Salt-induced alveolar weathering of rhyolite tuff on a building: causes and processes: Fid-Bau Portal

Salt-induced alveolar weathering of rhyolite tuff on a building: causes and processes

Siedel, Heiner

Studies of stone texture and spatial distribution of soluble salts were carried out at the historical façade of the church of St. Kunigunden in Rochlitz in order to explain the occurrence of alveolar weathering on the building stone, a local rhyolithe tuff. The rhyolithe tuff contains lapilli inclusions with porosities and water uptake quite different to those of the stone matrix. No alveolar weathering was observed in the walls near the ground where gypsum was the dominant salt. In the zone above, which, in addition to nitrates and chlorides, is loaded with magnesium sulphate and gypsum, the lapilli inclusions preferentially weather out due to salt cristallization, thus forming single, isolated pits. The formation of these pits leads to a distinct spatial distribution of magnesium sulphate and gypsum. Magnesium sulphate is accumulated in sheltered position at the bottom of the holes whereas gypsum is concentrated near the surface. Accelerated weathering due to hydration/dehydration activities of magnesium sulphate causes further material loss in the holes. The enlargement of the holes becomes a self-perpetuating process at this stage. Finally they coalesce, forming mature alveolar structures with holes and ridges. The heterogeneous structure of the building stone as well as the high content of magnesium sulphate in the affected zone are preconditions for alveolar weathering in the case investigated in this study. In: Lisbeth M. Ottosen and Inge Rörig-Dalgaard and Poul Klenz Larsen and Isabelle Brajer and Peder Bollingstoft and Mette Marciniak and Maja Svane (eds.): Salt Weathering on Buildings and Stone Sculpture. Proceeding of the conference in Copenhagen 22-24 October 2008, Danmarks Tekniske Universitet DTU, Kopenhagen 2008, pp. 79-88