Cement is a minor ingredient by weight in a cubic metre of concrete, yet it leaves the largest carbon footprint. Almost all of that footprint comes from a single component: clinker. Understanding what cement clinker is and why it emits so much explains why the cement and concrete industry is now trying to use less of it.
What clinker is
Clinker is the product that comes out of the kiln at a cement works: hard, grey nodules obtained by burning a mix of limestone and clay at around 1,450 °C. At that temperature the minerals partially melt and recombine into new compounds that can react with water and harden. (It has nothing to do with the clinker left behind in coal-fired furnaces, which is a combustion residue.)
Clinker is not sold as it is. It is ground very finely with a small amount of gypsum, which controls setting, and the result is Portland cement. When other main constituents, such as ground granulated blast-furnace slag (GGBS), fly ash or limestone, are ground or blended with it, the result is one of the other types of cement.
How clinker is made
- Quarry: limestone, which makes up most of the raw mix, is extracted together with clay or marl.
- Grinding and blending of the raw materials into a finely ground raw meal of tightly controlled composition.
- Preheater and precalciner: the raw meal falls through a tower of cyclones against the flow of hot gases, and the limestone starts to decompose.
- Rotary kiln: an inclined, rotating tube several tens of metres long that reaches around 1,450 °C. This is where clinker forms.
- Cooler: rapid cooling locks in the compounds that matter.
- Cement grinding: clinker, gypsum and, depending on the type, other constituents.
| Compound | Typical proportion | What it contributes |
|---|---|---|
| Tricalcium silicate (alite, C₃S) | 50–70 % | Setting and strength in the first few days |
| Dicalcium silicate (belite, C₂S) | 15–30 % | Long-term strength |
| Tricalcium aluminate (C₃A) | 5–10 % | Very fast reaction, controlled by the gypsum |
| Tetracalcium aluminoferrite (C₄AF) | 5–15 % | Melts at a lower temperature and gives the grey colour |
How these compounds react with water is explained in our article on concrete setting.
Why it emits so much CO₂
Clinker emits CO₂ in two ways:
- The limestone itself. When it is heated, calcium carbonate breaks down into calcium oxide and carbon dioxide. This is pure chemistry: the CO₂ comes from the raw material and cannot be avoided by switching fuel. It accounts for more than half of clinker emissions.
- The fuel that keeps the kiln at 1,450 °C. This part can be reduced with alternative fuels and better energy efficiency.
That is why the most effective way to lower the footprint of concrete is not only to make clinker differently but to use less of it: less clinker per tonne of cement and less cement per cubic metre.
How much clinker each cement contains
| Type | Clinker | What replaces it |
|---|---|---|
| CEM I · Portland cement | ≥ 95 % | Almost nothing: clinker and gypsum |
| CEM II · Portland cement with other constituents | 65–94 % | Slag, silica fume, pozzolana, fly ash, burnt shale or limestone |
| CEM II/C-M · Portland-composite cement | 50–64 % | A mix of constituents (EN 197-5) |
| CEM III · blast furnace cement | 5–64 % | Ground granulated blast-furnace slag |
| CEM IV · pozzolanic cement | 45–89 % | Pozzolanas and fly ash |
| CEM V · composite cement | 20–64 % | Slag with pozzolana or fly ash |
| CEM VI · composite cement | 35–49 % | Slag with other constituents (EN 197-5) |
Cements with less clinker usually gain strength more slowly and need more careful curing, but they produce concrete that is more durable against certain chemical attacks.
In the UK and Ireland, much of the GGBS and fly ash is added at the concrete plant rather than in the cement works. BS 8500 then treats the binder as a combination with its own designation (CIIIA, for example, for Portland cement combined with GGBS), so a low-clinker binder can be specified either way.
How to reduce it
Less clinker in cement
This is the classic route: replacing part of the clinker with other constituents. Its limit is supply. GGBS only comes from blast furnaces, and steelmaking is moving to electric arc furnaces, which do not produce it; fly ash comes from coal-fired power stations, and Britain’s last one closed in 2024. That is why alternatives such as calcined clays combined with limestone, which can be found almost anywhere, are growing.
Less cement in concrete
The other lever is at the concrete plant: achieving the same strength with fewer kilograms of cement per cubic metre. That takes a well-designed aggregate grading and admixtures that improve the performance of the cement.
This is what SmartADDITIVES graphene-based admixtures do. In the catalogue reference, the cement content drops from 300 to 210 kg/m³, 30 % less, with the same characteristic strength. With a cement emission factor of around 0.6–0.8 kg of CO₂ per kg, depending on the type, those 90 kg less are equivalent to roughly 50–70 kg less CO₂ per cubic metre. The admixtures also have a verified, product-specific Environmental Product Declaration (EPD), valid for tenders with environmental criteria.
Other routes
- Carbon capture at cement works, currently at the stage of large-scale projects.
- Clinker-free binders, such as alkali-activated materials: they exist, but their use is still limited and not always covered by standards.
- Designing for the strength actually needed: not specifying more cement or a higher strength class than the structure and its exposure require.
