The role of carbon dioxide in the formation of thaumasite

The development of the thaumasite form of sulfate attack (TSA) has received considerable attention since its discovery in the foundations of motorway bridges in England in 1998. When TSA occurs in siliceous aggregate concrete and mortars, particularly in samples from the field, it is normally assumed that the carbonate source necessary for the formation of thaumasite was either present as a minor component in the aggregate, as a limestone filler in the binder, or from the groundwater. Recent laboratory studies carried out by the authors have identified a further source of carbonate ions, and that is from atmospheric carbonation. However, in other studies, it appears that an initial air cure can improve the resistance of concretes to TSA. This apparent dichotomy suggests that there is insufficient understanding of the relationship between atmospheric carbonation and TSA.The performances of small mortar cubes made using fine aggregates of either high quality silica sand or limestone under different curing regimes have been compared. Detailed analyses of the nature of the thaumasite-ettringite solid solutions that formed have been carried out, and the mechanism of thaumasite formation, in particular, the role of calcium bicarbonate, is discussed in the light of the results obtained.