Transforming AOD slag toward a highly reactive mineral admixture with appreciable CO₂ sequestration: Hydration behavior, microstructure evolution, and CO₂ footprint

The use of argon oxygen decarbonization slag (AODS) is restricted due to its volume unsoundness and low hydration reactivity. In this study, the feasibility of transforming AODS toward a highly reactive mineral admixture with appreciable CO₂ sequestration was investigated, the hydration behavior, evolution of the compositions and microstructures of AODS with carbonation time were systematically studied, and CO₂ footprint of carbonated AODS was elucidated. Results indicated that γ-C₂S in AODS could quickly react with CO₂ to form CaCO₃. MgO and bredigite dissolved significantly when the pH value of the slurry dropped to 7.0, leading to a significant increase in the Mg²⁺ concentration of the slurry, and promoting the conversion of calcite to monohydrocalcite. However, due to the lower Ca/Si and Ca/Mg ratios, calcium silicate (CS) and akermanite exhibited extremely low carbonation reactivity. As the carbonation time increased, the particle size of CaCO₃ gradually increased, from approximately 150 nm at 5min to approximately 400 nm at 20min, [SiO₄] gradually transited from the Q⁰, Q¹ and Q² structures to the Q³ and Q⁴ structures, ultimately forming a large amount of amorphous SiO₂ gel, which led to an obvious increase in specific surface area of AODS. The compressive strength of cement mortar mixed with 20 wt% carbonated AODS (CAODS) was increased by 25.8 % compared with that of the mortar incorporating AODS. The CO₂ sequestration capacity of AODS can reach approximately 200 kg/t, and 193.4 kg CO₂ can be cut when 1 ton of CAODS-based composite cement with excellent cementitious properties is produced.