Occurrence of thaumasite in gypsum lime mortars for restoration

For restoration purposes, it is essential to have compatible mortars for historic masonry built originally with gypsum mortars. For that reason, gypsum lime mortars with high weathering resistance were developed. Due to the added hydraulic components and carbonate components of the used limes of these mortars, the occurrence of thaumasite can take place. Different mortar mixtures with variable binder contents, using lime with differing amounts of hydraulic components, were prepared and stored under two contrasting curing conditions. Over 90 days of curing, these mortars were investigated using physical–mechanical and chemical–mineralogical techniques. The results have shown that the potential for the growth of thaumasite depends on the composition of the raw materials, especially of the lime. In gypsum lime mortars which were used for restoration purposes, small amounts of thaumasite were also detected, but these small amounts of thaumasite do not reduce mechanical properties. This assumption was confirmed by investigations of gypsum lime mortars, which were exposed outdoors for 13 years.