What is the primary chemical process involved in sulfate attack on concrete?

The primary chemical process involved in sulfate attack on concrete is the formation of expansive minerals, primarily ettringite and, in some cases, gypsum, due to the reaction of sulfate ions with components of the hydrated cement paste. Sulfate attack occurs when sulfate ions from external sources, such as sulfate-rich soils or seawater, penetrate the concrete. These sulfate ions react with calcium hydroxide (CH), a byproduct of cement hydration, and tricalcium aluminate (C3A), a component of Portland cement. The reaction with C3A leads to the formation of ettringite, which is calcium sulfoaluminate hydrate ([Ca6Al2(SO4)3(OH)12·26H2O]). Ettringite is expansive, meaning it occupies a larger volume than the reactants. The formation of ettringite within the hardened cement paste causes internal stresses that lead to cracking, scaling, and disintegration of the concrete. In some cases, sulfate ions can also react with calcium hydroxide to form gypsum (CaSO4·2H2O), which is also expansive and contributes to the deterioration process. The expansion caused by ettringite and gypsum formation disrupts the cement matrix, weakening the concrete and making it more susceptible to further attack. For example, concrete foundations exposed to sulfate-bearing soils are prone to sulfate attack if the concrete mix is not designed to resist it. Using sulfate-resistant cement (Type V) with a low C3A content and incorporating supplementary cementitious materials (SCMs) like fly ash or slag can significantly improve the concrete's resistance to sulfate attack by reducing the amount of C3A available for reaction and decreasing the concrete's permeability, limiting sulfate ingress.