Infrastructure Materials Group

Fly Ash

Fly ashes are finely divided residue resulting from the combustion of ground or powdered coal. They are

generally finer than cement and consist mainly of glassy-spherical particles as well as residues of hematite

and magnetite, char, and some crystalline phases formed during cooling. Use of fly ash in concrete started

in the United States in the early 1930's. The first comprehensive study was that described in 1937, by R. E.

Davis at the University of California (Kobubu, 1968; Davis et al., 1937). The major breakthrough in using fly

ash in concrete was the construction of Hungry Horse Dam in 1948, utilizing 120,000 metric tons of fly ash.

This decision by the U.S. Bureau of Reclamation paved the way for using fly ash in concrete constructions.

In addition to economic and ecological benefits, the use of fly ash in concrete improves its workability,

reduces segregation, bleeding, heat evolution and permeability, inhibits alkali-aggregate reaction, and

enhances sulfate resistance. Even though the use of fly ash in concrete has increased in the last 20 years,

less than 20% of the fly ash collected was used in the cement and concrete industries (Helmuth 1987).

One of the most important fields of application for fly ash is PCC pavement, where a large quantity of

concrete is used and economy is an important factor in concrete pavement construction. FHWA has been

encouraging the use of fly ash in concrete. When the price of fly ash concrete is equal to, or less than, the

price of mixes with only portland cement, fly ash concretes are given preference if technically appropriate

under FHWA guidelines (Adams 1988).

Classifications and Specifications

Two major classes of fly ash are specified in ASTM C 618 on the basis of their chemical composition

resulting from the type of coal burned; these are designated Class F and Class C. Class F is fly ash

normally produced from burning anthracite or bituminous coal, and Class C is normally produced from the

burning of subbituminous coal and lignite (as are found in some of the western states of the United States)

(Halstead 1986). Class C fly ash usually has cementitious properties in addition to pozzolanic properties

due to free lime, whereas Class F is rarely cementitious when mixed with water alone. All fly ashes used in

the United States before 1975 were Class F (Halstead 1986: ACI Comm. 226 1987c).

Fly ash which is produced at base loaded electric generating plants is usually very uniform. Base loaded

plants are those plants which operate continuously. The only exception to uniformity is in the start-up and

the shut-down of these plants. Contamination may occur from using other fuels to start the plant, and

inconsistencies in carbon content occur until the plant reaches full operating efficiency. The ash produced

from the start-up and shut-down must be separated from what is produced when the plant is running

efficiently. In addition, when sources of coal are changed, it is necessary to separate the two types of fly

ashes. Peak load plants are subjected to many start-up and shut-down cycles. Because of this, these

plants may not produce much uniform fly ash.

The most-often-used specifications for fly ash are ASTM C 618 and AASHTO M 295. While some

differences exist, these two specifications are essentially equivalent. Some state transportation agencies

have specifications that differ from the standards (Admixtures and Ground Slag 1990). The general

classification of fly ash by the type of coal burned does not adequately define the type of behavior to be

expected when the materials are used in concrete.

There are also wide differences in characteristics within each class. Despite the reference in ASTM C 618

to the classes of coal from which Class F and Class C fly ashes are derived, there was no requirement that

a given class of fly ash must come from a specific type of coal. For example, Class F ash can be produced

from coals that are not bituminous. and bituminous coals can produce ash that is not Class F (Halstead

1986). It should be noted that current standards contain numerous physical and chemical requirements that

do not serve a useful purpose. Whereas some requirements are needed for ensuring batch-to-batch

uniformity, many are unnecessary (RILEM 1988).

Mix Design

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The substitution rate of fly ash for portland cement will vary depending upon the chemical composition of

both the fly ash and the portland cement. The rate of substitution typically specified is a minimum of 1 to 1

1⁄2 pounds of fly ash to 1 pound of cement. It should be noted that the amount of fine aggregate will have to

be reduced to accommodate the additional volume of fly ash. This is due to fly ash being lighter than the

cement.

The amount of substitution is also dependent on the chemical composition of the fly ash and the portland

cement. Currently, States allow a maximum substitution in the range of 15 to 25 percent.

Effects of fly ash, especially Class F, on fresh and hardened concrete properties has been extensively

studied by many researchers in different laboratories, including the U.S. Army Corps of Engineers, PCA,

and the Tennessee Valley Authority. The two properties of fly ash that are of most concern are the carbon

content and the fineness. Both of these properties will affect the air content and water demand of the

concrete.

The finer the material the higher the water demand due to the increase in surface area. The finer material

requires more air-entraining agent to five the mix the desired air content. The important thing to remember

is uniformity. If fly ash is uniform in size, the mix design can be adjusted to give a good uniform mix.

The carbon content, which is indicated by the loss of ignition, also affects the air entraining agents and

reduces the entrained air for a given amount of air-entraining agent. An additional amount of air-entraining

agent will need to be added to get the desired air content. The carbon content will also affect water demand

since the carbon will absorb water. Again uniformity is important since the differences from non-fly ash

concrete can be adjusted in the mix design.

Fresh Concrete Workability. Use of fly ash increases the absolute volume of cementitious materials

(cement plus fly ash) compared to non-fly-ash concrete; therefore, the paste volume is increased, leading

to a reduction in aggregate particle interference and enhancement in concrete workability. The spherical

particle shape of fly ash also participates in improving workability of fly ash concrete because of the socalled "ball bearing" effect (Admixtures and Ground Slag for Concrete 1990; ACI Comm. 226 1987c). It has

been found that both classes of fly ash improve concrete workability.

Bleeding. Using fly ash in air-entrained and non-air-entrained concrete mixtures usually reduces bleeding

by providing greater fines volume and lower water content for a given workability (ACI Comm. 226, 1987c;

Idorn and Henrisken, 1984). Although increased fineness usually increases the water demand, the

spherical particle shape of the fly ash lowers particle friction and offsets such effects. Concrete with

relatively high fly ash content will require less water than non-fly-ash concrete of equal slump (Admixtures

and ground slag for concrete, 1990).

Time of Setting. All Class F and most Class C fly ashes increase the time of setting of concrete

(Admixtures and ground slag 1990; ACI Comm. 226, 1987c). Time of setting of fly ash concrete is

influenced by the characteristics and amounts of fly ash used in concrete. For highway construction,

changes in time of setting of fly ash concrete from non-fly-ash concrete using similar materials will not

usually introduce a need for changes in construction techniques; the delays that occur may be considered

advantageous (Halstead 1986).

Strength and Rate of Strength of Hardened Concrete. Strength of fly ash concrete is influenced by type of

cement, quality of fly ash, and curing temperature compared to that of non-fly-ash concrete proportioned for

equivalent 28-day compressive strength. Concrete containing typical Class F fly ash may develop lower

strength at 3 or 7 days of age when tested at room temperature (Admixtures and ground slag for concrete,

1990; ACI Comm. 226 1987c). However, fly ash concretes usually have higher ultimate strengths when

properly cured. The slow gain of strength is the result of the relatively slow pozzolanic reaction of fly ash. In

cold weather, the strength gain in fly ash concretes can be more adversely affected than the strength gain

in non-fly-ash concrete. Therefore, precautions must be taken when fly ash is used in cold weather

(Admixtures and ground slag 1990).

Freeze-thaw Durability of Hardened Concrete. On the basis of a comparative experimental study of freezethaw durability of conventional and fly ash concrete (Soroushian 1990; Virtanen 1983; Lane and Best

1982), it has been observed that the addition of fly ash has no major effect on the freeze-thaw resistance of

concrete if the strength and air content are kept constant. The addition of fly ash may have a negative

effect on the freeze-thaw resistance of concrete when a major part of the cement is replaced by it. The use

of fly ash in air-entrained concrete will generally require an increase in the dosage rate of the air-entraining

admixture to maintain constant air. Air-entraining admixture dosage depends on carbon content, loss of

ignition, fineness, and amount of organic material in the fly ash (ACI Comm. 226, 1987c).

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Carbon content of fly ash, which is related to the coal burned by the producing utility of the type and

condition of furnaces in the production process of fly ash, influences the behavior of admixtures in

concrete. It has been found that high-carbon-content fly ash reduces the effectiveness of admixtures such

as air-entraining agents (Joshi, Langan, and Ward 1987: Hines 1985).

Alkali-silica Reaction of Hardened Concrete. One of the important reasons for using fly ash in highway

construction is to inhibit the expansion resulting from ASR. It has been found that 1) the alkalies released

by the cement preferentially combine with the reactive silica in the fly ash rather than in the aggregate, and

2) the alkalies are tied up in nonexpansive calcium-alkali-silica gel. Thus hydroxyl ions remaining in the

solution are insufficient to react with the material in the interior of the larger reactive aggregate particles

and disruptive osmotic forces are not generated (Halstead 1986; Olek, Tikalsky, and Carrasquillo 1986;

Farbiarz and Carrasquillo 1986).

In a paper presented at the 8th International Conference on alkali-aggregate reactivity held in Japan in

1989, Swamy and Al-Asali indicated that ASR expansion is generally not proportional to the percentage of

cement replacement by fly ash. The rate of reactivity, the replacement level, the method of replacement,

and the environment all have a profound influence on the protection against ASR afforded by fly ash.

Several investigators (Mehta, 1980; Diamond, 1981; Hobbs, 1982) have stated that ASR expansions

correlated better with water-soluble alkali-silica contents than with total alkali content. The addition of some

high-calcium fly ash containing large amounts of soluble alkali sulfate might increase rather than decrease

the alkali-aggregate reactivity (Mehta, 1983). The effectiveness of different fly ashes in reducing long-term

expansion varied widely; for each fly ash, this may be dependent upon its alkali content or fineness

(Soroushian, 1990).

Blended Cements

The following will discuss on the Type "IP", "P" and "I(PM)" cements. The specifications for these cements

are in AASHTO M-240 and ASTM C-595. Blended cements can be manufactured by either intimate

blending of portland cement and pozzolan or intergrinding of the pozzolan with the cement clinker in the

kiln. Type "I(PM)" (pozzolan modified cement) allows up to 15 percent replacement of cement with fly ash.

The Type "IP" and Type"P" are pozzolan-modified portland cements which allow 15-40 percent

replacement with pozzolans. The differences in the two types of cements is in the ultimate strength and the

rate of strength gain of the concretes. Most States specify limits on the pozzolanic content on Type "IP"

cement. These limits are between 15 and 25 percent.

Restraints on the Use of Fly Ash Concrete in Highway Constructions

It is well known now that both classes of fly ash improve the properties of concrete, but several factors and

cautions should be considered when using fly ashes especially in highway construction, where fly ash is

heavily used. In a report prepared by the Virginia Highway and Transportation Research Council (VHTRC)

and summarized by Halstead (1986), several restraints relating to the use of fly ash concrete for

construction of highways and other highway structures were discussed. These restraints include the

following: 1) special precautions may be necessary to ensure that the proper amount of entrained air is

present; 2) not all fly ashes have sufficient pozzolanic activity to provide good results in concrete; 3)

suitable fly ashes are not always available near the construction site, and transportation costs may nullify

any cost advantage; and 4) mix proportions might have to be modified for any chance in the fly ash

composition.

Since the cement-fly ash reaction is influenced by the properties of the cement, it is important for a

transportation agency not only to test and approve each fly ash source but also to investigate the properties

of the specific fly ash-cement combination to be used for each project (Halstead 1986).

Recommendations

1. The standard specifications for fly ash (ASTM C-618 or AASHTO M-295) should be used. The

included optional specification for uniformity as described below should also be required. This

concerns the variation in the amount of air entraining agent to maintain an 18 percent air content in

the mortar. A maximum variation in the amount of air entraining agent of 20 percent is specified.

2. The State highway agencies should develop certification programs similar to those in existence for

portland cement. This program should include testing by the supplier with check tests on grab

samples taken by the agency. The plan should also require that the supplier's laboratory participate

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in the Cement and Concrete Reference Laboratory (CCRL) program which includes inspection of

facilities and testing of comparative samples.

Until the certification programs are in place, it is suggested that the States test the fly ash and use

sealed silos and transports. Five tests per silo should be run to insure uniformity of the fly ash. Once

uniformity of a source is established, sampling could be reduced to one per 400 tons as specified in

ASTM C-311. It is recommended that 10,000 tons of fly ash be tested before reducing the testing

frequency.

3. The air content of each load of concrete should be monitored at least in the beginning of production.

This would indirectly monitor the uniformity of the fly ash.

4. Specifications should contain strength requirements with minimum substitution ratio and maximum

replacement. This would allow maximum substitution without sacrificing strength. The water cement

ratio should be based on the total cementious materials, i.e., the portland cement plus the fly ash

substituted.

5. Substitution ratios on a minimum of 1 to 1 on a mass basis with a maximum substitution should be

specified. A substitution rate of 15 to 25 percent maximum is currently being specified for typical

concrete production. These values should be established based on the actual fly ashes and portland

cements that are available.

6. Mix designs should be performed by the State on each combination of materials, or by the contractor

with the requirement to provide the test data to the State for verification with trial batches.

Since the chemical composition of fly ashes and portland cements vary considerably, substantial

problems could result if fixed rates and percentages of substitutions are used for all combinations of

fly ashes and cements.

Exemptions

The EPA guideline on the substitution of fly ash requires the State highway agency to document the

reasons for not allowing the substitution of fly ash for cement if it feels that it is technically inappropriate.

The following two cases will not require documentation.

A. Fly ash should not be substituted for a portion of Type "IP", Type "I" (PM) or Type "P".

B. Substitution should not be specified for high early strength concrete. In this case, concrete that

contains fly ash gains strength slower so it would not be capable of having high early strength.

References

Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway

Technology, David Whiting, et al, SHRP-C-345, Strategic Highway Research Program, National Research

Council.

ACI Committee 226. 1987a. Ground granulated blast furnace slag as a cementitious constituent in concrete

ACI 226.AR-87. Detroit: American Concrete Institute.

Adams, T. H. 1988. Marketing of fly ash concrete. In MSU seminar: Fly ash applications to concrete

(January), 1.10, 5.10. East Lansing: Michigan State University.

Admixtures and ground slag for concrete. 1990. Transportation research circular no. 365 (December).

Washington: Transportation Research Board, National Research Council.

Davis, R. E., R. W. Carlson, J. W. Kelly, and A. G. Davis. 1937. Properties of cements and concretes

containing fly ash. Proceedings, American Concrete Institute 33:577-612.

Diamond, S. 1981. Effects of two Danish fly ashes on alkali contents of pore solutions of cement fly ash

pastes. Cement and Concrete Research 11:383-94.

Diamond, S. 1985. Very high strength cement-based materials: A perspective. Materials Research Society

Symposia Proceedings 142:223-43.

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Farbiarz, J., and R. L. Carrasquillo. 1986. Effectiveness of fly ash replacement in the reduction of damage

due to alkali-aggregate reaction in concrete. Report no. FHWA/TX-87/15+450-1 (May). Texas State

Department of Highways and Public Transportation.

Halstead, W. J. 1986. Use of fly ash in concrete. NCHRP 127 (October). Washington: Transportation

Research Board, National Research Council.

Helmuth, R. 1987. Fly ash in cement and concrete. Skokie, III.: Portland Cement Association.

Hines, D. 1985. Fly ash use in lean concrete base. Colorado Department of Highways final report. Report

no. CDOH-SMB-R-85-13 (December).

Hobbs, D. W. 1982. Influence of pulverized-fuel ash and granulated blast furnace slag upon expansion

caused by the alkali-silica reaction. Magazine of Concrete Research 34:83-93.

Idorn, G. M., and K. R. Henrisken. 1984. State of the art for fly ash uses in concrete. Cement and Concrete

Research 14 (4):463-70.

Joshi, R. C., B. W. Langan, and M. A. Ward. 1987. Strength and durability of concrete with high proportions

of fly ash and other mineral admixtures. In Durability of building materials. Vol. 4, 253-70. Amsterdam:

Elsevier Science Publishers.

Kohubu, M. 1969. Fly ash and fly ash cement. In Proceedings, Fifth international symposium on the

chemistry of cement (1968). Part IV, 75-105. Tokyo: Cement Association of Japan.

Lane, R. O., and J. F. Best. 1982. Properties of fly ash in portland cement concrete. Concrete International.

Design and Construction 4 (7):81-92. RILEM. 1988. Siliceous by-products for use in concrete. Final report:

73-SBC RILEM Committee. Materials and Structures 21 (121):69-80.

Mehta, P. K. 1980. Performance test for sulfate resistance and alkali-silica reactivity of hydraulic cements.

In ASTM STP 691: Durability of building materials and components, 336-45.

Mehta, P. K. 1983. Pozzolanic and cementitious by-products as mineral admixtures for concrete: A

practical review. In ACI special publication SP-79: The use of fly ash, silica fume, slag and other mineral

by-products in concrete, ed. V. M. Malhotra, 1-46. Detroit: American Concrete Institute.

Olek, J., P. J. Tikalsky, and R. L. Carrasquillo. 1986. Production of concrete containing fly ash for

pavement applications. Research report 364-2 (May). Austin: University of Texas Center for Transportation

Research

Soroushian, P. 1990. Durability characteristics of fly ash concrete. In Recent advances in concrete

technology seminar, MSU-CTS 4 (February), 6.1-6.18. East Lansing: Michigan State University.

Virtanen, J. 1983. Freeze-thaw resistance of concrete containing blast-furnace slag, fly ash or condensed

silica fume. In ACI special publication SP-79: The use of fly ash, silica fume, slag and other mineral byproducts in concrete, ed. V. M. Malhotra, 923-42. Detroit American Conrete Institute

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