Origin of Salts

Author:Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser

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• Salts in building substrate and subsoils
• Road salts - deicing salt
• Salts of microbiological origin
• Salts Resulting from Restoration Materials

Abstract

Salts that cause damage to historic buildings, wall paintings and monuments made from inorganic porous materials such as stone and brick, can originate from natural sources or human activities. They are present in seawater, groundwater and even rainwater, as well as in some materials themselves, such as stones in flood plains or near the sea, or man made materials such as cement or brick, depending on their manufacture. They may also result from the natural weathering cycle of the materials, including microorganisms or originate from human activities. The latter can be linked to emissions from burning fossil fuels, combustion engines, agricultural activities, de-icing salts as well as historical storage of salt or gun powder storage, or the use of buildings as stables. Salts enter the building dissolved in water and the ions will migrate differently in the porous system. They will tend to accumulate near areas where water evaporates. Airborne salts can accumulate on surfaces of specific parts of a building, e.g., plinths.

Introduction

Every naturally occurring liquid water can be seen as a salt solution with different concentration. This can be confirmed by simple measurement of its electrical conductivity as the presence of salts will increase it. Porous building materials like stone, brick, mortar and plaster contain salts in their pore space. Where water flows through porous materials and evaporates from the surface, eventually some amount of salt efflorescence will be found.

Some salts or salt-forming ions are introduced into the monument by human activity. Monuments may be contaminated due to consolidants and and cleaning agents but also due to historical storage of salt or gun powder, alkaline building materials like water glass or Portland cement, were used extensively in some countries over the last century. Some damages previously referred to as a result of acid rain 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
Deicing salts should be added to the list of these materials, because they are still being used and they will affect mostly bridges, tunnels and foundations of buildings near roads. Other sources for damaging salts can be fertilizers and 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 used for brick masonry in some countries.

All natural materials are contaminated with salt-forming ions, varying from low to very high levels. These include building materials, e.g., natural stone or brick, and binding agents, such as lime, cement and renders. The substratum, subsoil consistency, ground- and surface water also play a role [Roesch.etal:1993]Title: Damage to Frescoes caused by sulphate-bearing salts: Where does the salt come from?
Author: Rösch, Heinrich; Schwarz, Hans-Jürgen
. Beside natural emissions by volcanoes ((H₂S, HCl, HF, SO₂ etc.) and sea- water aerosols (NaCl etc.), [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
[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
[Steiger.etal:2002]Title: Immissionsbelastung durch salzbildende Stoffe und Wirkung auf mineralische Baustoffe
Author: Steiger, Michael; Behlen, Andreas; Wiese, Utz
[Becker.etal:2005]Title: Luftschadstoffe und Natursteinschäden
Author: Becker, Karl-Heinz; Brüggerhoff, Stefan; Steiger, Michael; Warscheid, Thomas
, anthropogenic emissions are becoming more important.

The generation of energy, emissions from traffic and so forth, produce a great number of damaging substances, of which sulfur dioxide (with the secondary product sulphate)[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
and nitrogen oxides (with the secondary product nitrate), are the most familiar. It is still not fully understood, how ozone (O3) and the large number of organic compounds participate in the destruction of cultural property. 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
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, oxalates are usually formed in situ, 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.
[Klemm.etal:1999]	Klemm, Werner; Siedel, Heiner (1999): Schwefelisotopenanalyse von bauschädlichen Sulfatsalzen an historischen Bauwerken, Wiss. Mitt. Inst. Geol. TU Bergakademie Freiberg
[Roesch.etal:1993]	Rösch, Heinrich; Schwarz, Hans-Jürgen (1993): Damage to Frescoes caused by sulphate-bearing salts: Where does the salt come from?. In: Studies in Conservation, 38 (4), 224-230, Url
[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. In: Journal of Aerosol Science, 20 (8), 1229-1232
[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.
[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.
[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.
[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.

crystallization of a dissolved salt

Water glass is the common name for a liquid solution of alkaline silicates, also known as Sodium silicate Na2SiO3 or liquid glass.