Thaumasite is a mineral formed by calcium carbonates, silicates and sulfates Ca3Si(OH)6(SO4)(CO3)·12H2O and is morphologically and structurally very similar to ettringite 3CaOAl2O33CaSO4.32H2O

A certain degree of controversy still exists around the formation of thaumasite and its possible connection with ettringite. The current state of knowledge can roughly be grouped together around the three following theories:

1) According to the first theory, thaumasite is formed through ettringite by substitution of aluminum for silicon.

2) In the second case, ettringite acts as a nucleator for the thaumasite formation.

3) According to the third theory, thaumasite could be formed directly without influence of ettringite in the process at all.