On the hydration of ternesite and the formation of thaumasite

The calcium sulfosilicate phase ternesite, previously considered as an undesired product in cement kilns, has recently gained attention due to its potential replacement of ye'elimite in calcium sulfoaluminate clinkers and the contribution of ternesite hydration products to the strength development of such clinkers. However, the potential formation of thaumasite during ternesite hydration, in particular in the presence of ettringite, has not yet been fully explored. In the present study synthesized ternesite was hydrated in the presence of ettringite, carbonated ettringite, portlandite and calcite. Thaumasite formed, together with gypsum and C-S-H, only in samples initially containing ettringite, both at 6.5 °C and at ambient temperature. Portlandite and calcite did not have a noticeable impact on the hydration of ternesite and the formation of thaumasite. The dissolution and/or carbonation of ettringite, together with its surface template effect, is suggested to be key to initiate thaumasite nucleation via the solid solution route.