Impacts of hydration degree of steel slag on its subsequent CO2 capture behaviors and mechanical performances of prepared building materials

Mineral carbonation is an effective method to improve the large-scale utilization of steel slag. In this study, we explored the effects of the hydration degree of steel slag on its subsequent carbonation. To control the hydration degree, three factors were used, namely the water−solid ratio (w/s), gypsum addition, and hydration curing duration. The results showed that with increasing w/s, the compressive strength decreased and 3-d CO2 uptake increased, whereas the 7- and 28-d CO2 uptakes remained constant. Gypsum addition improved both the strength and CO2 uptake of the steel slag. Prolonging the hydration duration from 1 to 9 d before carbonation curing was beneficial for strength enhancement and effective CO2 capture. The quantitative X-ray diffraction, scanning electron microscopy, thermogravimetry differential thermal analysis, and Fourier transform infrared results showed that ettringite and C-S-H gels were generated by hydration reactions. Moreover, calcite, vaterite, and monocarbonate, which were generated during carbonation, filled the internal systems of the samples, improving their internal density and providing strength. Increasing w/s and gypsum addition and ensuring sufficient hydration curing improved the degree of steel slag hydration and increased the formation of needle-like ettringite, portlandite, and C-S-H gel during the hydration reactions. These compounds were easier to carbonate than the raw materials and were advantageous for CO2 capture in steel slag. This study provides theoretical guidance for the preparation of carbonation-cured steel slag-gypsum building materials.