Effects of sodium aluminate on fleeting semi-dry carbonation and properties of steel slag powders in low concentration CO₂ atmosphere

Accelerated carbonation suffers from the limitation of low efficiency when CO₂ supply is inadequate. This study aims at proposing a carbonation accelerator of sodium aluminate (SA) to stimulate the reaction between steel slag and CO₂. Additionally, a fleeting semi-dry carbonation is proposed and mixed gas with 20 vol% CO₂ was used to simulate the CO₂ concentration of flue gas. According to the MGC results, with only 0.07 wt% of SA added, steel slag obtained 3.55 wt% of CO₂ uptake after 10 min carbonation under 20 vol% CO₂ atmosphere, which is 36.02 % higher than that of the carbonated control steel slag. As a result of elevated carbonation efficiency, f-CaO (free calcium oxide) in SA incorporated steel slag (SA-SS) is decreased more substantially than that of the control steel slag after carbonation. According to autoclaving test, cement paste cast with carbonated SA-SS showed less expansion than the one cast with carbonated control steel slag. Mortar cast with carbonated SA-SS obtained a competitive compressive strength of 40.63 MPa at 28 d. SA plays the role as a pH buffer that slows down the alkalinity decrease during the fleeting carbonation and hence enables more CO₂ to dissolve in water. With enhanced carbonation efficiency, CaCO₃ particles in carbonated SA-SS are big cubes which is opposed to the small, shapeless grains in carbonated control steel slag.