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Fly ash, also known as pulverised fuel ash (pfa) 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 pfa is somewhat different from Portland cements. It will not hydrate with water directly but needs lime and water to hydrate. In concrete the lime required arrises from the hydration of the cement. Fly ash is always used in combination with Portland cement, typically in the range 80% Pc and 20% fly ash to 60% Pc and 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/pc blend mixes tend to be slower to hydrate than similar Pc only mixes, but may have improved durability.
Concrete Society Technical Report 61, Enhancing reinforced concrete durability, gives information on the effects of fly ash/pfa on the rates of carbonation and chloride ingress.
For further information see the following:
Research Information Digest 3, Fly ash, published in CONCRETE in July 2005, pp 28–30.
Research Information Digest 4, Conditioned fly ash, published in CONCRETE in September 2005, pp 66–68.
Current Practice Sheet 116, Pulverised-fuel ash, Part 1: Origins and properties, published in CONCRETE in April 1999, pp 33–35.
Current Practice Sheet 146, Fly ash, Part 2: Standards, published in CONCRETE in March 2006, pp 7–9.
Current Practice Sheet 148, Cement combinations, published in CONCRETE in May 2006, pp 24 & 26.
Copies are available as free downloads from the Members Area of the Concrete Society web site.
Acknowledgement: The Concrete Society
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