CaSi(OH)6·CaCO3·CaSO4·12H2O Thaumasite is a calcium silicate mineral, containing Si atoms in unusual octahedral configuration, with chemical formula Ca3Si(OH)6(CO3)(SO4)·12H2O, also sometimes more simply written as CaSiO3·CaCO3·CaSO4·15H2O.

It occurs as colorless to white prismatic hexagonal crystals, typically as acicular radiating groups.

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

[5] 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.

[4] The silicate structure of thaumasite is unusual due to the presence of non-tetrahedral silicon in its crystal lattice.