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Fly ash in concrete
Part I—What it is and how it works:
effect on properties of fresh and hardened concrete
W
ith escalating energy costs affecting cement costs,
the use of fly ash, a lower cost material, is rapidly
increasing. Surveys in 1979 of the American ready
mixed concrete industry showed that fly ash was used in
37 percent of the ready mixed concrete produced, and
an even more recent survey indicates that the total
amount of fly ash used equals almost 10 percent of the
total amount of portland cement used.
WHAT FLY ASH IS
fly ash gradually react with the calcium hydroxide released in the hydration of the portland cement. As the fly
ash combines with the calcium hydroxide it slowly converts it to calcium silicate and calcium aluminate
binders. In the process the amount of cement increases,
enhancing the strength and reducing the permeability of
the concrete. This chemical reaction occurs much more
slowly than the hydration of portland cement. Because
the reaction proceeds slowly, the full potential strength
of the fly ash concrete may not be attained in thin concrete members or on surfaces which lose moisture
quickly. Like the hydration of cement the fly ash-calcium
h yd roxide reaction stops if insufficient moisture is present.
Fly ash is a by-product of the burning of pulve ri ze d
coal in power plants. It is removed by mechanical collectors or electrostatic precipitators as fine particles from
the combustion gases before they are discharged into
the atmosphere. The parWHAT FLY ASH DOES
ticles are typically spheriBASIC CHEMICAL REQUIREMENTS FOR FLY ASH*
IN FRESH CONCRETE
cal, ranging in diameter
ASTM C 618
Class F Class C
Physically fly ash modfrom 0.00004 to 0.006
Silicon dioxide (SiO2) + aluminum
ifies
the plastic properinch. Electrostatic precipoxide (AI2O3) + ferric oxide
ties of fresh concrete beitators capture the prefer(Fe2O3), percent, not less than
70.0
50.0
cause the particles are
able, finer-sized particles
Sulfur trioxide (SO3), percent, not
very small and essentially
that escape mechanical
more than
5.0
5.0
spherical. Because it has
collectors. The chemical
Moisture content, percent, not
a lower rate of chemical
composition of the fly ash
more than
3.0
3.0
reactivity
than the ceis determined by the minLoss on ignition, percent, not
ment it replaces, it reeral matter in the coal.
more than
12.0
6.0
duces the early heat
There are two classifica* For optional chemical requirements and for physical requirements, refer to ASTM C 618, “Standard Specification
buildup. On the minus
tions for fly ash in ASTM C
for Fly Ash and Raw or Calcined Natural Pozzolan for Use As
side, it usually re q u i re s
618: Class F fly ash proa Mineral Admixture in Portland Cement Concrete.”
greater attention to the
duced from burning anachievement and control
thracite or bituminous
of proper air content when needed.
coal and Class C fly ash produced from lignite or subbituminous coal. Major national organizations generalPlastic properties
ly use ASTM C 618 Class F requirements as the basis for
Unlike other pozzolans, fly ash does not increase the
their specifications for fly ash used in concrete, and most
water requirement of the concrete mix. The small, round
state transportation departments cite it as well, except
particles act as tiny glass ball bearings that increase the
that they use a lower limit on loss on ignition, usually 6
degree of workability for a given water content or, conpercent but sometimes as low as 3 percent. Information
versely, reduce the amount of water required for a given
presented here is based mainly but not exclusively on
degree of workability. Fly ash concretes show less segreexperience with Class F, with which the majority of pubgation and bleeding as well as better finishability and
lished reports deal. Z
pumpability than plain concretes. These effects make its
HOW FLY ASH WORKS
use particularly valuable in lean mixes or in concretes
made with aggregates deficient in fines.
Added to concrete, fly ash plays the dual role of fine
aggregate and cementitious component. In the earliest
Heat of hydration
stages of curing, it acts as an inert fine aggregate, but in
Fly ash or some other pozzolan is used today in virtuthe presence of moisture, the silica and alumina of the
ally all mass concrete for dams to reduce the high heat
buildup in the interior of the structure (Figure 1). Mass
concrete, unlike structural concrete, is normally not reinforced against tensile failure. A temperature drop of 45
degrees F following the heat buildup is about as much as
unreinforced concrete could withstand without cracking.
Air entrainment
The use of fly ash in concrete usually calls for more airentraining agent to entrain a given amount of air. There
are two reasons for this. First and most important, carbon in the fly ash absorbs some of the air entraining
agent, thus decreasing the amount available for creating air bubbles. The amount of absorption varies with
the amount of carbon present and possibly also with the
form of such carbon. Second, fly ash is normally finer
than cement and is usually added in greater amounts
than the cement replaced. This produces a greater surface area within the concrete mix. Thus, a greater volume of air-entraining agent is needed to provide the
same surface concentrations of the air-entraining agent.
To determine the carbon content of a fly ash, a simple
laboratory test is made. Fly ash is burned, and the weight
loss on ignition represents the amount of carbon. There
is a direct relationship between the carbon content and
the amount of air-entraining agent absorbed by the carbon. Hence the extra amount of air-entraining agent required can be estimated and added to the mix. Once the
EPA ENCOURAGES THE USE OF
FLY ASH CONCRETE
Though fly ash can be used to improve the properties of concrete, fly ash is still not widely used for
this purpose. (It is usually used to reduce cost.) Of
the 16 states that permit the use of fly ash-cement
blends in concrete pavements, only two have constructed more than 100 lane-miles, and of the 19
that allow adding flay ash as an admixture, only
four have constructed over 100 lane-miles.
To encourage greater use of fly ash, the Environmental Protection Agency (EPA), acting under the
1976 Resource Conservation and Recovery ACT
(RCRA), has proposed a new guideline. If adopted
in its original form this guideline will require any
agency purchasing concrete with federal funds to
allow bidders to submit bids on any of three bases:
• concrete containing fly ash as an admixture
• concrete using a blended cement in which fly ash
is the pozzolan
• plain portland cement concrete without fly ash
Specifications would have to be of a performance
type and not a recipe type, and contracts would be
awarded to the lowest bidder regardless of the type
of concrete to be used. However, for equal bids,
the concrete containing the most fly ash would be
accepted.
Figure 1. Effect of fly ash on the temperature rise of mass
concrete containing a total of 282 pounds of cementing
material per cubic yard.
proper volume of air is entrained, characteristics of the
air void system meet generally accepted criteria.
To minimize the difficulties in air-entrainment due to
high-carbon fly ashes, all state transportation departments using the material have limited the maximum
permissible loss on ignition to 6 percent and many have
lowered that to 3 percent, this despite the 12 percent
ceiling allowed by ASTM Specification C 618 for Class F
fly ash.
WHAT FLY ASH DOES IN HARDENED CONCRETE
A common reason for using fly ash in concrete is to
achieve the needed compressive strength at a lower cement content. Replacement of portland cement by fly
ash on a one-for-one basis, either by volume or weight,
results in lower compressive strengths at ages up to
about 3 months, but greater strengths develop at 6
months and beyond (Figure 2). Some of the other properties modified by the use of fly ash in concrete are discussed below.
Durability
Under the heading of durability come all aspects of resistance of concrete to materials or conditions that
might reduce its longevity.
Freeze-thaw resistance—There are no apparent differences in freeze-thaw durability between fly-ash and
non-fly-ash concretes of equal strengths and equal air
contents. Fly ash does not affect the air-entrainment as
such, but rather the air-entraining-agent demand, as
discussed earlier.
greater volume than the combined volumes of the reactive materials. This leads to cracking and spalling. Fly
ash, if used in large enough doses, has been found to
help safeguard against this reaction. The minimum replacement of cement by fly ash to be effective is reportedly 36 to 48 percent by volume. It is questionable,
though, whether the early strength losses caused by replacing such high percentages of cement would be tolerable for more than a few applications.
Corrosion of reinforcing steel—The alkalinity of concrete tends to coat reinforcing or other steel with a protective film of ferrous hydroxide; this prevents the easy
penetration of water and oxygen. Fly ash does not
change this alkalinity significantly. The pozzolanic gel
seems to allow less lime to be leached out of the conc re t e, whether because of the lower permeability, the
chemical fixing of lime, or both.
Figure 2. Qualitative comparison of rates of strength gain of
plain cement concrete and a concrete in which part of the
cement has been replaced by fly ash.
Permeability—Given any combination of cement and
aggregate, the less permeable the concrete, the greater
will be its resistance to aggressive solutions or pure water and the better will be its durability. Tests show that
the permeability of fly ash concrete is directly related to
the quantity of hydrated cementitious material at any
given time. After 28 days curing, by which time little pozzolanic activity would have occurred, fly ash concretes
are more permeable than plain concretes, but after 6
months curing this comparison is reversed. By then considerable imperviousness has developed.
Chemical attack—The main causes of concrete deterioration by chemical action are leaching of calcium hydroxide, acidic dissolution of cementitious hydrates, the
action of atmospheric and dissolved carbon dioxide, and
the reactivity of cement components with a variety of aggressive agents. Fly ash reduces such deterioration by reducing the long-term permeability of the concrete and,
through the pozzolanic reaction, by tying up the calcium
h yd roxide chemically.
Both Class F and Class C fly ashes have been found to
provide greatly improved sulfate resistance. Since the action of seawater on concrete is similar to that of ground
waters containing sulfate, fly ash used in such concrete
is expected to perform suitably.
Alkali-aggregate reactions—Sodium and potassium alkalis in certain cements react with the siliceous constituents of certain aggregates to form products of
Creep, modulus of elasticity and drying shrinkage
The rate of creep with time is quite similar for plain
concrete and concretes with fly ash contents of 15 percent or less. Howe ve r, at fly ash contents higher than 15
percent, slightly higher creep occurs.
In concretes of equal strength, with and without fly
ash, the concrete containing fly ash usually has a higher
ultimate modulus of elasticity. Howe ve r, this value may
be lower at early ages.
Fly ash in commonly used proportions does not generally influence the drying shrinkage of concrete significantly. Howe ve r, since drying shrinkage is a function of
the paste volume and since the addition of fly ash usually increases cement-paste volume, the drying shrinkage may be increased by a small amount if the water
content remains constant.
Acknowledgment
Information in this article is taken largely from “Fly Ash for
Use in Concrete,” by E. E. Berry and V. M. Malhotra—”A
Critical Review,” ACI Journal, March-April 1980, pages 5973, with some added data from “Quality Control of Highway
Concrete Containing Fly Ash,” by Woodrow J. Halstead,
NRMCA Publication No. 164, 14 pages, May 1981. Copies
of the latter only are available from National Ready Mixed
Concrete Association, 900 Spring Street, Silver Spring,
Maryland 20910.
PUBLICATION #C820417
Copyright © 1982, The Aberdeen Group
All rights reserved