 They can be 'external' or 'internal'.This is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete.Sulfate ions that penetrate the concrete react with CH to form gypsum

ŜH+CH→CSH2 (Cement chemist notation)

C3A + 3CŜH2 + 26H→C3A.3CŜ.H32

tricalcium aluminate + gypsum → ettringite

when concentration of sulfate ions decrease the ettringite breaks down into monosulfates

2C3A + C3A.3CŜ.H32 → 3C3A.3CŜ.H12

When it reacts with concrete, it causes the slab to expand, lifting, distorting and cracking as well as applying pressure to surrounding walls which can mean structural movement, significantly weakening the concrete.