CO2 absorption of recycled concrete aggregates in natural conditions

Cement production is linked to a substantial CO2 emission contributing to about 5-8% of the man-made emissions. However, hardened cementitious materials can absorb CO2 in the process called carbonation, both during the service life of the structures and during their demolition and recycling phase. As experimental data of CO2 absorption during the recycling phase are scarce, the goal of the study is to experimentally determine the CO2 absorption of recycled concrete aggregates (RA) from the point of crushing until reuse in recycled aggregate concrete (RC). Samples from three plants producing both RA and RC were collected and stored both in the plants and in the laboratory for several months. The change in the amount of uncarbonated cement paste of the RA with time was determined by using a novel method: image analysis of samples embedded in epoxy, ground and sprayed with phenolphthalein. This allowed to calculate the CO2 absorption during the storage of RA. The amount of absorbed CO2 corresponds to about 5.4-12.6% of total CO2 emission originally stemming from the production of cement.