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 CO 2 from ambient air, and thus has the potential to be used for greener non-structural applications. The CO 2 sequestration during curing under high-concentration CO 2 (10 %), which is mainly via the RMC carbonation, is determined by acid digestion. The CO 2 sequestration during service under ambient CO 2 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 CO 2 in situ; particularly, the CO 2 sequestrations by the RMC and the biochar are determined separately. The effect of varying biochar content and CO 2 curing time on the overall CO 2 sequestering ability of the concrete are studied, together with the variation of the local CO 2 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 CO 2 during the curing phase (0.2–0.3 g per gram RMC), and constantly sequestrate noticeable amounts of CO 2 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 CO 2 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. [Display omitted] • Lightweight concrete developed with reactive MgO cement and high-dosage biochar. • Substantial CO 2 sequestration under curing (10% CO 2) and ambient service (0.1% CO 2). • Separately quantified CO 2 sequestration by MgO and biochar with new methodology. • Quantified relationship between CO 2 sequestration and biochar content, age, humidity. • Discovered unconventional depth-dependency of RMC carbonation induced by biochar. [ABSTRACT FROM AUTHOR]