Aqueous carbonation of steel slag with different CO₂concentration for the application of supplementary cementitious materials

Carbonation of steel slag (SS) offers a dual benefit of CO₂sequestration and enhancement of SS properties for the use as supplementary cementitious materials (SCMs). To address the challenges of the requirements on high pressure and concentration of CO₂, this study investigated the feasibility of aqueous carbonation of SS at varying CO₂concentrations and durations, and evaluated the performance of carbonated SS (CSS) in cement paste. The results showed that SS carbonated at low concentrations (10 % and 20 %) mainly produced irregular and fusiform calcium carbonate (Cc) crystals, while those carbonated at high concentrations (50 % and 100 %) mainly formed cubic Cc. High CO₂concentration led to rapid carbonation and high specific surface area (SSA) of CSS. By contrast, low CO₂concentrations resulted in a moderate carbonation reaction, which could achieve slightly higher CO₂absorption after 2h. Moreover, using low CO₂concentration increased the SSA of CSS and could improve the flow of blended cement paste significantly. The combined nucleation and filler effects of Cc, along with the pozzolanic activity of amorphous SiO₂in CSS enhanced both 3d and 28d compressive strength of blended cement paste. Higher CO₂concentrations and longer carbonation duration were favorable for 3d strength, but CSS carbonated at 20 % CO₂could achieve a comparable to or better 28d strength value of blended cement paste than that carbonated at higher CO₂levels.