Answer: When Type II or V cement is specified, it's probably needed to improve sulfate resistance, since that's the primary use for the Type V cement. For concrete exposed to sulfates, ACI 318-95, Building Code Requirements for Structural Concrete, requires the use of Type II cement for moderate exposure, Type V cement for severe exposure and Type V plus a pozzolan for very severe exposure.Currently, ACI 318 allows substitution of blended cements made with fly ash or slag for Type II cement, as long as the cement in the blended cement has a C3A content below 8%. However, it doesn't allow direct substitution of a Type I cement combined with Class F fly ash for either Type II or Type V cement. That matter is currently being discussed within ACI, and these requirements may change in the future. In any case, you'll need to run laboratory sulfate resistance tests using the specified Type II or V cement and the Type I with Class F fly ash. If the concretes have equal sulfate resistances, your local building officials may be able to approve a particular combination of materials.