Natural carbonation of unground steelmaking slag for assessment of direct carbon sequestration potential during outdoor storage

CO 2 sequestration using steelmaking slag remains challenging for practical applications: one of the main barriers is the energy and costs required for accelerated carbonation. In Japan, steelmaking slag is often stored in outdoor yards for several months, so natural carbonation of the slag during storage can reduce the energy and costs of accelerated carbonation. Therefore, optimization of the natural carbonation process is expected to contribute to achieving energy-saving and low-cost CO 2 sequestration. This paper aims to assess the CO 2 sequestration potential of steelmaking slag via natural carbonation during outdoor storage. Direct sequestration of atmospheric CO 2 by unground steelmaking slag under ambient temperatures and pressures is focused on, and the CO 2 sequestration capacity and mechanism are examined through long-term exposure experiments. It is found that the calcium carbonate layer is formed on the surface of the steelmaking slag and that the thickness of this layer has an upper limit. The average upper limit is 25 µm regardless of the particle size, which may constrain the maximum amount of direct CO 2 sequestration. In addition, CO 2 sequestration is 13–157 kg-CO 2 /t-slag depending on the particle sizes (150–4750 µm) and exposure period (7–154 days). While natural carbonation is a lengthy process, the amount of CO 2 sequestered is found to be not necessarily inferior to that under accelerated conditions. Although one type of steelmaking slag is tested in this study, utilizing the natural carbonation of the steelmaking slag during outdoor storage potentially provides an energy-saving and low-cost CO 2 sequestration method. • Steelmaking slag’s CO 2 sequestration potential via natural carbonation is evaluated. • A long-term exposure experiment is conducted, simulating storage in outdoor yards. • The maximum CO 2 sequestration is 157 kg-CO 2 /t-slag after 154 days of exposure. • The upper limit of CaCO 3 layer growth is around 25 µm regardless of particle size. • CaCO 3 layer on the surface of slag limits the potential amount of CO 2 sequestration.