On the formation of thaumasite CaSiO₃·CaSO₄·CaCO₃·15H₂O: Part II

The thermodynamics of the formation of thaumasite in concrete subjected to sulphate attack were investigated in this paper. This second part is intended to perform calculations based on the value of Gibbs free energy that wasestablished in the first part. The impact of phases that can be present in the hardened cement paste on the formation of thaumasite was evaluated by modelling the relevant reactions. It was determined that thaumasite can be formed from a mixture of gypsum, calcite, portlandite, quartz and water. C—S—H phases can also react with gypsum and calcite to form thaumasite. Pure carbon and cement pastes seem to be immune. It is indicated by the calculations that thaumasite can not be formed from a reaction of C—S—H phases with ettringite and calcite. A precipitation of thaumasite is estimated to be possible at very low sulphate concentrations from an aggressive environment, even below those that are specified as moderate sulphate attack.