The sands in our area are mainly river sands that are deficient in very fine particles (material that passes the No. 50 sieve). Concrete made with these sands is hard to finish, especially when a steel trowel is required. Our ready mixed concrete supplier recommends that we use a concrete with fly ash added to improve finishability. However, I've heard that fly ash retards the setting of concrete. Is it possible to add fly ash without affecting the concrete setting time?
Fly ash doesn't chemically retard setting. But many times fly ash is used in concrete as a cement replacement in combination with a water reducer. These concretes may set more slowly than a straight cement mix would. If fly ash is not being used as a cement replacement, but is added just to increase fines, it won't retard setting. It simply serves as an additional fine aggregate and makes the concrete easier to finish, easier to pump, and less likely to bleed excessively. If concrete is air-entrained, you'll probably need more air-entraining agent to get the same air content as a concrete that doesn't contain fly ash.