A. 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, bag houses 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.
Frequently Asked Questions
A. 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.
A. 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”.
A. 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.
A. 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.
A. 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.
A. 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.
A. 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.
A. 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.
A. 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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Eco Material Technologies Class C Fly Ash Brochure
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Safety Data Sheets
Celceram PV20A-5 Micron Specifications
Chemical Comparison of Fly Ash and Portland Cement
Class F Fly Ash Increases Resistance to Sulfate Attack
Fly Ash Decreases Alkali/Silica Reaction
Fly Ash Decreases the Permeability of Concrete
Fly Ash for Architectural Concrete
Fly Ash for Block Manufacturing
Fly Ash for Insulating Concrete Form Construction
Fly Ash for Pipe Manufacturing
Fly Ash for Precast Prestressed Concrete Products
Fly Ash for Stone Matrix Asphalt
Fly Ash for Structural Concrete
Fly Ash in Cold Weather Concrete
Fly Ash in Hot Weather Concrete
Fly Ash Increases Resistance to Freezing & Thawing
Fly Ash Reclaimed From Landfill
Fly Ash Reduces Vapor transmission in Concrete Floors
Gypsyn for Agricultural Fields
Harvested Fly Ash for Concrete Production
Higher Volume Fly Ash for Concrete Pavement
Proportioning Fly Ash Concrete Mixes
Ternary Concrete Mixes with Cement Slag and Fly Ash
American Concrete Pavement Association
American Concrete Pipe Association
American Institute of Architects
American Road & Transportation Builders Association
American Shotcrete Association
American Society of Civil Engineers
National Concrete Masonry Association
National Precast Concrete Association
National Ready Mixed Concrete Association
Precast/Prestressed Concrete Institute
Texas Coal Ash Utilization Group
United States Green Building Council
EPA Coal Combustion Residual Beneficial Use Evaluation: Fly Ash Concrete and FGD Gypsum Wallboard
National Energy Technology Laboratory
National Governors Association Publications
Office of Surface Mining “News Releases and Fact Sheets”
Coal Combustion Products Information and Publications
AECOM Coal Ash Materials Safety Study
American Public Power Assoc. “Public Power Statistics”
CCP Production and Use Statistics
Concrete, Fly Ash & The Environment
Economic Impacts of Prohibiting Coal Fly Ash Use in Transportation Infrastructure Construction (ARTBA)
EPA Fly Ash Concrete and FGD Gypsum Wallboard Beneficial Use Evaluation
Flyash Concrete – Sustainable Sources
USGS Coal Combustion Products Statistics & Information
Research, Education & Testing Programs
American Society for Testing & Materials
Center for By-Products Utilization – University of Wisconsin-Milwaukee
Coal Combustion Products Program – The Ohio State University
Electric Power Research Institute
Energy & Environmental Building Alliance
ICC Evaluation Service, LLC (a subsidiary of the International Code Council)
Institute of Concrete Technology
Office of Clean Coal and Carbon Management – US Department of Energy
Ohio State University Coal Combustion Products Extension Program
Recycled Materials Resource Center – University of Wisconsin-Madison
University of Kentucky Center for Applied Energy Research
University of North Dakota Energy Environmental Research Center