Q.: We must furnish concrete made with Type II cement having a tricalcium aluminate content of 4 to 5 percent. This is to be used in a floor that must be sulfate resistant. Would it make the floor more sulfate resistant to increase the concrete design strength from 3000 to 4000 psi? Would using fly ash in the Type II cement mix help improve the sulfate resistance still further?

A.: The answer to both questions is yes. The Portland Cement Association's Design and Control of Concrete Mixtures, Eleventh Edition, Figure 22, page 32, affirms the first answer. The U.S. Bureau of Reclamation Concrete Manual, Eighth Edition, affirms the second answer in its Table 2 on page 11.