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Classification of Fly ash

Classification of Fly ash ASTM C618 classifies Fly ash into two classes:

a) Class F

b) Class C

Two types of fly ash are commonly used in concrete: Class C and Class F. Class C are often high-calcium fly ashes with carbon content less than 2%; whereas, Class F are generally low-calcium fly ashes with carbon contents less than 5% but sometimes as high as 10%. In general, Class C ashes are produced from burning sub-bituminous or lignite coals and Class F ashes bituminous or anthracite coals. Performance properties between Class C and F ashes vary depending on the chemical and physical properties of the ash and how the ash interacts with cement in the concrete.

Class F Fly ash: Fly ash normally produced from burning anthracite or bituminous coal falls in this category. This class of Fly ash exhibits pozzolanic property but rarely if any, self-hardening property. In Class F Fly ash, total calcium typically ranges from 1 to 12 percent, mostly in the form of calcium hydroxide, calcium sulphate, and glassy components in combination with silica and alumina.

Class C Fly ash: Fly ash normally produced from lignite or sub-bituminous coal is the only material included in this category. This class of Fly ash has both pozzolanic and varying degree of self cementitious properties.