Ground granulated blast furnace slag (GGBS) is a co-product of the iron and steel industry, formed in the blast furnaces that create iron out of iron ore. It has been used as a supplementary cementitious material in concrete around the world since the end of the nineteenth century due to its technical properties (such as improving the concrete’s durability).
However, as the concrete industry increasingly considers its role in the climate crisis and looks at ways of decarbonising its operations and products, the idea that GGBS can be used as a substitute for carbon intensive Portland cement clinker (referred to as ‘clinker’ in this paper) in concrete to reduce emissions has gained traction.
To help to determine whether this idea has merit, a group of experts drawn from across the concrete and cement industry, construction, academia and civil society undertook a literature review to better understand global production and utilisation of GGBS and clinker, and how this balance could change in the near-future.
The review indicates that global clinker production is currently 8x to 12x higher than global GGBS production and will remain at this order of magnitude to 2030 and beyond. It is found that around 90% of all iron slag is already processed into GGBS each year. No references were found to demonstrate significant usable GGBS stockpiles.
This review demonstrates that GGBS is a limited and constrained resource that is almost fully utilised globally. Any increase in its use in one location is highly likely to result in a reduction in use elsewhere, balancing each other out overall.
This paper concludes that whilst global supplies of GGBS must continue to be fully utilised to reduce overall clinker demand, any local increase in the amount of clinker substituted with imported GGBS is unlikely to decrease global emissions.
GGBS should continue to be used where required technically. Beyond this, it may still be specified where well-established supply chains exist, but it should not be used in high proportions in the hope of reducing global greenhouse gas emissions. It should also be noted that very high proportion mixes can lead to a need to increase total binder content if not managed through the project programme.
Alternative options exist for reducing clinker usage and thus reducing global emissions, and designers should work with the supply chain to identify the best way to do this on each project. Concrete buyers and specifiers should recognise these issues and decide how best to align their ambitions to reduce emissions both on-site and globally.
- GGBS is an excellent supplementary cementitious material that helps displace clinker demand globally.
- However, GGBS is a limited and constrained resource that is almost fully utilised globally.
- Therefore whilst global supplies must continue to be fully utilised, locally increasing GGBS use is unlikely to decrease global emissions.
- Alternative options exist for reducing emissions when using concrete.
- GGBS should still be used where required technically.
- If GGBS is to be used beyond technical requirements, it should come from well-established supply chains, and be used in proportions cognisant of global constraints.