The role of calcium hydroxide in the formation of thaumasite

It has recently been derived by thermodynamic calculation that the presence or absence of calcium hydroxide plays a vital role in the resistance of cement paste or concrete against the formation of thaumasite. To obtain experimental data on this matter, special binders have been mixed and used for the preparation of mortar bars. These specimens were exposed to moderate sulphate attack for a period of 18 months at a temperature of 8 °C. Mortar bars containing calcium hydroxide showed visual signs of attack a few months after exposure, leading to expansion, mass loss and complete failure. In contrast to this, no signs of attack were observed when no calcium hydroxide was present in the microstructure. These results confirm the conclusions of earlier thermodynamic calculations that the presence of calcium hydroxide has an important impact on the formation of thaumasite. Calcium-rich C–S–H formed in the presence of calcium hydroxide is vulnerable against sulphate attack and the formation of thaumasite. In the absence of calcium hydroxide, C–S–H has a much lower calcium/silicon ratio and a higher resistance against the formation of thaumasite.