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:

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:

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.