Class F fly ash is designated in ASTM C 618 and originates from anthracite and bituminous coals. It consists mainly of alumina and silica and has a higher LOI than Class C fly ash. Class F fly ash also has a lower calcium content than Class C fly ash. Replacement When used in portland cement, Class F fly ash can be used as a portland cement replacement ranging from 20-30% of the mass of cementitious material. Advantages When used as a portland cement replacement, Class F fly ash offers the following advantages when compared to unmodified portland cement: Increased late compressive strengths (after 28 days) Increased resistance to alkali silica reaction (ASR) Increased resistance to sulfate attack Less heat generation during hydration Increased pore refinement Decreased permeability Decreased water demand Increased workability Decreased cost ($80/ton for portland cement vs. $30/ton for fly ash).