Origin of Salts

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Author:Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser

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Abstract

The salts that cause damage to buildings, including wall paintings, and monuments made of stone, brick and other inorganic building materials, can originate from both natural sources or human activities. Natural sources are, for example, salts present in the ground on which the building stands. But they can also be present in the material itself, for example brick or some cements, depending on their manufacture; in stones near flood plains or from the natural stone weathering cycle. They can be brought in by sea spray and wind, for buildings in coastal areas and some microorganisms growing on building materials may generate some. Salts originating from human activities can be linked to emissions from burning fossil fuels, agricultural activities, automotive combustion engines and the use of deicing salts. Historic buildings may also have been used to store salt or gun powder, or even as stables Over the years, salts will accumulate within or near the surface of the building as water evaporates. Since what migrates within the stone are the ions of the various salts present when water enters the porous system, the accumulation of ions upon water evaporation may result in the formation of different combinations of them,i.e., different salts to the ones entered the material originally.

Introduction

Every natural liquid water can be considered a more or less diluted salt solutions as confirmed by simple electrical conductivity measurements. When water enters porous building materials, such as stone, brick, mortar or plaster, salts will also enter their pore space. As water evaporates at the suface of these materials, salts will accumulate near that area and eventually, some salt efflorescence will result.

Some salts are introduced into the buildings or monuments by human activity. These may result from the use of consolidants and/or cleaning agents such as alkaline materials, such as water glass building materials; some Portland cement in mortars, as well as historical use of the buildings for storage of salt or gun powder, or as stables. Some damages previously attributed to air pollution are now known to be due to the use of these materials [Klemm.etal:1999]Title: Schwefelisotopenanalyse von bauschädlichen Sulfatsalzen an historischen Bauwerken
Author: Klemm, Werner; Siedel, Heiner
Link to Google Scholar
. Deicing salts should be added to the list of these materials, especially since they are still in use mostly for roads, bridges and tunnels, affecting these structures and the foundations of buildings near them. Other sources for damaging salts can be fertilizers and some detergents that are introduced through ground and surface water. Finally, cleaning interventions based on the use of alkaline materials followed by an acid rinse, can leave soluble salts behind. This cleaning approach is mostly used for brick masonry in some countries, although there are records of their application on stone facades.

Many building materials are contaminated with salt-forming ions, varying from low to high concentrations. These can include natural stone or man-made ones, such as brick, lime, cement and mortars. The soil substrate and subsoil consistency, as well as the ground- and surface water contribute to this contamination [Roesch.etal:1993]Title: Damage to Frescoes caused by sulphate-bearing salts: Where does the salt come from?
Author: Rösch, Heinrich; Schwarz, Hans-Jürgen
Link to Google Scholar
. So do natural emissions by volcanoes ((H2S, HCl, HF, SO2 etc.) and sea- water aerosols (mostly NaCl but other salts as well), [Steiger.etal:1994b]Title: Determination of wet and dry depostion of atmospheric pollutants on building stones by field exposure experiments
Author: Steiger, Michael; Dannecker, Walter
Link to Google Scholar
[Steiger.etal:1997]Title: Sea Salt in Historic Buildings: Deposition, Transport and Accumulation
Author: Steiger, Michael; Behlen, Andreas; Neumann, Hans-Herrmann; Willers, U.; Wittenburg, Christian
Link to Google Scholar
[Steiger.etal:2002]Title: Immissionsbelastung durch salzbildende Stoffe und Wirkung auf mineralische Baustoffe
Author: Steiger, Michael; Behlen, Andreas; Wiese, Utz
Link to Google Scholar
[Becker.etal:2005]Title: Luftschadstoffe und Natursteinschäden
Author: Becker, Karl-Heinz; Brüggerhoff, Stefan; Steiger, Michael; Warscheid, Thomas
Link to Google Scholar
, but anthropogenic activities are becoming the most important contributors to salt contamination.

Among these are the industrial and traffic emissions, which produce a great number of aggressive substances, such as sulfur dioxide (at the origin of sulfates) [Wittenburg.etal:1993]Title: Ein Vergleich von Schwefeldioxid-Depositionsgeschwindigkeiten auf Naturwerksteine aus verschiedenen experimentellen Ansätzen (Comparison of sulfur dioxide deposition velocities on building stones from different experiments)
Author: Wittenburg, Christian; Mangels, Henning; Wolf, Falk; Steiger, Michael; Bothmann, Thomas; Dannecker, Walter
Link to Google Scholar
and nitrogen oxides (originating nitrates), are the most common ones. It is still not fully understood, how ozone (O3) and the large number of organic compounds released participate in the deteroration mechanism of building materials. Immissions mostly reach the objects through dry or wet deposition [Steiger.etal:1989]Title: Variability of aerosol size distributions above the North Sea and its implication to dry deposition estimates
Author: Steiger, Michael; Schulz, Michael; Schwikowski, Margit; Naumann, K.; Dannecker, Walter
Link to Google Scholar
by rainwater or fog, but also through accumulation in the subsoil.

Nitrates and organic salts such as oxalates mostly are of microbiological origin. While nitrates are transported into the object by humidity, calcium oxalates are usually formed in situ by microorganisms, where they remain due to their low solubility.

Literature

[Becker.etal:2005]Becker, Karl-Heinz; Brüggerhoff, Stefan; Steiger, Michael; Warscheid, Thomas (2005): Luftschadstoffe und Natursteinschäden. In: Siegesmund, Siegfried; Auras, Michael; Snethlage, Rolf (eds.): STEIN. Zerfall und Konservierung, Edition Leipzig, 35-45.Link to Google Scholar
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[Roesch.etal:1993]Rösch, Heinrich; Schwarz, Hans-Jürgen (1993): Damage to Frescoes caused by sulphate-bearing salts: Where does the salt come from?. Studies in Conservation, 38 (4), 224-230, Url.Link to Google ScholarFulltext link
[Steiger.etal:1989]Steiger, Michael; Schulz, Michael; Schwikowski, Margit; Naumann, K.; Dannecker, Walter (1989): Variability of aerosol size distributions above the North Sea and its implication to dry deposition estimates. Journal of Aerosol Science, 20 (8), 1229-1232.Link to Google Scholar
[Steiger.etal:1994b]Steiger, Michael; Dannecker, Walter (1994): Determination of wet and dry depostion of atmospheric pollutants on building stones by field exposure experiments. In: Zezza, Fulvio; Ott, Heinrich; Fassina, Vasco (eds.): The conservation of monuments in the Mediterranean Bassin: Stone and Monuments: Methodologies for the Analyses of Weathering and Conservation, , proceedings of the 3rd international symposium, Venice, 22-25 June 1994, , 171-178.Link to Google Scholar
[Steiger.etal:1997]Steiger, Michael; Behlen, Andreas; Neumann, Hans-Herrmann; Willers, U.; Wittenburg, Christian (1997): Sea Salt in Historic Buildings: Deposition, Transport and Accumulation. In: Moropoulou, A.; Zezza, F.; Kollias, E.; Papachristodoulou, I. (eds.): 4th International Symposium on the Conservation of Monuments in the Mediterranean - Proceedings, , 325-335.Link to Google Scholar
[Steiger.etal:2002]Steiger, Michael; Behlen, Andreas; Wiese, Utz (2002): Immissionsbelastung durch salzbildende Stoffe und Wirkung auf mineralische Baustoffe. In: Institut für Steinkonservierung Mainz (eds.): Salze im historischen Mauerwerk, IFS, Mainz, 1-10.Link to Google ScholarFulltext link
[Wittenburg.etal:1993]Wittenburg, Christian; Mangels, Henning; Wolf, Falk; Steiger, Michael; Bothmann, Thomas; Dannecker, Walter (1993): Ein Vergleich von Schwefeldioxid-Depositionsgeschwindigkeiten auf Naturwerksteine aus verschiedenen experimentellen Ansätzen (Comparison of sulfur dioxide deposition velocities on building stones from different experiments). In:: VDI-Berichte 1060 (Materialien in ihrer Umwelt), , 383-392.Link to Google Scholar