It can—when limestone-containing cement replaces some Portland clinker in a concrete mix that still meets the project’s performance requirements. Limestone is not carbon-free, and there is no universal saving for every product: the result depends on the amount and type of substitute, the full formulation and the application.
How limestone-containing cement can lower emissions
Portland clinker is the main carbon-intensive ingredient in conventional cement. Replacing some clinker with limestone or other suitable materials can reduce emissions per tonne of cement. The key is substitution within a designed cement or concrete formulation, not simply adding limestone to a mix and assuming its footprint will fall.
That distinction matters because cement and concrete are different comparison units. A lower-emission tonne of cement does not automatically mean a lower-emission cubic metre of concrete if the mix proportions or performance differ. Compare products using the same declared unit, assessment boundary and functional requirements.
Why limestone itself is not carbon-free
Limestone has a carbon cost in conventional clinker production. The UK government’s 2021 Industrial Decarbonisation Strategy states that half of emissions from cement production arise from limestone calcination, which releases carbon dioxide as a process by-product. Improving kiln fuel efficiency alone therefore cannot eliminate cement emissions.
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The potential benefit comes from using less clinker in the finished cement, not from treating limestone as an emissions-free ingredient. The overall result depends on the materials and production process used and on how much cementitious material the concrete needs to meet its specified performance.
What historical figures tell us—and what they do not
A 2021 UK Parliament POST briefing reports that clinker substitution contributed to a 20–30% average decrease in CO2 emissions per tonne of cement since the 1980s. This is a historical global average attributed to substitution; it is not a current UK product figure or a guaranteed saving for a particular mix.
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The same briefing reports that secondary cementitious materials made up 26.2% of total cementitious materials used in the UK in 2018. That is a dated national figure, not a current market share. It also describes LC3—limestone calcined clay cement—as a formulation that can contain up to 60% clinker substitutes from blended calcined clay and limestone. That is an example, not a specification for every LC3 product. These figures are from the POST briefing, Reducing the whole life carbon impact of buildings.
Which UK standards and material routes matter?
UK industry guidance covers cements under BS EN 197-1 and BS EN 197-5, as well as plant combinations using ground granulated blast-furnace slag (GGBS), fly ash, limestone fines and calcined clay under BS 8500-2. The applicable route depends on the cement or combination and the intended use. Check current conformity and the project specification with the cement or concrete supplier and the designer; the existence of a standard route does not establish that every formulation suits every application.
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The Mineral Products Association’s Embodied CO2e of UK cements fact sheet page provides industry guidance on UK cement data and standards coverage. Cement properties vary with the type and proportion of substitute materials, and some cements are suited to specific applications rather than general use.
How to compare products for a project
Use equivalent products and performance requirements, then examine product-level emissions evidence. The UK government’s low-carbon industrial-products framework describes classifications and embodied-emissions reporting intended to help buyers compare products. Its technical annex includes concrete classification approaches, including UK-adapted ratings.
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- Set the functional requirement. Specify the required strength class, exposure conditions, intended application and any other relevant performance criteria before comparing carbon figures.
- Identify the formulation. Ask for the clinker percentage and substitute types, including limestone, calcined clay, GGBS or fly ash where relevant. Do not infer these from a generic label alone.
- Compare like-for-like emissions data. Request embodied-carbon figures for the declared unit and assessment boundary that match the comparison. Prefer a product-specific environmental product declaration (EPD) or other comparable verified product data where available.
- Check conformity and suitability. Confirm the relevant standard, the product’s intended application and its fit with the project specification with the supplier and designer.
- Use the same basis for every option. Do not compare cement emissions per tonne with concrete emissions per cubic metre, or figures with different boundaries, as if they measured the same thing.
The framework’s government response, updated 28 May 2026, says the government intends to publish voluntary guidance on embodied-emissions reporting, product classifications and green procurement, using existing sector-specific classifications rather than a bespoke cross-sector model. Those are announced intentions; consult the government response factsheet and technical annex for the framework and its approaches.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What LC3 means
LC3 stands for limestone calcined clay cement: a blended cement using limestone and calcined clay to replace part of the clinker. The POST briefing’s example of up to 60% clinker substitutes refers to blended calcined clay and limestone in an LC3 formulation; it should not be read as a universal product composition or as proof of a particular project-level carbon saving. Evaluate a named product against its declared composition, verified emissions data and required application.
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