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Blended cement can lower emissions by replacing part of the clinker in cement with other materials. That matters because making clinker releases carbon dioxide both from the fuel used to heat the kiln and from the chemical transformation of limestone. Roman concrete and the Shard illustrate two different reasons builders combine materials: useful binder properties then, and lower curing heat as well as environmental considerations in a modern foundation. Neither example means Roman concrete was Portland cement or that every modern blend delivers the same carbon savings.
How blended cement can reduce emissions
Cement and concrete production together account for around 8% of global CO₂ emissions, according to the United Nations Environment Programme (UNEP) in its 5 October 2026 article. Much of cement’s emissions are tied to clinker, the kiln-made ingredient in ordinary Portland cement. Producing it requires high-temperature heat, often supplied by fuel, and releases process CO₂ when limestone is chemically converted during calcination.
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A blended cement reduces the proportion of clinker by incorporating other materials. Depending on the mix and what is available, these may include fly ash, ground granulated blast-furnace slag (GGBS), natural pozzolans, calcined clay or limestone. Using less clinker can reduce both the fuel-related and calcination emissions associated with making it. It does not establish that every ingredient is impact-free or that a particular building has lower whole-life emissions; those outcomes depend on the materials, transport, construction and use of the project.
UNEP says blended cements available today can produce up to 50% fewer emissions per tonne than ordinary Portland cement while providing comparable strength, durability and cost. That is an upper-end claim, not a guaranteed saving for every blend or project. UNEP also reports that limestone calcined clay cement (LC3) has the potential to cut cement-production CO₂ emissions by up to 40% compared with traditional cement while meeting required construction performance.
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What Roman concrete has in common—and what it does not
Roman concrete, known as opus caementicium, combined lime mortar with volcanic ash called pozzolana. The Pantheon is a famous example of its longevity and structural ambition. The useful comparison is the broad principle of combining materials to create a binder with desired properties.
Roman concrete was not modern Portland blended cement. Its ingredients, chemistry and production differed from today’s clinker-based cement, and it was not made using modern clinker-replacement recipes. The historical parallel is about the long-standing use of complementary materials, not an unbroken technology or equivalent emissions performance.
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Why the Shard used a blended foundation mix
UNEP reports that the Shard’s extensive foundations used a cement blend containing 70% GGBS. In large foundation elements, the blend’s lower early heat helped manage thermal stresses as the concrete cured. The example shows how a mix can be chosen for engineering performance as well as environmental considerations; it does not establish that the same proportions suit other sites or structures.
Blended concrete is also used in demanding structures for performance reasons. UNEP notes high-performance mixes in the Burj Khalifa and One World Trade Center. The relevant lesson is that blend selection is a design decision: the required strength, durability, workability and heat behaviour matter alongside emissions.
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How standards and supply shape what can be used
In the UK, The Concrete Centre says BS 8500:2023 incorporates multi-component cements defined in BS EN 197-5:2021. Cement types covered by BS EN 197-5 can allow up to 65% Portland cement replacement using two or more supplementary cementitious materials (SCMs). This is a standards-specific allowance, not blanket permission for every mix or application; the concrete specification, intended exposure and project requirements still govern suitability.
Availability is another constraint. The UK Department for Energy Security and Net Zero says conventional SCM supplies such as fly ash and GGBS are becoming less available. A 2025 review in npj Materials Sustainability describes related supply pressures as coal-fired generation phases down and steelmaking changes. Emerging alternatives, including calcined clay and natural pozzolans, may therefore matter more, but any proposed material needs consistent supply and evidence that it meets the application’s requirements.
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How to assess a blend for a real project
There is no single best blend for every use. Designers and buyers can compare options using the project’s actual specification rather than a headline replacement percentage alone:
- Emissions: Compare verified emissions per tonne and the clinker replacement in the specified cement, using a clearly defined basis of comparison.
- Structural and exposure needs: Check strength and durability for the intended use and environment.
- Curing behaviour: Confirm setting and early heat characteristics where thermal control or construction timing matters.
- Supply: Establish that the chosen SCM can be supplied consistently for the project and location.
- Compliance: Verify that the cement and concrete specification meet applicable standards and project requirements.
The larger opportunity is significant because buildings and construction account for 34% of global energy-related CO₂ emissions, according to UNEP citing the Global Status Report for Buildings and Construction. UNEP also reports that around half of the buildings expected to exist in 2050 have yet to be built. Lower-clinker cement is one part of reducing construction’s impact, alongside using fewer materials where possible, extending building life, and increasing reuse.
“All materials have a role to play, and all materials need to reduce their environmental impact,” says Gulnara Roll, Head of GlobalABC at the United Nations Environment Programme (UNEP). “That means looking across the whole life cycle of buildings: using fewer materials where possible, extending the life of what we already have, increasing reuse and circularity, and expanding responsibly sourced local, bio-based and other low-carbon materials. Blended cement is one important part of that transition.”
Sources: UNEP, 5 October 2026; UK Department for Energy Security and Net Zero consultation; The Concrete Centre, Standards for concrete, BS 8500; npj Materials Sustainability, 2025.
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