The Addition of Fly Ash to Concrete

Over the last several years the use of fly ash in concrete has been fully documented to demonstrate a benefit in both the mechanical and durable properties in concrete structures according to the National Ready Mixed Concrete Association (NRMCA).  So what is fly ash and how wide-spread is its use in concrete?

Fly ash is defined by the Cement and Concrete Terminology (ACI Committee 116) as “the finely divided residue resulting from the combustion of ground or powdered coal, which is transported from the firebox through the boiler by flue gases.”  It is the ash that is produced from the combustion of coal-fired electric generating plants and there are two classifications of fly ash produced depending on the type of coal that is used. 

All fly ash has a significant amount of silicon dioxide (SiO2) and calcium oxide (CaO) but Class F fly ash is produced from anthracite and bituminous coal and Class C fly ash from burning younger lignite or subbituminous coal.  It is Class C fly ash that is preferred by the Green Building Guide and is the most common type offered in ready-mix concrete. 

Sources on Wikipedia cite the benefits of Class C fly ash as, “in addition to having pozzolanic properties, it also has some self-cementing properties. In the presence of water, Class C fly ash will harden and gain strength over time.”  Another benefit of Class C fly ash is it does not require an activator unlike Class F fly ash.

Not only does fly ash offer an environmental benefit of using a by-product of the coal industry but it also improves the performance and quality of concrete.  It is used in the range of 10 to 25% of the cementitious portion of concrete mixtures and more than 75% of concrete uses some portion of fly ash. 

Sources:  nrmca.org/research/cif%20spring%2008%20fly%20ash.pdf

flyash.sustainablesources.com/
precast.org/2013/10/future-fly-ash-use-concrete/

The Conco Companies are a leading supplier of concrete services for the Western United States. We got our start in the Bay Area in 1959 by offering clients the best value on a wide range of concrete services and products.  Since that time, we have taken a leading role as one of the top concrete contractors in the region. We have four regional offices serving California, Washington, Oregon, Nevada and Colorado.