Assessing the carbon capture capacity of South Wales’ legacy iron and steel slag

•Majority of samples are “Ca silicate-rich” dominated by the melilite group minerals.•Presence of carbonated mineral phases indicate occurrence of carbonation reactions.•Maximum potential reached by passive carbonation reactions under ambient conditions.•~17–25 Mt CO2 capture potential represent potential carbon sinks for future emissions. The industrial revolution was responsible for releasing over 1.5 trillion tonnes of carbon dioxide into the atmosphere resulting in a global temperature increase of ∼1 °C. It also produced billions of tonnes of alkaline materials that have the potential to react with some of that CO2, and this carbon capture potential may be used to help prevent climate change. Here we assess the carbon capture potential of legacy iron (Fe) and steel wastes in South Wales, the United Kingdom, and show that between 0 and 77% of total carbonation potential has been reached in the samples (equating to maximum potential between 40 and 608 kg CO2 /t of slag has been removed from the atmosphere by direct carbonation) across 10 historic works (out of 48 that operated in the region), even after > 140 years. The current work suggests that there is a capture potential of up to 17 Mt CO2 by direct carbonation which represents a potential carbon sink for future emissions. Iron and steel slag contains Ca silicate minerals, often dominated by melilite group phases (gehlenite, Ca2Al2SiO7 and akermanite, Ca2MgSi2O7), but also includes minor phases like olivine (fayalite, (Fe,Mn)2SiO4 and fosterite, Mg2SiO4). The presence of mineral carbonate phases such as calcite, magnesite, ankerite, and kutnohorite in the material demonstrates carbonation reaction has occurred after slag formation. Given that there is still carbonation potential to be realised, and that materials produced in the future from the iron and steel industry could play an important role in meeting our climate targets, then new management strategies are needed to maximise the use of this resource.