As recent extreme weather events in the northern hemisphere have demonstrated, global heating is so far advanced that we will have to rely on some forms of carbon capture to prevent the worst impacts of the climate emergency. Research presented at the Goldschmidt conference in Lyon, France, this month suggests that slag, the waste produced by the iron and steel industry, could be used to lock away carbon dioxide for thousands of years. Around the world about 400m tonnes of slag waste is generated every year. Some of it is used in construction, as road aggregate for example, but much of it remains heaped and unused. Jahmaine Yambing from the University of Cambridge has explored the potential of slag heaps to capture carbon by studying the 20m tonnes of slag produced by the former Consett steelworks in County Durham, UK. Since the steelworks closed 40 years ago rainwater has drained through the slag, dissolving minerals and depositing tufa (a form of calcite) on the nearby stream bed. By analysing the deposits Yambing has assessed what chemical reactions have occurred and how much carbon has been locked into the tufa. Yambing is using this data to understand the carbon capture potential of other slag heaps around the world.