Concrete is the second most consumed material on earth after water, and almost all of its carbon footprint
sits in one ingredient: Portland clinker. Making a tonne of it releases roughly 0.9 tonnes of CO2, about
60 percent of that from the chemistry of calcination itself rather than from the fuel.
Why replacement works
Fly ash is already a by-product. The carbon was emitted to generate electricity, not to make a binder, so the
ash arrives with essentially no additional process emissions - only collection, classification and transport.
Every tonne that displaces clinker therefore removes almost a full tonne of CO2 from the mix.
A worked example
Take a typical 30 MPa structural mix at 350 kg of binder per cubic metre:
| Mix | Clinker | Fly ash | Approx. CO2 per m3 |
|---|
| 100 percent OPC | 350 kg | 0 kg | 315 kg |
| 25 percent replacement | 263 kg | 87 kg | 240 kg |
| 35 percent replacement | 228 kg | 122 kg | 209 kg |
At 35 percent replacement that is roughly 106 kg of CO2 saved per cubic metre. On a project pouring
20,000 cubic metres it is a little over 2,100 tonnes - the same order as taking several hundred cars off the
road for a year.
What you gain besides carbon
- Lower permeability, so chlorides and sulphates penetrate more slowly
- Lower heat of hydration, which means less thermal cracking in thick pours
- Better workability at the same water content - the particles are spherical
- Effective mitigation of alkali-silica reaction with reactive aggregate
What you have to plan for
Early strength develops more slowly. In cold weather or on a fast formwork cycle that has to be designed
for, not discovered on site. Trial mixes are not optional.
If you are reporting embodied carbon under EN 15804 or a similar framework, ask us for the transport leg
data for your route - we can give you the numbers to put in the declaration.