An ingredient used to create ‘greener’ concrete might not be as effective in reducing greenhouse gas emissions as first hoped.
Ground granulated blast furnace slag (GGBS) is a by-product of the iron and steel industry and has become an increasingly popular replacement for the carbon intensive Portland cement clinker, which is produced in kilns with other materials when heated up to around 1,480°C.
But a report published this week by The Institution of Structural Engineers and others has found that increasing the use of the supplementary cementitious material in place of clinker in any one area is unlikely to have an impact on global emissions as GGBS is a “limited and constrained resource” that is already nearly fully utilised. Upping its use in one area would therefore reduce its use elsewhere as stockpiles of blast furnace slag around the world are “either small or unknown” and 90% of all iron slag is already being processed into GGBS annually.
Authors of the report encourage the continued use of GGBS though, calling it an “excellent supplementary cementitious material that helps displace clinker demand globally” which is currently eight to 12 times higher than the production of GGBS and will remain as such to 2030 and beyond, the review indicates.
It should not be used in high proportions in the hope of reducing greenhouse gas emissions though and should come from well-established supply chains. Alternative options to GGBS are also available, advises the report. It suggests that a study should be carried out to determine whether the use of GGBS in concrete for the sole purpose of reducing carbon intensity should be limited. An “acceleration in the development and scaling of other technologies is necessary,” it adds.
Tony Sheridan, group commercial director for Cemfree which uses GGBS in its products, says: “Cement accounts for around 80% of the embodied carbon within concrete. Alternatives to cement already exist and are being deployed successfully to dramatically reduce this carbon burden. Currently many of these products, like Cemfree, use waste materials such as GGBS because it is already widely understood within the industry and its availability is projected to increase until at least 2030.”
The supplier added that restricting GGBS use without a much greater understanding of how it is being currently used in construction and much more clarity on the dynamics of its international trade would be damaging. “Cemfree is developing the next generation of cement replacement binders using a wide range of alternative waste streams to further reduce the embodied carbon of concrete without using GGBS. Whilst these new technologies are developed and adopted, GGBS when used responsibly and efficiently remains an indispensable transitional solution. As experts in the use of AACM technology, we have demonstrated that GGBS holds immediate potential for significantly reducing the carbon footprint in construction.
“Using viable alternatives to cement is critical if we are to bring embodied carbon of concrete down in a significant way and have any impact on global warming. To quote the IPCC chair Jim Skea in his address to New York Climate Week: ‘Our message on agency is blunt: we have the technologies, the know-how and the money to tackle climate change. We need to put them to use. Now.’”