Aqueous carbonation of steel slag with Na2CO3 solution: Synthesis of amorphous silica gel and performance enhancement of carbonated steel slag blended cement paste

The low reactivity and bulk expansion limit the industry-scale application of steel slag (SS). Accelerated carbonation has been demonstrated as an efficient way to activate the reactivity of steel slag and reduce carbon emissions. However, the carbonation efficiency of steel slag is limited due to the influence of CO₂ penetration rate and carbonation environment in the existing carbonation process. Moreover, the novel upcycling techniques for the high-value utilization of steel slag are limited. This study proposed a two-step carbonation method with Na₂CO₃ solution to convert steel slag powder to a calcium-rich residue and silica gel separately. Then explored the properties evolution of cement paste incorporated with carbonated steel slag (CSS) varied in carbonation degree. The results indicated that carbonated steel slag achieved the maximum carbonation degree of 78.6 % at Na₂CO₃ solution of 2.5 mol/L, reaction temperature of 40 °C, L/S of 20 mL/g and carbonation time of 20 h. No obvious product layer was observed surrounding the steel slag due to the constant stirring dispersed the products, and nano-silica gel with a high purity of 98.8 % was successfully synthesized from steel slag by the two-step carbonation process followed by HNO₃ solution washing treatment. The incorporation of metakaolin (MK) induces the formation of carboaluminate phases and stabilizes the ettringite, eliminating the harmful effects of carbonated steel slag on cement paste and improving the mechanical properties of the ternary system. This study not only presents a promising way to increase the utilization of steel slag and offers an environmentally friendly approach to preparing nano-silica gel from steel slag, but also recycles the reaction solution, demonstrating great potential for industrial-scale application.