The Carbonation Kinetics of Alkaline Solids at Conditions Relevant to Direct Flue Gas Utilization

Alkaline solids (e.g., Ca(OH)₂ or fly ash) can be exploited as cement-alternative binders that also remove CO₂ from industrial flue gases (e.g., from coal/natural gas-fired power plants) via carbonation reactions. When these reactants are mixed with water and aggregates to form a concrete mixture, carbonation causes in situ precipitation of carbonate minerals, such as calcium carbonate (CaCO₃), that bind particles together and increase the concrete’s strength. The design and optimization of such carbonation processes for the production of concrete components requires detailed experimental datasets and analyses describing the reaction kinetics of the alkaline solids, and a method for scaling up kinetics data to the component level. Such data have not been rigorously evaluated within a parameter space relevant to the direct utilization of flue gases, i.e., temperature (T), relative humidity (RH), and CO₂ partial pressure (p_CO2), without substantial pre-processing of the gas stream. To this end, we (1) investigate the relationships between the carbonation kinetics of alkaline solid reactants and their theoretically predicted carbonation potentials and (2) experimentally assess the sensitivity of carbonation kinetics to RH, T, and p_CO2. Provided that a critical RH is exceeded, a reactant’s carbonation sensitivity relative to its theoretical carbonation potential can be described by a reduction factor that is correlated to its rate of calcium release, suggesting that carbonation kinetics in this regime are dissolution-limited. The carbonation kinetics of mixtures of powder reactants are found to be strongly predicted by a linear rule of mixtures, indicating negligible influences of concurrent pozzolanic reactions during the processing period. These findings provide critical inputs that inform the design of reactant mixture formulations and carbonation processes in relation to the properties of the constituent reactants. Future research is directed towards extending this approach for the scalable production of carbonated concrete components by direct flue gas utilization.