This document discusses the properties, classification, mechanism, and uses of fly ash. Fly ash is a byproduct of coal combustion in power plants. It is classified based on the type of coal burned and combustion temperature. Chemically, fly ash consists mainly of silicon dioxide, aluminum oxide, and iron oxide. Physically, it is a very fine spherical glassy powder. When mixed with cement, the silica in fly ash reacts with calcium hydroxide released during cement hydration to form additional calcium silicate hydrate, improving strength. Common uses of fly ash include as a cement additive, soil stabilizer, and in flowable fill, asphalt, bricks, and geopolymers.

1. Fly Ash – Properties, Types, Mechanism and Uses
Submitted By:-
Amit Payal-TO

2. Chemical Composition of Fly Ash
Physical Properties of Fly Ash
1. Fineness of Fly Ash
2. Specific Gravity of Fly Ash
3. Size and Shape of Fly Ash
4. Colour
Classification of Fly Ash
1. Type of Fly Ash as per IS Codes (IS 3812-1981)
A. Grade I
B. Grade II
2. Type of Fly Ash as per American Society for Testing and Materials (ASTM C618)
A. Type C
B. Type F
3. Type of Fly Ash based on boiler operations
A. Low temperature(LT) fly ash
B. High temperature(HT) fly ash
Mechanism of Fly Ash
Comparison of Requirements of Fly Ash in ASTM, EN and IS
Uses of Fly Ash
Contents

3. Chemical Composition of Fly Ash
Component
Bituminous
Coal
Sub bituminous
Coal
Lignite
Coal
SiO2 (%) 20-60 40-60 15-45
Al2O3 (%) 5-35 20-30 20-25
Fe2O3 (%) 10-40 4-10 4-15
CaO (%) 1-12 5-30 15-40
LOI (%) 0-15 0-3 0-5
The chemical composition of fly ash depends upon the type of coal used and the methods used for combustion of coal.
Table No 1: Chemical composition of fly ash of different coals.

4. Physical Properties of Fly Ash
1. Fineness of Fly Ash
As per ASTM, the fineness of the fly ash is to be checked in both dry n wet sieving. The fly ash sample is
sieved in 45 micron sieve and the percentage of retained on the 45 micron sieve is calculated. Further
fineness is also measured by LeChatelier method and Blaine Specific
Surface method.
2. Specific Gravity of Fly Ash
The specific gravity of fly ash ranges from a low value of 1.90 for a sub-bituminous ash to a high value of
2.96 for an iron-rich bituminous ash.
3. Size and Shape of Fly Ash
As the flyash is a very fine material, the particle size ranges in between 10 to 100 micron. The shape of the
fly ash is usually spherical glassy shaped.
4. Colour
The colour of the fly ash depends upon the chemical and mineral constituents. Lime content in the fly ash
gives tan and light colours where as brownish colour is imparted by the presence of iron content. A dark
grey to black colour is typically attributed to an elevated un-burned content.

5. Classification of Fly Ash
1. Type of Fly Ash as per IS Codes (IS 3812-1981)
A. Grade I
This grade of Fly ash is derived from bituminous coal having fractions SiO2+Al2O3+Fe2O3 greater than 70 %.
B. Grade II
This grade of Fly ash derived from lignite coal having fractions SiO2+Al2O3+Fe2O3 greater than 50 %.
2. Type of Fly Ash as per American Society for Testing and Materials (ASTM C618)
Depending on the type of coal and the resultant chemical analysis, ASTM has classified flyash into ,
A. Type C
Type C fly ash is produced from the combustion of lignite or sub bituminous coals, contains CaO higher than 10
percent and possesses cementitious properties in addition to pozzolanic properties.
B. Type F
Type F fly ash is produced from the combustion of bituminous or an anthracite coal contains CaO below 10 percent
and possesses pozzolanic properties.

6. Classification of Fly Ash
3. Type of Fly Ash based on boiler operations
A. Low temperature(LT) fly ash
It is produced when the combustion temperature is below 900o C
B. High temperature(HT) fly ash
It is generated out of combustion temperature below 1000o C

7. Mechanism of Fly Ash
Hydration
Process
Tricalcium
Silicate
+ Water =
Calcium
Silicate
Hydrate
+
Calcium
Hydroxide
Portland
cement
C3S + H = C-S-H + CH
Portland
Cement +
Fly Ash
S
(Silica+flya
sh)
+ CH = C-S-H
The chemistry of hydration of Portland cement is that about 50% of Portland cement is composed of the
primary mineral tri-calcium silicate, which on hydration forms calcium silicate hydrate and calcium
hydroxide.
If we have Portland cement, and the fly ash is the pozzolana , it can be represented by silica because non-
crystalline silica glass is the principal constituent of fly ash. The silica combines with the calcium hydroxide
released on hydration of Portland cement. Calcium hydroxide in hydrated Portland cement does not do
anything for strength, so therefore we use it up with reactive silica.
Slowly and gradually it forms additional calcium silicate hydrate which is a binder, and which fills up the
space, and gives us impermeability and more and more strength.
Table No 2: Hydration reaction of Portland cement and fly ash Portland cement

8. Comparison of Requirements of Fly Ash in ASTM,EN & IS
Properties ASTM C-618 En-450 En-197-I En-3892-I
IS 3812
2003- I
Sio2 minimum 35
Reactive/soluble Sio2, min. 25 25 20
Sio2+Al2O3+Fi2O3
minimum 70 70
MgO, Maximum 70
LOI(1hour)max 6 5-7 5-7 7 5
Total alkalis, max. 1.5 1.5
SO3, maximum 5 3 2 3
Free CaO, maximum 1 1
Total/reactive CaO,
maximum
10 10 10
Fineness, 45 micron,
maximum
34 40 12 34
Blaines fineness m2
/kg min.
320
Cement activity 28 days 75 75 80 80
Lime reactivity, N/mm2 4.5
Soundness, Le-Chatelier,
mm 10 10 10 10
Autoclave, Percent 0.8 0.8

9. Uses of Fly Ash
The major uses of fly ash are listed below-
•Used in the manufacture of Portland cement.
•Typically used for embankment construction.
•Used as a soil stabilisation material.
•Fly ash is also used as a component in the production of flow able fill.
•Used as the filler mineral in asphalt road laying to fill the voids.
•Fly ash is used as component in geoploymers.
•Used in Roller compacted concrete dams.
•Used in the manufacture of fly ash bricks
•When flyash is treated with silicon hydroxide, it acts as a catalyst.

10. Thanks