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Weathering of silicate-rich industrial wastes such as slag can reduce emissions from the steelmaking industry. During slag weathering, different minerals spontaneously react with atmospheric CO2 to produce calcite. Here, we evaluate the CO2 uptake during slag weathering using image-based analysis. The analysis was applied to an X-ray computed tomography (XCT) dataset of a slag sample associated with the former Ravenscraig steelworks in Lanarkshire, Scotland. The element distribution of the sample was studied using scanning electron microscopy (SEM), coupled with energy-dispersive spectroscopy (EDS). Two advanced image segmentation methods, namely trainable WEKA segmentation in the Fiji distribution of ImageJ and watershed segmentation in Avizo ® 9.3.0, were used to segment the XCT images into matrix, pore space, calcite, and other precipitates. Both methods yielded similar volume fractions of the segmented classes. However, WEKA segmentation performed better in segmenting smaller pores, while watershed segmentation was superior in overcoming the partial volume effect presented in the XCT data. We estimate that CO2 has been captured in the studied sample with an uptake between 20 and 17 kg CO2/1,000 kg slag for TWS and WS, respectively, through calcite precipitation.

<p>CO<sub>2</sub> mineralization is a natural process that occurs during weathering of silicate materials that are calcium/magnesium-rich and aluminum-poor (Kelemen et al., 2020). During this process, silicates convert to carbonates, making silicate-rich materials such as ultramafic rocks and alkaline wastes suitable for CO<sub>2</sub> removal from air.  Using slag to sequester CO<sub>2</sub> is particularly attractive as it is a by-product of a key industry, and it can utilize CO<sub>2</sub> from the emission source, therefore reducing the need for CO<sub>2</sub> and slag transportation, or draw down of CO<sub>2</sub> already in the atmosphere. It is estimated that steel slag has the potential to capture ~150-250 Mt CO<sub>2</sub> yr<sup>-1</sup> now, and ~320-870 Mt CO<sub>2</sub> yr<sup>-1</sup> by 2100 (Renforth, 2019).</p><p>Although the chemical composition of alkaline wastes shows that CO<sub>2</sub> capture can significantly offset emissions from corresponding industries, recent observations reveal that the CO<sub>2</sub> uptake in alkaline wastes in underutilized (Pullin et al., 2019). Here, we use image-based analysis to understand the microstructures of CO<sub>2</sub> mineralization in slag. We use X-ray Computed Tomography (XCT) to visualize slag internal structures and to calculate reactive surface area and pore connectivity. We then use scanning electron microscopy (SEM), coupled with energy dispersive spectroscopy (EDS) to study the distribution of elements within the studied sample.</p><p>In our study, we use a slag sample collected from the former Ravenscraig Steelworks in Lanarkshire, Scotland, where steelmaking took place from 1950s until 1992 (Stewart, 2008), leaving behind a slag heap that has been weathering since then. Our analysis demonstrates that calcium carbonate precipitates as pore-lining. Surface passivation and low surface-connected porosity were identified as processes that can cause reduction in CO<sub>2</sub> uptake.</p><p> </p><p>References</p><p> </p><p>Kelemen, P.B., McQueen, N., Wilcox, J., Renforth, P., Dipple, G., Vankeuren, A.P., 2020. Engineered carbon mineralization in ultramafic rocks for CO2 removal from air: Review and new insights. Chem. Geol. 550, 119628. https://doi.org/10.1016/j.chemgeo.2020.119628</p><p>Pullin, H., Bray, A.W., Burke, I.T., Muir, D.D., Sapsford, D.J., Mayes, W.M., Renforth, P., 2019. Atmospheric Carbon Capture Performance of Legacy Iron and Steel Waste. Environ. Sci. Technol. 53, 9502–9511. https://doi.org/10.1021/acs.est.9b01265</p><p>Renforth, P., 2019. The negative emission potential of alkaline materials. Nat. Commun. 10, 1401. https://doi.org/10.1038/s41467-019-09475-5</p><p>Stewart, D., 2008. Fighting for Survival: The 1980s Campaign to Save Ravenscraig Steelworks. J. Scottish Hist. Stud. 25, 40–57. https://doi.org/10.3366/JSHS.2005.25.1.40</p>

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