Unconsolidated artificial ground is an ever‐increasing feature on the Earth's surface but it poses various challenges such as pollutant release and ground instability. The process of lithification could be an important factor in changing the properties of artificial ground and ameliorating these challenges. In this study, a lithified deposit of a furnace slag associated with a former iron and steel works in Scotland was analysed to determine the mechanisms and drivers of lithification. Scanning electron microscope analysis showed that Ca leached from around the edges of clasts of slag through reaction of the chemically unstable slag with water from an adjacent water body. Dissolution of Ca (and OH−) from the slag caused the water in contact with the slag to become hyperalkaline, facilitating ingassing and hydroxylation of CO2 from the atmosphere (fingerprinted through carbon isotope analysis). Reaction of the dissolved Ca and CO2 led to precipitation of calcite. Scanning electron microscope analysis showed the calcite is distributed between slag clasts, forming rims around clasts and cementing clasts together into a solid rock‐like mass. Understanding the mechanisms and drivers of lithification in artificial ground will be important, given its widespread nature particularly in urban areas where artificial ground is the substrate of most development.

<p>Legacy deposits of by-product slag from iron and steel making create significant volumes of artificial ground around the world. Composed mainly of calcium-silicate mineral phases, experimental studies have shown the potential of slag for capturing atmospheric CO<sub>2</sub> by mineralisation (e.g. Huijgen et al. 2005). Renforth (2019) calculated that steel slag could capture ~370-400 kg CO<sub>2</sub> per tonne of slag, depending on the type of slag. ~0.5 Gt of steelmaking slag is produced every year (USGS 2018) and this could potentially reach ~2 Gt yr<sup>-1</sup> by the end of the century (Renforth 2019). In addition to new slag, there is an estimated 160 million m<sup>3</sup> of legacy slag in the UK alone (Riley et al. 2020), stockpiled or dumped from historical steelmaking.</p><p>Artificial ground poses challenges around ground stability but slag-dominated artificial ground also offers opportunities for atmospheric CO<sub>2</sub> drawdown. In this contribution, we document the lithification of legacy slag deposits – conversion of loose gravelly slag material into a rock-like mass through cementation of calcite via drawdown of atmospheric CO<sub>2</sub>.</p><p>Parts of slag heaps at our case study sites (Glengarnock and Warton, UK) have a lithified nature: gravel-to-cobble sized lumps of slag are visible but have been cemented together with a mineral cement, with an appearance not unlike a natural breccia rock. We present field, X-Ray Diffraction and δ<sup>13</sup>C data from these case study sites to document the lithification of slag-dominated artificial ground through mineralisation of atmospheric CO<sub>2</sub> as a cementing phase; we present scanning electron microscope data to show the microstructural evolution of this lithification. This understanding has implications for artificial ground stabilisation and how atmospheric CO<sub>2</sub> drawdown can be harnessed in this process.</p><p> </p><p>References</p><p>Huijgen et al., 2005, ES&T, 39, 9676-9682</p><p>Renforth, 2019, Nat. Comms., 10, 1401</p><p>Riley et al., 2020, J. Geochem. Exp., 219, 106630</p><p>USGS, 2018, USGS Minerals Yearbook</p>