Sulfides in concrete.
Damaging chemical reactions can occur with rock types containing sulfides, that lead to the formation of iron oxides and sulfate ions, which combine with the calcium-rich material of the cement paste to generate gypsum. If the concrete is extremely wet and sufficiently porous expansion and cracking may result.
Minerals which contain iron in the ferrous state and which readily undergo oxidation may occur in concrete aggregates. Iron pyrites is found in flint gravels and in some aggregates derived from mining waste. Oxidation takes place in the presence of moisture and atmospheric oxygen. This usually results in no greater damage than rust staining, and perhaps popping of concrete surfaces where the reactive aggregate is just below the surface. However, in extreme cases, disruption of the concrete may occur because of the attack on the cement paste by sulfates.