Q: We're going to be placing concrete slabs on grade in an area where the water soluble sulfate content of the soil is 0.4% (severe exposure). Is the use of Type V cement mandatory for this exposure?

A.: In selecting cements for sulfate resistance, a low C3A content is the main consideration. For severe exposures, Type V cement with a maximum C3A content of 5% is specified in Table 4.2.1 of the ACI Building Code (ACI 318-89). The code commentary also says that in certain areas the C3A content of other available types such as Type III or Type I may be less than 5% and that these cements are usable in severe exposures. Reducing permeability is the most effective way to increase concrete sulfate resistance and the simplest way to do this is to reduce water-cement ratio. A 1989 study showed that for concretes made with low (less than 0.40) or very high (greater than 0.60) water-cement ratios, C3A content of the cement didn't have a great effect on sulfate resistance. However, C3A content did affect sulfate resistance of concretes made with intermediate water-cement ratios (0.45 to 0.55).Reference David Stark, Durability of Concrete in Sulfate-rich Soils, Portland Cement Association, R&D Serial No. 097, 1990.