Fly ash is a byproduct of coal combustion that is captured by filters before flue gases reach chimneys. There are two main classes of fly ash - Class C and Class F. Class C contains more calcium and has self-cementing properties, while Class F is pozzolanic and requires an activator like cement. Using fly ash in concrete provides benefits like improved workability, reduced water demand and heat of hydration, and lower permeability. However, early strength is often reduced. Long-term strength is generally unaffected by fly ash use at typical replacement levels of 15-25%.

1. Assignment No 03
Subject :Properties ofConcrete
Submitted to: DR.ADNAN NAWAZ
Submitted by:ABDUL BAIS
Section: 5A
Reg No: FA15-CVE-025/WAH
Dated: 17-NOV-2017

2. FLY ASH:
Fly ash, also known as "pulverized fuel ash" in the United Kingdom, is a coal combustion
product composed of fine particles that are driven out of the boiler with the flue gases. Ash
that falls in the bottom of the boiler is called bottom ash. In modern coal-fired power plants, fly
ash is generally captured by electrostatic precipitators or other particle filtration equipment
before the flue gases reach the chimneys. Together with bottom ash removed from the bottom
of the boiler, it is known as coal ash. Depending upon the source and makeup of the coal being
burned, the components of fly ash vary considerably, but all fly ash includes substantial
amounts of silicon dioxide (SiO2) (both amorphous and crystalline), aluminum oxide (Al2O3)
and calcium oxide (CaO), the main mineral compounds in coal-bearing rock strata.
Differenttypesof Flyash are:
Accordingto ASTMFLY ASH are of two Classes:ClassCand ClassF.
Class F flyash:
The burningof harder,olderanthracite andbituminouscoal typically producesClassFflyash.Thisfly
ash ispozzolanicinnature,andcontainslessthan7% lime (CaO).Possessingpozzolanicproperties,the
glassysilicaandaluminaof ClassF flyashrequiresacementingagent,suchasPortlandcement,
quicklime,orhydratedlime—mixedwithwatertoreact andproduce cementitiouscompounds.

3. Alternatively,addingachemical activatorsuchassodiumsilicate (waterglass) toaClassF ash can form
a geopolymer.
Class C flyash:
Flyash producedfromthe burningof youngerlignite orsub-bituminouscoal,inadditiontohaving
pozzolanicproperties,alsohassome self-cementingproperties.Inthe presence of water,ClassCflyash
hardensandgets strongerovertime.ClassCflyash generallycontainsmore than20% lime (CaO). Unlike
ClassF, self-cementingClassCflyashdoesnot require anactivator.Alkali andsulfate (SO4) contentsare
generallyhigherinClassCflyashes.
Effects of Fly Ash on Properties of Fresh Concrete:
Workability:
The spherical shaped particles of fly ash act as miniature ball bearings within the concrete mix,
thus providing a lubricant effect. This same effect also improves concrete pump ability by
reducing frictional losses during the pumping process and flat work finish ability.
Decreased water demand. The replacement of cement by fly ash reduces the water demand
for a given slump. When fly ash is used at about 20 percent of the total cementitious, water
demand is reduced by approximately 10 percent. Higher fly ash contents will yield higher water
reductions. The decreased water demand has little or no effect on drying shrinkage/cracking.
Some fly ash is known to reduce drying shrinkage in certain situations.
Reduced heat of hydration:
Replacing cement with the same amount of fly ash can reduce the heat of hydration of concrete. This
reduction in the heat of hydration does not sacrifice long-term strength gain or durability. The reduced
heat of hydration lessens heat rise problems in mass concrete placements.
Lower early strength:
Fly ash concrete mixes typically result in lower strengths at early ages. The slower strength
gain may require forms to be strengthened to mitigate hydraulic loads. It should be noted that
form removal and opening to traffic may be delayed due to the slower strength gains. Lower
early strengths can be overcome by using accelerators.

4. Bleeding and Segregation:
Bleeding and Segregation is highly reduced by using Fly Ash. Bleeding is caused by
extra water in concrete mix but when we use fly ash less amount of water is required for
same workability so it reduce bleeding and as there is large amount of fines so
segregation is also prevented.
Effect of Fly Ash on Temperature Rise of Fresh Concrete:
The hydration or setting of Portland cement paste is accompanied by an evolution
Of heat that causes a temperature rise in fresh concrete. Replacement of cement by
Fly ash results in a reduction in the temperature rise in fresh concrete. This is of
Particular importance in mass concrete, where cooling following a large temperature
Rise can lead to cracking. The first major use of fly ash in concrete was in the
Construction of a gravity dam, where it was employed principally to control
Temperature rise
Effects on Hardened Concrete:
Effects of Fly Ash on Development of Concrete Strength
Amount of cement utilized in concrete and water to cement ratio are
considered major factors specify or control the strength of concrete. These
significant components of concrete are computed in practice based on cost,
hardened concrete durability and strength, and fresh concrete workability
which is required for conveying, placing, and costing the mixture of
concrete.

5. Fly ash concrete strength development is mainly affected by the degree and
way fly as influence the workability.
Moreover, it is demonstrated that using fly ash in concrete decrease
concrete water requirements and it neither cause issues in the selection of
mixture proportion nor prevent development of concrete strength at any
rate. There are different factors which influence fly ash-concrete strength
development such as particle size, chemical composition, fly ash properties,
reactivity, temperature and other condition of curing.
Effects of fly ash type on concrete strength
There are various types of fly ashes which their main difference is in amount
of calcium content that is an indicator to whether it is cementitious not.

6. ASTM Class C (high calcium) fly ash, which manufactured commonly from
burning lignitic sub-bituminous coal in power plants, can replace cement on
equal volume or weight basis and its effect on strength development of fly-
ash concrete is proven to be similar to cement controlled concrete.
ASTM Class F or low calcium fly ash is another type that is the first category
to be used as cement replacement fly ash. Fly ashes that were produced
from old power plants contained large grain size and unburned coal particles
that led fly ash to be less reactive and strength development rate were slow.
Long term concrete strength is not influenced by fly and it is shown that
Class F fly ash requires longer moist curing time compare with high calcium
fly ash.
Effects of Temperature and Curing Regime on Strength Development in
Fly-Ash Concretes
At standard temperature of 20Co
, early strength development of both cement
controlled and fly ash is not affected and lower strength is achieved by fly
ash concrete.
At high temperature curing, early strength development of both cement
controlled and fly ash concrete is increased significantly because of
increasing rate of hydration reaction.
However, high strength obtained at early ages is decrease at later for both
concrete and cement controlled concrete severely affected and its strength
decreased substantially by higher temperature. This might be due to uneven
distribution of hydrated products that increased pore sizes which in return
decrease strength.
Effects of Fly-Ash on Elastic Properties of Concrete
It is estimated that fly ash is slightly impact on concrete elastic properties.
There is small increase in modulus of elasticity of fly ash concrete compare
with cement controlled concrete provided the concrete have the same

7. strength with and without fly ash. Moreover, elastic modulus of fly ash
concrete is small at early ages but it is high at later ages.
Effects of Fly Ash on Creep Properties of Concrete
Creep is defined as internal strain which is related to the continuous applied
stress. It is demonstrated that the percentage of fly ash replacement
influence the effect of fly ash on hardened concrete for example, concrete
with up to 25 percent fly ash do not show considerable changes in fly ash
concrete compare with conventional concrete.
However, replacing cement of more than 25 percent will lead to increase the
total creep noticeably. It is thought that creep characteristic of concrete is
changed when percentage of fly ash replacing cement affect concrete
obtaining strength.
More importantly, there is a small reduction in basic creep of fly ash
concrete when loads are applied at early ages.
Effects of Fly Ash on Volume Changes of Concrete
It is claimed that, for the case where the concrete has normal sections such
walls and frame in buildings, the drying shrinkage at exposed surfaces of fly
ash concrete up to one year of age is slightly lesser or like conventional
concrete. However, at a short distance from the exposed surface the drying
shrinkage up to the age of one year is considerably less for cement
controlled concretes. For extremely thin sections, drying shrinkage of
concrete, which contained finely ground high-early-strength cements with
normal fineness, could be decreased by employing fly ash replacing cement
affect concrete obtaining strength.

9. Effects of Fly Ash on Permeability of Concrete:
The movement of aggressive solutions into a concrete mass or the removal
from

10. Concrete of dissolved reaction products must play a primary role in
determining
The rate of progress of concrete deterioration caused by chemical attack.
Permeability
Of a concrete mass is, therefore, fundamental in determining the rates of
Mass transport relevant to destructive chemical action. It should be
recognized that
All the cementitious hydrates and some of the aggregates from which
concretes are
Made are inherently subject to attack, not only by sulphates, chlorides,
acids, and
Organic agents, but by water alone. That concrete survives aqueous
environments
At all is attributable to (a) the low equilibrium solubility of the hydrated
components
And (b) the low rate of mass transfer in well-compacted, cured concrete.
Given any combination of cement and aggregate, it is generally observed
that the less permeable the concrete. The greater will be its resistance to
aggressive solutions
Or pure water.