Controls of Polymorphism and Morphology of Calcium Carbonate Compounds Formed by Crystallizing Amorphous Calcium Carbonate Hydrate

In this report, controls of polymorphism and morphology of calcium carbonate compounds were studied by suspending amorphous calcium carbonate hydrate (ACC) powder in water or MgCl2 solution. Characteristics of calcium carbonate compounds formed from ACC were determined by means of X-ray diffraction, thermal analysis (TG-DTA) and scanning electron microscopic observation. ACC was synthesized by adding 0.1mol⋅dm-3 CaCl2 solution into a mixed solution of 0.1mol⋅dm-3 NaOH and 0.1mol⋅dm-3 Na2CO3 at 0°C. The calcium carbonate compounds formed easily by suspending ACC in water and was affected remarkably by temperature (0-80°C) and pH (1.7-14.0). Thus ACC changed into hexagonal plate-like calcium carbonate hexahydrate at 0°C, rhombohedral calcite at 15-20°C, spherical vaterite at 30-50°C and needle-like aragonite above 80°C after aging for 1h. ACC was also changed into vaterite in the pH region of 7.0-9.5 and to basic calcium carbonate above pH 12.8 at 20-40°C. On the other hand, spherical calcium carbonate monohydrate was formed from ACC in MgCl2 solution (0.03-0.50mol⋅dm-3) and then changed finally to aragonite after long aging. For example, the amount of spherical monohydrate with a diameter of 30μm reached a maximum after aging for 3d, and it changed to needle-like aragonite with an average length of 50μm after 10d in 0.10mol⋅dm-3 MgCl2 solution. Accordingly, ACC changed easily to calcium carbonate anhydrides (calcite, aragonite, vaterite), calcium carbonate hydrates (monohydrate, hexahydrate) and basic calcium carbonate, when ACC was suspended in solutions of different conditions such as temperature, pH and concentration of MgCl2.