The role of different ratios of biochar in the artificial lightweight cold-bonded aggregates (ALCBAs) containing high volume of red mud (RM)

To explore new channels for the effective recycling of red mud (RM) in the field of construction-related materials, and study the feasibility of applying biochar (BC) and RM into capturing CO2, this study developed the artificial lightweight cold-bonded aggregates mixed with high volume of RM (RM-ALCBAs). Different amount of BC (0%, 5%, 10% and 15%) was also added to the RM-ALCBAs, and a set of quantitative devices for assessing carbon absorption of RM-ALCBAs was innovatively developed. Meanwhile, its compressive strength, water absorption rate and apparent density were tested, and then the microscopic properties of RM-ALCBAs were also investigated by using TGA, XRD, SEM-EDS, MIP. Finally, the sustainability and heavy metal leaning behavior of RM-ALCBAs were assessed. It was found that the total carbon uptake of RM-ALCBAs was about 30.58–33.06 kg/tons, indicating that RM-ALCBAs show the obvious carbon-capturing potential, and when the biochar dosage was increased from 0% to 15%, its compressive strength was 1.28 MPa-1.98 MPa, and the water absorption rate and apparent density of RM-ALCBAs before carbonation were 17.96%-20.46% and 1.85 g/cm3-1.98 g/cm3, while the water absorption rate and apparent density of RM-ALCBAs after carbonation were 16.71%-17.04% and 2.02 g/cm3-1.83 g/cm3, respectively, which is in accordance with the specification of LWAs in China. With the increase of BC doping, the porosity in RM-ALCBAs gradually increased, the proportion of micropores gradually decreased, and the proportion of macropores gradually increased. More importantly, converting RM into RM-ALCBAs can effectively lower the toxic leaching and the concentration of heavy metals is also meet the Chinese requirements. RM-ALCBAs mixed with 5% BC has the lowest energy consumption per-unit compressive strength, carbon emission per-unit compressive strength with about 26.0–29.1 kg/t, and cost per-unit compressive strength with much lower than that of FA-ALCBAs and GGBFS-ALCBAs. Overall, the RM-ALCBAs have obvious eco-friendly potentials and belong to the carbon-negative construction materials that can effectively recycle solid waste of RM, which can provide some new insights and useful data for the development path of low-carbon ALCBAs.