What is Fly Ash Concrete?

Jessica Reed

Fly ash concrete is a type of concrete constructed using a byproduct, known as fly ash, created when coal is burned. When coal combusts, it produces three different byproducts, known as coal combustion products or CCB, one of which is a very fine powder known as fly ash. This powder is harmful to both the people who might breathe it and to the environment. Often the byproducts are dumped into landfills where they harm the environment around them.

Fly ash concrete requires less cement mix than conventional concrete.
Fly ash concrete requires less cement mix than conventional concrete.

To help prevent this, some fly ash left over from coal combustion is turned into fly ash concrete. Fly ash concrete was discovered in 1929 by engineers who were working on the Hoover Dam. The engineers realized they could use the fly ash to form a concrete mixture that needed less cement than the normal concrete mixture.

Fly ash concrete was discovered by engineers working on the Hoover Dam.
Fly ash concrete was discovered by engineers working on the Hoover Dam.

There are several advantages to using fly ash concrete aside from helping the environment. Fly ash concrete is typically easier to work with than typical concrete and requires less water to make. The qualities of the fly ash concrete allow it to be transported longer distances than normal concrete, meaning a company can travel farther to a work site. It also reduces bleeding, a problem caused when too much water is used and the excess water floats to the top of the concrete.

While it does provide these advantages, not all companies use fly ash concrete for a number of reasons. Certain manufacturers may have a harder time getting fly ash to put in their concrete and will choose the easily accessible materials available in their area instead. Others are worried that the mineral makeup may vary depending on where it was shipped from, producing inconsistent mixtures, or that it has poor freeze-thaw performance.

The concrete is classified into two categories: Class F and Class C. Class F fly ash is produced from anthracite or bituminous coal while Class C comes from lignite or sub-bituminous coal. The key difference is whether or not the concrete is classified as "pozzolan."

Pozzolan materials can act like cement when water and certain materials are added. Class C fly ash is considered a pozzolan material because it can form a cement-like substance to make concrete. Class F, on the other hand, cannot and therefore requires a cementing agent to help it stick together and form concrete. Class C fly ash concrete is typically considered the best type, but both will work when the proper materials are added to the mixture.

Fly ash concrete uses less water than conventional concrete.
Fly ash concrete uses less water than conventional concrete.

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Discussion Comments


@pastanaga - One of the problems with evaluating the quality and risks of using fly ash in concrete is that the quality of the ash varies depending on where they get it from. Not just with how they made it, but also from what kind of coal they made it from.

So, it's difficult to know whether observations that it is less durable and possibly dangerous to health is true of all kinds of fly ash for concrete, or only the bad quality kinds. More research must be done to evaluate this.

Although, frankly, I'd prefer it if we just didn't have to burn so much coal in the first place.


@indigomoth - Actually, and unfortunately, there are some who believe that fly ash concrete is an inferior kind of concrete in terms of lasting power and safety. As it says in the article, there are worries about how it would last under freeze-thaw conditions, which is quite important, as being able to expand and contract to temperature is vital to building materials in quite a few places.

Additionally, I've seen studies which show that the air quality in buildings made with fly ash concrete is much worse than the air in buildings made with ordinary concrete.

Since fly ash is extremely carcinogenic it might not be a risk we should be willing to make. It's just a shame, because fly ash is really difficult to dispose of safely and putting it in concrete materials seems like an ideal solution.


It's really interesting that they discovered this while making the Hoover Dam. The history of that dam is quite fascinating.

It almost seems like they should list it as a wonder of the world.

I was watching a documentary the other day which went through what would happen to various places in the world if all the people disappeared.

And Las Vegas would continue to have power for several years, just because the Hoover Dam would keep going, whether people maintained it or not.

And that concrete would last for another thousand years or so, I think was the estimate.

Very few things we have made would last that long. Particularly when they were holding back a river at the same time.

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