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Author: [[user:HSiedel|Heiner Siedel]]
 
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Author: [[user:HSiedel|Heiner Siedel]]
<br>English Version by [[user:SLeithaeuser|Sandra Leithäuser]]<br><br>
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back to [[building Materials]]
back to [[stone|decay pattern stone]]
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== Salts and Alveolar Weathering ==
== Salze und Alveolarverwitterung ==


Die auch als auch „Steingitter“ oder „Wabenverwitterung“ (engl. „honeycomb weathering“) bezeichnete Verwitterungsform ist ein Phänomen, das bereits im 19. Jahrhundert von Naturforschern an natürlichen Gesteinsaufschlüssen beschrieben wurde. Die bizarren, aber zugleich häufig relativ regelmäßig ausgebildeten Formen, bei denen tief ausgewitterte Höhlen (Alveolen) im cm- bis dm-Bereich von unverwitterten oder nahezu unverwitterten Wänden oder Stegen getrennt werden, haben früh das Interesse der Geowissenschaften erregt und Fragen nach der Entstehung aufgeworfen. Klassische Untersuchungsgebiete im Gelände waren und sind in Deutschland Buntsandsteingebiete des Pfälzer Waldes <bib id=Haeberle:1915/> und das Elbsandsteingebirge (Kreidesandstein, <bib id=Beyer:1911/>). Bereits Häberle 1915 <bib id=Haeberle:1915/> verweist auf Beispiele, wo Wabenverwitterung auch von Baugesteinen beschrieben wird.
Alveolization is also known as honeycomb weathering and was first described by natural scientists of the 19th century, on natural outcrops of rocks. These bizarre, but often regularly composed shapes  consist of deeply eroded cavities (alveoli), ranging from centimeters to decimeters. The alveoli are separated by walls or ridges of stone, which have been unaffected or nearly unaffected by weathering. The phenomenon of alveolization has attracted the interest of geo-scientists very early on, who aimed to study the question of its origin. In Germany, the typical areas for research were the Palatine Forest (Pfälzer Wald) <bib id="Haeberle:1915"/> and the Elbe Sandstone Mountains (Kreidesandstein, <bib id="Beyer:1911"/>). As early as 1915 Häberle <bib id="Haeberle:1915"/> points to examples of honeycomb weathering on building stone.  


[[file:Rauenstein7.JPG|thumb|300px|right|Abbildung 1: Alveolarverwitterung am Oberkreide-Sandstein des Rauensteins, Sächsische Schweiz]]
[[file:Rauenstein7.JPG|thumb|300px|right|Figure 1: Alveolar weathering on cretaceaus sandstone , Rauenstein, Saxon Switzerland, Germany]]
[[file:Caen, St Pierre 063.jpg|thumb|300px|right|Abbildung 2: Alveolarverwitterung am Kalkstein von Caen, Kirche St. Pierre in Caen, Normandie, Frankreich. ]]
[[file:Caen, St Pierre 063.jpg|thumb|300px|right|Figure 2: Alveolar weathering on the limestone of Caen, church St. Pierre in Caen, Normandy, France ]]


Als eine mögliche Ursache für die Herausbildung des Reliefs wird die chemische Verwitterung genannt <bib id=Beyer:1911/>, <bib id=Haeberle:1915/>. Besonders Otto Beyer, der schon im Titel seiner Arbeit die Salzminerale Alaun und Gips als Ursache für die Sandsteinverwitterung benennt, misst im Ergebnis seiner Untersuchungen der Salzverwitterung große Bedeutung bei. Er sieht die unterschiedliche Wirkung von leichter löslichem [[Alaun]] und schwerer löslichem, häufig Krusten bildendem [[Gips]] als Ursache für die Ausbildung von loch- und wabenförmigen Strukturen an Sandstein-Felswänden. Während Alaun im Porenraum des Sandsteins kristallisiert, Sprengdruck erzeugt und das Gefüge zerstört, verstopft Gips die Poren, „füllt ebenso Sickerrisse … aus, überrindet wulst- oder krustenförmig seine Austrittsstellen und wirkt durch die dadurch gebildeten festen Rippen, Rinden und sonstigen Zementierungen konservierend für den Sandstein“ (<bib id=Beyer:1911/>, S. 466). An vielen weiteren Aufschlüssen in aller Welt wurden in Alveolen später lösliche Salze nachgewiesen und Vermutungen angestellt, dass sie den Verwitterungsprozess befördern. So treten z. B. in Küstengebieten wie auch in ariden Vorkommen häufig Chloride in solchen Strukturen auf. Daneben werden aber nach wie vor auch andere Einflussfaktoren diskutiert, wie Oberflächenverhärtung z.B. durch lokale Ausfällung von umgelagerter Kieselsäure an der Gesteinsoberfläche. Solche Prozesse der Silikatverwitterung laufen an Felsoberflächen in der Natur über sehr lange Zeiträume ab.
A possible cause for the development of this pattern is the chemical decomposition of the stone <bib id="Beyer:1911"/>, <bib id="Haeberle:1915"/>. Otto Beyer, as a result of his investigations, suggests that salt induced weathering plays an important role, naming the salt minerals alum (KAl(SO<sub>4</sub>)<sub>2</sub>·12H<sub>2</sub>O) and gypsum (CaSO<sub>4</sub>*2H<sub>2</sub>O) in the title of his paper. He sees the different effects of the more readily soluble [[alum]] and the poorly soluble, often crust forming salt [[gypsum]] as a cause for the development of cavity and honeycomb shaped structures on sandstone rocks. While alum crystallizes in the pores of the sandstone, giving rise to a blasting pressure that destroys the fabric, gypsum blocks up the pores, “fills drip holes and fissures while covering seepage points with a crust, or bulges over them, and achieves with the hereby developed strong ribs, rinds and other cementations, the conservation of the sandstone”(<bib id="Beyer:1911"/>, S. 466).


An Bauwerken neigen verschiedene Arten von Baugesteinen ebenfalls zur Alveolarverwitterung. Dazu zählen vor allem poröse Sedimentgesteine (bestimmte Sandsteine, Kalksteine, Tuffe). In vielen Fällen wirken - schon makroskopisch offensichtlich - Inhomogenitäten im Natursteingefüge auslösend für diese Verwitterung. So können Schichtstrukturen, Fremdeinschlüsse, Bioturbationen und Ähnliches eine unterschiedlich schnelle Reliefbildung durch Rückverwitterung in verschiedenen Gesteinsbereichen verursachen. Die gesteinseigenen, strukturellen Einflussfaktoren auf die Ausbildung von Alveolen bilden sich in der Regel dann auch im jeweils entstehenden Verwitterungsmuster ab. Ursächliche Unterschiede im Mikrogefüge können dabei beispielsweise im Gehalt wenig verwitterungsresistenter Minerale oder in der Porengrößenverteilung auch quantitativ festgemacht werden. Sie bilden gewissermaßen die Voraussetzung, dass sich auf einer ursprünglich ebenen Oberfläche überhaupt ein Relief ausbilden kann. Eine entscheidende Rolle für den Verwitterungsfortschritt spielen aber praktisch in allen bisher genauer untersuchten Fällen Salze, die meist durch Umweltbelastung und Reaktionen mit dem Mörtel ins Gestein gelangt sind.
In the 70s, Pauly was one of the pioneers in developing this weathering pattern in laboratory studies, to try to understand the formation mechanism given the high incidence of this deterioration in the limestone buildings in such areas as La Rochelle and Arles, France. He found that the presence of salt, wet-dry cycling and wind conditions could induce this type of weathering pattern.<bib id="Pauly:1976"/><bib id="Pauly:1976a"/>


Neuere Untersuchungen, die die Art und genaue räumliche Verteilung der Salze in Alveolen und benachbarten Wänden zum Gegenstand hatten (<bib id=Siedel:2008/><bib id=Siedel:2010/>), haben an Sandsteinen und Tuffen das Zusammenwirken von leicht- und schwerer löslichen Salzen bei der Alveolarverwitterung demonstriert. Zunächst konnten sich durch lokales Auswittern von Fossilstrukturen bzw. diagenetisch mit Eisenmineralen imprägnierten Gesteinsabschnitten im Sandstein oder von Lapillieinschlüssen im Tuff geringfügige initiale Reliefunterschiede ausbilden. Unterschiedlich gut lösliche Salze, in den untersuchten Fällen [[Magnesiumsulfat]] und Gips, wurden daraufhin im Verlauf von zahlreichen Feuchte-Trocknungs-Wechseln in den Vertiefungen und an der übrigen Gesteinsoberfläche ungleichmäßig angereichert. Der schwerer lösliche Gips verblieb überwiegend nahe an der Gesteinsoberfläche, während sich die noch in Lösung befindlichen Magnesiumsulfate bei langsamer Abtrocknung der Oberfläche vor allem in den deutlich länger feuchten Abschnitten hinter den Vertiefungen konzentrierten.
Soluble salts in alveoli have later been detected on many other outcrops in the world, and suggestions have been made that they advance the weathering process. For instance in coastal, but also in arid regions, chlorides often appear in such structures.  Alongside, other influencing factors like the induration of the surface are still being discussed, e.g., due to local precipitation of reworked silicic acid on the surface of the rock. In nature such silica weathering processes on rock surfaces proceed very slowly and take a very long time to develop.  


Die hohe Salzkonzentration in den Vertiefungen führt bei weiterer Trocknung dann zur Kristallisation, bei Wiederbefeuchtung auch zur Hydratation der Salze und zur oberflächigen Absprengung von Gesteinskörnern. Damit wird die anfänglich kleine Vertiefung erweitert. In größeren Hohlräumen finden sich oft lose Häufchen von abgesprengten Gesteinskörnchen und Salz am Boden, die auch nach dem Ausräumen immer wieder neu entstehen. Auf diese Weise können sich Alveolen vertiefen und auch mit benachbarten Höhlchen „zusammenwachsen“, indem die Wände / Stege dazwischen unterhöhlt werden. Die vergipste Oberfläche bleibt dagegen aufgrund der weniger ausgeprägten Verwitterungsaktivität des Gipses verhältnismäßig stabil. Dieses durch Detailuntersuchungen an Fassaden nachgewiesene Muster der Verwitterung in der „reiferen“ Phase der Alveolenbildung entspricht weitgehend dem bereits von Beyer 1911 aufgrund von Beobachtungen an natürlichen Aufschlüssen vorgeschlagenen kombinierten Wirken leicht- und schwerlöslicher Salze.
In buildings or monuments, several types of stones are prone to alveolar weathering. Amongst these are particularly porous sedimentary rocks (such as sandstones, limestones and tuff). Often visible by the naked eye, the inhomogeneity in the fabric of natural stone effectively causes this weathering pattern.
The structure of the sedimentary layers, foreign inclusions, bioturbations or other such disruptions, can cause during the weathering process the relief development at differing speeds. The structurally influencing factors relating to the specific stone, which are responsible for the formation of alveoli, are generally displayed in the developed weathering patterns. Causal differences in the micro-structure can be determined quantitatively, for example, in the contents of minerals with a low resistance to weathering or in the pore size distribution. These differences generate the preconditions for a relief to develop on a previously level surface. An essential role for the progress of weathering in nearly all investigated cases, is played by salts. Salts are usually introduced into the stone via a contaminated environment and by reactions with mortar.


Weitere Detailuntersuchungen an anderen Fallbeispielen sind erforderlich, um die Dynamik der Verwitterung und die Einflussfaktoren auf die Bildung von Alveolen an Baugesteinen noch umfassender zu verstehen.
Recent studies on the appearance of alveolization, explored the kind of salts present and their exact spatial distribution in alveoli and the adjoining walls (<bib id="Siedel:2008"/><bib id="Siedel:2010"/>). They showed the interaction of readily and poorly soluble salts in sandstones and tuffs, beginning with localized erosion that resulted in minor differences in relief due to the presence of fossil structures, or, in the case of sandstone, the diagenetically conjunction with ferrous mineral impregnated stone sections, or due to lapilli inclusions in tuff. It was observed that salts with significantly differing solubility, such as [[magnesium sulfate]] and gypsum, were heterogeneously distributed in the stone following wet-dry cycling. The poorly soluble gypsum stayed mainly near the surface of the stone while the more soluble magnesium sulfate, mostly still in solution, concentrated in the subsurface of the depressions (alveoli) that remained damp for longer times<bib id="Huinink.etal:2004"/> .  


== Literatur ==
The high concentration of salts in the depressions will crystallize upon further drying, and when rewetted it leads to the hydration of salts and to detachment of grains from the stone´s surface. This process enlarges the initially small depression. Often loose accumulations of detached stone grains and salts can be found inside the larger cavities. They repeatedly reappear after removal. Now the alveoli deepen and connect with neighboring cavities, because walls or ridges are eroded.  The gypsum encrusted surface, meanwhile, remains stable due to a higher resistance to weathering. The decay pattern of advanced alveolization has been established through detailed investigations on facades. The findings are in accordance with the suggestions made by Beyer in 1911, based on his observations about the combined effect of readily and poorly soluble salts on natural outcrops.


<bibprint/>
Further detailed investigations, considering other case studies are necessary in order to fully understand the dynamic of weathering and the influencing factors of alveoli formation on building stone.


-->
== Literature ==


[[Category:Alveolarweathering]] [[Category:HSiedel]] [[Category:R-CBlaeuer]] [[Category:inProgress]]
<biblist />
[[Category:Alveolarweathering]] [[Category:Siedel,Heiner]] [[Category:R-CBlaeuer]] [[Category:approved]]

Latest revision as of 13:31, 16 December 2013

Author: Heiner Siedel


English Version by Sandra Leithäuser

back to building Materials

Salts and Alveolar Weathering

Alveolization is also known as honeycomb weathering and was first described by natural scientists of the 19th century, on natural outcrops of rocks. These bizarre, but often regularly composed shapes consist of deeply eroded cavities (alveoli), ranging from centimeters to decimeters. The alveoli are separated by walls or ridges of stone, which have been unaffected or nearly unaffected by weathering. The phenomenon of alveolization has attracted the interest of geo-scientists very early on, who aimed to study the question of its origin. In Germany, the typical areas for research were the Palatine Forest (Pfälzer Wald) [Haeberle:1915]Title: Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine
Author: Häberle, Daniel
Link to Google Scholar and the Elbe Sandstone Mountains (Kreidesandstein, [Beyer:1911]Title: Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets
Author: Beyer, O.
Link to Google Scholar). As early as 1915 Häberle [Haeberle:1915]Title: Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine
Author: Häberle, Daniel
Link to Google Scholar points to examples of honeycomb weathering on building stone.

Figure 1: Alveolar weathering on cretaceaus sandstone , Rauenstein, Saxon Switzerland, Germany
Figure 2: Alveolar weathering on the limestone of Caen, church St. Pierre in Caen, Normandy, France

A possible cause for the development of this pattern is the chemical decomposition of the stone [Beyer:1911]Title: Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets
Author: Beyer, O.
Link to Google Scholar, [Haeberle:1915]Title: Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine
Author: Häberle, Daniel
Link to Google Scholar. Otto Beyer, as a result of his investigations, suggests that salt induced weathering plays an important role, naming the salt minerals alum (KAl(SO4)2·12H2O) and gypsum (CaSO4*2H2O) in the title of his paper. He sees the different effects of the more readily soluble alum and the poorly soluble, often crust forming salt gypsum as a cause for the development of cavity and honeycomb shaped structures on sandstone rocks. While alum crystallizes in the pores of the sandstone, giving rise to a blasting pressure that destroys the fabric, gypsum blocks up the pores, “fills drip holes and fissures while covering seepage points with a crust, or bulges over them, and achieves with the hereby developed strong ribs, rinds and other cementations, the conservation of the sandstone”([Beyer:1911]Title: Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets
Author: Beyer, O.
Link to Google Scholar, S. 466).

In the 70s, Pauly was one of the pioneers in developing this weathering pattern in laboratory studies, to try to understand the formation mechanism given the high incidence of this deterioration in the limestone buildings in such areas as La Rochelle and Arles, France. He found that the presence of salt, wet-dry cycling and wind conditions could induce this type of weathering pattern.[Pauly:1976]Title: Le Rôle des chlorures dans les maladies alvéolaire et desquamante
Author: Pauly, J.-P.
Link to Google Scholar[Pauly:1976a]Title: Maladie Alveolaire Conditions de Formation et d'Evolution
Author: Pauly, Jean-Pierre
Link to Google Scholar

Soluble salts in alveoli have later been detected on many other outcrops in the world, and suggestions have been made that they advance the weathering process. For instance in coastal, but also in arid regions, chlorides often appear in such structures. Alongside, other influencing factors like the induration of the surface are still being discussed, e.g., due to local precipitation of reworked silicic acid on the surface of the rock. In nature such silica weathering processes on rock surfaces proceed very slowly and take a very long time to develop.

In buildings or monuments, several types of stones are prone to alveolar weathering. Amongst these are particularly porous sedimentary rocks (such as sandstones, limestones and tuff). Often visible by the naked eye, the inhomogeneity in the fabric of natural stone effectively causes this weathering pattern. The structure of the sedimentary layers, foreign inclusions, bioturbations or other such disruptions, can cause during the weathering process the relief development at differing speeds. The structurally influencing factors relating to the specific stone, which are responsible for the formation of alveoli, are generally displayed in the developed weathering patterns. Causal differences in the micro-structure can be determined quantitatively, for example, in the contents of minerals with a low resistance to weathering or in the pore size distribution. These differences generate the preconditions for a relief to develop on a previously level surface. An essential role for the progress of weathering in nearly all investigated cases, is played by salts. Salts are usually introduced into the stone via a contaminated environment and by reactions with mortar.

Recent studies on the appearance of alveolization, explored the kind of salts present and their exact spatial distribution in alveoli and the adjoining walls ([Siedel:2008]Title: Salt-induced alveolar weathering of rhyolite tuff on a building: causes and processes
Author: Siedel, Heiner
Link to Google Scholar[Siedel:2010]Title: Alveolar weathering of Cretaceous building sandstones on monuments in Saxony, Germany
Author: Siedel, Heiner
Link to Google Scholar). They showed the interaction of readily and poorly soluble salts in sandstones and tuffs, beginning with localized erosion that resulted in minor differences in relief due to the presence of fossil structures, or, in the case of sandstone, the diagenetically conjunction with ferrous mineral impregnated stone sections, or due to lapilli inclusions in tuff. It was observed that salts with significantly differing solubility, such as magnesium sulfate and gypsum, were heterogeneously distributed in the stone following wet-dry cycling. The poorly soluble gypsum stayed mainly near the surface of the stone while the more soluble magnesium sulfate, mostly still in solution, concentrated in the subsurface of the depressions (alveoli) that remained damp for longer times[Huinink.etal:2004]Title: Simulating the growth of tafoni
Author: Huinink, H. P.; Pel, L.; Kopinga, K.
Link to Google Scholar .

The high concentration of salts in the depressions will crystallize upon further drying, and when rewetted it leads to the hydration of salts and to detachment of grains from the stone´s surface. This process enlarges the initially small depression. Often loose accumulations of detached stone grains and salts can be found inside the larger cavities. They repeatedly reappear after removal. Now the alveoli deepen and connect with neighboring cavities, because walls or ridges are eroded. The gypsum encrusted surface, meanwhile, remains stable due to a higher resistance to weathering. The decay pattern of advanced alveolization has been established through detailed investigations on facades. The findings are in accordance with the suggestions made by Beyer in 1911, based on his observations about the combined effect of readily and poorly soluble salts on natural outcrops.

Further detailed investigations, considering other case studies are necessary in order to fully understand the dynamic of weathering and the influencing factors of alveoli formation on building stone.

Literature

[Beyer:1911]Beyer, O. (1911): Alaun und Gips als Mineralneubildungen und als Ursachen der chemischen Verwitterung in den Quadersandsteinen des sächsischen Kreidegebiets. In: Zt. dt. geol. Ges., 63 (4), 429-467Link to Google Scholar
[Haeberle:1915]Häberle, Daniel (1915): Die gitter-, netz- und wabenförmige Verwitterung der Sandsteine. In: Geologische Rundschau, 6 (4), 264-285, Url, 10.1007/BF01797404Link to Google Scholar
[Huinink.etal:2004]Huinink, H. P.; Pel, L.; Kopinga, K. (2004): Simulating the growth of tafoni. In: Earth Surface Processes and Landforms, 29 (10), 1225--1233, Url, 10.1002/esp.1087Link to Google Scholar
[Pauly:1976]Pauly, J.-P. (1976): Le Rôle des chlorures dans les maladies alvéolaire et desquamante. In: Skoulikidis, T. (eds.): Proceedings of the 2nd International Symposium on Deterioration of Building Stones, Athens 1976,Universite Technique Nationale 79-91.Link to Google Scholar
[Pauly:1976a]Pauly, Jean-Pierre (1976): Maladie Alveolaire Conditions de Formation et d'Evolution. In: Rossi-Manaresi, Raffaella (eds.): The Conservation of Stone I, Proceedings of the International Symposium, Bologna, June 1975,Centro per la conservazione delle sculture all'aperto, Bologna 55-80.Link to Google Scholar
[Siedel:2008]Siedel, Heiner (2008): Salt-induced alveolar weathering of rhyolite tuff on a building: causes and processes. In: Ottosen, Lisbeth M.; Rörig-Dalgaard, Inge; Larsen , Poul Klenz; Brajer, Isabelle; Bøllingtoft, Peder; Marciniak, Mette; Svane, Maja (eds.): Salt Weathering on Buildings and Stone Sculptures, Technical University of Denmark, Lyngby, Denmark, 79-88.Link to Google ScholarFulltext link
[Siedel:2010]Siedel, Heiner (2010): Alveolar weathering of Cretaceous building sandstones on monuments in Saxony, Germany. In: Prikryl, R; Torok, A. (eds.): Natural Stone Resources for Historical Monuments. Geological Society, London, Special Publications, Geological Society, London, 11-23, Url, 10.1144/SP333.2.Link to Google Scholar
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