Carbon dioxide sequestration characteristics of concrete mixtures incorporating high-volume cement kiln dust

Abstract

The production of Portland cement is a significant source of carbon emission into the atmosphere. Therefore, the utilization of a cement byproduct to enhance carbon sequestration in concrete contributes to carbon and waste cycling, thus promotes cleaner production. This study evaluated the impact of cement kiln dust (CKD), an industrial waste from carbon dioxide-generating cement manufacturing process, on the carbon dioxide (CO2) uptake characteristics of concrete mixtures. Two variants of CKD having remarkably different fineness were individually used as cement replacement (0–60%) in the mortar phase of two base concrete mixtures at water-binder ratio (w/b) of 0.5 and 0.425. Following 12 or 18 h in-mold pre-curing, mortar specimens were subjected to 12-h accelerated carbonation curing (ACC) at 414 kPa and 24 ± 1 ℃, and then moist-cured until 28 days. Generally, the total CO2 uptakes in CKD-modified mixtures were less than those of control mixtures. However, expressed as percentage of cement content, the ‘relative’ CO2 uptake was higher in mixtures incorporating finer-grained CKD at higher w/b and at low and high CKD content than in the control mixtures. The relative CO2 uptake of CKD-modified mixtures exhibited an inverse correlation with pre-ACC strength. Further, the control and mixtures containing fine and coarse CKD variants sequestered CO2 of up to 11%, 14% and 8% by mass of cement content, respectively. Additionally, mixtures with w/b of 0.5 sequestered significantly more carbon dioxide than 0.425 w/b counterparts. In all cases, it was found that 12-h pre-ACC curing offered better carbon dioxide sequestration performance. Values of pH increased with CKD content. Thus, CKD served as a hyper-alkaline constituent offering viable remedy to ACC-induced dealkalization of concrete and, consequently, the freedom to apply full-depth ACC in reinforced concrete without concerns of low corrosion protection of embedded rebars.