Crystallization of amorphous calcium carbonate

Abstract

Amorphous precipitates of CaCO₃ were prepared by reacting a mixed solution of Na₂CO₃–NaOH with a CaCl₂ solution at various pH (≥11.2) at an ambient temperature of 278 K. The precipitates prepared at pH ranging from 11.2 to 13.0 were identified as amorphous calcium carbonate (ACC) by means of TG–DTA, FT-IR spectroscopy and powder X-ray diffractometry. This showed loss of absorbed water until ca. 530 K and subsequent crystallization to calcite. The crystallization temperature of ACC varied depending on the pH value of sample preparation. The crystallization processes of ACC prepared at various pH values were investigated by DSC. The enthalpy change and activation energy of the crystallization process increased with increasing pH value of sample preparation, ranging from 3.99 to 12.26 kJ (mol CaCO₃)⁻¹ and from 151.6 to 304.1 kJ mol⁻¹, respectively. The morphologies of ACC and its crystallization product were observed by scanning electron microscopy. It was shown that finely dispersed particles of crystalline calcium carbonate can be prepared through the crystallization of ACC by heating up to ca. 630 K.