Properties of Carbonated Steel Slag Admixture in the Cementitious System

Global warming caused by carbon dioxide (CO2) emissions has emerged as an undeniable environmental concern. While advocating for energy conservation and emissions reduction, the challenge of addressing the substantial CO2 emissions cannot be underestimated. Currently, steel slag is utilized in carbon capture and storage technology due to its potential for carbonation. However, the carbonation of steel slag necessitates a stable and cost-effective carbon source. Industrial exhaust gases are considered a viable option, but they often have low CO2 concentrations, resulting in sluggish carbonation rates. Therefore, this study focuses on directly converting steel slag powder into concrete mineral admixtures to enhance the carbonation rate at low CO2 concentrations. Experimental results reveal that a carbonation time of 3–7 days, a liquid–solid ratio of 50%, and the selection of sodium silicate as the alkali activator yield the optimal carbonation conditions. Under these conditions, the CO2 uptake can reach 15.3%–16.0%, and the f-CaO content can be reduced to 0.2%–0.3%. Mixing 30% carbonated steel slag powder with P·Ⅰ 42.5 cement in mortar samples yields a compressive strength of 32.1 MPa at 7 days and 47.5 MPa at 28 days, along with a flexural strength of 6.2 MPa at 7 days and 8.0 MPa at 28 days. The addition of carbonated steel slag powder not only enhances the mechanical properties but also reduces the pore diameter in the hardened cementitious system. In 7 days, the pore size decreases from being concentrated around 349 nm to approximately 282 nm, and in 28 days, the pore size decreases from being concentrated around 62 nm to roughly 55 nm. This transformation is primarily attributed to the role played by calcite grains in the carbonated steel slag powder, which facilitates nucleation and filling effects.