How the two differ in kind
There are two ways to reduce the embodied carbon of a structure, and they sit at opposite ends of a scale running from 'change the mix' to 'change the building'.
The first works because of where concrete's carbon actually is. Aggregate, water and transport contribute relatively little; the overwhelming majority sits in the CEMENT, whose manufacture releases carbon dioxide both from the fuel used and from the chemical decomposition of limestone itself — the second of which is inherent to the process rather than a fuel-switching problem. So replacing a proportion of the cement with a supplementary cementitious material — ground granulated blast-furnace slag, fly ash, calcined clay, limestone powder — reduces the carbon roughly in proportion to what was replaced, while the concrete remains concrete. Nothing about the design, the trades, the detailing or the procurement changes.
That gives it the best ratio of carbon saved to disruption caused of anything available, and it is why it is the first move on almost every project. It is also bounded in two directions. Technically, higher replacement levels slow the early strength gain, which affects striking times, post-tensioning and the programme — and cold weather makes that worse. Structurally, the supply is finite: these are industrial by-products, and the industries producing them are themselves changing, so availability varies by region and is not something a design can assume indefinitely.
MATERIAL SUBSTITUTION — timber, mass timber, steel with high recycled content, or simply a different structural arrangement using less of everything — can save a great deal more. It also changes the project: a different design, different consultants, different trades, different fire and acoustic strategies, different procurement, and a different set of things that go wrong. It has to be decided early, because by the time the frame is designed the option has closed.
The factors that actually differ
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| Lower-carbon concrete (SCM replacement) | Changing the structural material | |
|---|---|---|
| Where the saving comes from | Less cement. Carbon falls roughly in proportion to the replacement level. | A different material, or less material — a structurally different building. |
| Size of the saving | Substantial and bounded by how much cement can be replaced. | Potentially much larger, particularly with timber where sequestration is counted. |
| Disruption to the project | Almost none. Same design, same trades, same programme apart from strength gain. | Total. Different design, consultants, trades, detailing and procurement. |
| When it must be decided | Late is still possible — it is a specification change to the mix. | Early. Once the frame is designed the option has effectively closed. |
| Effect on the programme | Real at high replacement: slower early strength means later striking and later loading, and cold weather compounds it. | Can go either way — prefabricated timber or steel erects fast; an unfamiliar system on an unfamiliar team does not. |
| Durability | Generally improved. Lower heat of hydration, denser microstructure, better resistance to chloride and sulphate attack. | Different, not better or worse — timber needs moisture and durability detailing concrete does not. |
| Availability | Regional and finite. These are industrial by-products, and the industries producing them are changing. | Depends on supply chains and local capability — mass timber in particular is not available everywhere. |
| Fire and acoustics | Unchanged. | A different strategy entirely, and one of the main sources of cost and risk in a substitution. |
| How the number is verified | Product declarations for the mix, with the replacement level stated. | A whole-structure comparison across a defined boundary, and the boundary is where the arguments happen. |
| Can you do both | Yes — a timber building still has concrete in its foundations, and that concrete can use SCMs. | Yes, and this is the normal answer on a real project. |
Which one, and when
Choose lower-carbon concrete (scm replacement) when…
- The structure is already designed in concrete, which is the most common situation.
- A meaningful reduction is wanted without changing the team, the design or the procurement.
- The elements suit it: foundations, substructure, mass pours, and anything where early strength is not on the critical path.
- Durability matters — marine, sulphate ground, or anything where chloride ingress is a concern.
Choose changing the structural material when…
- The project is early enough that the structural option is genuinely open.
- The carbon target is large enough that mix optimisation alone cannot reach it.
- The building type suits an alternative — residential and low-to-mid-rise commercial for mass timber, for example.
- There is local capability: designers, contractors and a supply chain who have done it before.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why is nearly all the carbon in the cement?
- Because of how cement is made. Producing it involves heating limestone to a high temperature, which releases carbon dioxide twice over: once from burning fuel to reach that temperature, and again from the limestone itself decomposing chemically as it converts. The second of those — the process emissions — is inherent to the chemistry rather than a consequence of which fuel is used, which is why decarbonising cement is so much harder than decarbonising a process that merely needs heat. The other ingredients of concrete contribute comparatively little: aggregate is quarried and crushed, water is water, and transport matters only where distances are long. So the carbon intensity of a concrete mix tracks its cement content closely, and that is what makes replacement such a direct lever — it acts on the part where the carbon actually is.
- How much cement can be replaced?
- It depends on the material, the element and the programme, and the binding constraint is usually early strength rather than final strength. Concrete with a high replacement level typically reaches a similar or higher strength eventually — often better, along with improved durability — but it gets there more slowly, and construction depends on early strength for striking formwork, for stressing tendons and for loading the structure. Cold weather slows it further, because these reactions are more temperature-sensitive than ordinary cement hydration. The practical consequence is that replacement levels are chosen element by element rather than set once for a project: foundations, substructure and mass pours tolerate high levels comfortably because nothing is waiting on them, while a suspended slab on a fast cycle does not. Specifying strength at a later age than the conventional one is the other lever, where the programme allows it.
- Is lower-carbon concrete weaker?
- Not at maturity, and frequently the opposite — the difference is in the rate. High-replacement mixes commonly reach equal or greater long-term strength, with a denser microstructure that improves resistance to chloride ingress and sulphate attack and a lower heat of hydration that reduces thermal cracking in large pours. Those are genuine durability benefits, not consolation. What changes is the early-age behaviour: slower strength gain, which affects when formwork can be struck and when the element can be loaded, and greater sensitivity to curing and to temperature. So the honest framing is that it is a different material with a different curing profile rather than a compromised one, and the project has to accommodate the profile — proper curing in particular, since these mixes are less forgiving of being left to dry out.
- Will these materials keep being available?
- Not indefinitely at current volumes, and that is worth knowing because it shapes what a long-term strategy can rely on. Ground granulated blast-furnace slag is a by-product of iron making and fly ash of coal-fired power generation, and both of those industries are contracting or changing in many regions — coal generation most obviously. Availability is already regional rather than universal, and prices reflect it. That is one reason attention has shifted toward calcined clays and limestone powder, which are not by-products of a declining industry and are far more widely available. For a project being designed now, the practical step is to confirm what the local supply chain can actually deliver rather than specifying a replacement level from a document, and to write the specification in terms of performance where possible so the supplier can meet it with what exists.
- How should the two options be compared?
- Over the same boundary, for the same building, doing the same job — and the boundary is where most disagreements actually live. A comparison that counts one option's manufacturing emissions and another's manufacturing plus transport plus end-of-life is not a comparison. The questions to settle explicitly are which life-cycle stages are included, how biogenic carbon in timber is treated, what happens at end of life, and whether the substructure is in scope — which matters because a lighter frame may need less foundation, and that saving belongs to the frame decision. It is also essential to compare structures that perform equivalently: the same spans, loads, fire rating and acoustic performance, since a comparison of two structures that are not equivalent is measuring the specification rather than the material.
- Does timber really store carbon?
- It contains carbon that the tree removed from the atmosphere, and how that is accounted for is a genuine methodological question rather than a settled fact — which is why timber comparisons vary so much between studies. The carbon is in the material and stays there while the material exists; what differs is whether the accounting credits it at the point of construction, defers it, or nets it against what happens at end of life, since timber that is burned or that decays releases it again while timber reused or retained keeps holding it. The other condition is the forestry: the benefit rests on the forest being managed so that harvesting is matched by regrowth, which is what certification is intended to demonstrate. Timber's advantage in most assessments is real; the size of it depends on assumptions that should be stated rather than assumed.
- What about simply using less?
- It is the option that beats both and gets the least attention, because it is design work rather than a product choice. Structures are routinely heavier than they need to be — grids chosen for convenience rather than efficiency, slabs at a standard thickness across spans that vary, elements sized to a round number, load assumptions carried forward conservatively from a previous project. Optimising the structural arrangement, using post-tensioning or voided systems to reduce material in slabs, rationalising the grid, and challenging conservative loading assumptions can remove a substantial fraction of the material before any question of what the material is made of. It also saves cost in the same proportion, which makes it the easiest of these arguments to win. The constraint is that it has to happen at concept stage, alongside the material decision rather than after it.
- Is reusing the existing structure better than either?
- Usually by a wide margin, because the embodied carbon of a structure that already exists has already been spent and does not have to be spent again. Retaining a frame and re-cladding, re-servicing and re-planning around it avoids not only the new structure's manufacturing emissions but the demolition, the disposal and the transport in both directions. The obstacles are practical rather than theoretical: the existing grid may not suit the new use, floor-to-floor heights may be wrong, loading capacity has to be assessed and often strengthened, and the structure's condition has to be surveyed and understood. That assessment costs money early in a project, which is exactly when budgets are tightest — which is the real reason it is skipped more often than it should be, rather than any finding that it does not work.