CO2-sequestering ability of lightweight concrete based on reactive magnesia cement and high-dosage biochar aggregate

Lightweight concrete is widely used in non-structural components of buildings, e.g., partition wall bricks. Most current lightweight concretes engage Portland cement and lightweight aggregates. In this study, lightweight concretes based on reactive magnesia cement (RMC) and high amount of biochar aggregates (up to 508 kg/m3, 61 vol% of the concrete) are developed. The new material demonstrates the ability to sequestrate massive amounts of CO2 from ambient air, and thus has the potential to be used for greener non-structural applications. The CO2 sequestration during curing under high-concentration CO2 (10 %), which is mainly via the RMC carbonation, is determined by acid digestion. The CO2 sequestration during service under ambient CO2 concentration (0.1%, or 1000 ppm), which is via the continued carbonation of RMC and the adsorption of biochar, is determined by monitoring the removal of CO2 in situ; particularly, the CO2 sequestrations by the RMC and the biochar are determined separately. The effect of varying biochar content and CO2 curing time on the overall CO2 sequestering ability of the concrete are studied, together with the variation of the local CO2 sequestration with carbonation depth in the concrete. Based on the experimental results, the new concretes are seen to achieve adequate mechanical properties for non-structural application. They can permanently sequestrate a significant amount of CO2 during the curing phase (0.2–0.3 g per gram RMC), and constantly sequestrate noticeable amounts of CO2 during the service stage (1–12 mg per gram RMC, 1 mg per gram biochar). The incorporation of porous biochar not only decreases the material density, but also improves the RMC carbonation by accelerating CO2 penetration, while the RMC barely influences the biochar adsorption. However, the RMC carbonation ability permanently vanishes with curing age, whilst the biochar adsorption decreases marginally with the curing age.