Synthesis and Characteristics of Amorphous Calcium Carbonate

A study was made to prepare amorphous calcium carbonate by drying the gelatinous phase which precipitated from the reaction ofCaO-H2O-CO2-CH3OH system. Formation region of the gelatinous phase, conditions for converting to amorphous phase from the gelatinous phase without any crystallizing and characteristics of the amorphous phase were investigated by means of X-ray diffraction, thermal analysis (TG-DTA), electron microscopic observation, chemical analysis, measurements of specific surface area and solubility. The gelatinous phase was prepared by passing CO2 at a flow rate of above 50 cm3/min into the suspensions containing less than 10 g CaO or 5 g Ca(OH)2 in 100 cm3 of methanol. By rapid drying at 40°C under reduced pressure, the gelatinous phase converted to the amorphous phase. Also the gelation rate was accelerated with increasing flow rate of CO2 and existence of water below 3 cm3. It seemed that the amount of CO2 in the amorphous phase (stabilized at 230°C) was more than that of stoichiometric composition of CaCO3 in order to absorb a small amount of CH3O- and HCO3- on the surface of CaCO3 crystal nucleus. The amorphous phase converted rapidly to calcite by heating above 250°C but was stable under reduced pressure at room temperature over a long time. In comparison with ordinary calcium carbonate, the amorphous calcium carbonate has unique characteristics such as porous ultrafine particles (below 100 Å) and a high activity like converting rapidly to calcite with releasing partial CO2 in water as a result of hydrolysis.