Frequently Asked Questions about Fly Ash

1. Where does fly ash come from?
2. What is the difference between Class C and Class F fly ash?
3. How do you include fly ash in a "concrete mix design"?
4. What are the effects of fly ash on air entrainment?
5. Why does concrete made with fly ash last longer?
6. How much fly ash should be put into concrete?
7. Can fly ash be used for soil stabilization?
8. How does the use of fly ash help the environment?
9. Is fly ash safe to use?
10. Does everyone allow the use of fly ash?

1. Where does fly ash come from?

Fly ash is produced through the combustion of coal used to generate electricity. After coal is pulverized, it enters a boiler where flame temperatures reach up to 1500 degrees Celsius. Upon cooling, the inorganic matter transforms from a vapor state to a liquid and solid state. During this process individual, spherical particles are formed. This is fly ash. It is then collected by either using electrostatic precipitators, baghouses or a combination of both. Fly ash from these systems is collected in hoppers and then transferred to storage silos. Fly ash is tested for physical properties such as fineness, loss on ignition, and moisture, before it is allowed to be shipped to its end user.
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2. What is the difference between Class C and Class F fly ash?

The primary difference between Class C and Class F fly ash is the chemical composition of the ash itself. While Class F fly ash is highly pozzolanic, meaning that it reacts with excess lime generated in the hydration of portland cement, Class C fly ash is pozzolanic and also can be self cementing. ASTM C618 requires that Class F fly ash contain at least 70% pozzolanic compounds (silica oxide, alumina oxide, and iron oxide), while Class C fly ashes have between 50% and 70% of these compounds. Typically, Class C fly ash also contains significant amounts of calcium oxide - over 20%. Most Class F fly ash contains little calcium oxide; however, some Class F fly ash sources may contain intermediate levels (8% to 16%) of calcium oxide. While both classes of fly ash greatly reduce concrete permeability as compared to the cement only mixes, Class F tends to give proportionately greater permeability reduction. Due to the higher levels of pozzolanic compounds, Class F fly ash mitigates against sulfate attack, alkali silica reaction, corrosion of reinforcement, and chemical attack. While Class C fly ash generally improves concrete durability as related to these forms of attack, higher replacement percentages may be necessary to effectively mitigate them.
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3. How do you include fly ash in a "concrete mix design"?

For mix design purposes, fly ash itself should be considered like portland cement, except that the specific gravity for fly ash is different. The specific gravity of portland cement is typically 3.15, while the specific gravity of fly ash may range from 2.2 to 2.8, depending on fly ash composition. Therefore, if a certain percentage of cement is replaced with fly ash on a mass basis, simply multiply the initial portland cement quantity by the percent replacement. For some fly ashes, particularly low calcium Class F fly ashes, higher replacement rates (1.2:1 up to 2:1) are required to maintain equivalent early concrete strength. For example, in a mix originally containing 500 pcy of portland cement, if 20% of cement is replaced with fly ash with a 1.3 replacement rate, then 100 pcy of cement is removed and 130 pcy (100 * 1.3) of fly ash is included. The next modification to the concrete mix design involves the water content. Due to the particle shape of fly ash, the water demand is typically reduced, up to 5% less with Class F fly ash, and up to 10% less with Class C fly ash (this may also be accomplished by a partially lowered chemical admixture dosage). The final step, as with traditional mix designs, involves adjusting the aggregate content for proper yield. Simply sum up the aggregate, cementitious materials, water, and air volumes, subtract from 27.0 ft³, and that amount should be added (or subtracted) with aggregate volume (typically fine aggregate is used for small modifications to mix design).

For a completed example of a concrete mix design including fly ash, see our "Mix Designs".
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4. What are the effects of fly ash on air entrainment?

The impact of fly ash on air entraining admixture (AEA) is primarily due to the carbon, represented as loss-on-ignition (LOI), remaining in the fly ash after combustion of the coal fuel. Due to the high rate of absorption of surfactants such as air entrainment into carbon, the loss-on-ignition generally has a positive correlation with AEA dosage demand. When using fly ash with a relatively high LOI, care should be taken to dose the concrete for the desired air content, and perform quality control checks using volumetric/pressure air testing and/or unit weight testing.
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5. Why does concrete made with fly ash last longer?

By reacting with free lime to form additional binder material, the permeability of concrete made with fly ash is lower than that made without it. Since virtually all durability aspects of concrete are improved with a reduction in permeability, the use of fly ash improves concrete durability. In addition, concrete water demand is typically reduced when using fly ash, leading to a lower water/cementitious ratio thus increasing strength. Both of these key properties lead to concrete with a longer life expectancy. Corrosion of reinforcement is slowed dramatically with a reduction in permeability. Concrete resistivity is increased leading to a slowed corrosion propagation rate. By consuming free lime, fly ash lessens the potential for sulfates from soil and/or groundwater to attack concrete. Finally, fly ash will consume excess alkalis, reducing potential for deleterious alkali silica reaction.
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6. How much fly ash should be put into concrete?

Typically, concrete designers use fly ash a partial replacement for portland cement at values up to 30 percent of the total cementitious composition. The use of high percentages (high volumes) of fly ash in concrete have been studied extensively over the last 15 years and the benefits have been well documented. When properly designed and constructed, the increased benefits of concrete made with 40, 50, and 60 percent fly ash replacement include dramatically reduced concrete permeability, and excellent resistance to all forms of premature deterioration. From both an environmental standpoint and life cycle cost approach, use of high volume fly ash concrete has gained increasing acceptance among architects and engineers. When designing and specifying concrete for strength and durability, the proper selection of constituent materials depends on the exposure conditions, type of structure, and intended use. For applications such as footings, columns, walls, and beams, where surface exposure is minimal, high volume fly ash concrete mixes may be used effectively. For mass concrete placements such as mat or raft foundations, the use of even higher quantities of fly ash is recommended.
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7. Can fly ash be used for soil stabilization?

Fly ash, particularly Class C, can effectively be used to stabilize soil for various types of construction. High calcium fly ash (Class C) acts as a good source of calcium hydroxide which "self activates", reacting with silica and alumina in the fly ash and soil to form a cementitious hydration product. In addition, C₃A in fly ash (Class C) can react with sulfates to gain strength relatively quickly. Class F fly ash typically requires outside activation, either from lime or portland cement. The pozzolanic reaction binds up excess free lime, leading to higher soil strengths and lower soil swell potential.
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8. How does the use of fly ash help the environment?

The use of fly ash in concrete improves the environment in a variety of ways. By using fly ash as a partial replacement for portland cement, the production of carbon dioxide emissions is reduced. Every ton of portland cement produced creates approximately one ton of carbon dioxide emissions. Therefore, the use of 20 percent fly ash in a structure containing 500 cubic yards of concrete would reduce CO₂ emissions by approximately 25 tons. The use of fly ash improves the durability of concrete and thus improves the expected life cycle, leading to a reduced structure demolition and replacement rate. The use of fly ash also avoids putting the ash into a landfill.
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9. Is fly ash safe to use?

Fly ash is classified by the EPA as a non-hazardous product. As indicated by the MSDS, it is a relatively inert material. Fly ash is a product of coal combustion thus possesses no significant risk of fire or explosion. Fly ash is similar to sand in composition and consistency. When transporting and handling fly ash, the recommended precautions for safe handling as outlined on the product MSDS should be followed.
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10. Does everyone allow the use of fly ash?

Currently, the United States government dictates that all major concrete construction projects should include recycled materials such as fly ash. Nearly all State Departments of Transportation allow or specify the use of fly ash for their projects. In areas where fly ash is available for marketable use in concrete, project specifications generally allow for its use. The number of specifying agencies and private firms allowing the use of fly ash continues to increase every day.
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