• Air-cooled blast furnace slag exhibits higher CO2 uptake capacity (0.04 g/g) than water-quenched slag under direct aqueous carbonation conditions.
• Both slag types show CO2 uptake capacities comparable to previous studies, demonstrating feasibility for CO2 sequestration with low-energy input.
• The solid–liquid ratio and CO2 concentration significantly influence the carbonation efficiency of blast furnace slags.
• The findings support the use of blast furnace slag for simultaneous waste management and CO2 emission reduction in the iron and steel industry.