Deterioration Mechanisms

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Authors: Michael Steiger, Hans-Jürgen Schwarz

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Abstract[edit]

Models that can lead to salt-induced damage are presented.

Introduction[edit]

Salts can cause damage in porous inorganic materials such as sandstone, brick and renders, as well as to any paint layers applied to them. Damaging processes always occur in connection with water. When no water is present - in any phase - no damage processes will occur.

Damage on metallic materials are usually based on different principles and are dealt with in a separate chapter.

The deterioration mechanisms are based on models of the crystallization process and the changes in the physical and chemical conditions in the pore space. In some cases more than one deterioration process may be active.

Overview to the topic on deterioration mechanisms[edit]

In the literature (e.g., [Correns:1926]Title: Über die Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.
Link to Google Scholar, [Winkler:1975]Title: Stone: Properties, Durability in Man´s Environment
Author: Winkler, Erhard M.
Link to Google Scholar, [Duttlinger.etal:1993]Title: Salzkristallisation und Salzschadensmechanismen
Author: Duttlinger, Werner; Knöfel, Dietbert
Link to Google Scholar) damage mechanisms are mentioned that are directly associated with salts or enhanced by the presence of salts. The following types of damage mechanisms are mentioned that are directly associated with salts or be enhanced by salts, which will be discussed briefly below:


Linear Crystallisation Pressure[edit]

The linear growth pressure is the maximum pressure under which a crystal can still grow or, vice versa, the maximum pressure that a growing crystal can exert.

The linear growth pressure is particularly dependent on the degree of supersaturation of the salt solution. The calculations in most of the literature go back to Correns and Steinborn [Correns.etal:1939]Title: Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.; Steinborn, W.
Link to Google Scholar .


Pore Size Dependent Crystallisation Pressure[edit]

Crystals in large pores grow at the expense of those in small pores. The crystals in the large pores grow at a lower chemical potential until they fill up the pore space and then even further until a higher chemical potential is reached by the increased pressure in the large pore that equals that of crystal growth in the small pore. Consequently, a pore size distribution with two distinct maxima crystallization of salts can be expected to cause damage [Snethlage:1984]Title: Steinkonservierung, Forschungsprogramm des Zentrallabors für Denkmalpflege 1979-1983, Bericht für die Stiftung Volkswagenwerk. Arbeitshefte des Bayerischen Landesamtes für Denkmalpflege
Author: Snethlage, Rolf
Link to Google Scholar.

Hydrostatic Crystallisation Pressure[edit]

For most salts [Correns.etal:1939]Title: Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.; Steinborn, W.
Link to Google Scholar, the volume of 'salt + saturated solution' is greater than that of the supersaturated solution. When salt crystallizes out in pores sealing them, a hydrostatic pressure will develop at the pore walls.

Hydration Pressure[edit]

Zahlreiche bauschädliche Salze liegen abhängig von Temperatur und relativer Luftfeuchte in verschiedenen Hydratstufen vor, d.h. es existieren von ihnen Salzphasen mit zum Teil deutlich unterschiedlichem Wassergehalt. Die Bildung der neuen Phase ist sowohl nach kompletter Auflösung des alten Kristalls, als auch durch Umbau des Kristalls im festen Zustand vorstellbar. Der Einbau von Wasser in das Kristallgitter ist mit einer Volumenzunahme verbunden und übt somit einen Druck auf eine Begrenzung aus.


This article will be released soon.


Thermic Effects[edit]

Since wet materials are much better heat conductor than dry materials, a higher moisture content lead to a change of temperature differences in a building material and thus to a change in the thermal stretching and shrinkage. Since a temperature compensation is faster, the thermal stresses will decrease normally.

New findings according to decay mechanisms[edit]

In recent years, the previous models of the mechanisms of damage by salts were checked and had to be modified to some extent.

Literature[edit]

[Becker.etal:1916]Becker, G.F.; Day, A.L. (1916): Notes on the Linear Force of Growing Crystals. In: Journal of Geology, 24 (4), 313-333, UrlLink to Google Scholar
[Bruhns.etal:1913]Bruhns, W.; Mecklenburg, W. (1913): Über die sogenannte "Kristallisationskraft". In: Sechster Jahresbericht des Niedersächsischen Geologischen Vereins zu Hannover, (), 92-115Link to Google Scholar
[Buil:1983]Buil, Michel (1983): Thermodynamics and Experimental Study of the Crystallization Pressure of Water Soluble Salts. In: F.H. Wittmann (eds.): Materials Science and Restoration, Lack und Chemie, Filderstadt, 373-377.Link to Google Scholar
[Correns.etal:1939]Correns, Carl W.; Steinborn, W. (1939): Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft. In: Zeitschrift für Kristallografie, (101), 117-133Link to Google Scholar
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A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.

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