Applications for Fly Ash
Fly ash can be used as prime material in many cement-based products, such as poured concrete, concrete block, and brick. One of the most common uses of fly ash is in Portland cement concrete pavement or PCC pavement. Road construction projects using PCC can use a great deal of concrete, and substituting fly ash provides significant economic benefits. Fly ash has also been used for embankment and mine fill, and it has increasingly gained acceptance by the Federal Highway Administration. The rate of substitution of fly ash for Portland cement typically specified is 1 to 1 1/2 pounds of fly ash for 1 pound of cement. Accordingly, the amount of fine aggregate in the concrete mix must be reduced to accommodate the additional volume of the fly ash.
Types of Fly Ash
There are two common types of fly ash: Class F and Class C. Class F fly ash contains particles covered in a kind of melted glass. This greatly reduces the risk of expansion due to sulfate attack, which may occur in fertilized soils or near coastal areas. Class F is generally low-calcium and has a carbon content that is usually less than 5 percent. Class C fly ash is also resistant to expansion from chemical attacks. It has a higher percentage of calcium oxide than Class F and is more commonly used for structural concrete. Class C fly ash is typically composed of high-calcium fly ashes with a carbon content of less than 2 percent. 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, while Class C fly ash is used at dosages of 15 to 40 percent.
Benefits
Fly ash can be a cost-effective substitute for Portland cement in many markets. Fly ash is also recognized as an environmentally friendly material because it is a byproduct and has low embodied energy, the measure of how much energy is consumed in producing and shipping a building material. By contrast, Portland cement has a very high embodied energy because its production requires a great deal of heat. Fly ash requires less water than Portland cement. Other benefits include:
Produces various set times High strength gains, depending on use Can be used as an admixture Considered a low-shrink material Produces dense concrete with a smooth surface and sharp detail Great workability Reduces crack problems, permeability, and bleeding Reduces heat of hydration Allows for a lower water-cement ratio for similar slumps when compared to no-fly-ash mixes Reduces CO2 emissions
The EPA has regulations for the proper disposal of coal ash, aka fly ash, as it does contain arsenic, mercury, and cadmium, which are contaminants that can cause water and air pollution. Using it in concrete and other products is beneficial for reducing greenhouse gas emissions, and dumping in landfills.
Disadvantages
Smaller builders and housing contractors may not be familiar with fly ash products, which can have different properties depending on where and how it was obtained. Additionally, fly ash applications may face resistance from traditional builders due to its tendency to effloresce along with concerns about freeze/thaw performance. Other concerns about using fly ash in concrete include:
Slower strength gainSeasonal limitationIncreased need for air-entraining admixturesIncrease in salt scaling produced by higher proportions of fly ash
