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.
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.
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.