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Fly ash, also known as pulverised fuel ash is the ash resulting from the burning of pulverised coal in coal-fired electricity power stations. The ash is very fine and it is removed from the flue gases by electrostatic precipitators.
The chemical composition of fly ash is somewhat different from Portland cement. It will not hydrate with water directly but needs lime and water to hydrate. In concrete the lime required arises from the hydration of the cement. Fly ash is always used in combination with Portland cement, typically in the range 20 to 40% fly ash, depending on the application.
The blended cement can be supplied as a pre-blended factory produced product or be prepared in the mixer by adding each material separately from its own silo. Fly ash/Portland cement blend mixes tend to be slower to hydrate than similar Pc only mixes, but may have improved durability.
Acknowledgement: The Concrete Society
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