Deterioration Mechanisms

Autoren: Amelie Stahlbuhk

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Abstract

The different salt crystallization processes that can lead to deterioration of porous inorganic materials are discussed.

Introduction

Much of our cultural heritage is constituted by porous inorganic materials such as stone, brick, mortars and renders. Salts will enter these porous materials, e.g., from rising damp, and their crystallization in the pores will lead to their deterioration; a process that depends on the relative humidity (RH) and temperature. When the RH decreases, water from the salt solution will be released into the atmosphere until the deliquescence relative humidity (DRH) of the salt in question is reached. At this point, all the salt will be crystallized. If the RH increases, surpassing the DRH, the salt will absorb water vapor from the air and tend to form a saturated solution and, if the RH continues to increase, it will be further diluted. This shows that changes in RH around the DRH of the salt will lead to alternating crystallization and deliquescence cycles. This changes are responsible for the deterioration induced by these cycles that can be attributed to crystallization pressure; repeated cycling inducing more damage. One of the critical factors in developing an effective crystallization pressure is the formation of a supersaturated solution [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
. Crystallization from a supersaturated solution will not occur when reaching the saturation concentration or temperature, where a saturated solution should be formed, but at a lower RH or temperature. Supersaturated solutions form through water vapor evaporation, if for example, the RH drops, or by lowering temperature (note did not include oder nach Flüssigwassereintrag because it does not make sense to me). Dissolution of a metastable phase can result in a supersaturated solution, which will be discussed a following section [Steiger.etal:2008]Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Author: Steiger, Michael; Asmussen, Sönke
.

Whether the presence of salts will result in a deterioration process depends on the nature of the salt, or saltmixture, and its properties, but mainly by the environmental condition including the amount of water vapor available. These mechanisms, based on theoretical models, are only applicable for porous inorganic materials.

Crystallization within a pore

The concentration of a salt solution within a pore will depend on environmental factors such as relative humidity (RH) and temperature (T). In the RH drops, the concentration will increase as water vapor will be released from the solution; if the amount evaporated is such that the solubility of the salt in question is attained, then crystallization of the salt will occur. This RH is known as the deliquescence RH (DRH). Capillary transportation of the solution to the material's surface, where the main evaporation occurs, will increase the evaporation rate. As drying continues, the evaporation front will move into the interior of the porous material, as the evaporated moisture will move faster than the capillary movement to the evaporation front [Hall.etal:1984]The entry doesn't exist yet.. Therefore, salts transported by the solution can crystallize both on the material's surface, i.e., efflorescence, as well as in the area below the surface as a subflorescence. Where crystallization will actually occur depends on the drying conditions, the composition of the salt solution in question, its concentration and the material's properties [Espinosa-Marzal.etal:2010]Title: Advances in Understanding Damage by Salt Crystallization
Author: Espinosa-Marzal, Rosa M.; Scherer, George W.
.

Historical Background

Currently, our understanding of the models to explain the deterioration of porous materials and crystallization pressure from salts resulted in some controversial discussions since the 20th century. The growth of a crystal in the presence of obstacles was understood as a linear crystallization pressure (u.a. [Becker.etal:1916]Title: Notes on the Linear Force of Growing Crystals
Author: Becker, G.F.; Day, A.L.
, [Taber:1916]Title: The Growth of Crystals under External Pressure
Author: Taber, Stephen
, [Correns.etal:1939]Title: Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.; Steinborn, W.
, [Duttlinger.etal:1993]Title: Salzkristallisation und Salzschadensmechanismen
Author: Duttlinger, Werner; Knöfel, Dietbert
). A growing crystal, when it crystallizes from a supersaturated solution, can generate a maximum pressure when it grows along a crystal surface and reaches an obstacle. A film solution should be present between the crystal surface and the obstacle to allow its ongoing growth [Taber:1916]Title: The Growth of Crystals under External Pressure
Author: Taber, Stephen
. Important studies have been carried out by [Correns:1926]Title: Über die Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.
, who classified the crystallization pressure as a function of the volume increase of the crystal (hydrostatic crystallization pressure), hydration pressure as well as those referred to as linear crystallization pressure.

However, others ([Taber:1916]Title: The Growth of Crystals under External Pressure
Author: Taber, Stephen
), pointed out the influence of the interfacial energy that should have a given value so that the crystallization could take place. He pointed out that a growing crystal is not attached to the surface, there being a solution film between it and the surface, to allow for an ion exchange to take place, and that can be attributed to the inter-facial energy between crystal and pore-wall ([Correns:1926]Title: Über die Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.
, [Bruhns.etal:1913]Title: Über die sogenannte "Kristallisationskraft"
Author: Bruhns, W.; Mecklenburg, W.
, [Weyl:1959]Title: Pressure Solution and the Force of Crystallisation - A Phenomenological Theory
Author: Weyl, Peter K.
.

Some experiments, where both a loaded and an unloaded alum crystal in a saturated solution, showed that only the unloaded crystal grew thus requiring a re-examination of the linear crystallization pressure, and that furthermore the pressure developed could be correlated better with a volume increase rather than the pressure of the crystal. This could also be applied to the hydration pressure, since an anhydrous or a low hydrated phase would have a smaller volume than the fully hydrated one ([Bruhns.etal:1913]Title: Über die sogenannte "Kristallisationskraft"
Author: Bruhns, W.; Mecklenburg, W.
). The hydrostatic pressure is given by the increased volume resulting from the crystallization of the salt, since the volume of salt and saturated solution is greater than that of the supersaturated solution. Relevant contributions regarding the hydration pressure can be found [Mortensen:1933]Title: Die 'Salzprengung' und ihre Bedeutung für die regionalklimatische Gliederung der Wüsten
Author: Mortensen, Hans
, who found that the hydration pressure of an unhydrous crystal could be calculated as follows;

Δp_hydr=(ΔnRT/ΔV_m)∙ln(RH/RH_eq) (Eq.1)

where Δn is the difference in water molecules per moles of salt of the lower hydrate n₁ with the higher hydrate n₂; ΔV_m is the difference between the molar volume of both phases; RH the relative humidity at which the hydration reaction occurs, and RH_eq the equilbrium moisture at the temperature T corresponding to the hydration-dehydration equilibriumt. The hydration pressure corresponds to the maximum developed pressure that a growing crystal of the higher hydration can exert on the pore wall, since at a higher pressure, dehydration would result [Mortensen:1933]Title: Die 'Salzprengung' und ihre Bedeutung für die regionalklimatische Gliederung der Wüsten
Author: Mortensen, Hans
, [Steiger.etal:2014]Title: Weathering and Deterioration
Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja
.

Both Correns and Steinborn ([Correns.etal:1939]Title: Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.; Steinborn, W.
) also studied the "crystallization pressure". For the case of the "linear growth pressure" they gave a formula for the chemical potentials for which the degree of supersaturation of the solution defined the intensity of the developed pressure, as follows:

∆p=(RT/V_m)∙lnS=(RT/V_m)∙ln(c/c₀) (Eq. 2)

where ∆p is the crystallization pressure; R the gas constant; T the temperature; V_m the molecular volume of the salt; S the supersaturation; c the concentration of the supersaturated solution, and c₀ the concentration of the saturated solution.

Since their various experiments using different crystal surfaces always produced lower values than the theoretical ones, they attributed this to the different inter-facial energies. In subsequent equations, the crystallization pressure was also given as a function of the supersaturation, which allowed different ways of expressing them (e.g., [Goranson:1940]The entry doesn't exist yet., [Buil:1983]Title: Thermodynamics and Experimental Study of the Crystallization Pressure of Water Soluble Salts
Author: Buil, Michel
).

Everett ([Everett:1961]Title: The thermodynamics of frost damage to porous solids
Author: Everett, D.H.
considered the crystalization of ice and suggested a theory through which the crystallization pressure was the consequence of stability differences of the larger crystals. These stability differences result from differences in curvature of the interface between the solid and the liquid phases, which lead to different chemical potential for crystals with different sizes. However, he did not consider the supersaturation, his equation:

∆p=γ_cl∙(dA/dV) (Eq. 3)

describes the pressure differences between two crystals of different size; where γ_cl is the interfacial energy between the solid and the liquid phase, A is the surface, and V is the volume. For spherical crystals this can be rewritten

∆p=2γ_cl∙[(1/r₁)-(1/r₂)] (Eq. 4)

The pressure that develops through crystal growth in the larger pores with the radius r₂ is appropriate, since crystal growth in the smaller pores is not thermodynamically favored. The crystal developing in larger pores will grow as long as the chemical potential of the crystal corresponds to that of a crystal in the smaller pore r₁ ([Everett:1961]Title: The thermodynamics of frost damage to porous solids
Author: Everett, D.H.
, [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
).

The two approaches of Correns and Everett were considered contradictory deterioration mechanisms for many years, since one considered supersaturation and the other the crystal curvature as responsible for the crystallization pressure developed. Both approaches were followed in parallel, and that of Correns was criticized by its very high supersaturation. In the case of the Everett theory, because data for the reliable pore sizes were available, which is not the case for the supesaturation [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
.

Crystallization Pressure

Deterioration of a porous material from salt crystallization can only occure when the crystal(s) grows in a supersaturated solution against an obstacle, such as a pore wall (e.g. [Correns.etal:1939]Title: Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.; Steinborn, W.
). As the crystal grows further, a mechanical load, i.e., tension, is applied to the porous material. Another requirement for crystal growth against a wall is the presence of a solution film between the crystal and the wall [Weyl:1959]Title: Pressure Solution and the Force of Crystallisation - A Phenomenological Theory
Author: Weyl, Peter K.
(a thickness of about 1 nm [Scherer:1999]Title: Crystallization in pores
Author: Scherer, George W.
) through which the ion transport can occur. The reason for the presence of this solution film are the repulsive forces between the crystal and the pore wall, otherwise there would be no further growth [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
. In tests where a crystal was clamped between two glass-plates to evaluate the developed force, the presence of the solution film between crystal and glass-plates could be visually observed and its relevance confirmed [Desarnaud.etal:2016]The entry doesn't exist yet.. The repulsive pressure between the two components defines the maximal crystallization pressure, since if this value is increased a direct contact between crystal and wall would result in ending the crystal growth. Since the repulsive pressure has a slight dependence on the wetting properties and the surface charge, this implies that different salts and porous materials will develop diverse crystallization pressures [Desarnaud.etal:2016]The entry doesn't exist yet..

A crystal growing in a pore is exposed to an anisotropic pressure. Those faces/surfaces of the crystal which grow towards the pore-wall, are subjected to a higher pressure than the free surfaces in contact with the solution and subjected to its hydrostatic pressure. The difference between these two pressures is the crystallization pressure. Since crystal solubility is dependent on the applied pressure and increases with it, the free, unloaded surfaces/faces and those that are loaded, i.e., under pressure, will have different solubilities. This means, that comparing the concentration of the solution at the loaded face with that found by the free surface, the latter will be supersaturated [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
. The equation for the crystallization pressure can be given as:

∆p=(RT/V_m)∙ln(a/a₀) (Eq. 5).

where ∆p is the crystallization pressure, i.e., the difference between the pressure on a loaded face p_c and the hydrostatic pressure of the free faces p_l; a is the activity of the supersaturated solution and a₀ that of the saturated solution. By using activities the non-ideal behaviour of the salt solutions is considered [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
.

This equation shows a significant similarity to that suggested by Correns and Steinborn ([Correns.etal:1939]Title: Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.; Steinborn, W.
), however, we are considering the non-ideal behaviour of salt solutions by using the activity instead of concentrations. Furthermore, Correns neglected the anisotropic pressure to which the crystal is subjected.

A pressure develops on the pore structure when the unloaded crystal face is in contact with the supersaturated solution, since the supersaturation will be decreased by the crystal growth of the unloaded crystal face (as long as they are available). This reduces the supersaturation and the pressure on the loaded face cannot be sustained. Since the original solution was saturated with regards to the loaded face, it will fall below saturation which will result in the dissolution of the loaded face thus reducing the pressure to maintain the equilibrium. Both these effects lead to short-term high crystallization pressures that only can operate under supersaturated conditions. Therefore, the pressure build-up through crystallization does not occur under equilibrium conditions; it depends on a kinetic and dynamic process that is subjected to diffusion and crystal growth rate of unloaded crystal faces ([Steiger:2009]Title: Mechanismus der Schädigung durch Salzkristallisation
Author: Steiger, Michael
, [Scherer:2004]Title: Stress from crystallization of salt
Author: Scherer, George W.
).

Expansion

When a salt dissolves, the following equation applies:

M_νMX_νX∙ν₀H₂O⇌ν_MM^zM++ν_XX^zX-+ν₀ H₂O (Eq. 6)

where M are the cations; X the anions, ν the number of ions M and X; z the charge of the corresponding ions and ν₀ the number of water molecules.

The activity of the dissolved salt is a=a_M^ν_M∙a_X^ν_X∙a_w^ν₀, the ion activity product, where a_w is the water activity.

Expressing the activity as the activity coefficient and the molality a_i=γ_i∙(m_i/m⁰)

where a_i corresponds to the ion activity; γ_i the acitivity coefficient of the ions; m_i the molaliry of the ions in the solution, and, where m⁰=1 mol/kg, then the crystallization pressure of a crystal in contact with the solution can be calculated, as long as the activity coefficients and the water activity are known. Δp It follows then that a more then a more detailed equation for Δp. In case of a single salt, it is possible to introduce an average activity coefficient γ^±=(γ_M^ν_Mγ_X^ν_X)^(1/ν) but taking into consideration the simplifciation m_i=ν_i∙m. This then corresponds to the activity a=(ν_M^ν_Mν_X^ν_X)∙(mγ_±)^ν∙a_w^ν₀ where ν corresponds to the number of ions that result from the salt dissolution. From this results the following equation:

Δp=νRT/V_m∙[ln(m/m₀)+ln(γ_±/γ_±,0)+(ν₀/ν)∙ln(a_w/a_w,0)] (Eq. 7)

Next the influencing factors and the individual terms of the equation will be considered in more detail. The first term in brackets is the supersaturation express as concentration. The second and third term in brackets consider the non-ideal behaviour of the concentrated salt solution(s). If one considers an ideal behaviour, the second term is not required, since the average activity coefficient would be 1, however, the third term will remain. The water activity decreases with increasing molality and is always smaller in salt solutions than in pure water. However, since ν₀, i.e., the number of water molecules is relevant, it will not contribute in the case of anhydrate salts. Next to ν₀ in the equation, is the composition of the salt with various components (such as ions and the mentioned water molecules). If the crystallization water is discarded, and with it the term that includes the water activity, the calculated crystallization pressure will not be too large ([Steiger:2005b]Title: Crystal growth in porous materials: II. The influence of crystal size
Author: Steiger, Michael
). However, if the influence of the ions is also discarded the calculated crystallization pressure will be significantly influenced, since ν also appears in the term before the brackets. Hence, the number of ions is directly proportional to the crystallization pressure, and if this is not taken into account, the calculation will be considerably lower, about a factor of 2 or more ([Steiger:2005b]Title: Crystal growth in porous materials: II. The influence of crystal size
Author: Steiger, Michael
, [Flatt.etal:2007]Title: A commented translation of the paper by C.W. Correns and W. Steinborn on crystallization pressure
Author: Flatt, Robert J.; Steiger, Michael; Scherer, George W.
, [Coussy:2006]The entry doesn't exist yet.). If the non-ideal behaviour is neglected, the influence on the crystallization pressure will depend on the salt and can be greater or smaller. With increasing supersaturation, the activity coefficient increase and with it the pressure, and vice-versa. Considering a hydrated salt, the term with the water activity will decrease the calculated pressure, so that a pressure increase from the activity coefficients will be compensated ([Steiger:2005b]Title: Crystal growth in porous materials: II. The influence of crystal size
Author: Steiger, Michael
). On the other hand, the molar volume of the salt is inversely proportional to the crystallization pressure. This means, that salts with a low molar volume by equivalent supersaturation will develop higher pressures than those with larger molar volumes. To be considered is that sombe salts cannot reach high supersaturations [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
.

These considerations make it clear that Correns and Steinborn ([Correns.etal:1939]Title: Experimente zur Messung und Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.; Steinborn, W.
)in their study did not consider the non-ideal behaviour of the salt, but more relevantly, they neglected the composition of the salt, since the influence of the ions number was also neglected so that the calculated crystallization pressures were too low ([Steiger:2005b]Title: Crystal growth in porous materials: II. The influence of crystal size
Author: Steiger, Michael
, [Flatt.etal:2007]Title: A commented translation of the paper by C.W. Correns and W. Steinborn on crystallization pressure
Author: Flatt, Robert J.; Steiger, Michael; Scherer, George W.
, [Coussy:2006]The entry doesn't exist yet.).

Crystallization in small pores

In the case of small pores the effect of the crystal size on their solubility has to be considered. The smaller the crystal size, the higher the solubility, so that crystals in small pores require higher concentrations for their growth. The influence of the size of spherical crystals on their solubility can be calculated by the following equation (Ostwald-Freundlich equation):

lna₀/a_∞=(2γ_clV_m)/rRT (Eq. 8)

where a₀ and a_∞ are the thermodynamic solubility products of the smaller crystal with regards to an infinitely large crystal; γ_cl is the interfacial energy between the crystal and the solution, and r the radius of the crystal [Steiger:2005b]Title: Crystal growth in porous materials: II. The influence of crystal size
Author: Steiger, Michael
.

In the case of a round pore having a small cylindrical entrance, a model for two dissimilar sized pores is required. If the solution is saturated with regards to the smaller crystal at the pore entry r₁, then it will be supersaturated with regards to the larger crystal in the spherical pore r₂. Therefore the larger crystal will continue its growth and apply pressure to the pore wall, until equilibrium is reached. Since the required concentration at the free surface (the small crystal) is greater than for the large crystal, then growth can only proceed under pressure against the pore-wall. Applying the crystallization equation for pressure using the saturation activity of the larger crystal (instead of the activity of the saturated solution), with that of the smaller crystal at the pore entry (instead of the solution activity), and the last equation for solubility dependence, the following equation for calculating the crystallization pressure is obtained:

∆p=2γ_cl∙[(1/r₁)-(1/r₂)] (Eq. 4)

The equation corresponds to that of [Everett:1961]Title: The thermodynamics of frost damage to porous solids
Author: Everett, D.H.
; it is a special case for a given geometry based on a pressure build-up following supersaturation. Thus, the equations of Correns and Everett are interconvertible, i.e., exchangeable, and describe the same deterioration mechanism for specific geometries, and therefore are not contradictory [Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
. In the case of the described geometry, the supersaturation will increase with respect to a decreasing pore entry. For other geometries, other equations for the crystallization pressure can be developed ([Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
, [Scherer:1999]Title: Crystallization in pores
Author: Scherer, George W.
, [Flatt:2002]Title: Salt damage in porous materials: how high supersaturations are generated
Author: Flatt, Robert J.
, [Steiger:2006]Title: Crystal growth in porous materials: Influence of supersaturation and crystal size
Author: Steiger, Michael
.

Also for smaller pores, the pressure will only develop when the pore solution is supersaturated with respect to free surfaces, i.e., an unloaded surface. For smaller pores this is the consequence of the different solubility presented by the curving of the surfaces. The higher the curvature, the higher will be solubility and therefore the supersaturation with respect to the less curved faces where crystal growth is occurring. In this case, the supersaturation and the crystallization pressure will be in equilibrium, so that this can operate for longer periods. Nonetheless, it is critical to take into account that the influence of the crystal size of its solubility will only be relevant for radius <0.1 µm ([Steiger:2005]Title: Crystal growth in porous materials: I. The crystallization pressure of large crystals
Author: Steiger, Michael
) and to attain higher pressures a pore diameter of <10-50 nm ([Steiger:2009]Title: Mechanismus der Schädigung durch Salzkristallisation
Author: Steiger, Michael
) is required, and this occurrence happens only for some construction materials. In the case of larger pores the crystallization pressure is not in equilibrium (see above). In this case, the slow evaporation will result in highly concentrated solutions that are not in contact with all crystal faces. Thus, highly concentrated solutions can remain that are not in contact with all crystal faces. If there is no sufficient contact with a free surface, the supersaturation can work for longer time, and the pressures maintained ([Steiger:2005b]Title: Crystal growth in porous materials: II. The influence of crystal size
Author: Steiger, Michael
). High supersaturation can be achieved through fast evaporation. If the diffusion of ions in the solution is interrupted, this may result in growth at loaded faces, when free faces are no longer in contact with the solution ([Flatt:2002]Title: Salt damage in porous materials: how high supersaturations are generated
Author: Flatt, Robert J.
).

Special case of hydration pressure

Some salts can crystallize with different degrees of hydration, for example, magnesium sulfate (MgSO₄∙nH₂O), where n ranges from .

Einige Salze können in unterschiedlichen Hydratphasen vorliegen, beispielsweise verschiedene Phasen des Magnesiumsulfats (MgSO₄∙nH₂O). Durch den unterschiedlichen Wassergehalt in den Phasen ergibt sich ein für die jeweils höher hydratisierte Phase höheres molares Volumen, was lange Zeit für die Druckausübung auf das Porengefüge verantwortlich gemacht wurde (z.B.[Correns:1926]Title: Über die Erklärung der sogenannten Kristallisationskraft
Author: Correns, Carl W.
). Inzwischen werden zwei unterschiedliche Arten der Hydratationsreaktion unterschieden.

Zum einen kann eine Hydratation über Wasserdampf, also über die relative Luftfeuchte der Umgebung, erfolgen. Liegt der Wert der RH dabei unterhalb der Deliqueszenzfeuchte des geringeren Hydrats beziehungsweise der wasserfreien Phase, so geschieht die Hydratation über eine Festphasenreaktion. Für den dabei maximal durch das Wachstum eines Kristalls der hydratisierten Phase wirkenden Druck gegen die Porenwand kann die Gleichung für den Hydratationsdruck nach Mortensen [Mortensen:1933]Title: Die 'Salzprengung' und ihre Bedeutung für die regionalklimatische Gliederung der Wüsten
Author: Mortensen, Hans
verwendet werden (s.o.). Während bei der Kristallisation aus einer Lösung die Übersättigung der Lösung Triebkraft für den ausgeübten Druck ist, ist es bei der Hydratation über Wasserdampf (RH<DRH des niedrigeren Hydrats) die Übersättigung des Wasserdampfs der Umgebungsluft gegenüber des Gleichgewichtsdampfdrucks der Hydratphase bei der gegebenen Temperatur. Im Falle einer solchen Hydratation und der Verwendung der Gleichung nach Mortensen muss beachtet werden, dass es eine maximale relative Luftfeuchtigkeit gibt, oberhalb derer die Anwendung der Gleichung nicht mehr legitim ist. Überschreitet die relative Luftfeuchtigkeit die Deliqueszenzfeuchte der höheren Hydratphase, liegt nur noch eine Lösung vor, wodurch auch kein Hydratationsdruck wirken kann. [Steiger.etal:2014]Title: Weathering and Deterioration
Author: Steiger, Michael; Charola A. Elena; Sterflinger, Katja

Liegt die RH bei der Befeuchtung oberhalb der Deliqueszenzfeuchte der niedriger hydratisierten Phase, so erfolgt die Hydratation über einen anderen Mechanismus. Das geringere Hydrat bildet eine (gesättigte) Lösung, aus welcher die höher hydratisierte Phase auskristallisiert (Auflösungs- und Rekristallisationsvorgang). Auch hierbei ist nicht die Volumenzunahme Grund für den Druckaufbau im porösen Gefüge, sondern die Kristallisation der Hydratphase aus einer ihr gegenüber übersättigten Lösung. Somit stellt dieser Mechanismus einen Sonderfall des Kristallisationsdrucks in Folge der Kristallisation aus übersättigten Lösungen dar. Ebenfalls dieser Mechanismus ist bei einer Flüssigwasserbefeuchtung eines porösen Systems mit einer wasserfreien oder geringer hydratisierten Phase für den Druckaufbau verantwortlich. [Steiger:2003b]Title: Salts and Crusts
Author: Steiger, Michael

Beispielhaft lässt sich dies gut anhand des Systems Natriumsulfat-Wasser zeigen. Wird ein poröser Prüfkörper mit Thenardit im Porenraum bei Raumtemperatur getränkt, so bildet sich eine in Bezug auf Thenardit gesättigte Lösung. Ein Blick auf das Phasendiagramm des Systems zeigt jedoch, dass diese Lösung in Bezug auf Mirabilit übersättigt ist und der Grad der Übersättigung mit sinkender Temperatur zunimmt. Bei der Kristallisation von Mirabilit aus einer stark übersättigten Lösung können bei ausreichend gefüllten Poren hohe Drücke auf das Porengefüge wirken. Dies kann nicht nur bei der Flüssigwasserbefeuchtung beobachtet werden, sondern auch bei der Befeuchtung der wasserfreien bzw geringer hydratisierten Phase bei relativen Luftfeuchtigkeiten bis zur oder oberhalb der Deliqueszenzfeuchte, wobei die hydratisierte Phase dann aus dieser in Bezug auf sie übersättigten Lösung gebildet wird. Verallgemeinert ausgedrückt, kommt es durch das Auflösen einer metastabilen Phase zur Bildung einer in Hinblick auf die stabile Phase übersättigten Lösung, so dass während der Kristallisation der stabilen Phase hohe Drücke wirken können. [Steiger.etal:2008]Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Author: Steiger, Michael; Asmussen, Sönke

Consideration of crystallization pressure within building deterioration

Schädigungen von realen Bauwerken aus porösem Material, hervorgerufen durch Salzkristallisation, werden häufig beobachtet. Die Salze beziehungsweise Salzlösungen in den porösen Materialien stehen im direkten Austausch mit der Umgebung. Somit können Schwankungen der relativen Luftfeuchtigkeit zu zyklischen Wechseln zwischen Kristallisation und Auflösung führen, wenn die Schwankungen in Bereichen ober- und unterhalb der Deliqueszenzfeuchte erfolgen. Auch der Eintrag von Regenwasser oder das Auftreten von Kondensation kann zur Befeuchtung oder Hydratation von vorliegenden Salzen führen. Einige Salze können auch bei alleinigen Temperaturschwankungen Phasenwechsel durchlaufen, wenn in dem Salzsystem Phasen vorliegen, die nur in einem bestimmten Temperaturbereich stabil sind (beispielsweise Mirabilit). An Bauwerken liegen aber in den meisten Fällen geringere Konzentrationen und weniger drastische Bedingungen vor als in den im Labor für die Untersuchung der Schädigungsmechanismen durchgeführten Experimenten.

Zudem gelten in Bezug auf den Kristallisationsdruck die bereits im Abschnitt „Vertiefung“ erwähnten Punkte. Kleine Poren, die für einen Kristallisationsdruck als Gleichgewichtszustand notwendig sind, liegen in den meisten Baumaterialien nicht vor, sind aber in Zement möglich. Demnach ist der schädigende Kristallisationsdruck an realen Objekten in der Regel kein Gleichgewichtszustand, sondern ein dynamischer Prozess ([Steiger:2009]Title: Mechanismus der Schädigung durch Salzkristallisation
Author: Steiger, Michael
, [Scherer:2004]Title: Stress from crystallization of salt
Author: Scherer, George W.
. Die hohen Übersättigungen liegen dann nur über kurze Zeiträume vor und werden durch die Kristallisation an freien Flächen abgebaut.

Literature

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relative humidity

Deliquescence Relativ Humidity

crystallization of a dissolved salt

concentration given as amount of substance (mol) of the dissolved substance per kilogram solvent; unit: mol•kg-1.

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.