High-calcium fly ash with CaO above 20 percent - pozzolanic and self-cementing, for faster early strength.
Class C ash carries enough free lime to hydrate on its own, so it contributes to strength from day one instead of waiting for the calcium hydroxide released by the cement. That makes it the right choice where a project cannot afford the slower early gain of a Class F mix.
The trade-off is a shorter working window and sensitivity to sulphate exposure, so it is generally not recommended for marine or sulphate-rich ground without a durability assessment.