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COAL FLY ASH Material Description

ORIGIN

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 wetbottom (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

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CURRENT MANAGEMENT OPTIONS

Recycling

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)

Disposal

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

Million Metric Used

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

7.2

1.9

1.7

0.63

0.27

0.15

8.0

2.2

1.9

0.7

0.3

0.2

60

17

14

5

2

2

Approximate Total 11.85 13.3 100

MARKET SOURCES

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

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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.

HIGHWAY USES AND PROCESSING REQUIREMENTS

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

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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 pozzolanictype 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 pozzolanstabilized 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.

MATERIAL PROPERTIES

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.

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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

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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.

REFERENCES

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.

[ Asphalt Concrete

]

[ Portland Cement Concrete

]

[ Embankment or Fill

]

[ Stabilized Base

]

[ Flowable Fill

]

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