The Efficient Use of GGBS in Reducing Global Emissions: A Balanced Approach
By James Morton
Ground granulated blast furnace slag (GGBS) has long been recognised as a valuable supplementary cementitious material (SCM) in the construction industry. As a by-product of the iron and steel manufacturing process, GGBS offers the potential to reduce the carbon footprint of concrete by partially replacing Portland cement clinker, which is a significant contributor to global CO₂ emissions. However, recent insights highlight the complexities surrounding its availability and optimal use, emphasising the need for a balanced and strategic approach to truly achieve global emission reductions.
Understanding GGBS and Its Role in Concrete
GGBS is produced when molten iron slag, a by-product of iron production in a blast furnace, is rapidly cooled using water or steam, resulting in a glassy, granular material. This granulated slag is then dried and ground into a fine powder, which can be used as a partial replacement for Portland cement in concrete. Incorporating GGBS into concrete mixtures enhances durability, reduces permeability, and improves resistance to chemical attacks, making it particularly suitable for structures exposed to aggressive environments.Wikipedia
The environmental advantage of GGBS lies in its ability to displace clinker in concrete production. Clinker manufacturing is energy-intensive and releases a substantial amount of CO₂, both from the combustion of fossil fuels and the calcination of limestone. By substituting a portion of clinker with GGBS, the embodied carbon of concrete can be significantly reduced.
Global Availability and Constraints of GGBS
Despite its benefits, the global availability of GGBS is inherently limited. Its production is directly tied to the output of the iron and steel industry, with approximately 90% of iron slag already being processed into GGBS annually. Current estimates place global GGBS production between 330 and 407 million tonnes per year, whereas global clinker production ranges from 3.34 to 3.84 billion tonnes annually—a disparity that underscores the limited capacity for GGBS to replace clinker on a global scale.
This constrained supply means that any increase in GGBS usage in one region is likely to result in a corresponding decrease elsewhere, leading to no net reduction in global emissions. Moreover, over-specifying GGBS in areas where clinker production is already more carbon-efficient could inadvertently shift clinker production to regions with higher emissions, potentially exacerbating global CO₂ output.
A Strategic Framework for Efficient GGBS Utilisation
To ensure that GGBS is used effectively and contributes meaningfully to global emission reductions, a strategic, three-step approach is recommended:
Assess Technical Necessity: GGBS should be specified where its technical properties are essential, such as in environments exposed to chlorides (e.g., marine structures or areas using de-icing salts) or where temperature and crack control are critical. In such cases, GGBS enhances durability and extends the lifespan of concrete structures.
Evaluate Supply Chain Robustness: When considering GGBS for applications beyond technical requirements, it's crucial to source it from well-established and sustainable supply chains. Using GGBS in high proportions solely to reduce carbon intensity, without a reliable supply chain, can lead to resource misallocation and may not yield the intended environmental benefits.Institution of Structural Engineers+1Institution of Structural Engineers+1
Explore Alternative SCMs and Clinker Reduction Strategies: To complement GGBS use, other SCMs such as fly ash, calcined clays, or natural pozzolans should be considered, especially in regions where they are more abundant. Additionally, implementing clinker efficiency measures—like optimising aggregate grading, adjusting early strength requirements, and utilising admixtures—can further reduce the overall clinker content in concrete, contributing to substantial emission reductions.
Embracing a Holistic Perspective
While GGBS plays a valuable role in lowering the carbon footprint of concrete, it is not a panacea. Its limited global availability necessitates a judicious and context-specific application. Over-reliance on GGBS in regions where it is scarce or where alternative SCMs are viable may undermine broader emission reduction efforts.
The construction industry is encouraged to adopt a holistic perspective, focusing not only on material substitution but also on innovative design, efficient construction practices, and the development of new, sustainable materials. By doing so, the industry can make meaningful strides toward achieving global emission reduction goals.
Conclusion
The efficient use of GGBS in concrete is a critical component of sustainable construction practices. However, its role must be understood within the broader context of global material availability and emission reduction strategies. By following a balanced approach that considers technical requirements, supply chain robustness, and alternative measures, the construction industry can optimise the use of GGBS and make meaningful progress toward global emission reduction goals.