Fly Ash Concrete


Fly Ash

What is Fly Ash Historical Background Chemical Nature Classification
Chemical Composition Mechanical properties Benefits High Volume
Fly Ash
Need for extending Strength Specifications to beyond 28 days – Restraints on use of Fly Ash in Highway Constructions

Classification of Fly Ash

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 sub-bituminous coal and lignite.

Class F is generally low in lime, usually under 15 percent, and contains a greater combination of silica, alumina and iron (greater than 70 percent) than Class C fly ash.

Class C fly ash normally comes from coals which may produce an ash with higher lime content — generally more than 15 percent often as high as 30 percent. Elevated CaO may give Class C unique self-hardening characteristics.

Both types react in concrete in similar ways. Both Class F and Class C fly ashes undergo a “pozzolanic reaction” with the lime (calcium hydroxide) created by the hydration (chemical reaction) of cement and water, to create the same binder (calcium silicate hydrate) as cement.

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.

Although both types of fly ash impart a wide range of qualities to many types of concrete, they differ chiefly in the following ways:

Class F Fly Ash:

1. Most effectively moderates heat gain during concrete curing and is therefore considered an ideal cementitious material in mass concrete and high strength mixes. For the same reason, Class F is the solution to a wide range of summer concreting problems.

2. Class F is often recommended for use where concrete may be exposed to sulfate ions in soil and ground water.

Class C Fly Ash:

1. Most useful in “performance” mixes, prestressed applications, and other situations where higher early strengths are important.

2. Especially useful in soil stabilization since Class C may not require the addition of lime.

IS 3812 (Part 1):2003 ( Pulverized Fuel Ash — Specification for use as Pozzolana in Cement, Cement Mortar And Concrete) defines two types of Pulverized Fuel Ash viz.

(i) Siliceous Pulverized Fuel Ash — Pulverized fuel ash with reactive calcium oxide less than 10 %, by mass. Such fly ash is normally produced from burning anthracite or bituminous coal and has pozzolanic properties.

(ii) Calcareous Pulverized Fuel Ash — Pulverized fuel ash with reactive calcium oxide not less than 10 % by mass. Such fly ash is normally produced from lignite or sub-bituminous coal and have both pozzolanic and hydraulic properties.


Concrete-Techgroup thanks Headwaters Resources , USA for granting them permission to use their resources in developing this article.