Effects of temperature, pH and calcium-to-carbonate ratio on the calcium carbonate polymorphs
Mohd Hanifa, U. Sharma, Prakash Chander Thapliyal, L.P. Singh
Abstract
Abstract The precipitation of calcium carbonate (CaCO 3 ) polymorphs (calcite, vaterite, and aragonite) plays a crucial role in tailoring the physicochemical properties of construction materials during mineral carbonation. This study systematically explores the influence of pH, temperature, and the [Ca 2+ ]: [CO 3 2− ] ratio on the formation, quantitative composition, mineralogy, and morphology of these polymorphs. Results demonstrate that calcite predominates at pH < 7 and > 10, while vaterite forms primarily between pH 7 and 10, with the highest yield (83%) at pH 8.4. Calcite reaches a maximum yield (98%) at pH 12. No aragonite was detected at any pH. Temperature significantly influences the formation of calcium carbonate polymorphs. Calcite and vaterite are primarily formed at temperatures below 50 °C. Aragonite begin to appear between 50 °C and 85 °C, with the highest yield of 96% observed at 80 °C. However, at temperatures above 90 °C, aragonite gradually transforms into calcite. A higher [Ca 2+ ]: [CO 3 2− ] ratio favors calcite formation. Morphological analysis using FE-SEM reveals spherical vaterite, rhombohedral calcite, and needle-like aragonite. A nanoscale calcite (< 200 nm) with rhomboidal shapes formed at pH 12.5. TGA confirms that vaterite and aragonite are thermodynamically less stable, eventually converting to calcite, attributed to the higher coordination numbers and ligand-ligand repulsion in [CaO 12 ] (vaterite) and [CaO 9 ] (aragonite) versus [CaO 6 ] (calcite).