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Publication Number: FHWA-RD-97-148

User Guidelines for Waste and Byproduct Materials in Pavement Construction


COAL FLY ASH Material Description


The fly ash produced from the burning of pulverized coal in a coal-fired boiler is a fine-grained, powdery particulate material that is carried off in the flue gas and usually collected from the flue gas by means of electrostatic precipitators, baghouses, or mechanical collection devices such as cyclones.

In general, there are three types of coal-fired boiler furnaces used in the electric utility industry. They are referred to as dry-bottom boilers, wet-bottom boilers, and cyclone furnaces. The most common type of coal burning furnace is the dry-bottom furnace.

When pulverized coal is combusted in a dry-ash, dry-bottom boiler, about 80 percent of all the ash leaves the furnace as fly ash, entrained in the flue gas. When pulverized coal is combusted in a wet-bottom (or slag-tap) furnace, as much as 50 percent of the ash is retained in the furnace, with the other 50 percent being entrained in the flue gas. In a cyclone furnace, where crushed coal is used as a fuel, 70 to 80 percent of the ash is retained as boiler slag and only 20 to 30 percent leaves the furnace as dry ash in the flue gas.(1) A general flow diagram of fly ash production in a dry-bottom coal-fired utility boiler operation is presented in Figure 5-1.


Figure 5-1. Production of fly ash in a dry-bottom utility boiler with electrostatic precipitator.

During 1996, the most recent year for which ash statistics are currently available, the electrical utility industry in the United States generated approximately 53.5 million metric tons (59.4 million tons) of coal fly ash. Until 1996, the amount of fly ash produced annually had remained roughly the same since 1977, ranging from 42.9 to 49.7 million metric tons (47.2 to 54.8 million tons).(2)

Additional information on coal ash use in the United States can be obtained from:

American Coal Ash Association (ACAA)

2760 Eisenhower Avenue, Suite 304 Alexandria, Virginia 22314

Electric Power Research Institute

3412 Hillview Road Palo Alto, California 94304

Edison Electric Institute

1701 Pennsylvania Avenue, N.W. Washington, D.C. 20004-2696




In 1996, approximately 14.6 million metric tons (16.2 million tons) of fly ash were used. Of this total, 11.85 million metric tons (13.3 million tons), or approximately 22 percent of the total quantity of fly ash produced, were used in construction-related applications. Table 5-1 lists the leading construction applications in which fly ash was used during 1996.

Between 1985 and 1995, fly ash usage has fluctuated between approximately 8.0 and 11.9 million metric tons (8.8 and 13.6 million tons) per year, averaging 10.2 million metric tons (11.3 million tons) per year.(3) Fly ash is useful in many applications because it is a pozzolan, meaning it is a siliceous or alumino-siliceous material that, when in a finely divided form and in the presence of water, will combine with calcium hydroxide (from lime, Portland cement, or kiln dust) to form cementitious compounds.(4)


Approximately 70 to 75 percent of fly ash generated is still disposed of in landfills or storage lagoons.(2) Much of this ash, however, is capable of being recovered and used.

Table 5-1. Fly ash construction-related applications (1996).

Applications Quantity Used Percent of Total Used
Million Metric Tons Million Tons
Cement production and/or concrete products
Structural fills or embankments
Stabilization of waste materials
Road base or subbase materials
Flowable fill and grouting mixes
Mineral filler in asphalt paving
Approximate Total 11.85 13.3 100



Although electric utility companies produce ash at their coal-fired power plants, most utilities do not handle, dispose of, and/or sell the ash that they produce. There are approximately 40 to 50 commercial ash marketing firms operating throughout the United States, in all states except Hawaii. In addition to commercial ash marketing organizations, certain coal-burning electric utility companies have formal ash marketing programs of their own. Most coal-burning electric utility companies currently employ an ash marketing specialist whose responsibility it is to monitor ash generation, quality, use or disposal, and to interface with the ash marketers or brokers who are under contract to the utility companies.

Because of variations in coals from different sources, as well as differences in the design of coal-fired boilers, not all fly ash is the same. Although there may be differences in the fly ash from one plant to another, day-to-day variations in the fly ash from a given power plant are usually quite predictable, provided plant operation and coal source remain constant. However, there can be a substantial variation in fly ash obtained from burning coal with other fuels (such as natural gas or wood) or with other combustible materials (such as municipal solid waste, scrap tires, etc.). As long as the basic operating parameters at the power plant do not change, fly ash from a known source that is supplied by a reputable ash marketing organization should be a consistent, quality-controlled product.

Fly ash to be used in Portland cement concrete (PCC) must meet the requirements of ASTM C618.(5) Two classes of fly ash are defined in ASTM C618: 1) Class F fly ash, and 2) Class C fly ash.

Fly ash that is produced from the burning of anthracite or bituminous coal is typically pozzolanic and is referred to as a Class F fly ash if it meets the chemical composition and physical requirements specified in ASTM C618. Materials with pozzolanic properties contain glassy silica and alumina that will, in the presence of water and free lime, react with the calcium in the lime to produce calcium silicate hydrates (cementitious compounds).

Fly ash that is produced from the burning of lignite or subbituminous coal, in addition to having pozzolanic properties, also has some self-cementing properties (ability to harden and gain strength in the presence of water alone). When this fly ash meets the chemical composition and physical requirements outlined in ASTM C618, it is referred to as a Class C fly ash. Most Class C fly ashes have self-cementing properties.

Fly ash is typically stored dry in silos, from which it can be used or disposed of in a dry or wet form. Water can be added to the fly ash to allow for stockpiling or landfilling in a conditioned form (approximately 15 to 30 percent moisture), or for disposal by sluicing into settling ponds or lagoons in a wet form. Approximately 75 percent of the fly ash produced is handled in a dry or moisture-conditioned form, making it much easier to recover and use. The main advantage to the conditioning of fly ash is the reduction of blowing or dusting during truck transport and outdoor storage.



Portland Cement Concrete – Supplementary Cementitious Material

Fly ash has been successfully used as a mineral admixture in PCC for nearly 60 years. This is the largest single use of fly ash. It can also be used as a feed material for producing Portland cement and as a component of a Portland-pozzolan blended cement.

Fly ash must be in a dry form when used as a mineral admixture. Fly ash quality must be closely monitored when the material is used in PCC. Fineness, loss on ignition, and chemical content are the most important characteristics of fly ash affecting its use in concrete. Fly ash used in concrete must also have sufficient pozzolanic reactivity and must be of consistent quality.

Asphalt Concrete – Mineral Filler

Fly ash has been used as a substitute mineral filler in asphalt paving mixtures for many years. Mineral filler in asphalt paving mixtures consists of particles, less than 0.075 mm (No. 200 sieve) in size, that fill the voids in a paving mix and serve to improve the cohesion of the binder (asphalt cement) and the stability of the mixture. Most fly ash sources are capable of meeting the gradation (minus .075 mm) requirements and other pertinent physical (nonplastic) and chemical (organic content) requirements of mineral filler specifications.

Fly ash must be in a dry form for use as a mineral filler. Fly ash that is collected dry and stored in silos requires no additional processing. It is possible that some sources of fly ash that have a high lime (CaO) content may also be useful as an antistripping agent in asphalt paving mixes.

Stabilized Base – Supplementary Cementitious Material

Stabilized bases or subbases are mixtures of aggregates and binders, such as Portland cement, which increase the strength, bearing capacity, and durability of a pavement substructure. Because fly ash may exhibit pozzolanic properties, or self-cementing properties, or both, it can and has been successfully used as part of the binder in stabilized base construction applications. When pozzolanic-type fly ash is used, an activator must be added to initiate the pozzolanic reaction. Self-cementing fly ash does not require an activator. The most commonly used activators or chemical binders in pozzolan-stabilized base (PSB) mixtures are lime and Portland cement, although cement kiln dusts and lime kiln dusts have also been used with varying degrees of success. Sometimes, combinations of lime, Portland cement, or kiln dusts have also been used in PSB mixtures.

The successful performance of PSB mixtures depends on the development of strength within the matrix formed by the pozzolanic reaction between the fly ash and the activator. This cementitious matrix acts as a binder that holds the aggregate particles together, similar in many respects to a low-strength concrete.

Flowable Fill – Aggregate or Supplementary Cementitious Material

Flowable fill is a slurry mixture consisting of sand or other fine aggregate material and a cementitious binder that is normally used as substitute for a compacted earth backfill. Fly ash has been used in flowable fill applications as a fine aggregate and (because of its pozzolanic properties) as a supplement to or replacement for the cement. Either pozzolanic or self-cementing fly ash can be used in flowable fill. When large quantities of pozzolanic fly ash are added, the fly ash can act as both fine aggregate and part of the cementitious matrix. Self-cementing fly ash is used in smaller quantities as part of the binder in place of cement.

The quality of fly ash used in flowable fill applications need not be as strictly controlled as in other cementitious applications. Both dry and reclaimed ash from settling ponds can be used. No special processing of fly ash is required prior to use.

Embankment and Fill Material

Fly ash has been used for several decades as an embankment or structural fill material, particularly in Europe. There has been relatively limited use of fly ash as an embankment material in this country, although its use in this application is becoming more widely accepted.

As an embankment or fill material, fly ash is used as a substitute for natural soils. Fly ash in this application must be stockpiled and conditioned to its optimum moisture content to ensure that the material is not too dry and dusty or too wet and unmanageable. When fly ash is at or near its optimum moisture content, it can be compacted to its maximum density and will perform in an equivalent manner to well-compacted soil.



Physical Properties

Fly ash consists of fine, powdery particles that are predominantly spherical in shape, either solid or hollow, and mostly glassy (amorphous) in nature. The carbonaceous material in fly ash is composed of angular particles. The particle size distribution of most bituminous coal fly ashes is generally similar to that of a silt (less than a 0.075 mm or No. 200 sieve). Although subbituminous coal fly ashes are also silt-sized, they are generally slightly coarser than bituminous coal fly ashes.(2)

The specific gravity of fly ash usually ranges from 2.1 to 3.0, while its specific surface area (measured by the Blaine air permeability method)(6) may range from 170 to 1000 m2/kg.

The color of fly ash can vary from tan to gray to black, depending on the amount of unburned carbon in the ash. The lighter the color, the lower the carbon content. Lignite or subbituminous fly ashes are usually light tan to buff in color, indicating relatively low amounts of carbon as well as the presence of some lime or calcium. Bituminous fly ashes are usually some shade of gray, with the lighter shades of gray generally indicating a higher quality of ash.

Chemical Properties

The chemical properties of fly ash are influenced to a great extent by those of the coal burned and the techniques used for handling and storage. There are basically four types, or ranks, of coal, each of which varies in terms of its heating value, its chemical composition, ash content, and geological origin. The four types, or ranks, of coal are anthracite, bituminous, subbituminous, and lignite. In addition to being handled in a dry, conditioned, or wet form, fly ash is also sometimes classified according to the type of coal from which the ash was derived.

The principal components of bituminous coal fly ash are silica, alumina, iron oxide, and calcium, with varying amounts of carbon, as measured by the loss on ignition (LOI). Lignite and subbituminous coal fly ashes are characterized by higher concentrations of calcium and magnesium oxide and reduced percentages of silica and iron oxide, as well as a lower carbon content, compared with bituminous coal fly ash.(7) Very little anthracite coal is burned in utility boilers, so there are only small amounts of anthracite coal fly ash.

Table 5-2 compares the normal range of the chemical constituents of bituminous coal fly ash with those of lignite coal fly ash and subbituminous coal fly ash. From the table, it is evident that lignite and subbituminous coal fly ashes have a higher calcium oxide content and lower loss on ignition than fly ashes from bituminous coals. Lignite and subbituminous coal fly ashes may have a higher concentration of sulfate compounds than bituminous coal fly ashes.

The chief difference between Class F and Class C fly ash is in the amount of calcium and the silica, alumina, and iron content in the ash.(6) In Class F fly ash, total calcium typically ranges from 1 to 12 percent, mostly in the form of calcium hydroxide, calcium sulfate, and glassy components in combination with silica and alumina. In contrast, Class C fly ash may have reported calcium oxide contents as high as 30 to 40 percent.(8) Another difference between Class F and Class C is that the amount of alkalis (combined sodium and potassium) and sulfates (SO4) are generally higher in the Class C fly ashes than in the Class F fly ashes.

Table 5-2. Normal range of chemical composition for fly ash produced from different coal types(expressed as percent by weight).

Component Bituminous Subbituminous Lignite
SiO2 20-60 40-60 15-45
Al2O3 5-35 20-30 10-25
Fe2O3 10-40 4-10 4-15
CaO 1-12 5-30 15-40
MgO 0-5 1-6 3-10
SO3 0-4 0-2 0-10
Na2O 0-4 0-2 0-6
K2O 0-3 0-4 0-4
LOI 0-15 0-3 0-5

Although the Class F and Class C designations strictly apply only to fly ash meeting the ASTM C618 specification, these terms are often used more generally to apply to fly ash on the basis of its original coal type or CaO content. It is important to recognize that not all fly ashes are able to meet ASTM C618 requirements and that, for applications other than concrete, it may not be necessary for them to do so.

The loss on ignition (LOI), which is a measurement of the amount of unburned carbon remaining in the fly ash, is one of the most significant chemical properties of fly ash, especially as an indicator of suitability for use as a cement replacement in concrete.



  1. Babcock and Wilcox Company. Steam. Its Generation and Use. New York, NY,1978.

  2. DiGioia, Anthony M., Jr. and William L. Nuzzo. "Fly Ash as Structural Fill," Proceedings of the American Society of Civil Engineers, Journal of the Power Division, New York, NY, June 1972.

  3. American Coal Ash Association. 1996 Coal Combustion Product-Production and Use. Alexandria, Virginia, 1997.

  4. Federal Highway Administration and American Coal Ash Association. Fly Ash Facts for Highway Engineers. Report No. FHWA-SA-94-081, Washington, DC, December, 1995.

  5. ASTM C618-92a. "Standard Specification for Fly Ash and Raw or Calcined Natural Pozzolan for Use as Mineral Admixture in Portland Cement Concrete," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1994.

  6. ASTM C204. "Test Method for Fineness of Portland Cement by Air Permeability Apparatus," American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1994.

  7. Meyers, James F., Raman Pichumani and Bernadette S. Kapples. Fly Ash. A Highway Construction Material. Federal Highway Administration, Report No. FHWA-IP-76-16, Washington, DC, 1976.

  8. McKerall, W.C., W.B. Ledbetter, and D. J. Teague. Analysis of Fly Ashes Produced in Texas. Texas Transportation Institute, Research Report No. 240-1, Texas A&M University, College Station, Texas, 1982.


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