Fly Ash Applications

Fly Ash

Fly ash is a fine powder recovered from coal-fires electric generation power plants. Fly ash could be an expensive replacement for Portland cement in concrete and using it, improves strength, segregation and ease of pumping concrete. Fly ash could be added using a rate of 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.

The rate of substitution typically specified is a minimum of 1 to 1 ½ pounds of fly ash to 1 pound of cement. Fly ash is a pozzolan, substance containing aluminous and silicious material that forms cement in the presence of water. When mixed with lime and water it forms a compound similar to Portland cement.

Fly Ash Applications

Fly ash could be used as prime material in blocks, paving or bricks, however, on the most important applications is PCC pavement. PCC pavements use a large amount of concrete and using fly ash provide great economic benefits. Coal-fired power plants produce fly ash, providing an excellent prime material used in blended cement, mosaic tiles, and hollow blocks among others. Fly ash has also been used recently for paving roads, and as embankment and mine fills.

Fly Ash Drawbacks

Fly ash has been gaining acceptance by Federal Government, specifically FHWA, however, smaller builders and housing contractors are not that familiar with fly ash products. Fly ash could have different properties depending on where it was obtained or how it was obtained. For this reason, fly ash applications are encountering resistance on traditional builders due to tendency to effloresce and major concerns about freeze/thaw performance.

Other major concerns about using fly ash concrete are:

• Slower strength gain
• Seasonal Limitation, and
• Increase in air entraining admixtures.
• Higher fly ash contents can produce an increase of salt scaling.

Fky Ash Benefits

Fly ash could 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 by-product and has low embodied energy. Fly ash is available in two colors, and coloring agents can be added at the job site. Fly ash also requires less water than Portland cement, and it is easier to use in cold weather. Other benefits are:

• Produces various set times.
• Cold weather resistance.
• Depending on its use fly ash could produce higher strength gains.
• Can be used as an admixture.
• It can substitute Portland cement.
• It is considered as a non-shrink material.
• Produces denser concrete and a smoother surface with sharper detail.
• Great workability.
• Reduces crack problems.
• Reduce permeability.
• Reduce bleeding.
• Reduced heat of hydration.
• Produces lower water/cement ratio for similar slumps when compared to no fly ash mixes.
• Reduces CO2 emissions.
• Set timeSummer – Typically 20-30% fly ash is found to be acceptable and during Winter – Typically 10-20% fly ash is found to be acceptable.

Fly Ash Types

Currently, more than 50% 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% by mass of cementitious material and Class C fly ash at 15% to 40%.

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 are generally low-calcium fly ashes with carbon contents less than 5% but sometimes as high as 10%.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 normally is composed of high-calcium fly ashes with carbon content less than 2%.