How to use GGBS efficiently

GGBS has a range of benefits when used in concrete including: 

• Lower early-age temperature rise, reducing the risk of thermal cracking in large pours 
• Minimizes the risk of Alkali Silica Reaction 
• High resistance to chloride ingress, reducing the risk of steel reinforcement corrosion 
• High resistance to attack by sulphates and other chemicals 
• Better workability, making placing and compaction easier 
• Off-white colour 
• Reduction of carbon footprint 

As GGBS replaces a proportion of the carbon-intensive clinker within concrete, it lowers the carbon footprint of concrete.​ However, since the 1980s, the average proportion of GGBS used in concrete is 40% - 50% and this has not changed due to the increased awareness about decarbonisation. 

There is currently no direct replacement for GGBS that gives the same performance at the same carbon reduction. Moving away from GGBS before new technologies are available or standardised will result in embodied carbon of UK construction increasing in the short term. 

GGBS is produced by grinding granulated blast furnace slag (GBS), a co-product of blast furnaces that produce iron for steel production.  

Until recently, the UK had four operational blast furnaces producing iron (for steel production) and GBS. In 2024 the two blast furnaces at Port Talbot ceased operations.  

There are now two remaining operational blast furnaces at Scunthorpe. This has prompted concerns about whether there is enough GGBS available to meet demand and whether importing GGBS is reliable since changes in the global market can affect availability and pricing.  

Before the closure of the UK blast furnaces, GBS and GGBS were both imported and exported as part of global trade, now both are only imported to meet market demands.  

Although the MPA is not able to collect specific figures, due to competition law, combined data shows the use of GGBS and fly ash is currently steady ​(Mineral Products Association, 2026)​.  

In addition, MPA members report anecdotally that there is currently no shortage; it is supplied through many plants across the UK, and the suppliers have indicated they have long term supply contracts in place.  

Therefore, considering the current and historic demand and current forecast of needs in the UK, there is no prospect of shortage any time soon. 

There is little public data on the availability globally, and some of it appears contradictory. 

The ‘efficient use of GGBS’ paper published by The Institution of Structural Engineers, Climate Group, The Concrete Centre and the UK Low Carbon Concrete Group did not identify a shortage of GGBS globally. It did state that it is almost fully utilised and that it should continue to be fully utilised. 

Using more, or less, GGBS in the UK is likely to have little effect on global availability. This is because the UK is a relatively small user of GGBS on the global scale.  

The latest figures from MPA Cement showed that the UK used around 2.5 million tonnes of GGBS and fly ash combined in concrete in 2024. The ‘efficient use of GGBS’ paper estimated that global production was in the region of 330-407 million tonnes. Therefore, the UK currently uses approximately 0.6-0.7% of the global supply.  

In addition, the UK is particularly set up to use GGBS efficiently as: 

• The UK has the grinding capacity to produce GGBS optimised for use in concrete, which many countries do not have, and 
• the codes in the UK are particularly tailored to support the efficient use of GGBS in concrete (due to the historic supply), which is not the case in all other countries. 

Therefore, it is not currently practical for all countries to use GGBS in concrete the way we do in the UK. Any use must be considered at a local level; the proportion the producer is offering, and what effect increasing the proportion above that will have on their stocks. This can only be understood by discussing this with suppliers at the early stage. 

There are applications which require the specification of GGBS for its technical properties which can enhance the long-term strength and durability of concrete, such as in aggressive ground or to resist the ingress of chlorides. However, specifiers should avoid specifying particular cements or combinations. This should be addressed through specifying the required exposure class in accordance with BS 8500-1. 

Instead of specifying GGBS to reduce carbon, it is more appropriate to specify a carbon range or limit for the concrete than to specify specific proportions of individual constituents. 

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%. At low levels of replacement other SCMs such as fly ash, calcined clay or silica fume provide better durability performance. 

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. 

No. If you are offered concrete that uses GGBS from a supplier, it is because they have it available and it is an effective way to meet your specification.  

While there is currently no shortage of GGBS in the UK, it is a valuable material and should be used efficiently. 

The first step is to optimise the concrete design to minimise the overall quantity required. Specifying a carbon target, rather than a particular cement type, allows the supply chain to deliver the most efficient solution. Early engagement with concrete suppliers can also help determinewhat is appropriate for the project. The Concrete Centre produces a range of resources to support the efficient design and sustainable specification of concrete. 

Combining GGBS with other supplementary materials, such as limestone fines or calcined clay, can achieve comparable strength, carbon, and durability performance to a purely GGBS mix. However, the final design should be confirmed with the designer and concrete supplier on a case-by-case basis. 

No. Availability of GGBS on the global market is already indirectly factored into the carbon accounting and the carbon factors used for GGBS; as a co-product in blast furnace steel production, according to BS EN 15804, the method of impact allocation used in calculating the embodied carbon of GGBS is based on its economic value, which increases as the demand and use of GGBS increases. 

The carbon factors for concrete should not be manipulated outside of the rules defined by BS EN 15804 and BS EN 16757 the Product category rules for concrete. These rules have been developed to ensure consistency and transparency in carbon reporting both within product categories and between them.  

Carbon accounting for projects should be carried out in accordance with BS EN 15978, with additional requirements coming from complementary standards such as the RICS Whole Life Carbon Professional Statement, where necessary. These standards set the boundaries and rules to be applied consistently across projects, making it easier to understand and interpret the results. They do not attempt to quantify the global impacts of a project, which would require a different calculation method.