Why Is Fly Ash Put in Concrete?

Why Is Fly Ash Put in Concrete?
Why Is Fly Ash Put in Concrete? (Image: Fly ash particles seen at 2000x magnification. Courtesy of Wikimedia Commons.)

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What is Fly Ash?

Fly ash is a by-product of coal burning in power plants that has a use of its own. Since it was considered a waste product of the burning, fly ash was traditionally disposed of in landfills. Now, fly ash is captured at the boiler exhaust and sold, creating an additional revenue stream for coal plants that helps them control their costs. Nearly a third of the fly ash produced in the United States is used as a replacement for carbon-dioxide-producing Portland cement in concrete.

Benefits of Using Fly Ash in Concrete

Concrete with fly ash in it is easier to work and finish because it can be mixed in proportions that make summer concrete finishing easier and don't affect winter finishing. Concrete mixed with fly ash needs less water. This mean that there is less shrinkage and cracking. Fly ash creates concrete with a higher compressive strength over time than concrete without fly ash, however, the difference is not significant. Since fly ash costs less than cement, using fly ash in the mixture lowers the cost of concrete.

The Green Choice

Concrete with fly ash could also be called green concrete. By using a by-product from coal burning, concrete with fly ash saves virgin material and energy. The fly ash isn't taking up valuable landfill space and it helps reduce pollution. By replacing Portland cement, fly ash reduces carbon dioxide emissions. Because the concrete is more durable, it means that there is less concrete debris created over time, which will add to the savings of energy, landfill space and virgin materials. A ton of fly ash taken out of the waste stream will conserve enough landfill space for 455 of solid waste produced by and average American and reduce carbon dioxide emissions equal to two months of emissions by a car.

Some Drawbacks

There are a few drawbacks with using fly ash, though they are far outweighed by the benefits. The compressive strength gains of concrete with fly ash happen slowly over a longer period of time than non-fly ash concrete. Some reports have said it is more difficult to finish. It also adds one more product that needs to mixed in a certain proportion to the others when batching the concrete. A few states don't allow fly ash in concrete or have restrictions on the type of concrete it can be used in. For companies who do work across state lines, this can force the need for creating different mixtures for those states that restrict the use of fly ash.


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