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

Abstract

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.