The slab in the base of a power plant cooling tower will be continuously subjected to sulfate water of concentrations that vary between 900 and 1500 parts per million. By reference to the U.S. Bureau of Reclamation Concrete Manual [Table 2, page 11] we see that Type II cement will be adequate for this exposure and Type V cement will not be needed. Our question is, will a float finish be adequate or must we steel trowel the slab for better resistance to sulfates?
Steel troweling would undoubtedly reduce the permeability of the concrete at the surface. If the concrete were to be subjected to only occasional and brief contact with sulfate water this lower permeability might be of some significance. However, the concrete will be continuously moist, and under this circumstance it would seem that the concrete will soon become saturated no matter whether the permeability at the surface is high or low. For this reason steel troweling would not seem to have any important effect on the sulfate resistance of the concrete slab.