Cement, the essential "glue" used to make concrete, contributes about 8 per cent of global carbon dioxide emissions. "Portland" cement is made by heating limestone to very high temperatures, combining it with clay to form clinker, then grinding it down with gypsum. About two-fifths of emissions are generated by the kilns used for heating, and three-fifths are generated from the release of carbon by the heated limestone itself. New, alternative formulations of cement, such as LC3 (limestone calcined clay cement), require less heat and reduce the share of clinker needed in the final product.

Other supplementary cement-like materials including recycled concrete, ash and slag can also be used to cut the proportion of clinker compared with standard Portland cement. "Non-Portland cements have been around for over a hundred years, but now with pressure on achieving net zero and technological advances, there is more interest," says Susan Bernal, Professor of Sustainable Construction Materials at the University of Bath. "It's a low-hanging fruit," she says. Alternatively, so-called "carbon-capture utilisation and storage" (CCUS) systems are able to trap carbon emissions, compress and then transport them to sites where they can be injected into stable geological formations for long-term storage. Another option is to reinject the captured carbon into the concrete during the manufacturing process.

On the bright side, the low-emission cement formulations do not require the extensive carbon-capture infrastructure. But they have a more limited record, different stability and moisture tolerance levels and not all are yet recognised in established and widely used standards. The biggest constraint is cost, with most technologies only likely to be adopted at scale in response to regulations that mandate emission reductions, and when backed by financial incentives and subsidies.

Stuart Gilfillan, professor of geochemistry at the University of Edinburgh, says: "It's cheaper just to emit.".