Enhancing the reactivity of calcium carbonate in CO₂ mineralized steel slag by ultrasonic-assisted carbonation

Abstract

The reactivity of calcium carbonate (Cc), as the dominant product in carbonated steel slag, plays a pivotal role in determining its performance as a supplementary cementitious material (SCM). This study investigates an ultrasonic-assisted carbonation strategy to enhance the reactivity of Cc formed during CO<inf>2</inf> mineralization of steel slag. Compared to conventional wet carbonation, ultrasonic treatment significantly enhanced the reactivity of Cc, making it more readily reactive with C<inf>3</inf>A when used as an SCM, due to pronounced physicochemical modifications. These modifications include a reduced crystallite size, an increased specific surface area, and a higher proportion of metastable or poorly crystalline phases. Such changes result in a higher density of chemically active sites on the Cc surface. These enhanced properties stem from the combined effects of ultrasonic cavitation and acoustic streaming. Cavitation created localized high-temperature and high-pressure microenvironments that accelerated nucleation and inhibited excessive crystal growth, while acoustic streaming improved mass transport by thinning the diffusion boundary layer, thus facilitating rapid CO<inf>2</inf> dissolution and ion exchange. Together, these mechanisms yield defect-rich, nano-structured Cc with superior reactivity suitable for cementitious applications.