Some aspects and mechanisms of sulphate attack

Consideration of the concrete durability should take into account sulphate attack which may be significantly various. The destruction of concrete under sulphate attack is particularly related to expansion, however, the dissolution of cement matrix may by considerable. The way of destruction is dependent on the type of salt, its cation and solution pH. The most reactive phases of cement paste are alumunoferrite hydrate phases (AFm) and calcium hydroxide. The expansion is directly connected with formation of gypsum and ettringite. The ultimate product of total destruction of concrete is gypsum. Mechanism of sulphate attack is a subject of discussion and hypotheses. The test results of expansion of mortar immersed in 5% Na₂SO₄ and SEM analysis indicate that ettrinite formed in air pores may not affect the value of the expansion. Ettringite formation inside small pores of the microstructure (eg C-S-H) can cause expansive force and destruction of mortars.