Concrete with recycled aggregates from construction: Properties, emissions and carbon capture assessment

The cement industry is responsible for approximately 7 % of carbon dioxide global emissions into the atmosphere. However, cementitious materials and construction and demolition waste (CDW) can capture CO₂ through carbonation, making accurate quantification essential for effective carbon compensation strategies. This study aims to quantify the CO2 capture potential of concrete produced with fine (FA) and coarse (CA) mixed recycled aggregate (RA) from construction and demolition waste (CDW) using thermogravimetric analysis (TGA). The materials were characterized by XRF, XRD, BET, Laser granulometry, and TGA. Concrete mixtures with different water/cement ratios (0.45 and 0.65) and RA replacement levels (0 %, 50 %, and 100 %) were made and studied. Physical (specific mass and capillary absorption) and mechanical tests (compressive strength, tensile strength, static and dynamic elastic modulus) were performed to evaluate concrete properties. The accelerated carbonation test, conducted at a 5 % CO2 concentration, was used for carbon capture studies by thermogravimetry, measuring carbonation depth over time. The emissions balance was calculated, excluding the CO₂ captured through the carbonation reaction of emissions associated with concrete production. Finally, an integrative analysis was performed using Shapley Additive Explanations (SHAP) with Random Forest models. The results show that the concrete carbon capture varied between 12 and 94 kg.CO2/m³ with the highest values coming from concrete with FA and less w/b ratio. Concretes with 100 % RA exhibit lower CO₂ emissions and have the potential to capture up to 19 % of the emissions from cement production. As the replacement rate of AR increased, the concrete's mechanical properties decreased, and the carbonation depth and capillary water absorption increased, which is undesirable from a durability perspective. Compressive strength losses ranging from 16 % to 38 % were recorded, depending on the replacement content and concrete’s w/b ratio. Since recycled aggregate concrete has lower emissions per cubic meter and enhanced CO₂ capture potential, optimizing its mix design with admixtures is essential to mitigate losses in physical and mechanical properties, thereby contributing to the carbon market's circularity.