What is pulverised fuel ash?
Pulverised fuel ash (PFA), also called coal fly ash, is the very fine ash powder captured from the flue gases of a pulverised-coal power station. It consists largely of spherical glassy particles and is used in the concrete industry as a pozzolanic addition that replaces part of the cement.
On its own, fly ash does not react with water — it is not a binder like cement. But in the presence of calcium hydroxide (released by cement hydration) and water it does form new, strength-giving compounds. That is the definition of a pozzolan.
Why it is in concrete
- Less cement, less COâ‚‚. Producing Portland cement clinker accounts for a large share of concrete’s carbon footprint. Fly ash replaces part of it without sacrificing ultimate strength.
- Better workability. The spherical particles act like ball bearings in the wet mix. Concrete with fly ash flows more easily at the same water content — or needs less water for the same consistency.
- A denser structure over time. The pozzolanic reaction continues more slowly than cement hydration. The pores in the concrete become finer over the years, which slows the ingress of water, chlorides and oxygen. That is good for durability and against reinforcement corrosion.
- Lower heat of hydration. In mass pours that reduces the risk of cracking from temperature differentials.
The drawbacks
- Slower strength gain. Particularly at low temperatures, strength develops later. That governs striking times: in winter it can mean days.
- Carbonation. Because part of the calcium hydroxide is consumed, the reserve against carbonation is smaller. This matters for exposed concrete and for members with tight cover.
- Availability. With coal-fired stations closing across Europe, fly ash is becoming scarcer and more expensive. The industry is therefore looking at alternatives such as ground granulated blast furnace slag, calcined clay and finely ground limestone.
Requirements and quality
Fly ash for concrete and mortar must comply with the European standard EN 450-1, which sets requirements for loss on ignition, chloride and sulphate content and fineness, among others. The extent to which fly ash counts as binder in the water/binder ratio calculation is expressed through the k-value.
Related terms
- Cement
- Ready-mixed concrete
- Concrete cover
- Latent hydraulic
- Concrete corrosion
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