Effects of Carbonation–Hydration Curing on CO2 Uptake, Mechanical Properties, and Leaching Behavior of Heavy Metals in Stainless Steel Slag

This study aimed to study the effects of different carbonation–hydration processes and carbonation degrees on the mechanical properties and leaching behavior of heavy metals in stainless steel (SS) slag paste. SS slag powders were carbonated at different water/solid ratios, and cement paste containing 50% by weight carbonated SS slag were prepared and employed in different carbonation–hydration approaches. CO2 uptake of SS slag powders seriously depended on water/solid ratio, and the optimum water/solid ratio for carbon sequestration was 8% in the experimental range. The long-term cumulative leaching fractions of Cr and Ni in carbonated SS slag powder decreased with carbonation progress, whereas excessive carbonation showed negative effects on the immobilization of heavy metal ions. The 30-day cumulative leaching fraction of Cr and Ni of SS slag powder with carbonation degrees of 36.2% were 65.8% and 43.2% lower than raw SS slag. Carbonation–hydration curing approaches showed obvious effects on mechanical property development and leaching characteristics of heavy metals of SS slag paste. The CO2 uptake of SS paste were positively correlated to the hydration period before carbonation curing, and CO2 uptakes of 14.5% and 17.2% were obtained for H3C6 and H28C6 (H3C6 and H28C6 refer to SS paste specimens first cured in a concrete-curing room for 3 and 28 days, respectively, and then subjected to carbonation curing for 6 h). Carbonation curing for 2 h after hydration first for 3 days can increase the 28-day compressive strength by 27.7%, and the cumulative leaching fractions of Cr and Ni of H3C6 decreased by 58.2% and 44.4%, respectively.