Sustainable upcycling of artificial lightweight cold-bonded aggregates (ALCBAs) designed by biochar and concrete slurry waste (CSW) into porous carbons materials for CO2 sequestration

CSW (Concrete Slurry Waste) is a hazardous solid waste due to its high alkalinity, and traditional landfill disposal wastes a lot of land. To effectively recycle CSW, this project applied it, ordinary Portland cement (OPC), and Biochar (BC) into designing artificial lightweight cold-bonded aggregates (ALCBAs) by disc granulation. A new test device for evaluating the amount of carbon absorption in ALCBAs was designed and ALCBAs adding ultra-high volume of CSW (CSW-ALCBAs) with BC dosage of 0%, 5%, 10%, and 15% were designed, respectively. The compressive strength, water absorption rate, carbon-sequestration efficiency, pores, hydration phase, and ecological properties of CSW-ALCBAs were also investigated under standard, carbonation, and sealing curing conditions. The results showed that CSW-ALCBAs had obvious carbon-sequestration potential, and the maximum carbon absorption was 18.2 wt% at 15% BC dopsage and the minimum carbon absorption was 16.2 wt% at 5% BC doping. Also, the carbon absorption of CSW-ALCBAs increases with the rise in BC dosage. Besides, the compressive strength in standard and sealed curing conditions peaked at 5% BC dosage, about 2.76 MPa, and then decreased rapidly with increasing BC dosage, and the compressive strength drops to 1.86 MPa at 15% BC dosage. Carbonization condition can improve the mechnaical and physical properties of CSW-ALCBAs with water absorption rate in the range of 6%− 13%, and the maximum compressive strength was about 6.18 MPa. And the compressive strength with the lowest and highest increasing rate in the groups under 28-d carbonized condition were in groups B5 and B10, where the compressive strength increased by 56% and 215%, respectively. Additionally, the total porosity of CSW-ALCBAs is in 13%− 23%, and the BC can provide abundant harmless pores (<50 nm) and store sufficient water. The BC could result in the internal curing effect, and the hydration products such as CH, CC, C-S-H, and AFt could be clearly observed. The choice of 5% BC doping for developing green CSW-ALCBAs is very promising. Overall, this study provides a novel method for the production of carbon-negative aggregates, which not only helps to address the recycling of massive CSW, but also opens up new channels for CSW recycling and CO2 sequestration. Meanwhile, compared with the previous reports about the recycling of CSW has been explored in various ways of utilization and curing regimes, this project explores its potential for carbon capture and investigates the changes in various properties of CSW-ALCBAs after carbonization.