Author:

Keywords:

lime, carbonation, calcite, crystal morphology, scalenohedron, rhombohedron

Abstract:

The morphology of calcium carbonate crystals has been studied at 20ºC during accelerated carbonation using 20% and 100 % volume CO2 concentrations at ~95% R.H. for lime pastes. Accelerated carbonation resulted in precipitation of calcite crystals with a habit and morphology similar on the sample surface but different along the sample thickness, depending on the lime type and CO2 concentration. Micrometer-sized rhombohedral calcite crystals precipitated on the sample surface in 100% CO2 atmosphere while the crystals were sub-micrometer-sized rhombohedral in the 20% CO2 atmosphere. Through the sample thickness, the carbonated profile was composed of scalenohedral calcite crystals with cracked/corroded surfaces that were disintegrated into nanometer-sized rhombohedra. It has been found that scalenohedral calcite undergoes a significant modification to rhombohedral when exposed to high CO2 concentrations. This can be explained with a dissolution-reprecipitation process under excess CO32- ions, leading to re-precipitation of nanometer-sized rhombohedral calcite crystals. Despite high CO2 concentrations, carbonation was not complete due to the heat released during carbonation and precipitated calcite crystals hindering the diffusion of CO2.