Q.: A new source of Class C fly ash is available. Would we use the same amount as the Class F that we have been using?
A.: It would be best to make some trial mixes. In general, the weight of Class F fly ash is usually more than the weight of cement replaced, whereas the weight of Class C fly ash used is usually less than the weight of cement replaced. Thus you'll probably need less Class C than the amount of Class F you've been using.