Development of cement-free bio-based cold-bonded lightweight aggregates (BCBLWAs) using steel slag and miscanthus powder via CO₂ curing

Bio-based lightweight aggregates are a novel type of lightweight aggregates. The use of plant-based aggregates in concrete effectively decreases its bulk density, the environmental impact and improve the thermal insulation property. However, the application of plant-based aggregate in ordinary cement-based concrete still faces critical problems, for instance, the polysaccharide leached from the plant retards the hydration of clinker and lead to degradation of concrete strength. In this study, a novel bio-based cold-bonded lightweight aggregates (BCBLWAs) was developed with the use of converter steel slag and miscanthus powder via CO2 curing. The aggregates using steel slag as a binder show relatively high strength and low density with the use of miscanthus. The polysaccharide leached from miscanthus powder, which negatively influences the cement-based aggregates, has no influence on the artificial carbonated steel slag aggregate. The developed BCBLWAs obtained compressive strengths from 0.58 MPa to 5.3 MPa and loose bulk densities from 550 kg/m3 to 1300 kg/m3. The reaction products and microstructure of BCBLWAs after carbonation curing were characterized by TGA, XRD, SEM and nitrogen physisorption. The CO2 uptake capacity of BCBLWAs was also evaluated. Furthermore, the prepared BCBLWAs were utilized to prepare a lightweight concrete, which obtained optimal compressive strength (28.5 MPa), dry bulk density (1630 kg/m3) and thermal insulation (0.439 W/(m⋅K)). The effect of BCBLWAs on the hydration of cement was also evaluated to be minor and only showed a slight retardation effect. The lowest thermal insulation of BCBLWAs incorporated lightweight concrete reached 0.255 W/(m⋅K).