Specifying lower-carbon concrete using BS 8500

The updated standard now includes a much wider range of lower-carbon mixes, potentially reducing the UK’s annual carbon footprint by 1 million tonnes. Gareth Wake explains how specifiers can use it.

The key UK standard for the specification of concrete is BS 8500, which complements the overarching European standard BS EN 206 and providing UK-specific guidance. On 30 November 2023, a revised edition was published, substantially increasing the range of lower-carbon concretes available to specifiers. In particular, it now includes a wider range of multicomponent cements that combine Portland cement with two additions or supplementary cementitious materials (SCMs). Since the 1980s, SCMs such as fly ash and ground granulated blast-furnace slag (GGBS) have been included in concrete, both for the specific properties they bring and to lower its embodied carbon.

Cement production accounts for around 90% of concrete’s CO2 emissions, because the raw materials must be heated to high temperatures, and because of the chemical reaction involved in the process. Replacing a proportion of the Portland cement with another cementitious material is therefore currently a key strategy for reducing embodied carbon. The UK concrete and cement industry has set out a roadmap to fully decarbonise by 2050 – for example by phasing out fossil fuels and introducing carbon capture and storage at cement manufacturing plants. But specifiers can make an important contribution, now and in the future, by always choosing the lowest-carbon mix that can meet their project’s requirements. With the 2023 update to BS 8500, the options have increased considerably.

How multicomponent blends reduce embodied carbon

In 2021, the cement standard BS EN 197-5 was updated to include products in which up to 65% of the Portland cement (CEM I) content was replaced by two SCMs – adding multicomponent equivalents to the binary combinations that are already well-established in the UK. These new “ternary” combinations consist of a range of proportions of CEM I and limestone fines with either GGBS or fly ash, or a range of proportions of CEM I with GGBS and fly ash.

Extensive testing was carried out, leading to the update to BS 8500:2023 that will enable the specification of multicomponent blends as part of the available lower-carbon concretes. Now, up to 20% of the binary combination can be replaced with limestone powder, an SCM that can be sourced locally across the UK. For every 5% of limestone powder used, there is a corresponding carbon reduction of approximately 5% per tonne of cement. If the new concretes were used across all mainstream applications, this would add up to an annual carbon saving of about 1 million tonnes.

Research commissioned by the Mineral Products Association found that limestone fines could be used as a material-efficient replacement for both Portland cement and GGBS in a CEM I/ GGBS concrete, resulting in an equivalent performance but with a lower carbon footprint, while reducing consumption of GGBS.

Standard methods for concrete specification

BS 8500 is published in two parts. While Part 2 is intended for use by producers of concrete, it is Part 1 to which the specifier should refer. This provides detailed guidance on compiling specifications, and on exposure classes, aggregate classes, intended working life, and properties of the fresh concrete such as consistence. BS 8500:2023-1 offers five approaches to the specification of concrete:

Designated concretes are a range of predetermined mixes for common applications, split into five categories: GEN concretes for unreinforced applications, RC concretes for reinforced applications, PAV concretes for external paving, FND concrete for foundations in aggressive ground conditions, and CB cement bound concretes for highway reinstatement. For example, FND2 is a concrete suitable for ground assessed as DC-2; RC28/35 is a concrete in the strength class C28/35 suitable for use in an internal suspended floor. Designated concretes can only be supplied by ready-mixed producers with third-party conformity certification. They are not suitable where there is a risk of chloride corrosion, where a designed concrete should be specified instead.

Designed concretes are for the informed specifier, where the designer considers all the requirements for the hardened concrete to derive the necessary strength class and other properties such as cement type, minimum cement content and maximum water/cement ratio. Normally the designer will assess the exposure conditions and consider the recommendations set out in BS 8500-1 to determine the concrete properties and minimum cover to the reinforcement. The flexibility of designed concretes makes them suitable for specifying the most sustainable concrete, using lower-carbon cements alongside other considerations such as the use of recycled or secondary aggregates. Once the designer has completed their sections of the specification,
it is passed onto the contractor to add requirements for the fresh concrete, such as consistence. The producer is then responsible for meeting the requirements of the specification through provision of the mix design.

Prescribed concretes allow the more informed designer to specify concrete by prescribing the composition. This method is rarely used but is useful where a particular ratio of constituents is required. The specifier is responsible for the strength and
other performance characteristics of the concrete.

Standardised prescribed concretes are intended for small building sites where concrete is either mixed by hand or in a small mixer less than 150 litres. They are denoted ST, and have no requirement for strength demonstration, but BS 8500-1 provides some indicative values for the strength class that may be assumed for structural design. To ensure ST concrete is safe for the indeterminate range of materials and site supervision, the cement content is very high, resulting in much higher embodied carbon compared to designated concretes. Their use should therefore be avoided where a ready- mixed concrete, either designated or designed, can be used instead.

Proprietary concretes are developed by the producer and marketed based on their enhanced fresh or hardened properties. The producer will normally guarantee the performance of these products and provide test certificates. They may be covered by third-party product conformity certification. The range of proprietary concretes include lower-carbon concretes and concretes for high-performance applications, where a lower-carbon solution can be produced by a reduction in the total volume of material.

Understanding cement classification

Cements that are produced in a cement works are denoted CEM. Portland cement (CEM I) consists of at least 95% Portland cement clinker. Other common cements are designated using CEM II, CEM III, CEM IV, CEM V or CEM VI, with further qualifying letters to indicate the proportions and types of constituents.

The first letter indicates the proportion of the clinker that has been replaced, as shown in Table 1. For example, a Portland-composite cement designated CEM II/A contains between 6-20% clinker replacement, whereas a Blast furnace cement designated CEM III/C contains between 81% and 95%.

Table 1: Allowable proportions of cement replacement by different designations

CEM VI only has one range of Portland cement replacement proportions (51-65%) so does not have a designation letter. A further letter indicates the type of addition(s) in the cement combination:

For example, CEM II/A-V is a Portland-fly ash cement with 6-20% fly ash, while CEM II/B-M (S-L) is a multicomponent cement where GGBS and limestone fines replace between 21% and 35% of the Portland cement clinker. Where more than one addition is listed, the first has the higher proportion. Cements that meet the requirements for sulfate resistance have the additional notation “+SR”, as in CEM III/A +SR.

CEM designations refer only to products made in the cement factory. Equivalent combinations to these cements are produced at the concrete batching plant from a cement and one or more additions in the mixer that conform to the requirements of BS EN 197. These are denoted slightly differently, with C instead of CEM. So, CEM III/A would be CIIIA, and CEM II/C-M (S-L) becomes CIIC-SL.

Table 2: Indicative embodied carbon (modules A1-A3) for different cements and combinations

Specification simplified

BS 8500:2023 aims to increase the use of lower-carbon mixes by removing unnecessary barriers. Using the methodology of the previous edition for specifying the increased number of cements and combinations would have been impractical, so this edition simplifies specification by introducing the “combined performance category”, which covers resistance to sulfates and chloride. Cements and combinations of cements are categorised according to their relative resistance to chlorides (graded from 1-4) and sulfate attack (graded from A-G). This is based on Concrete Society Technical Report 61: TR61 Enhancing reinforced concrete durability, and BRE Special Digest 1, Concrete in Aggressive Ground.

In exposure classes XD and XS where reinforcement is at risk of corrosion due to chloride ingress, the update has removed the minimum recommended characteristic strength. It has gone further with regard to corrosion due to carbonation, removing limiting values of minimum cement content and maximum water/cement ratio, to leave just a minimum recommended characteristic strength for each nominal cover to the reinforcement. 

Gareth Wake is director of the British Ready-Mixed Concrete Association, part of the MPA

For more details of combined performance categories, as well as minimum concrete strengths and depth of cover to reinforcement, refer to The Concrete Centre document “How to design concrete structures using Eurocode 2: BS 8500 for building and civil structures”


The first CEM VI cement to be certified in France is being used for the construction of the Grand Angle apartment building in Villefranche-sur-Saone. The ternary blend contains limestone and GGBS and is produced by Lafarge, which says it has 40% lower embodied carbon than CEM I



Tarmac used a Portland Limestone Ternary cement C VI at the Hexham Flood Alleviation Scheme in Northumberland. This concrete has three cementitious components – cement clinker, GGBS and up to 20%of limestone filler to replace some of the GGBS. Compared to a standard CEM I it offers a 64% reduction in embodied carbon to 119kg/m3 CO e