Control of aragonite formation and its crystal shape in CaCl₂-Na₂CO₃-H₂O reaction system

Calcium carbonate (CaCO₃) was synthesized in the CaCl₂-Na₂CO₃-H₂O reaction system at the reaction temperature of 40, 50, 60, and 70℃. The initial pH was changed to 2.4–11.0 at 50–70 ℃. The initial pH was adjusted by changing the pH of CaCl₂ aqueous solution using HCl and NH₃ aqueous solution. The CaCO₃ precipitates obtained were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), and specific surface area measurement. In the case of no adding the pH adjustment solution, the product was a mixture of calcite and vaterite at 40 ℃. The formation of aragonite was observed at ≥50 ℃, and single-phase aragonite was obtained at ≥60 ℃, according to XRD measurements. Thus, Aragonite could be synthesized by changing the reaction temperature. At 70 ℃, the synthesis of CaCO₃ was performed by changing the initial pH of 2.4–11.0. The formation of single-phase aragonite was detected in the initial pH range of 2.4–10.4. However, a small amount of calcite was contained in aragonite at the initial pH ≥ 10.8. The crystal shape of obtained aragonite was needle-like, and its particle size became finer as the initial pH increased. At an initial pH 2.4, the length and width of the crystals were 2–7 μm and 0.5–1.0 μm, respectively. The finest particles were samples synthesized at the initial pH 11.0, its length and width were 1–2 μm and 0.1–0.2 μm, respectively. The specific surface area of the products was 4.12–14.88 m² g⁻¹ at the initial pH of 2.4–11.0. The particle size of aragonite could be controlled by changing the initial pH at 70 ℃.