Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca₃ Si(O H)₆( C O₃)( SO₄)·12 H₂O, also sometimes more simply written as CaSiO₃·CaCO₃·CaSO₄·15H₂O. It occurs as colorless to white prismatic hexagonal crystals, typically as acicular radiating groups. It also occurs as fibrous masses. Its Mohs hardness is 3.5 and it has a specific gravity of 1.88 to 1.90. Optically it is uniaxial negative with indices of refraction of nω = 1.507 and nε = 1.468. Thaumasite-251460

It occurs as a hydrothermal alteration mineral in sulfide ore deposits and geothermal alteration of basalt and tuff. It occurs with zeolites, apophyllite, analcime,

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

, gypsum and pyrite. Thaumasite can also be formed in man-made concrete structures at the detriment of calcium silicate hydrates (C-S-H, with dashes denoting the non-

stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equal ...

of this hydrated cement phase acting as the "glue" in hardened cement paste) during cement alteration, especially when

sulfate attack Cement hydration and strength development mainly depend on two silicate phases: tricalcium silicate (C3S) (alite), and dicalcium silicate (C2S) (belite). Upon hydration, the main reaction products are calcium silicate hydrates (C-S-H) and calciu ...

develops. The reaction consuming the silicates of the "cement glue" can lead to harmful decohesion and softening (more rarely to expansion and cracking) of concrete. Unlike conventional

, in which the

calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...

( portlandite) and

calcium aluminate Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

hydrates react with sulfates to form gypsum and ettringite (an expansive phase) respectively, in the case of the thaumasite form of sulfate attack (TSA) the calcium silicate hydrates ensuring the cohesion in the hardened cement paste are also destroyed. As a consequence, even concrete containing sulfate-resisting Portland cement may be affected. It was first described in 1878 in Sweden and named from the Greek, "thaumazein", ''to be surprised,'' in reference to its unusual composition with carbonate, sulfate and hydroxysilicate anions. The silicate structure of thaumasite is unusual due to the presence of non-tetrahedral silicon in its

crystal lattice In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by : \mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n ...

. Indeed, an atypic octahedral configuration is observed for Si present in thaumasite in the form of hexahydroxysilicate: i(OH)₆sup>2−, a species exhibiting a geometry similar to that of the hexafluorosilicate iF₆sup>2−.

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Thaumasite form of sulfate attack (TSA)
Calcium minerals Carbonate minerals Cement Concrete Hexagonal minerals Minerals in space group 173 Hydrates Luminescent minerals Silicate minerals Sulfate minerals

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