Mechanism of the internal sulfate reaction
Delayed ettringite formation in concrete structures - Mechanism, pathology and prevention

The phenomenon of internal sulfate reaction (ISR) results from the delayed formation of a mineral called "ettringite" with the chemical formula 3CaO.Al ₂ O ₃ .3CaSO ₄ .32H ₂ O.

This delayed formation of ettringite can lead to expansion, manifested by the appearance of relatively large-mesh, multidirectional cracking on the concrete surface.

However, ettringite is not systematically harmful to concrete, as it is a normal product of cement hydration. That's why we've provided the following information on the different types of ettringite found in concrete.