Selected Class F with loss on ignition held below 2 percent - for air-entrained concrete and any mix where carbon is a problem.
Unburnt carbon in fly ash adsorbs air-entraining admixture, which is why a high-LOI ash can wreck an air-void system and, with it, freeze-thaw durability. This grade is drawn only from units running high combustion efficiency and is screened lot by lot to keep LOI under 2 percent.
It is the grade to specify for air-entrained pavement concrete, architectural concrete where colour consistency matters, and any mix where admixture dosage has to stay predictable.