Carbon dioxide transport and uptake in concrete during accelerated carbonation curing

Carbon dioxide (CO2) is the dominant greenhouse gas resulting from many anthropogenic activities, mainly combustion of fossil fuels. One of the strategies to mitigate CO2 emissions is considered to be carbon dioxide capture and storage (CCS). The current storage methods focus on enhanced oil recovery, underground geological storage, disposal in deep oceans, and ex situ mineral carbonation of abundant metal oxide minerals such as olivine, serpentinite and wollastonite.During mineral carbonation, a gas stream rich in CO2 is reacted with mineral metal oxides to form thermodynamically stable carbonates. These carbonated minerals, however, store CO2 but do not produce any materials that are of value. Accelerated carbonation curing of concrete can be used as a mineral sequestration method with the advantage of producing a value-added concrete product. During accelerated carbonation curing of concrete, CO2 is reacted with cement and stored as a solid calcium carbonate in concrete construction products. Among the concrete products, non-reinforced precast concrete, such as blocks and bricks, can be used for carbonation curing. In previous studies, pressurized chambers have been used for accelerated CO2 curing of concrete, where a high pressure of CO2 is required for sufficient gas diffusion in concrete and homogeneous carbonation. In this research, a flow-through carbonation reactor was used for concrete curing and the rate and extent of CO2 uptake by concrete was studied. One of the advantages of the carbonation reactor applied in this study is that significantly less energy for gas mixture compression is required compared to a CO2 pressure chamber.The overall objective of this thesis was to develop and assess the performance of an accelerated carbonation curing reactor for concrete using an advective flow of flue gas. The rate and extent of CO2 uptake by concrete in a 1-D flow-through carbonation reactor were studied and compared with the published results on CO2 uptake in pressurized chambers using diffusive flow of CO2. The factors limiting the CO2 uptake were studied through experimental observation as well as mathematical modeling of CO2 transport and reaction in concrete during accelerated carbonation curing. Carbonation efficiencies of 16-20% attained in the flow-through reactor were comparable to those obtained for static CO2 pressure chambers. The extent of CO2 uptake was limited by formation of solid calcium carbonate in micro-scale pores. Intermittent carbonation experiments showed that the carbonation efficiency was limited in part by slow dissolution and/or diffusion of dissolved reactive components in the concrete matrix. The electron microprobe imaging technique used in this study also confirmed formation of solid calcium carbonate which filled up the narrow pores (<4 µm). The uptake efficiency reached 67% when cement was carbonated in an aqueous suspension in a completely mixed flow-through reactor where the effect of pore blockage was eliminated and a higher percentage of reacting surface area was exposed to dissolved CO2. However, formation of a calcium carbonate layer still inhibited diffusion of dissolved calcium and CO2 through this layer. In the presence of the calcium carbonate layer and other carbonation products like silica (SiO2 gel), and at partial pressure of CO2 used for carbonation, the aqueous solution reached a chemical equilibrium and carbonation ceased before the maximum theoretical uptake could be achieved. The effect of physico-chemical processes on CO2 uptake during carbonation curing was also studied using a mathematical model. Equations describing the CO2 transport by advection and dispersion in concrete pore space, dissolution in pore water and reaction with reactive cement species were solved numerically. The initial concentration of cement species were calculated based on a hydration model which was developed to simulate the 4 hours of hydration time before carbonation starts.

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