Synopsis
The current approach to the carbon assessment of concrete uses a system boundary limited to a given project. Where cement replacements such as ground granulated blast-furnace slag (GGBS) are used to reduce carbon, the benefit to the project relies on the assumption that GGBS was a waste product. However, estimation of the historical production and use of GGBS in the UK shows that it has been used in concrete to its maximal extent for two decades or more, mainly due to its low cost and beneficial effects on the physical properties of concrete.
Therefore, while specifying GGBS on individual projects gives a carbon saving within the project boundary, it does not reduce emissions at a national level. Given the importance of GGBS from a technical perspective, it is proposed that its use should be focused on these technical benefits, rather than as a low-carbon cement replacement. This, along with likely future supply issues, illustrates the urgency of moving past our dependency on GGBS and focusing on alternatives such as calcined clays, pozzolanas and limestone fines to reduce the carbon intensity of concrete.