Pretreatment of alkali activation and carbonation of steel slag for using as binding material

Abstract

Steel slag (SS) exhibits low hydration activities and expansive phases, which should be pretreated prior to using as binding materials. Thus, these properties of SS were improved through a combined treatment of alkali and then wet carbonation (A_C) in this study. Effect of sodium silicate modulus (M = 1.0–2.5) and carbonation temperature (50–200 °C) on cementitious property of SS was investigated. Results showed that the optimum modulus and temperature were 2.0 and 150 °C under single alkali and single carbonation treatment of SS, respectively. In case of combined treatment, the optimal modulus was 2.0 with an optimum temperature of 100 °C (A2.0_C100°C-SS). The 28 d compressive strength increased by 27.1% and expansion rate decreased by 17.7% of A2.0_C100°C-SS-PC compared to untreated SS-PC. This was attribute to the fact that the newly formed CaCO3 with micro-nano size and high activity existed in A_C-SS can react with Al-containing minerals in OPC to form monocarboaluminate, and Si-gels produced during carbonation can react with CH to form C-S-H phases. The early hydration activity of OPC was more obviously improved by adding A_C-SS compared to adding A_SS or C_SS. This was due to the micro-nano size effect of CaCO3 and Si-gels can provide nucleation sites for C-S-H formation. Alkali pretreatment can improve the SS carbonation, reaching the maximum carbonation degree of 53% at 100 °C compared to carbonation alone (47% at 150 °C). The embodied CO2 index of A2.0_C100-SS-OPC system decreased to 9.843 compared to OPC (10.511), indicating that the A_C treatment for SS was a more sustainable and environmentally friendly method.