Thaumasite–ettringite solid solutions in degraded mortars

The thaumasite form of sulfate attack (TSA) has been observed in mortar prisms made from Portland–limestone cements after laboratory storage in 1.8% magnesium sulfate solution at 5 °C for 5 years. The prisms all showed evident signs of degradation, which increased with increasing limestone content. X-ray powder diffraction indicated that a solid solution was formed in all the prisms, which was based on the crystal structure of thaumasite, but extended towards the chemical composition of ettringite. The prism made from the cement with the highest level (35%) of limestone replacement gave the greatest amount of the thaumasite solid solution and had a composition close to the thaumasite end member, whereas those cements with lower (15%, 5% and 0%) levels of replacement gave reduced amounts of the solid solution and had greater lattice parameters. The solid solutions that have been observed are compared with those reported by Barnett et al. for synthetic thaumasite samples grown from sucrose solution.