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Concrete Compass: How to use GGBS efficiently

Ground Granulated Blast Furnace Slag (GGBS) has become a familiar and valuable constituent of UK concrete. This is due to its ability to enhance durability and long-term performance, while significantly reducing embodied carbon through the displacement of Portland cement.

Like all resources, however, GGBS should be used efficiently. Efficient use does not mean avoiding GGBS, nor does it mean maximising its proportion in every mix. Instead, it requires a more considered approach to specification and use —one that focuses on performance, whole-life outcomes and appropriate application.

Guidance

What is GGBS?

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How to use GGBS efficiently

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Specifying Sustainable Concrete: Supplementary Cementitious Materials

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Material Efficiency Design Guide

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Webinars

The changing face of supplementary cementitious materials

Supplementary cementitious materials (SCMs) are an established way of reducing the embodied carbon of concrete in manufacture today but the availability, use and standards related to their use is evolving.

Concrete in Practice: Reducing carbon emissions at Bristol’s Temple Quarter

This event focuses on how early collaboration in the design stage and engagement with supply chain partners is enabling Temple Quarter Enterprise Campus to achieve a 25% reduction in embodied carbon.

Using GGBS (from Spring Seminar or post-recording)

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Projects

Sidcup Storyteller

At Sidcup Storyteller, the efficient use of GGBS supported a light-coloured exposed concrete finish that performs multiple roles — structural, acoustic and architectural.

Colne Valley Viaduct

At the Colne Valley Viaduct, reducing embodied carbon wasn’t driven by a single product choice, but by joined-up thinking from outline design through to construction. The project team optimised the amount of material used, removed finishes where possible, and took advantage of the inherent durability of concrete, complemented with the efficient use of GGBS.

Emmanuel College

Emmanuel College had ambitious carbon targets for the project, with structural engineer Smith and Wallwork charged with assessing embodied carbon at every RIBA stage. In addition to utilising material efficiently, embodied carbon was further reduced through the use of 50% GGBS in the structural concrete.

Salvation Army HQ

High levels of GGBS and a lean ribbed-slab structure have helped to reduced embodied carbon by 30% at Tatehindle’s admirably restrained HQ for the Salvation Army, writes Tony Whitehead

FAQs

Why are there concerns around the availability of GGBS in the UK?

GGBS is produced by grinding granulated slag from blast furnaces used for steel production.

Until recently, the UK had two operational blast furnaces producing both steel and granulated blast furnace slag. In 2024 the blast furnace in Port Talbot ceased operations. There is now one remaining operational blast furnace at Scunthorpe. This has prompted concerns about whether there is enough GGBS available to meet demand and whether the supply is reliable since changes in the global market can affect availability and pricing

Should I specify a maximum GGBS proportion is used in my concrete?

No. Where the use of GGBS is needed to meet technical requirements, this should be specified using the appropriate combined performance categories in BS 8500-1. This allows a range of cements or combinations to be used that can meet that performance. Specifiers should avoid specifying particular cements or combinations, as most concrete plants will not stock all cementitious materials. Also, not restricting cement types will also allow suppliers to provide the lowest carbon option.

Additional restrictions on the maximum proportion of GGBS permitted to utilise a ‘fair share’ uses GGBS inefficiently. This is because its use in small proportions (e.g. 20-25%) will not bring any significant benefit in either carbon emissions or durability performance and the material could be better used in different applications. GGBS is most efficiently used at replacement levels of around 35–55%.

A more effective approach is to set a carbon target for the concrete package as a whole. This allows higher GGBS contents to be used where they deliver the greatest technical and environmental benefit, while avoiding its use where it is less suitable, or deliver fewer benefits.

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