Passive CO2 mineralisation in slag: evidence from a slag heap in Lanarkshire, Scotland.
Authors: Khudhur Faisal W. K. , Macente Alice , MacDonald John , Daly Luke 
Published: March 2022

&lt;p&gt;CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt; removal from air. &amp;#160;Using slag to sequester CO&lt;sub&gt;2&lt;/sub&gt; is particularly attractive as it is a by-product of a key industry, and it can utilize CO&lt;sub&gt;2&lt;/sub&gt; from the emission source, therefore reducing the need for CO&lt;sub&gt;2&lt;/sub&gt; and slag transportation, or draw down of CO&lt;sub&gt;2&lt;/sub&gt; already in the atmosphere. It is estimated that steel slag has the potential to capture ~150-250 Mt CO&lt;sub&gt;2&lt;/sub&gt; yr&lt;sup&gt;-1&lt;/sup&gt; now, and ~320-870 Mt CO&lt;sub&gt;2&lt;/sub&gt; yr&lt;sup&gt;-1&lt;/sup&gt; by 2100 (Renforth, 2019).&lt;/p&gt;&lt;p&gt;Although the chemical composition of alkaline wastes shows that CO&lt;sub&gt;2&lt;/sub&gt; capture can significantly offset emissions from corresponding industries, recent observations reveal that the CO&lt;sub&gt;2&lt;/sub&gt; uptake in alkaline wastes in underutilized (Pullin et al., 2019). Here, we use image-based analysis to understand the microstructures of CO&lt;sub&gt;2&lt;/sub&gt; 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.&lt;/p&gt;&lt;p&gt;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&lt;sub&gt;2&lt;/sub&gt; uptake.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;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&lt;/p&gt;&lt;p&gt;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&amp;#8211;9511. https://doi.org/10.1021/acs.est.9b01265&lt;/p&gt;&lt;p&gt;Renforth, P., 2019. The negative emission potential of alkaline materials. Nat. Commun. 10, 1401. https://doi.org/10.1038/s41467-019-09475-5&lt;/p&gt;&lt;p&gt;Stewart, D., 2008. Fighting for Survival: The 1980s Campaign to Save Ravenscraig Steelworks. J. Scottish Hist. Stud. 25, 40&amp;#8211;57. https://doi.org/10.3366/JSHS.2005.25.1.40&lt;/p&gt;