Class C fly ash is designated in ASTM C 618 and originates from subbituminous and lignite coals. Its composition consists mainly of calcium, alumina, and silica with a lower loss on ignition (LOI) than Class F fly ash. Additional chemical properties are listed in Table 1.

Table 1. Class C fly ash Chemical Composition
Requirements (ASTM C618), %
SiO2 plus Al2O3 plus Fe2O3, min
SO3, max
Moisture content, max
Loss on Ignition, max

When used in portland cement, Class C fly ash can be used as a portland cement replacement ranging from 20-35% of the mass of cementitious material.


When used as a portland cement replacement, Class C fly ash offers the following advantages when compared to unmodified portland cement:


When using Class C fly ash as a portland cement replacement, it is important to know several precautions. The time of set may be slightly delayed. Also, the fine aggregate fraction of the concrete will need to be modified because fly ash has a lower bulk specific gravity than does portland cement and therefore occupies more volume for the same mass. Class C fly ash must replace at least 25% of the portland cement to mitigate the effects of alkali silica reaction. If using any organic admixtures such as air entrainment, the amount added must be modified since the carbon (LOI) in the fly ash adsorbs organic compounds. Finally, if the fly ash has a high calcium content, it should not be used in sulfate exposure applications. When using this or any other alternative cementing material with portland cement, it is necessary to create trial mixtures to ensure proper proportioning for the desired properties.