Decarbonising concrete: validation of lower-carbon cement blends for incorporation in the standards
This project aims to enable the use of lower-carbon concretes by building evidence on lower-carbon cement blends to support future inclusion in industry standards.
National Highways needs to reduce construction and maintenance emissions, but current UK concrete standards limit the use of certain lower‑carbon cement blends because the durability evidence needed to update those standards is missing. Without a coordinated programme to generate this evidence, the adoption of wider lower‑carbon concretes would remain slow, fragmented and inconsistent across the Strategic Road Network (SRN).
This challenge reflects a wider issue across the construction sector, where reliance on PC remains a major source of carbon emissions and the absence of durability data restricts the use of some alternative lower-carbon cement blends.
This project addresses both challenges by creating the evidence base needed to inform potential updates to relevant standards and specifications, enabling the safe, confident and scalable use of lower‑carbon concretes across the SRN and the wider construction industry.
It explores alternative binary and ternary cement blends that incorporate lower-carbon supplementary cementitious materials (SCMs), such as calcined clays and limestone, at higher replacement levels and in combinations not currently permitted under the UK concrete standard (BS 8500).
The primary objective is to reduce reliance on traditional Portland cement (PC) and support National Highways’ Construction and Maintenance Net Zero 2040 ambition and the UK’s 2050 net-zero commitments.
The scope includes understanding the fresh, mechanical, and long-term durability performance of lower-carbon concretes, identifying key variables that require testing, and preparing the ground for future validation and standard updates.
Objectives
Increase flexibility in adopting credible, sustainable and readily available cementitious alternatives, such as calcined clays and limestone, to reduce dependence on constrained materials like Ground Granulated Blast Furnace Slag (GGBS) and fly ash.
Enable the development of lower-carbon concrete solutions by exploring cement blends with higher levels of SCM replacement, including combinations not currently permitted under UK standards.
Improve understanding of the performance of lower-carbon concretes, with a particular focus on durability and long-term asset resilience.
Provide the robust technical evidence required to inform future updates to national concrete standards, as well as National Highways standards and specifications.
Success criteria
Identified viable lower-carbon cement blends with higher SCM replacement levels.
Demonstrated viable lower‑carbon alternative SCMs to currently limited cementitious materials.
Defined performance requirements for durable lower-carbon concrete.
Established the evidence needed to inform updates to BS 8500 and National Highways standards and specifications.
How the project supports National Highways KPIs
Carbon emissions reduce with adoption of wider lower-carbon cement blends across construction and maintenance activities.
A well‑maintained and resilient network improves with proven, durable lower-carbon concretes that extend asset life and reduce future renewal needs.
Efficient delivery improves with lower-carbon concretes that reduce reliance on scarce materials and support better whole‑life cost and performance outcomes.
The approach
The project followed a structured programme of research, standards review and technical development to build the evidence required for future validation of lower‑carbon concretes.
The work began with a comprehensive desktop study, including a review of UK and European standards and a comparison of BS 8500 with the EN 197 to identify where existing limits constrain the adoption of alternative cement blends. An extensive literature review examined national and international research on lower‑carbon technologies, including binary and ternary cement blends incorporating calcined clays and limestone.
In parallel, a detailed feasibility study was undertaken, which helped to identify key concrete variables requiring further investigation, such as suitable alternative SCM sources, replacement levels, binder content, and water-to-binder ratios.
In addition, a ranking methodology based on a multi-criteria decision-making process was applied to prioritise lower-carbon cement blends for further investigation. As part of this process, a carbon analysis framework was developed to enable comparison of embodied carbon performance between proposed lower-carbon mixes and conventional PC-based concretes.
These activities collectively informed the development of a laboratory testing specification designed to assess fresh properties, mechanical behaviour and long‑term durability performance of the proposed concrete mixes and generated preliminary mix designs ready for future validation.
Throughout the project, National Highways worked with suppliers, researchers, standards bodies and wider industry partners to ensure alignment with ongoing initiatives, support knowledge exchange and avoid duplication. This coordinated approach created a robust evidence base and a clear route towards future validation and the safe, confident adoption of durable lower‑carbon concretes across the SRN.
Project obstacles and how they were addressed
The project encountered several obstacles during delivery as it worked to build the evidence base for lower-carbon concretes. Limited long-term durability evidence for certain alternative cement blends created uncertainty around performance, while existing standards restricted the combinations that could be explored. Market pressures on materials such as GGBS and fly ash added further complexity, and the scope of durability testing required careful planning to ensure a robust and efficient programme.
In addition, variability in resources such as calcined clays may affect the performance of certain lower-carbon concretes, and existing admixtures, developed for conventional cements and SCMs, may not perform reliably with novel lower-carbon blends, creating a further barrier to adoption.
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Limited durability evidence was managed by conducting a comprehensive evidence review to identify the most critical durability mechanisms requiring detailed testing.
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Restrictions in current standards were addressed by assessing UK and European standards side-by-side to identify permitted combinations and ensure future testing aligned with areas where evidence gaps exist.
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Material availability challenges were mitigated by focusing on viable lower-carbon alternatives and ensuring the proposed testing programme accounted for long term supply constraints.
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Complexity of durability testing was managed through early development of a detailed test specification, together with a staged laboratory testing programme to provide clarity, reduce delivery risks and ensure a structured pathway into the next stage of work.
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Variability in calcined clays sources was managed through early engagement with suppliers to source calcined clays from different origins and assess its impact on concrete properties within a defined Quality Control (QC) and Quality Assurance (QA) framework.
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Admixture incompatibility was mitigated through early engagement with admixture suppliers to reduce programme risk and secure suitable admixtures for the proposed lower-carbon cement blends. The test specification included clear selection criteria and rheology/workability requirements.
Outcomes and impact
The project strengthened National Highways’ understanding of how lower-carbon concretes could be adopted safely and consistently. It delivered a clear evidence base on material performance, clarified durability risks and defined the testing approach needed to progress promising cement blends toward future standards updates.
This work provides a stronger foundation for shifting from traditional PC‑based mixes to durable, lower‑carbon alternatives. Additionally, this work potentially helps to accelerate uptake of new lower-carbon cements and to send the required demand signal to industry and increase commercial viability.
Key outcomes include:
- A consolidated evidence base on global research, standards constraints and opportunities for adoption higher SCM replacement levels in concrete.
- Priority lower‑carbon cement blends identified for further validation.
- A defined fresh, mechanical and durability testing specification to guide future laboratory work.
- Early insights on durability considerations informing where testing should focus.
- A practical guideline to inform implementation of validated lower-carbon cement blends into standards such as BS 8500 and Design Manual for Roads and Bridges (DMRB)/Manual of Contract Documents for Highways Work (MCHW).
User and stakeholder outcomes
Materials and structures specialists gained clearer insight into the potential of lower-carbon cement blends, improving confidence in how these alternatives could progress toward safe use on the network.
Standards and specifications teams benefited from greater clarity on the evidence required to update BS 8500 and National Highways documentation, helping shape future revisions.
Supply chain partners gained early visibility of likely material requirements and testing expectations, strengthening preparation for future changes in cement use.
Findings were also shared with wider industry groups, including relevant BSI Committee (e.g. BSI B/517), Mineral Products Association (MPA), Transport Research and Innovation Board (TRIB), and academic partners, supporting shared understanding and collaboration across the concrete and materials community.
Key insights and learning
The work confirmed that wider lower-carbon cement blends offer strong potential but highlighted several factors that must be addressed before they can be adopted safely and at scale. The research clarified where durability risks remain, how current standards limit progress, and what future testing and supply considerations must be understood to support wider use. Several important lessons emerged:
- Lower-carbon cement blends such as binary and ternary systems with calcined clays and limestone in combinations and higher replacement levels that currently permitted under UK standards show strong potential but need robust durability evidence.
- Current standards sometimes limit innovation, underscoring the need for data driven updates to BS 8500.
- Durability mechanisms (e.g. carbonation, corrosion resistance, freeze-thaw, etc.) require targeted investigation.
- Growing industry demand means evidence-based updates will accelerate safe adoption.
- Testing multiple clay sources will help future‑proof specifications as supply chains evolve.
Future value and next steps
The next stage will validate the performance of priority lower-carbon concrete mixes through a structured programme of laboratory testing and durability assessment.
This work will generate the evidence needed to support updates to BS 8500, MCHW and DMRB, enabling the safe adoption of new lower-carbon cements in concretes across the SRN. It will provide comparative insight on carbon reduction and structural resilience, helping National Highways transition to more sustainable and reliable concrete materials.
By strengthening the case for specification changes and wider industry adoption, the next phase will support progress toward National Highways’ Construction and Maintenance Net Zero 2040 ambition and contribute to the UK’s 2050 net-zero commitments, reinforcing National Highways’ position as a leader in sustainable construction.
