Microstructural analysis of slag properties associated with calcite precipitation due to passive CO₂ mineralization

CO₂ mineralization in slag has gained significant attention since it occurs with minimal human intervention and energy input. While the amount of theoretical CO₂ that can be captured within slag has been quantified based on slag composition in several studies, the microstructural and mineralogical effects of slag on its ability to capture CO₂ have not been fully addressed. In this work, the CO₂ uptake within legacy slag samples is analyzed through microstructural characterization. Slag samples were collected from the former Ravenscraig steelmaking site in Lanarkshire, Scotland. The collected samples were studied using X-ray Computed Tomography (XCT) to understand the distribution and geometry of pore space, as well as with scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) to visualize the distribution of elements within the studied samples. Electron backscatter diffraction (EBSD) was used to study the minerals distribution. The samples were also characterized through X-ray diffraction (XRD) and X-ray fluorescence (XRF), and the amount of captured CO₂ was quantified using thermogravimetric analysis (TGA). Our results demonstrate that CO₂ uptake occurs to the extent of ∼9-30 g CO₂/ kg slag. The studied samples are porous in nature, with pore space occupying up to ∼30% of their volumes, and they are dominated by åkermanite-gehlenite minerals which interact with the atmospheric CO₂ slowly at ambient conditions. EDS and EBSD results illustrate that the precipitated carbonate in slag is calcite, and that the precipitation of calcite is accompanied by the formation of a Si-O-rich layer. The provided analysis concludes that the porous microstructure as well as the minerals distribution in slag should be considered in forecasting and designing large-scale solutions for passive CO₂ mineralization in slag.