Magnesium sulfate's attack on cement‐based materials results in massive deposition of gypsum and brucite accompanied by decalcification of calcium silicate hydrate (CSH) (the binding material of the cement). Magnesium silicate hydrate (MSH) appears as a late‐stage product, mainly on deteriorated surfaces, and replaces the decalcified CSH. MSH appears as an inhomogeneous phase forming an intimate mixture with brucite. The Mg:Si ratio of the MSH is lower than the equivalent Ca:Si ratio of the CSH and is probably close to 1. The deterioration itself is primarily attributed to the decalcification of the CSH rather than its replacement by