This document discusses the use of fly ash as a partial replacement for cement in concrete. Fly ash is a byproduct of coal combustion in thermal power plants. Using fly ash in concrete can reduce costs, improve workability and durability, and provide environmental benefits by reducing the amount of cement needed. The document examines the chemical properties of fly ash and how it reacts with cement. It recommends using fly ash to replace up to 50% of cement in high-volume fly ash concrete, which can further improve sustainability and concrete performance.

1. Reciept No. 133
Utilisation of Fly Ash in Cement Concrete
Pramey M. Zode
S.E. (Civil),
Sinhgad Academy of Engineering,
Kondhwa(Bk.), Pune-48 (India)
E-mail: pramey.17@gmail.com
Abstract - To meet the ever increasing demand of electricity,
Thermal Power Plants (TPPs) are being set up all over the
world, thereby resulting into more consumption of the coal
in these plants. The disposal of ash derived from combustion
has become a major issue now-a-days. The study of Fly Ash,
as it is called, has found that it can be used in various civil
engineering applications such as bricks and concrete
making. This paper reviews the utilisation of Fly Ash as the
admixture in partial replacement of ordinary Portland
cement to upto 35%, and even more upto 50% in High-
Volume Fly Ash (HVFA) concrete which reduces the water
demand, improves the workability, minimizes cracking due
to thermal and drying shrinkage, and enhances durability to
reinforcement corrosion, sulphate attack, and alkali-silica
expansion. This admixing proves to be a best filler material
which also reduce overall cost of construction and act as an
eco-friendly material.
I.INTRODUCTION
Ash is a residue resulting from combustion of pulverised
coal or lignite in Thermal Power Plants (TPPs). About 80% of
total ash is in finely divided form which is carried away with
flue gases and is collected by electrostatic precipitator or other
suitable technology. This ash is called Fly Ash or chimney
Ash or Hopper Ash. In an industrial context, fly ash usually
refers to ash produced as an industrial by-product during
combustion of coal in TPPs.
Fly ash is a fine (85% of its mass passing through a 45µm
screen), pozzolaneous or a siliceous and/or aluminous glassy
powdered material having micron-sized earth elements, which
in the presence of water and lime, will react to form a
cementitous material. It consists of inorganic materials mainly
silica and alumina with some quantum of organic material in
the form of unburnt carbon. Fly ash also contains
environmental toxins in significant amounts, including arsenic
(43.4 ppm); barium (806 ppm); beryllium (5 ppm); boron (311
ppm); cadmium (3.4 ppm); chromium (136 ppm); chromium
VI (90 ppm); cobalt (35.9 ppm); copper (112 ppm); fluorine
(29 ppm); lead (56 ppm); manganese (250 ppm); nickel
(77.6ppm); selenium (7.7 ppm); strontium (775 ppm);
thallium (9 ppm); vanadium (252 ppm); and zinc (178 ppm).
II.SOURCES OF FLY ASH IN INDIA
According to National Thermal Power Corporation
(NTPC), coal is used for approximately 62.3% of electric
power generation in India. According to Central Electricity
Authority of India, there are around 83 major coal fired
thermal power plants in India. As per the Ministry of Power
Statistics, the total installed generating capacity (TPPs) is
about 79838 MW. In addition to this, there are more than
1800 selected industrial units which have TPPs of >1MW
capacity. These are the chief sources of fly ash in India.
III.ASH CONTENT IN INDIAN COAL
The quality of coal depends upon its rank and grade. The
coal rank arranged in an ascending order of carbon contents is:
Lignite --> sub-bituminous coal --> bituminous coal -->
anthracite
Indian coal is of mostly sub-bituminous rank, followed by
bituminous and lignite (brown coal). Thus, the ash content in
Indian coal ranges from 35% to 50%.
IV.CURRENT FLY ASH GENERATION IN INDIA
The current electricity generation in India is about 1,12,058
MW, 65-70% of which is thermal (mostly coal based).
According to an estimate 100,000 MW capacity or more
would be required in the next 10 years due to continually
increasing demand for electricity. Thus, the present fly ash
generation in India is around 110 million tonnes / year and is
set to continue at a high rate into the foreseeable future.
V.CURRENT FLY ASH UTILISATION
According to the MOEF Gazette Notification dated Sept.
14, 1999, the then existing power stations were to achieve
20% ash utilization within three years and 100% utilization in
15 years from the date of notification. New Stations were to
achieve 30% ash utilization within 9 years at the rate of 10%

2. ash utilization within 3 years. Presently, out of 110 million
tonnes of total ash generated, only about 30% is being
utilized. Therefore thermal power stations are under great
pressure to find useful applications of fly ash. The technology
utilizing fly ash in high volume fly ash concrete can provide
an avenue for utilization of fly ash on a bulk scale.
VI. PROBLEMS DUE TO FLY ASH
Fly ash is a very fine powder and tends to travel far in the
air. When not properly disposed, it is known to pollute air and
water, and causes respiratory problems when inhaled. When it
settles on leaves and crops in fields around the power plant, it
lowers the yield. The conventional method used for disposal
of both fly ash and bottom ash is to convert them into slurry
for impounding in ash ponds around the thermal plants. This
method entails long-term problems. The severe problems that
arise from such dumping are:
1) The construction of ash ponds requires vast tracts of land.
This depletes land available for agriculture over a period of
time.
2) When one ash pond fills up, another has to be built, at great
cost and further loss of agricultural land.
3) Huge quantities of water are required to convert ash into
slurry. During rains, numerous salts and metallic content in
the slurry can leach down to the groundwater and contaminate
it.
Taking into account these facts, fly ash is being used in
various construction activities as a raw material. In this paper,
detailed study of use of fly ash in raw materials like Portland
cement is considered. Further this paper reviews the use of
high volume fly ash in cement making for better yield.
FLY ASH BASED POZZOLANA PORTLAND
CEMENT
I.POZZOLANS
Pozzolans are defined as silicious and aluminous materials
which in themselves possess little or no cementitious value
but in finely divided form and in the presence of moisture, it
chemically react with calcium hydroxide at ordinary
temperature to form compounds possessing cementitious
properties.
II.CLASS F FLY ASH
The burning of harder, older anthracite and bituminous
coal typically produces Class F fly ash. This fly ash is
pozzolanic in nature, and contains less than 20% lime (CaO).
Possessing pozzolanic properties, the glassy silica and
alumina of Class F fly ash requires a cementing agent, such
Portland cement, quicklime, or hydrated lime, with the
presence of water in order to react and produce cementitious
compounds.
Most of the state and federal specifications allow, and even
encourage, the use of Fly Ash; especially, when specific
durability requirements are needed. Fly Ash has a long history
of use in concrete. Fly Ash is used in about 50% of ready
mixed concrete. Class C Fly Ash is used at dosages of 15 to
40% by mass of the cementitious materials in the concrete.
Class F is generally used at dosages of 15 to 30%.
III.FLY ASH IN PORTLAND CEMENT
Owing to its pozzolanic properties, fly ash is used as a
replacement for some of the Portland cement content of
concrete. The use of fly ash as a pozzolanic ingredient was
recognized as early as 1914, although the earliest noteworthy
study of its use was in 1937.Before its use was lost to the Dark
Ages, Roman structures such as aqueducts or the Pantheon in
Rome used volcanic ash (which possesses similar properties to
fly ash) as pozzolan in their concrete. As pozzolan greatly
improves the strength and durability of concrete, the use of
ash is a key factor in their preservation.
Use of fly ash as a partial replacement for Portland cement
is generally limited to Class F fly ashes. It can replace up to
30% by mass of Portland cement, and can add to the final
strength of concrete and increase its chemical resistance and
durability. Recently concrete mix design for partial cement
replacement with High Volume Fly Ash (50 % cement
replacement) has been developed. For Roller Compacted
Concrete (RCC) [used in dam construction] replacement
values of 70% have been achieved with processed fly ash at
the Ghatghar Dam project in Maharashtra, India. Due to the
spherical shape of fly ash particles, it can also increase
workability of cement while reducing water demand. The
replacement of Portland cement with fly ash is considered by
its promoters to reduce the greenhouse gas "footprint" of
concrete, as the production of one ton of Portland cement
produces approximately one ton of CO2 as compared to zero
CO2 being produced using existing fly ash. New fly ash
production, i.e., the burning of coal, produces approximately
twenty to thirty tons of CO2 per ton of fly ash. Since the
worldwide production of Portland cement is expected to reach
nearly 2 billion tons by 2012, replacement of any large portion
of this cement by fly ash can significantly reduce carbon
emissions associated with construction.

3. Inclusion of Fly Ash in Portland cement based plastic
concrete mixes improves concrete workability by reducing the
water content for a given consistency. The spherical particles
create a ‘ball bearing’ effect in the mix – thus improving
workability. Fly Ash particles also fill voids in the mix which
reduces the water requirement for a given plastic consistency.
Workable Fly Ash concrete, places easier, finishes better and
produces better ‘off-form’ surfaces than plain Portland cement
concrete. For use in concrete, Fly Ash is referred to as a
‘supplementary cementitious material’.
IV.CHEMICAL COMPARISION OF FLY ASH AND
PORTLAND CEMENT
The chemical composition of fly ash is very similar to that
of portland cement.
TABLE I
TYPICAL CHEMICAL COMPOUNDS
IN POZZOLANIC CLASS F FLY ASH AND PORTLAND CEMENT
Chemical
compound
Class F fly ash Cement
SiO 54.90 2.60
Al2O3 25.80 4.30
Fe2O3 6.90 2.40
CaO 8.70 64.40
MgO 1.80 2.10
SO2 0.60 2.30
Na2O & K2O 0.60 0.60
The table above shows typical compound analysis for
Class F fly ash and ordinary portland cement. A glance at the
table reveals:
1. The same compounds exist in fly ash and portland cement.
Those of fly ash are amorphous (glassy) due to rapid cooling;
those of cement are crystalline, formed by slower cooling.
2. The major difference between fly ash and portland cement
is the relative quantity of each of the several compounds in
them. Portland cement is rich in lime (CaO) while fly ash is
low. Fly ash is rich in reactive silicates while Portland cement
has smaller amounts.
Portland Cement + Water Calcium Silicate Hydrate
Free Lime (CaOH)
Portland Cement + Water
+ Fly Ash Calcium Silicate Hydrate
Portland cement is manufactured with CaO, some of which
is released in a free state during hydration. As much as 20
pounds of free lime is released during hydration of 100
pounds of cement. This liberated lime forms the necessary
ingredient for reaction with fly ash silicates to form strong and
durable cementing compounds no different from those formed
during hydration of ordinary Portland cement. A review of the
chemistry of both materials makes it apparent that a blend of
the two will enhance the concrete product and efficiently
utilize the properties of both.
V.ADVANTAGES OF FLY ASH BASED PORTLAND
CEMENT
A.Fly Ash improves concrete workability and lowers water
demand
Fly Ash particles are mostly spherical tiny glass beads.
Ground materials such as Portland Cement are solid angular
particles. Fly Ash particles provide a greater workability of
the powder portion of the concrete mixture which results in
greater workability of the concrete and a lowering of water
requirement for the same concrete consistency. Pump ability
is greatly enhanced.
B.Fly Ash generally exhibit less bleeding and
segregation than plain concretes
This makes the use of Fly Ash particularity valuable in
concrete mixtures made with aggregates deficient in fines.
C.Sulphate and Alkali Aggregate Resistance
Class F and a few Class C Fly Ashes impart significant
sulphate resistance and alkali aggregate reaction resistance to
the concrete mixture.
D.Fly Ash has a lower heat of hydration
Portland cement produces considerable heat upon
hydration. In mass concrete placements the excess internal
heat may contribute to cracking. The use of Fly Ash may
greatly reduce this heat build up and reduce external cracking.
F.Fly Ash generally reduces the permeability and adsorption
of concrete
By reducing the permeability of chloride ion, corrosion of
embedded steel is greatly decreased. Also, chemical resistance
is improved by the reduction of permeability and adsorption.

4. G.Fly Ash is economical
The cost of Fly Ash is generally less than Portland Cement
depending on transportation. Significant quantities may be
substituted for Portland Cement in concrete mixtures and thus
increase the long term strength and durability. Thus, the use of
Fly Ash may impart considerable benefits to the concrete
mixture over a plain concrete for less cost.
HIGH-VOLUME FLY ASH (HVFA) CONCRETE
Fly Ash has a vast potential for use in High Volume Fly
Ash (HVFA) concrete especially due to its physic-chemical
properties. Considerable amount of research has already been
done in India and abroad on its strength and other requisite
parameters. In commercial practice, the dosage of fly ash is
limited to 15%-20% by mass of the total cementitious
material. Usually, this amount has a beneficial effect on the
workability and cost economy of concrete but it may not be
enough to sufficiently improve the durability to sulphate
attack, alkali-silica expansion, and thermal cracking. Thus,
from theoretical considerations and practical experience it is
established that, with 50% or more cement replacement by fly
ash, it is possible to produce sustainable, high performance
concrete mixtures that show high workability, high ultimate
strength, and high durability.
I.CHARACTERISTICS DEFINING HVFA CONCRETE
MIXTURE
The characteristics defining a HVFA concrete mixture are
as follows:
1) Minimum of 50% of fly ash by mass of the cementitious
materials must be maintained.
2) Low water content, generally less than 130 kg/m3
is
mandatory.
3) Cement content, generally no more than 200kg/m3
is
desirable.
4) For concrete mixtures with specified 28-day compressive
strength of 30 MPa or higher, slumps greater than 150 mm,
and water-to-cementitious materials ratio of the order of 0.30,
the use of high range water-reducing admixtures
(superplasticizers) is mandatory.
5) For concrete exposed to freezing and thawing
environments, the use of an air-entraining admixture resulting
in adequate air-void spacing factor is mandatory.
6) For concrete mixtures with slumps less than 150 mm and
28-day compressive strength of less than 30 MPa, HVFA
concrete mixtures with a water-to-cementitious materials ratio
of the order of 0.40 may be used without superplasticizers.
II.MECHANISMS BY WHICH FLY ASH IMPROVES THE
PROPERTY OF CONCRETE
A good understanding of the mechanisms by which fly ash
improves the rheological properties of fresh concrete and
ultimate strength as well as durability of hardened concrete is
helpful to insure that potential benefits expected from HVFA
concrete mixtures are fully realized. These mechanisms are
discussed next:
A.Fly ash as a water reducer
There are two reasons why typical concrete mixtures
contain too much mixing-water. Typical concrete mixtures do
not have an optimum particle size distribution, and this
accounts for the undesirably high water requirement to
achieve certain workability. Secondly, to plasticize a cement
paste for achieving a satisfactory consistency, much larger
amounts of water than necessary for the hydration of cement
have to be used because portland cement particles, due to the
presence of electric charge on the surface, tend to form flocs
that trap volumes of the mixing water. It is generally observed
that a partial substitution of portland cement by fly ash in a
mortar or concrete mixture reduces that water requirement for
obtaining a given consistency. Experimental studies have
shown that with HVFA concrete mixtures, depending on the
quality of fly ash and the amount of cement replaced, up to
20% reduction in water requirements can be achieved. This
means that good fly ash can act as a superplasticizing
admixture when used in high-volume. The phenomenon is
attributable to three mechanisms. First, fine particles of fly ash
get absorbed on the oppositely charged surfaces of cement
particles and prevent them from flocculation. The cement
particles are thus effectively dispersed and will trap large
amounts of water, that means that the system will have a
reduced water requirement to achieve a given consistency.
Secondly, the spherical shape and the smooth surface of fly
ash particles help to reduce the interparticle friction and thus
facilitates mobility. Thirdly, the “particle packing effect” is
also responsible for the reduced water demand in plasticizing
the system. It may be noted that both portland cement and fly
ash contribute particles that are mostly in the 1 to 45 µm size
range, and therefore serve as excellent fillers for the void
space within the aggregate mixture. In fact, due to its lower
density and higher volume per unit mass, fly ash is a more
efficient void-filler than portland cement.

5. B.Drying shrinkage
Perhaps the greatest disadvantage associated with the use
of neat portland-cement concrete is cracking due to drying
shrinkage. The drying shrinkage of concrete is directly
influenced by the amount and the quality of the cement paste
present. It increases with an increase in the cement paste-to-
aggregate ratio in the concrete mixture, and also increases
with the water content of the paste. Clearly, the water-
reducing property of fly ash can be advantageously used for
achieving a considerable reduction in the drying shrinkage of
concrete mixtures. Table 2 shows mixture proportions of a
conventional 25 MPa concrete compared to a superplasticized
HVFA concrete with similar strength but higher slump. Due to
a significant reduction in the water requirement, the total
volume of the cement paste in the HVFA concrete is only
25% as compared to 29.6% for the conventional portland-
cement concrete which represents a 30% reduction in the
cement paste-to-aggregate volume ratio.
TABLE 2
COMPARISION OF CEMENT PASTE VOLUMES
Conventional
concrete
HVFA
concrete
kg/m3
m3
kg/m3
m3
Cement 307 0.098 154 0.149
Fly ash - - 154 0.065
Water 178 0.178 120 0.120
Entrapped air
(2%)
- 0.020 - 0.020
Course aggregate 1040 0.385 1210 0.448
Fine aggregate 825 0.305 775 0.287
Total 2350 0.986 2413 0.989
w/cm 0.58 - 0.39 -
Paste volume - 0.296 - 0.254
Paste percent - 30.0% - 25.7%
C.Thermal cracking
Thermal cracking is of serious concern in massive concrete
and reinforced concrete structures. For unreinforced mass-
concrete construction, several methods are employed to
prevent thermal cracking, and some of these techniques can be
successfully used for mitigation of thermal cracks in massive
reinforced-concrete structures. For instance, a 40-MPa
concrete mixture containing 350 kg/m3
portland cement can
raise the temperature of concrete by approximately 55-60o
C
within a week if there is no heat loss to the environment.
However, with a HVFA concrete mixture containing 50%
cement replacement with a Class F fly ash, the adiabatic
temperature rise is expected to be 30-35o
C.
D.Water-tightness and durability
In general, the resistance of a reinforced-concrete structure
to corrosion, alkali aggregate expansion, sulphate and other
forms of chemical attack depends on the water-tightness of the
concrete. The water-tightness is greatly influenced by the
amount of mixing-water, type and amount of supplementary
cementing materials, curing, and cracking resistance of
concrete. High-volume fly ash concrete mixtures, when
properly cured, are able to provide excellent water-tightness
and durability. The mechanisms responsible for this
phenomenon are discussed briefly below.
When a concrete mixture is consolidated after placement,
along with entrapped air, a part of the mixing-water is also
released. As water has low density, it tends to travel to the
surface of concrete. However, not all of this “bleed water” is
able to find its way to the surface. Due to the wall effect of
coarse aggregate particles, some of it accumulates in the
vicinity of aggregate surfaces, causing a heterogeneous
distribution of water in the system. Obviously, the interfacial
transition zone between the aggregate and cement paste is the
area with high water/cement and therefore with more available
space that permits the formation of a highly porous hydration
product containing large crystals of calcium hydroxide and
ettringite. Microcracks due to stress are readily formed
through this product because it is much weaker than the bulk
cement paste with a lower water/cement. It has been suggested
that microcracks in the interfacial transition zone play an
important part in determining not only the mechanical
properties but also the permeability and durability of concrete
exposed to severe environmental conditions. This is because
the rate of fluid transport in concrete is much larger by
percolation through an interconnected network of microcracks
than by diffusion or capillary suction. The heterogeneities in
the microstructure of the hydrated portland-cement paste,
especially the existence of large pores and large crystalline
products in the transition zone, are greatly reduced by the
introduction of fine particles of fly ash. With the progress of
the pozzolanic reaction, a gradual decrease occurs in both the
size of the capillary pores and the crystalline hydration
products in the transition zone, thereby reducing its thickness
and eliminating the weak link in the concrete microstructure.
In conclusion, a combination of particle packing effect, low
water content, and pozzolanic reaction accounts for the
eventual disappearance of the interfacial transition zone in
HVFA concrete, and thus enables the development of a highly
crack-resistant and durable product.

6. III.PROPERTIES OF HVFA CONCRETE
Based on field experience and laboratory tests, the
properties of HVFA concrete, when compared to conventional
portland cement concrete, can be summarized as follows:
1) Easier flowability, pumpability, and compactability.
2) Better surface finish and quicker finishing time when
power finish is not required.
3) Slower setting time, which will have a corresponding effect
on the joint cutting and lower power-finishing times for slabs.
4) Early-strength up to 7 days, which can be accelerated with
suitable changes in the mix design when earlier removal of
formwork or early structural loading is desired.
5) Much later strength gain between 28 days and 90 days or
more. (With HVFA concrete mixtures, the strength
enhancement between 7 and 90-day often exceeds 100%,
therefore it is unnecessary to over design them with respect to
a given specified strength.)
6) Superior dimensional stability and resistance to cracking
from thermal shrinkage, autogenous shrinkage, and drying
shrinkage.
7) After three to six months of curing, much higher electrical
resistivity, and resistance to chloride ion penetration,
according to ASTM Method C1202.
8) Very high durability to the reinforcement corrosion, alkali-
silica expansion, and sulphate attack.
9) Better cost economy due to lower material cost and highly
favorable lifecycle cost.
10) Superior environmental friendliness due to ecological
disposal of large quantities of fly ash, reduced carbon-dioxide
emissions, and enhancement of resource productivity of the
concrete construction industry.
CONCLUSION
The study of Fly Ash has shown that owing to its numerous
advantageous physical and chemical properties, the material is
found to be one of the best admixtures in Portland cement
concrete and High Volume Fly Ash (HVFA) concrete making
which improves not only the quality but also its workability
subjected to various parameters. Moreover, the fly ash
concrete offers a holistic solution to the problem of fly ash
disposal which is one of the major issues now-a-days that too
in a sustainable manner, at a reduced or no additional cost and
at the same time reducing the environmental impact of two
industries that are vital to economic development namely the
cement industry and the coal-fired power industry. Thus it
also favours the Green Technology and waste management
which in turn helps in sustainable development.
ACKNOWLEDGMENT
I wish to acknowledge the instructive guidance of Prof.
R.B. Bajare, Asst. Professor and Prof. Dr. S. R. Parekar,
HOD, Dept. of Civil Engineering, Sinhgad Academy of
Engineering.
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[2] 'High Volume Fly-Ash Concrete Technology', Fly Ash Summary Report in
India by Canadian International Development Agency (CIDA)
[3] P. Kumar Mehta, "HIGH-PERFORMANCE, HIGH-VOLUME FLY ASH
CONCRETE FOR SUSTAINABLE DEVELOPMENT", International
Workshop on Sustainable Development and Concrete Technology, University
of California, Berkeley, USA
[4] http://www.ashgroveresources.com/
[5] Fly ash, From Wikipedia, the free encyclopedia
[6] C. N. Jha & J. K. Prasad “FLY ASH: A RESOURCE MATERIAL FOR
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