Evaluation of CO2 Uptake of Fly-Ash-Blended High-Strength Concrete Due to Carbonation

Carbonation of concrete can cause the uptake of CO2 and alleviate the CO2 emissions burden of the concrete industry. This study shows a procedure for evaluating the CO2 uptake of fly ash (FA)-blended high-strength concrete, considering both the service and recycling stages. First, a blended cement hydration model is proposed to evaluate the contents of carbonizable substances, porosity, and CO2 diffusivity. In the service stage, a one-dimensional carbonation model is proposed for evaluating carbonation depth. In the recycling stage, an unreacted core model is proposed for evaluating the carbonation process of spherical crushed concrete. Second, CO2 uptake models are proposed for the service and recycling stages, considering concrete materials, structural elements, and environmental exposure. The total CO2 uptake ratio is determined as the sum of CO2 uptake in the service and recycling stages. The calculation results show that, as the FA replacement ratio increases from 0 to 40%, the CO2 uptake ratio in the service stage increases from 2.78 to 4.72%, and the total CO2 uptake ratio increases from 18.9 to 20.8%. As the surface to volume ratio of the structural element increases, or the size of particles of crushed concrete decreases, the rate of CO2 uptake increases but the total CO2 uptake ratio does not change.