Morphological controls of calcium carbonate hexahydrate and its dehydrating substance

Abstract

The morphological control of calcium carbonate hexahydrate and its dehydrate was investigated. Synthetic calcium carbonates were characterized by means of X-ray diffraction pattern, thermal analysis (TG-DTA), infrared spectrum and scanning electron microscopic observation. The hexahydrate was synthesized by aging amorphous calcium carbonate which formed by adding aqueous ammonia into calcium hydrogen carbonate solution at 0-9 °C. The crystal shape and particle size of the hexahydrate were remarkably affected by synthetic conditions such as temperature (0-9 °C), calcium ion concentration (2.94-11.34×10-3 mol·dm-3) and stirring speed (150-250 rpm). For example, the particle size of hexahydrate having hexagonal platelike crystals was controlled to approximately 5-25 μm below 3 °C. The hexahydrate unstable above room temperature was easily changed to spherical vaterite at 30-60 °C and to needlelike aragonite above 90 °C by dipping the hexahydrate in water at 30-100 °C. Moreover, an amorphous calcium carbonate hydrate containing 5% methoxyl group in skeleton structure of original hexagonal platelike particles was formed by dipping the hexahydrate in methanol for 1 min. This compound was converted to hexagonal platelike calcite by heating at 300 °C.