Uses, Benefits and Drawbacks of Fly Ash in Construction

fly ash concrete curb construction
••• SimplyCreativePhotography/E+/Getty Images

Fly ash is a fine powder which is a byproduct of burning pulverized coal in electric generation power plants. Fly ash is a pozzolan, a substance containing aluminous and siliceous material that forms cement in the presence of water. When mixed with lime and water it forms a compound similar to Portland cement.

The fly ash produced by coal-fired power plants provides an excellent prime material used in blended cement, mosaic tiles, and hollow blocks among others. Fly ash can be an expensive replacement for Portland cement in concrete although using it improves strength, segregation, and ease of pumping concrete. The rate of substitution typically specified is 1 to 1 ½ pounds of fly ash to 1 pound of cement. Nonetheless, the amount of fine aggregate should be reduced to accommodate fly ash additional volume.

Fly Ash Applications

Fly ash can be used as prime material in blocks, paving or bricks; however, one of the most important applications is PCC pavement. PCC pavements use a large amount of concrete and substituting fly ash provides significant economic benefits. Fly ash has also been used for paving roads and as embankment and mine fill, and it's gaining acceptance by the Federal government, specifically the Federal Highway Administration.

Fly Ash Drawbacks

Smaller builders and housing contractors are not that familiar with fly ash products which could have different properties depending on where and how it was obtained. For this reason, fly ash applications are encountering resistance from traditional builders due to its tendency to effloresce along with major concerns about freeze/thaw performance.

Other major concerns about using fly ash concrete include:

  • Slower strength gain.
  • Seasonal limitation.
  • Increase in air-entraining admixtures.
  • An increase of salt scaling produced by higher fly ash.

Fly Ash Benefits

Fly ash can be a cost-effective substitute for Portland cement in some markets. In addition, fly ash could be recognized as an environmentally friendly product because it is a byproduct and has low embodied energy. It's also is available in two colors, and coloring agents can be added at the job site. In addition, fly ash also requires less water than Portland cement and it is easier to use in cold weather. Other benefits include:

  • Produces various set times.
  • Cold weather resistance.
  • Higher strength gains, depending on its use.
  • Can be used as an admixture.
  • Can substitute for Portland cement.
  • Considered a non-shrink material.
  • Produces denser concrete and a smoother surface with sharper detail.
  • Great workability.
  • Reduces crack problems, permeability, and bleeding
  • Reduces heat of hydration.
  • Produces lower water/cement ratio for similar slumps when compared to no fly ash mixes.
  • Reduces CO2 emissions.

Fly Ash Types

Currently, more than 50 percent of the concrete placed in the U.S. contains fly ash. Dosage rates vary depending on the type of fly ash and its reactivity level. Typically, Class F fly ash is used at dosages of 15 to 25 percent by mass of cementitious material, and Class C fly ash at 15 to 40 percent.

Class F fly ash, with particles covered in a kind of melted glass, greatly reduces the risk of expansion due to sulfate attack as may occur in fertilized soils or near coastal areas. Class F is generally low-calcium fly ashes with carbon contents less than 5 percent but sometimes as high as 10 percent. Class C fly ash is also resistant to expansion from chemical attack, has a higher percentage of calcium oxide, and is more commonly used for structural concrete. Class C fly ash is typically composed of high-calcium fly ashes with carbon content less than 2 percent.