Heterogeneous lithification across a legacy coastal slag bank: the creation of new sedimentary rock from anthropogenic material

Lithification of artificial ground comprising by-products of legacy iron and steel workings presents a range of opportunities including atmospheric carbon dioxide (CO2) storage. The natural environmental processes altering these waste sites can also pose challenges such as ecotoxic metal leaching, and so it is important to characterise these largely undocumented anthropogenically-derived rocks. This study documents the lithification mechanisms, as well as mineralogical and geochemical characteristics across a legacy coastal iron and steel slag deposit (in Warton, England). X-Ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS) analysis of the slag deposit, as well as thermogravimetric analysis (TGA) of the cream-coloured material covering the deposit, shows lithification both on the top surface and the seaward side above the mean high-water mark (MHWM), which is the result of carbonate mineralisation. This process is driven by water weathering slag minerals (gehlenite, åkermanite, and pseudowollastonite), which release calcium (Ca). Ingassed and hydroxylated atmospheric CO2 reacts with the leached Ca to form calcite that is slightly to strongly depleted in 13C (δ13C values: -6.4 ‰ to -22.7 ‰), following partial dissolved inorganic carbonate (DIC) equilibrium. Calcium-silicate-hydrate (CSH) precipitation was responsible for lithifying the deposit where more frequent and abundant seawater washing prevents subsequent slag mineral dissolution and carbonate precipitation. This work shows that legacy iron and steel slag deposits are prone to lithification, particularly in coastal settings. This lithification can draw down atmospheric CO2 and has the potential to slow the release of toxic metals from CSH precipitation, enhancing the possibility for repurposing legacy industrial waste for CO2 storage and coastal defence applications.