Fabrication of calcite blocks from gypsum blocks by compositional transformation based on dissolution–precipitation reactions in sodium carbonate solution

Calcium carbonate (CaCO₃) has been used as a bone substitute, and is a precursor for carbonate apatite, which is also a promising bone substitute. However, limited studies have been reported on the fabrication of artificial calcite blocks. In the present study, cylindrical calcite blocks (ϕ6 × 3 mm) were fabricated by compositional transformation based on dissolution–precipitation reactions using different calcium sulfate blocks as a precursor. In the dissolution–precipitation reactions, both CaSO₄·2H₂O and CaSO₄ transformed into calcite, a polymorph of CaCO₃, while maintaining their macroscopic structure when immersed in 1 mol/L Na₂CO₃ solution at 80 °C for 1 week. The diametral tensile strengths of the calcite blocks formed using CaSO₄·2H₂O and CaSO₄ were 1.0 ± 0.3 and 2.3 ± 0.7 MPa, respectively. The fabrication of calcite blocks using CaSO₄·2H₂O and CaSO₄ proposed in this investigation may be a useful method to produce calcite blocks because of the self-setting ability and high temperature stability of gypsum precursors.