday. Diametral tensile test, X ray diffraction, infrared absorption spectroscopy and scanning electron microscopic observation were carried out to characterize the carbonate apatite formed. DTS of the set specimens before the treatment decreased from 7.3 MPa to 0.6 MPa with the amount of calcite content up to 90%. On the other hand, DTS after the treatment was between 1.3 MPa and 3.9 MPa depending on the calcite content. DTS showed a decrease after the treatment at lower calcite content and did an increase at higher calcite content. Results of X-ray diffraction analysis showed gypsum disappeared and calcite always remained after the treatment in both of the solutions. With 0.5 mol/L Na3PO4 solution, a main product was II-CaSO4 in the specimens of lower calcite content (<40%) and apatite over 60% of calcite. With 1 mol/L Na3O 4 solution, a main product was apatite irrespective of calcite content. An a-axis in hexagonal apatite crystal decreased (9.314 and 9.329å) and the c-axis increased (6.908 and 6.917Å). From the values of the lattice parameter, it was estimated that the synthesized carbonate apatite contain about 15-20 wt% of CO32- ion. FT-IR spectrum showed the absorption peaks around 1450 and 1410cm-1 in the specimen converted into apatite and confirmed the formation of B-type carbonate apatite in which CO32- ion substitutes for PO 43- ion. It was concluded that the hydrothermal treatment of a set gypsum containing calcite in Na3PO4 gave carbonate apatite monolith, although calcite remained in part.
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