Don’t rely on GGBS to cut carbon in concrete, warn ICE, IStructE, Climate Group and Concrete Centre experts, as global demand outstrips supplies
UK construction projects rely heavily on waste Ground Granulated Blast-Furnace Slag (GGBS) to cut embodied carbon, using the waste material to replace energy intensive Portland cement (clinker) in concrete without detrimental impacts on quality or strength.
But a draft research paper written by experts from IStructE, the Institution of Civil Engineers, Climate Group and MPA The Concrete Centre, questions the benefits for decarbonisation by revealing that global supplies have reached their limit with no significant unused stockpiles or new sources likely to appear.
Will Arnold, head of climate action at IStructE, said: ‘A project using GGBS will reduce its emissions by using less cement, but that means another project somewhere else in the world will have to use more cement, so overall, on a global scale, nothing changes.’
Replacing clinker with GGBS locally is therefore ‘highly likely’ to reduce its use elsewhere, balancing out overall use and ‘making no additional impact on current global emissions,’ the report states. An official version of the paper will launch at the Climate Week event in New York later this month.
Arnold added: ‘We're saying don't go asking for more GGBS in the hope that it will reduce global emissions, because it won't. If you want to make your concrete less carbon intensive, you can only get so far by putting GGBS in it.’
The research group conducted a review of existing literature to ascertain how much GGBS is typically produced each year, the amount used, and whether there is any spare.
It found that global clinker production is in the range of 3340 to 3840 Mt per year, which is eight to 12 times higher than global GGBS production, estimated at 330 to 407 Mt per year. The ratio is unlikely to change much based on predictions up to 2030.
A project using GGBS will reduce its emissions by using less cement, but that means another project somewhere else in the world will have to use more cement, so overall, on a global scale, nothing changes
There were no references demonstrating significant usable GGBS stockpiles, leading the authors to conclude that there is little opportunity for global GGBS production to increase significantly for use in concrete in the short term, making it a limited resource.
The report notes that if GGBS was globally abundant, a local increase in its use would be likely to decrease global emissions because local usage can be increased without requiring a reduction in use elsewhere.
The authors say GGBS should continue to be used where it is required technically, such as for durability or to reduce early heat generation. In addition, it should be considered by specifiers ‘where there is adequate local supply, and where GGBS can be used efficiently to reduce carbon as part of a holistic approach.’
Projects can reduce reliance on GGBS by specifying proven low carbon clinker substitution materials where they exist in local abundance, the report notes, which will result in a decrease in global emissions when part of a low carbon mix design. Global emissions can also be reduced through local clinker and concrete efficiency measures, such as setting maximum clinker limits, better aggregate grading, more relaxed requirements for early strength gain, and using admixtures or performance enhancers.
‘The British standard for concrete [BS 8500:2015+A2:2019] is due to be updated at the end of this year and will allow projects to specify three materials in cement, where the current version of the standard only allows you to mix two materials together, such as GGBS and clinker or limestone and clinker,’ said Arnold. ‘It means you will be able to remove more clinker from mixes.’
GGBS is a by-product of iron and steel manufacturing obtained through the heating of iron ore, limestone, and coke at temperatures of about 1500°C in a blast furnace. When added to concrete it can improve properties such as workability, strength, and durability.