Cement and Concrete Decarbonisation Data

Concrete is the most used manufactured material on earth and cement production accounts for a large share of global industrial emissions. The reason it is hard to decarbonise is chemical rather than industrial, and understanding that changes what data matters.

Burning cleaner fuel in a cement kiln helps and does not solve the problem, because most of the carbon dioxide does not come from the fuel at all.

Where do cement emissions come from?

That first row is the whole difficulty. A cement plant running entirely on renewable electricity and zero-carbon fuel would still emit the majority of its current carbon dioxide, because the emission is a product of the chemistry rather than of the energy.

What actually reduces the footprint?

1. Clinker substitution: replacing Portland clinker with slag, fly ash, calcined clay or limestone filler.
2. Mix optimisation: using no more cement than the structural requirement demands.
3. Specification age: allowing strength to be assessed at 56 or 90 days rather than 28.
4. Carbon capture at the plant, which is the only route that addresses calcination directly.

The first three are available today and are constrained by availability, standards and specification habits rather than by technology. The fourth addresses the chemistry but is at limited deployment and adds significant cost.

Why clinker substitution is constrained

Ground granulated blast furnace slag comes from steelmaking and fly ash from coal-fired power generation. Both are byproducts of industries that are themselves decarbonising, which means the supply of the main substitution materials is shrinking as demand for them grows.

Calcined clay is the leading alternative because clay is abundant and calcining it emits less than producing clinker. It is not a drop-in replacement, and availability depends on plant investment that takes years.

Why the footprint is per mix, not per producer

A ready-mix plant produces dozens of mix designs with substantially different cement contents. A high-strength structural mix and a low-strength fill mix from the same plant on the same day can differ in embodied carbon by a factor of two or more.

A producer-level average therefore tells a designer almost nothing actionable. What they need is the declared figure for the mix they are specifying, which means the declaration has to be per mix design and available at specification time rather than after pour.

The specification problem

Concrete is usually specified by strength class and exposure class, both of which are performance requirements. Carbon is not part of that specification language, so a designer asking for lower carbon is asking outside the framework everyone else is working in.

Where carbon limits are set per cubic metre by strength class, the specification becomes expressible and the supplier can respond. That is the pattern emerging in the member states regulating embodied carbon, and it works because it fits the existing specification structure rather than replacing it.

Strength at 28 days is a carbon decision

Concrete strength is conventionally specified at 28 days. Mixes with high substitution levels gain strength more slowly and reach comparable or better strength at 56 or 90 days.

Specifying 28-day strength therefore excludes the lower-carbon mixes by convention rather than by requirement. Where the construction programme genuinely permits a longer assessment age, allowing it unlocks substitution levels that are otherwise unavailable, at no cost beyond programme flexibility.

What data has to be declared?

The clinker factor is the field that reaches upstream. A ready-mix producer knows how much cement is in the mix; the emissions per tonne of that cement depend on the cement producer’s clinker factor and kiln fuel, which is a separate declaration.

Why locality matters more here

Concrete is produced close to where it is used because it has a working life of hours. A designer cannot specify a lower-carbon mix from a plant two hundred kilometres away, which is a constraint that does not apply to steel or timber.

The practical consequence is that available substitution levels are a local question. What is achievable near a steelworks with slag supply differs from what is achievable elsewhere, and a national average conceals that variation entirely.

What about carbonation?

Concrete reabsorbs some carbon dioxide over its life through carbonation, and more when crushed and exposed at end of life. The quantity is real and modest relative to the calcination emission, and it is legitimately accounted at a later life cycle stage.

It should be reported separately rather than netted off the production figure, because netting makes two declarations incomparable and obscures the emission that design decisions can actually influence.

How does the passport help?

By making per-mix declarations retrievable at specification time rather than assembled per project. A designer comparing three suppliers needs three comparable figures with their boundaries stated, and currently obtains three documents in different formats.

It also records what was actually poured, which is not always what was specified. Where the delivered mix differs, an as-built record of the delivery tickets tied to the structure is the only way an embodied carbon claim about the finished building can be verified.

Who is accountable for the declared figure?

The ready-mix producer declares the mix, but the largest term in that declaration comes from the cement producer upstream. If the cement’s clinker factor or kiln fuel figure is wrong, so is every mix declaration built on it.

That makes the cement declaration the load-bearing document in the chain, and it is one the specifier never sees directly. Requiring the ready-mix declaration to reference the cement declaration it relies on is the practical way to keep the provenance intact rather than losing it in a transcription.

It also matters for verification. A verifier checking a building’s embodied carbon claim needs to reach the cement figure, and a chain of signed declarations reaches it where a chain of retyped numbers does not.

What about reinforcement?

Reinforced concrete is two materials, and the steel is frequently a larger share of the embodied carbon than its volume suggests. A heavily reinforced structural element can carry more emissions in its rebar than in its concrete.

Declaring the concrete alone therefore describes part of the element. Where reinforcement is significant, the useful declaration covers the reinforced element with both materials identified, which requires the steel declaration discussed in iron and steel: the first ESPR delegated act.

Reinforcement also affects recovery. Rebar is recovered from crushed concrete magnetically and is generally recycled well, while the concrete itself is downcycled to aggregate — which is the honest end-of-life picture for the composite rather than for either material alone.

Does low-carbon concrete cost more?

Usually less than expected, and sometimes nothing at all. High-substitution mixes replace clinker, which is the expensive component, with byproducts that are cheaper. Where slag or fly ash is locally available, a lower-carbon mix can be cost-neutral or marginally cheaper than a standard one.

The costs that do arise are indirect: slower strength gain extends formwork cycles, which lengthens programme, and unfamiliar mixes require trial batching and additional testing. Both are real, and both are programme costs rather than material costs, which is why they are often absorbed silently on projects with schedule pressure.

Calcined clay and captured-carbon cements are genuinely more expensive today because the plant investment behind them has not yet been amortised across volume. That is a different situation from substitution, and conflating the two produces the widespread belief that all low-carbon concrete carries a premium.

What should a specifier do?

1. Ask for per-mix declarations rather than plant averages or generic database figures.
2. Specify a carbon limit per cubic metre by strength class, so suppliers can respond within their framework.
3. Allow 56 or 90 day strength assessment wherever the programme permits it.
4. Check locally available substitution materials before setting a target that no local plant can meet.
5. Record delivered mix data against the structure, not only the specified mix.

The third item is the cheapest carbon reduction available in most projects and the one most often refused, because programme certainty is valued more highly than a reduction nobody is measured on. Where an embodied carbon limit exists, that balance changes.