Using fly ash in concrete is an environmentally conscious construction technique that utilizes manufacturing waste, which otherwise would be added to the landfill. Fly ash is a by-product of coal, when burned to generate electricity. The majority of the electricity in the U.S. comes from fossil fuel sources, with coal currently being the most prevalent fuel used, according to the US Department of Energy. Fly ash is considered a hazardous waste, which is normally disposed of in landfills and ash ponds. However, when used as an additive in concrete mixtures, fly ash can enhance the strength and durability of concrete.
“Hazardous waste” is a term that applies to everything from perfume to spent fuel rods. Like with any hazardous by-product, proper storage, transportation, and disposal are important to prevent the ash from polluting the air. Fly ash is a pozzolanic material, which mean that it reacts with water and forms a cementitious bond with the Portland Cement used in concrete mixes. This reaction, in addition to air entraining methods, is what increases the strength and durability of the concrete. Fly ash is also readily available from coal-burning power plants around the world.
Coal is a complex resource that contains varying amounts of minerals. This means that no two pieces of coal are the same, and therefore, no two pieces of coal will produce the same grade of fly ash. Currently, there are two grades of fly ash used in concrete mixes: grade C and grade F. The efficiency and technology of the coal-burning facility, as well as the grade of burned coal, are what determines the grade of the fly ash by-product.
The amount of fly ash used in a concrete mix depends on the strength required for the concrete, and the outside temperature for curing. Fly ash with a higher calcium content will produce a stronger concrete. Fly ash content of 50% is usually safe to use, ensuring the concrete will cure properly. The reactions between the concrete mixture and the minerals and other content found in the fly ash, such as alkalis, carbon, and sulfates, is also a factor in how much fly ash is used in a concrete mix.
Ashes to Ashes… Not Anymore.
Utilizing fly ash in concrete mixes is an environmentally conscious way to repurpose a manufacturing by-product that otherwise would end up in a landfill. Architects and Designers can specify that fly ash is used in the concrete for construction. In the future, it will be exciting to see how the use of fly ash in concrete mixes develops, along with other advancements in concrete mix design.