CO2 uptake model of limestone-powder-blended concrete due to carbonation

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In this study, a model for evaluating the CO2 uptake rate of plain concrete and limestone-powder-blended concrete in both the service and recycling phases was proposed. First, a blended cement hydration model was proposed to evaluate the content of carbonatable substances, porosity and carbon dioxide diffusivity. In the service phase, a one-dimensional carbonation model was proposed to evaluate the carbonation depth. In the recycling stage, an unreacted core model was proposed to evaluate the carbonation process of spherical recycled concrete. Secondly, considering the concrete materials, structural elements and environmental exposure, a CO2 uptake model at the service stage and recycling stage was proposed. The total CO2 uptake rate is the sum of the CO2 uptake rates in the service stage and the recycling stage. The analysis results showed (1) as the limestone replacement rate increased from 0% to 20% (water–binder ratio is 0.3), the CO2 uptake rate in the service phase increased from 3.12% to 3.84%, and the total CO2 uptake rate increased from 22.82% to 26.11%. (2) Given a certain concrete mixture, as the surface area to volume ratio of structural units increased, or the amount of recycled concrete decreased, the CO2 uptake rate increased, but the total CO2 uptake rate did not change. (3) Given a certain limestone powder replacement ratio, as the water/binder ratio increased, the total CO2 uptake ratio increased. The contributions of this study are 1) propose an integrated hydration-carbonation-CO2 uptake model for limestone blended concrete, 2) clarify the differences of carbonation equations between service stage and recycling stage, and 3) determine the effects of concrete mixtures, structural element sizes and types, life cycle stages, and recycled concrete sizes on CO2 uptake.