How does concrete resist sulfate attack?

Concrete resists sulfate attack through a combination of material composition and chemical resistance. The primary defense lies in using sulfate-resistant cement, which contains low levels of tricalcium aluminate (C3A) – typically less than 5%. This reduces the formation of expansive ettringite when sulfates penetrate the concrete.

The permeability of concrete plays a crucial role. Dense, low-porosity concrete with a low water-cement ratio (below 0.45) significantly limits sulfate penetration. Supplementary cementitious materials like fly ash or slag can enhance resistance by refining pore structure and consuming calcium hydroxide.

Chemical reactions also contribute to protection. In high-sulfate environments, concrete forms a protective layer of gypsum on its surface, slowing further sulfate ingress. Some modern concrete mixes incorporate mineral admixtures that chemically bind sulfates before they can cause damage.

Proper curing and construction practices ensure complete hydration of cement particles, creating a more impermeable matrix. In extreme conditions, surface treatments or waterproof coatings provide additional protection against sulfate penetration. These combined strategies make modern concrete remarkably resistant to sulfate attack in most environments.