Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula CaSi(OH)(CO)(SO)·12HO, also sometimes more simply written as CaSiO·CaCO·CaSO·15HO.
Thaumasite is formed under special conditions in the presence of sulfate ions in concrete containing, or exposed to, a source of carbonate anions such as limestone aggregates, or finely milled limestone filler (). Bicarbonate anions () dissolved in groundwater may also contribute to the reaction. The detrimental reaction proceeds at the expense of calcium silicate hydrates (C-S-H, with dashes denoting here their non-stoichiometry) present in the hardened cement paste (HCP). The thaumasite form of sulfate attack (TSA) is a particular type of very destructive sulfate attack. C-S-H are the "glue" in the hardened cement paste filling the interstices between the concrete aggregates. As the TSA reaction consumes the silicates of the "cement glue", it can lead to a harmful decohesion and a softening of concrete. Expansion and cracking are more rarely observed. Unlike the common sulfate attack, in which the calcium hydroxide (portlandite) and calcium aluminate hydrates react with sulfates to respectively form gypsum and ettringite (an expansive phase), in the case of TSA the C-S-H ensuring the cohesion of HCP and aggregates are destroyed. As a consequence, even concrete containing low- sulfate-resisting Portland cement may be affected.
TSA is sometimes easily recognizable on the field when examining the altered concrete. TSA-affected concrete becomes powdery and can be dug with a scoop, or even scrapped with the fingers. Concrete decohesion is very characteristic of TSA.
TSA was first identified during the years 1990 in England in the United Kingdom in the foundation piles of bridges of the motorway M5 located in the Kimmeridgian marls. These marls are a mixture of clay and limestone sedimented under anoxic conditions and are rich in pyrite (, a  disulfide). Once these marls were excavated, pyrite was exposed to atmospheric oxygen or oxygen-rich infiltration water and rapidly oxidized. Pyrite oxidation produces sulfuric acid. In its turn,  reacts with portlandite (present in the hardened cement paste, HCP) and calcite ( (present in limestone aggregates or in carbonated HCP). The strong acidification of the medium caused by pyrite oxidation releases bicarbonate ions () or carbon dioxide () along with calcium () and sulfate ions (). 
Full pyrite oxidation can be schematized as: 
The sulfuric acid released by pyrite oxidation then reacts with portlandite ()) present in the hardened cement paste to give gypsum: 
When concrete also contains limestone aggregates or a filler addition,  reacts with calcite () and water to also form gypsum while releasing : 
Gypsum is relatively soluble in water , so there is plenty of calcium and sulfates ions available for TSA.
Simultaneously, carbonic acid () dissolves calcite to form soluble calcium bicarbonate: 
So, when all the chemical ingredients necessary to react with C-S-H from the hardened cement paste in concrete are present together the TSA reaction can occur. When grounds rich in pyrite, such as many clays or marls, are excavated for civil engineering works, the strong acidification produced by pyrite oxidation is the powerful driving force triggering TSA because it frees up and mobilizes all the ions needed to attack C-S-H and to form thaumasite ().
TSA is favored by a low temperature, although it can be encountered at higher temperature in warm areas. The reason is to be found in the retrograde solubility of most of the ingredients needed for the TSA reaction. Indeed, the solubility of dissolved carbon dioxide (), portlandite (), calcite (), and gypsum (·2), increases when the temperature is lowered. This is because the dissolution reactions of these mineral species are exothermic and release heat. A lower temperature facilitates the heat release and therefore favors the exothermic reaction. Only the solubility of silica (from C-S-H) increases with temperature because silica dissolution is an endothermic process which requires heat to proceed.