Carbon storage through dynamic carbonation curing of fresh lightweight concrete

Carbonation curing of fresh concrete is a chemical reaction between carbon dioxide and calcium silicates in cement. It is a mineral carbon sequestration process in which CO₂ gas is consumed to produce calcium silicate hydrate and calcium carbonate. The technique can be used to recycle CO₂ and reduce emissions in the cement industry. The degree of reaction is characterized by the carbon uptake and is highly influenced by water content. While insufficient water can hinder ionic dissolution, excess water would block carbon dioxide gas diffusion. To maximize the uptake, it is necessary to remove enough free water to create space for carbonate precipitation. Accordingly, a dynamic carbonation system was developed to remove surface free water whilst simultaneously circulating carbon dioxide. The gas circulation was designed to efficiently control the relative humidity carried by the flow and prevent water accumulation on the surface. The proposed curing process effectively combined preconditioning and carbonation into one mechanism. Carbonation was carried out at a gauge pressure of 0.5 bar in a sealed chamber for 4 and 18 hours. Carbon uptake was measured using thermal analysis. Based on cement mass, the resulting uptake reached 13% and 20% in 4-hour and 18-hour carbonation, respectively. Water compensation by means of surface spray was applied after 4-hour carbonation to promote subsequent hydration. The early strength by fresh carbonation was comparable to that of steam curing, while the late strength was higher in a much reduced process time. The carbonation products were characterized by X-Ray diffraction (XRD), thermogravimetric (TG) analysis, and scanning electron microscopy (SEM). Carbonation-induced carbonates were categorized as poorly crystalline and highly crystalline polymorphs. Through recrystallization, a transformation of the former to the latter was recorded during subsequent hydration as the dominant polymorph of calcium carbonate is stable calcite. The co-existence of poorly crystalline carbonates and low temperature hydrates was also reported. It suggested the presence of a hybrid compound of amorphous calcium silicate hydrocarbonate. Dynamic carbonation thus created a carbonate-bond matrix composed of 41 - 64% carbonates. The process provides an excellent means to recycle carbon dioxide, with economic and environmental benefits.