(698f) Direct Carbonation of Ca(OH)2 Using Super Critical CO2 at Different Temperatures Along with the Introduction of SiO2 Aggregate

The production of portland cement is responsible for about 7% of the global carbon dioxide (CO2) emissions. Roughly one ton of CO2 is produced for every ton of cement. The direct carbonation of calcium hydroxide Ca(OH)2 has recently been suggested as a possible CO2-neutral replacement for portland cement in some applications, e.g. prefabricated cement-based construction components. The art of burning limestone (CaCO₃) to form lime (CaO), the formation of slaked lime (calcium hydroxide) and subsequent carbonation has been practiced since at least Roman times as a form of cement. The carbonation process, however, is generally too slow to be practical for most modern applications. This study examines the use of supercritical carbon dioxide at three different temperatures at a constant pressure. The different temperatures were chosen to study how temperature effects the rate of carbonation while in the supercritical state. Discs of Ca(OH)2 were compacted at various pressures and thicknesses (different mass) to produce samples of different density and size. The induction of an aggregate, sand (SiO₂), was also studied to test the effectiveness of formed CaCO3 as a binder. The aggregate was mixed and compacted along with the mass of the Ca(OH)₂ into pellets. The main goals of this study were to determine the practical limits of carbonation of Ca(OH)2 at different temperatures within the supercritical state, with the introduction of an aggregate, and to collect relevant morphological (microstructural) information. X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and scanning electron microscopy (SEM) was used to characterize the carbonated samples. A full-scale heat and material balance for a pilot plant that utilizes this technology was also conducted in order to determine its feasibility on the industrial scale.