Salt efflorescence as indicator for sources of damaging salts on historic buildings and monuments: a statistical approach

The results of XRD analyses of salt types found in more than 300 samples of efflorescence on buildings and monuments in Saxony (Germany) have been statistically evaluated. The major salt types are gypsum, magnesium sulphates, and sodium sulphates. The frequency of other alkali sulphates as well as of alkali carbonates, which are normally not detected in routine chemical analyses, is also remarkable. Nitrates (niter, nitratine) were found in about 15% of all investigated samples, whereas the chloride halite was only detected in 6% of all samples. The results correspond well to those for other regions in southern and central Germany. Efflorescing salts are only to a limited extent representative for the mixture of ions in a substrate. A thorough analysis of the salt system with respect to its damage potential needs quantitative (destructive) chemical analysis of anions and cations. However, analysis of efflorescence offers a non-destructive approach to assess the potential sources of frequently crystallising salts. The high frequency of sulphate salts indicates the long-term influence of air pollution, whereas the frequent occurrence of alkali salts gives hints towards the dominating role of hydraulic binders supplying soluble alkalis for salt formation.