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Fly Ash – more important than you think

Coal Fly Ash (from CSI Section 03 05 13)
Coal fly ash is a waste product of coal-fired power plants and can be used as a substitute for some of the portland cement in a concrete mixture-up to 60% is common, but even higher levels of substitution are possible. It makes the concrete stronger and gives it improved workability compared with a conventional mix.

The environmental advantages include reducing the use of high-embodied-energy portland cement and reusing an otherwise landfilled waste product. At low levels (up to about 20% substitution) fly ash can be used instead of concrete with little or no impact on curing and finishing. At higher levels, care must be used to ensure that mixtures are properly engineered for the application and that appropriate finishing procedures are applied.

Fly ash is generally supplied in bulk to ready-mix plants which do the custom-mixing. Other industrial and agricultural waste products, including ground blast-furnace slag and rice-hull ash, can also be used to replace some of the portland cement in concrete.

For a detailed article on keeping fly ash out of landfills while improving your concrete, see "The Fly Ash Revolution:  Making Better Concrete with Less Cement" in Environmental Building News, June 1999.


According to the Coal Ash Research Center at the University of North Dakota at http://www.undeerc.org/carrc/html/WhatisCoalAsh.html:

Fly ash is the finest of coal ash particles. It is called "fly" ash because it is transported from the combustion chamber by exhaust gases. Fly ash is the fine powder formed from the mineral matter in coal, consisting of the noncombustible matter in coal plus a small amount of carbon that remains from incomplete combustion.

Fly ash is generally light tan in color and consists mostly of silt-sized and clay-sized glassy spheres. This gives fly ash a consistency somewhat like talcum powder. Properties of fly ash vary significantly with coal composition and plant-operating conditions.

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