Cement containing less tricalcium aluminate resists sulfate attack. Type II cement, for moderate sulfate conditions, can contain no more than 8% tricalcium aluminate by weight. Type V cement, for severe sulfate conditions, can be no more than 5% tricalcium aluminate. Theoretically, if sulfates do penetrate the concrete, they will have less tricalcium aluminate to react with, minimizing damage.
Denser concrete with fewer and smaller pores is less permeable to groundwater and sulfates that may be dissolved in it. Using a lower water/cement ratio makes the hardened product denser and less permeable, therefore “mechanically” resistant to attack.
Pozzolans like fly ash, silica fume, or high reactivity metakaolin, also help resist sulfate attack. Fly ash binds with free calcium hydroxide, thereby making it unavailable for sulfate reactions. It also makes the concrete denser.
Isolating concrete from groundwater also helps. Sulfate attack typically only occurs where the groundwater table is high or where there is improper drainage, so dissolved sulfates have access to the concrete. Standard construction practices for drainage and a moisture barrier under slabs help isolate concrete from external sulfates.