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<bibimport/>
Authors:  [[user:Hschwarz|Hans-Jürgen Schwarz ]], [[user:NMainusch|Nils Mainusch]], [[user:TMueller|Tim Müller]]
<br>English Translation by Sandra Leithäuser
<br>back to [[Sulfate]]
 
 
{{Infobox_Salt
{{Infobox_Salt
|Footnote            = <ref>http://webmineral.com/data/Gypsum.shtml seen on 30.07.2010</ref><ref>http://www.mindat.org/min-1784.html seen on 30.07.2010</ref>
|Footnote            = <ref>http://webmineral.com/data/Gypsum.shtml seen on 30.07.2010</ref><ref>http://www.mindat.org/min-1784.html seen on 30.07.2010</ref>
|photo                = [[File:SA101 1.jpeg|300px]]
|photo                = [[File:SA101 1.jpeg|300px]]
|Mineralogical_name   = Gypsum, Selenite
|mineralogical_Name   = Gypsum
|Chemical_name       = Calcium sulfate dihydrate
|chemical_Name       = Calcium sulfate dihydrate
|Trivial_name         = Gypsite, Sulfate of Lime
|Trivial_Name         = Alabaster, Satin Spar, Selenite,  
|Chemical_Formula     = Ca[SO<sub>4</sub>]•2H<sub>2</sub>O  
|chemical_Formula     = CaSO<sub>4</sub>•2H<sub>2</sub>O  
|Hydratforms          = Anhydrite (CaSO<sub>4</sub>)<br>Hemihydrate (CaSO<sub>4</sub>•0.5H<sub>2</sub>O)  
|Hydratforms          = CaSO<sub>4</sub> ([[Anhydrite]])<br> CaSO<sub>4</sub>•0.5H<sub>2</sub>O ([[Bassanite]])  
|Crystal_System      = monoclinic
|Crystal_System      = monoclinic
|Crystal_Structure    =
|Crystal_Structure    =  
|Deliqueszenzhumidity =
|Deliqueszenzhumidity = > 99% RH at 20°C
|Solubility          = 2.14 g/l
|Solubility          = 2.14 g/l
|Density              = 2.2-2.4 g/cm³  
|Density              = 2.31 g/cm³  
|MolVolume            = 74.69 cm<sup>3</sup>/mol  
|MolVolume            = 74.69 cm<sup>3</sup>/mol  
|Molweight            = 172.17g /mol  
|Molweight            = 172.17g /mol  
|Transparency        = transparent to opaque
|Transparency        = transparent to opaque
|Cleavage            = perfect
|Cleavage            = perfect, clearly visible formation of fibre
|Crystal_habit       =
|Crystal_Habit       = flat, prismatic, needle- like crystal,  granular, massive aggregate
|Twinning            =
|Twinning            = very common
|Refractive_indices   = α = 1.519-1.521<br>β = 1.522-1.523<br>γ = 1.529-1.530
|Refractive_Indices   = α = 1.5207<br>β = 1.5230<br>γ = 1.5299
|Birefringence        = Δ = 0.010
|Birefringence        = Δ = 0.0092
|Optical_orientation = biaxial positive
|optical_Orientation = biaxial positive
|Pleochroism          =  
|Pleochroism          = colorless
|Dispersion          = 58°  
|Dispersion          = 58°  
|Phase_transition     =
|Phase_Transition     =
|chem behavior         =
|chemBehavior         = hardly soluble in water
|Comments            = hardly soluble in water
|Comments            =
|Literature          =<bib id="Robie.etal:1978"/> <bib id="Dana:1951"/>
}}
}}
Authors:  [[user:Hschwarz|Hans-Jürgen Schwarz ]], Nils Mainusch, [[user:TMueller|Tim Müller]]
<br>back to [[Sulfate]]


=Calciumsulfate and Gipsum =


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=Calcium sulfate and gypsum =
 
== Abstract  ==
== Abstract  ==


An dieser Stelle wird das System CaSO<sub>4</sub>/H<sub>2</sub>O behandelt und im Speziellen auf Gips eingegenagen. Gips ist eines der wichtigsten Salze, die an z. B. Bauwerken und Wandmalereien für Schäden verantwortlich sind. Vor allem außen exponierte Objekte leiden unter Gipsschäden. Die Eigenschaften, die Schadenswirkung, das Vorkommen und auch der Nachweis von Gips werden behandelt. Abbildungen, Mikroaufnahmen und Beispiele aus der Praxis ergänzen und veranschaulichen das Dargelegte.<br>
The article discusses the system CaSO<sub>4</sub>-H<sub>2</sub>O, in reference to gypsum. Gypsum is one of the most important salts in the deterioration of building materials and, particularly, wall-paintings. Objects exposed to exterior conditions where air pollution is present are the most prone to damage from gypsum. The appearance and mechanism of the damage, as well as the examination methods are described. Images, microphotographs and examples from practical experiences illustrate the subject.<br>
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== Einleitung ==
== Introduction ==


Gips ist eines der heute am häufigsten vorkommenden bauschädlichen Salze. Er kommt in unterschiedlichen Formen und Ausprägungen an fast allen Objekten am Außenbau vor. Auch in Innenräumen ist er häufig zu finden.<br>
Gypsum is one of the most common salts causing deterioration of inorganic porous building materials. It is present on most exterior exposed surfaces and even in interior environments, under different shapes and inducing various deterioration patterns. <br>
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Gypsum, one of the most prevalent minerals, forms by precipitation from aqueous solution at temperatures under approximately 40°C. When the solution reaches higher temperatures (&gt; 60°C) anhydrite precipitates out. Calcium sulfate and calcium sulfate dihydrate are often present in rocks. Hemihydrate does not occur naturally.<br>


== Vorkommen von Gips  ==
Gypsum occurs naturally in salt deposits and deserts, where "desert rose" crystals form in combination with quartz inclusions. In natural salt deposits, gypsum and anhydrite sometimes form a caprock, i.e., a massive layer of material covering the deposit.  
 
Synthetic gypsum is produced in coal-fired power plants, as a by-product of flue-gas desulfurization.
Als eines der am meisten verbreiteten Minerale entsteht Gips beim Ausfall aus wässrigen Lösungen bei Temperaturen unter ca. 40°C. Liegen erhöhte Temperaturen (&gt; 60°C) einer Lösung vor, so wird direkt Anhydrit gebildet. In Form von Gesteinen sind beide Calciumsulfatformen häufig anzutreffen. Natürliche Vorkommen des Halbhydrates existieren nicht. <br>
Gips findet sich als Vorkommen in Salzlagerstätten und in Salzwüsten, wo durch den Einschluss von Quarzpartikeln bei der Formierung des Kristalls häufig sogenannte Wüstenrosen entstehen. In Salzlagerstätten bilden Gips und Anhydrit zuweilen einen "Salzhut" aus, d.h. eine mächtige Materialschicht, die sich über anderen Salzvorkommen eines natürlichen Lagers befindet. Künstlich hergestellter Gips entsteht u.a. im Zuge der Entschwefelung von Rauchgasen in Kraftwerken, in denen fossile Brennstoffe verwertet werden.  


== Herkunft und Bildung von Gips an Baudenkmalen ==
== Origin and formation of gypsum on monuments ==


Die hohe Schadensrelevanz von Gips für Denkmale aus mineralischer Bausubstanz steht wesentlich in Zusammenhang mit der Umwandlungsreaktion von Kalk zu Gips. Unter der Einwirkung von SO<sub>x</sub>-haltiger Luft in Verbindung mit Feuchte können auf diese Weise wichtige Materialkomponeneten von Bauwerken (Kalkmörtel, Verputz, calcitische Gesteine etc.) zu Gips entsprechend dem Chemismus:  
On monuments made from porous inorganic building materials, particularly in urban environment where anthropogenic air pollutants are present, such as sulfur oxides (SO<sub>x</sub>) that eventually convert to sulfuric acid in the presence of moisture, the reaction of these gases with any calcium carbonate present in limestone, sandstone, mortars, renders, results in the formation of gypsum. The reaction taking place can be simplified as follows:


CaCO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub> → CaSO<sub>4</sub> + H<sub>2</sub>O + CO<sub>2</sub>
CaCO<sub>3</sub> + H<sub>2</sub>SO<sub>4</sub> → CaSO<sub>4</sub> + H<sub>2</sub>O + CO<sub>2</sub>


umgewandelt werden. Gips stellt darüberhinaus einen wichtigen Baustoff für die Erstellung von Mörtel und Verputzen dar und kann bereits als Gestein und somit originäres Baumaterial Eingang in das Gefüge eines Denkmals gefunden haben. Ähnliches gilt für Anhydrit.  
It is to be remembered that gypsum can be a major component of some mortars, plasters and even some building stones (e.g., selenite, used for the base of the Garrisenda Tower in Bologna, Italy) thus being an integral part of the monuments fabric.  
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==  Schadenspotential und Verwitterungsaktivität ==
==  Damage potential and weathering activity ==
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=== Solubility properties  ===
=== Solubility properties  ===
[[file:CaSO4-Steiger-1.jpg|thumb|350px|right|'''Figure1:''' Solubility of CaSO4 in water (diagram: Michael Steiger)]]
Gypsum belongs to the group of salts with a low solubility in aqueous solution and therefore is less mobile than the more soluble ones. However, when other ions are present, its solubility can be significantly increased. For example, when halite, NaCl, is present, the  solubility of gypsum can be increased by a factor four depending on the concentration ratio of the two salts. <br>


[[file:CaSO4 sol.JPG|thumb|350px|left|'''Figure1:''' Solubility of CaSO4 in water (diagram: Michael Steiger)]]
[[file:Loeslichkeit Gips 02.JPG|thumb|right|350px|'''Figure 2:'''Solubility of gypsum compared with other salts (after <bib id="Stark.etal:1996"/>)]]
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<br>Gips zählt zur Gruppe der "gering" wasserlöslichen Salze und kann somit als wenig mobil bezeichnet werden. Allerdings ist der Fremdioneneinfluss auf die Gipslöslichkeit vergleichsweise groß. So wird die Löslichkeit von Gips durch Halit je nach Konzentrationsverhältnis bis um den Faktor vier erhöht.<br>
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[[file:Loeslichkeit Gips 02.JPG|thumb|left|350px|'''Figure 2:'''Solubility of gipsum compared with other salte (after <bib id=Stark.etal:1996/>)]]
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=== Hydratationsverhalten  ===


Das System CaSO<sub>4 </sub>– H<sub>2</sub>O: <br> Calciumsulfat kann in drei unterschiedlichen Hydratstufen auftreten, dem oben bezeichneten kristallwasserlosen Anhydrit, einem Halbhydrat, welches unter Normalbedingungen die instabilste Form darstellt, und Gips. Anhydrit existiert in verschiedenen Modifikationen, wodurch je nach Abhängigkeit der Modifikation des vorliegenden Anhydrit unterschiedliche chemische Eigenschaften (z.B. variierende Löslichkeit in Wasser) feststellbar sind. Das gleiche gilt auch für die Modifikationen des Halbhydrates. <br>  
=== Hydration behavior  ===
The system CaSO<sub>4 </sub>–H<sub>2</sub>O: <br> Calcium sulfate can appear in three different hydrate phases:
*Anhydrite (CaSO<sub>4</sub>)- the above mentioned anhydrous form.  
*Bassanite (hemihydrate) (CaSO<sub>4</sub>•0.5H<sub>2</sub>O)- a metastable form.
*Gypsum (CaSO<sub>4</sub>•2H<sub>2</sub>O)- calcium sulfate dihydrate. 


Als Wert für die Übergangstemperatur (in wässriger Lösung) kann der Bereich 40°C-66°C angegeben werden. Unter normalen Klimabedingungen an Denkmalen entsteht somit beim Ausfall von Calciumsulfat aus einer wässrigen Lösung in erster Linie Gips. Liegen die Temperatur einer Lösung höher als 40°C-60°C, bildet sich v.a. Anhydrit. Parallel hierzu kommt es zur Bildung des Halbhydrates, welches zwar metastabil ist, beim Ausfall aber zunächst in großer Menge auftritt und dann in eine der stabileren Hydratstufen umgebildet wird. <br>  
Anhydrite exists in different varieties with different chemical properties, such as different solubilities in water, depending on the conditions of its formation. The same applies to bassanite, the hemihydrate. <br>
The transition temperature in aqueous solution for the gypsum-bassanite (dihydrate to hemihydrate) is in the 40°C-66°C range. Under normal climatic conditions, on  monuments, the precipitation of calcium sulfate from aqueous solution will therefore be predominantly gypsum. Anhydrite forms when the temperature in solution is higher than 40°-60°C. However, in parallel with this reaction, metastable hemihydrate can also precipitate being subsequently transformed into the more stable dihydrate form. <br>
When heating the dihydrate (as a solid and dry) to approximately 50°C, the chemically combined water is lost leaving the hemihydrate. However, a complete transition to hemihydrate only takes place at temperatures of around 100°C. If the dihydrate is heated to 500-600°C, the anhydrous calcium sulfate is formed. At temperatures above 1000°C the thermal decomposition into calcium oxide and SO<sub>3</sub> is effected.  
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=== Hygroscopicity ===


Bei der Erhitzung des Dihydrates (als Feststoff in Abwesenheit von wässrigem Lösungsmittel) kommt es bei einer Temperatur ab etwa 50°C zum Austreiben von Kristallwasser und es entsteht das Halbhydrat. Die vollständige Überführung zum Halbhydrat findet erst bei Temperaturen von ca. 100°C statt. Wird das Dihydrat längere Zeit auf 500-600°C erhitzt, liegt völlig entwässertes Calciumsulfat vor. Bei Temperaturen über 1000°C erfolgt die Zersetzung in Calciumoxid und SO<sub>3</sub>.
[[File:Veraenderung Loeslichkeit durch Fremdionen Gips .JPG|thumb|right|350px|'''Figure 3''':
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The modification of the solubility of gypsum in water, in the presence of [[halite]], is shown in Figure 3. If the concentration of halite is approximately 140 g/l in aqueous solution, around 8 g gypsum is dissolved (Angaben nach <bib id="DAns:1933"/>)]]The pure gypsum salt has no defined [[deliquescence|deliquescence]] point. If, in the presence of halite, the relative humidity levels exceed 90% RH, gypsum crystals may dissolve, due to the deliqusecent behavior of halite. A decrease of humidity levels to approximately 75% RH will result in the recrystallization of gypsum.


=== Hygroskopizität ===
=== Crystallization pressure ===


[[Datei:Veraenderung Loeslichkeit durch Fremdionen Gips .JPG|thumb|right|350px|'''Abbildung 3''': Dargestellt ist die Veränderung der Löslichkeit von Gips in Wasser unter Anwesenheit von [[Halit]] Liegt Halit in einer Konzentration von ca. 140 g/l in wässriger Lösung vor, so lösen sich hierin etwa 8 g Gips (Angaben nach <bib id=DAns:1933/>)]]
At crystallization in aqueous solution, with the saturation at a ration of 2:1, gypsum produces a linear growth pressure of 28,2-33,4 N/mm<sup>2</sup> within a temperature range of 0-50°C.   In comparison with other damaging salts, these values lie in the middle range of a calculated  scale of values reaching from 7,2 to 65,4 N/mm<sup>2</sup> [according to <bib id="Winkler:1975"/>].
Gips besitzt als Reinsalz keinen definierten und durch die relative Feuchte beeinflussten Deliqueszenzpunkt. Bei Überschreiten von 90 % r.F. kann es in Gegenwart von [[Halit]] allerdings (durch die Feuchtesorption von [[Halit]]) zum Auflösen von Gipskristallen kommen; ein Absinken der Feuchtewerte auf ca.75 % r.F. bewirkt die Rekristallisation des Gipses.


=== Kristallisationsdruck ===
=== Hydration pressure ===


Bei der Kristallisation aus wässriger Lösung, die eine Übersättigung im Verhältnis 2:1 aufweist, läßt sich für Gips ein linearer Wachstumsdruck von 28,2-33,4 N/mm<sup>2</sup> im Temperaturbereich 0-50°C angeben. Im Vergleich mit anderen bauschädlichen Salzen liegen diese Werte im mittleren Bereich einer berechneten Werteskala, die insgesamt von 7,2 bis 65,4 N/mm<sup>2</sup> reicht [nach <bib id=Winkler:1975/>].
Gypsum as a constituent of an object, can only release the water of crystallization (chemically combined water) at temperatures of approx.  50°C, i.e. it will normally not dehydrate. In contrast the enclosure of the water of crystallization is possible, if anhydrite or hemihydrate are present in a monument. Both processes are associated with a change in volume (of 31.9 % at the conversion from  hemihydrate- gypsum) and the emergence of [[Schadensmechanismen|hydration pressure]] [values according to <bib id="Sperling.etal:1980"/>]. In the instance of a conversion from hemihydrate- gypsum (keyword Gipstreiben)  at temperatures ranging from 0-20°C and an RH or 80%, a hydration pressure of 114 –160 N/mm<sup>2</sup> can result -an extremely high value [according to <bib id="Stark.etal:1996"/>].


=== Hydratationsdruck ===
== Conversion reaction ==
 
The hazardous character of gypsum to historic substance, is connected to the conversion reaction [[calcite]]- gypsum. The gypsum molecules formed by calcite hold a volume, that exceeds the volume of the original calcite molecule by about 100%. In this context a relevant damage factor is the modification of the water solubility. Calcite has a water solubility of approx. 0,014g/l (20°C) and is therefore more difficult to dissolve than gypsum. When a conversion to gypsum takes place the result is a more water sensitive system. N.B. The research by Snethlage and Wendler <bib id="Snethlage.etal:1998"/> analyses the influence of gypsum on the linear hygroscopic expansion of certain sandstone materials. The damages and the change in swelling behavior of the material was explained through the influence of gypsum.
An einem Objekt vorliegender Gips kann das im Kristallgitter enthaltene Kristallwasser nur bei Temperaturen ab ca. 50°C abgeben, wird also in der Regel nicht dehydrieren. Umgekehrt ist die Einlagerung von Kristallwasser bei Vorliegen von Anhydrit oder Halbhydrat an einem Denkmal aber durchaus möglich. Beide Vorgänge sind mit Volumenveränderungen (von 31,9% beim Übergang Halbhydrat-Gips) und dem Entstehen von [[Schadensmechanismen|Hydratationsdrücken]] verbunden [Zahlenwerte nach <bib id="Sperling.etal:1980"/>]. Für den Fall des Überganges Halbhydrat-Gips (Stichwort Gipstreiben) kann bei einer Temperatur im Bereich 0-20°C und einer r.F. von ca. 80% ein [[Schadensmechanismen|Hydratationsdruck]] von 114 –160 N/mm<sup>2</sup> angegeben werden, was extrem hohe Werte darstellt [nach <bib id=Stark.etal:1996/>].
 
== Umwandlungsreaktionen  ==
 
Wie erläutert hängt der substanzgefährdende Charakter von Gips v.a. mit der Umwandlungsreaktion Calcit-Gips zusammen. Aus Calcit gebildete Gipsmoleküle besitzen ein Volumen, welches das der ursprünglichen Calcitmoleküle um etwa 100% übersteigt. In diesem Zusammenhang ist als relevanter Schadensfaktor die Veränderung der Wasserlöslichkeit zu nennen. Calcit ist mit einer Wasserlöslichkeit von ca. 0,014g/l (20°C) schwerer löslich als Gips, so dass nach Umwandlung zu Gips ein deutlich wasserempfindlicheres System vorliegt. Hingewiesen sei auf die Untersuchungen von Snethlage und Wendler [<bib id=Snethlage.etal:1998/>], die den Einfluss von Gips auf die hygrischen Längenänderungen eines bestimmten Sandsteinmaterials analysiert haben und die beobachtete Schadensbildung in erster Linie auf das veränderte Quellverhalten durch den Gipseinfluss erklären.
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== Analytischer Nachweis ==
== Analytical identification ==
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=== Mikrochemie ===
=== Micro-chemistry ===
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=== Microscopy ===
=== Mikroskopie ===


'''Laboruntersuchung:'''  Gips ist gering wasserlöslich, so dass gipshaltiges Probematerial beim Versetzen mit Aquadest nur geringfügig in Lösung geht. Wird gipshaltiges Probematerial in Lösung gebracht, entstehen bei vorsichtigem Einengen des Lösungmitteltropfens im Zuge der Rekristallisation zunächst Einzelnadeln und zunehmend nadelige Gipsaggregate im Bereich des Saumes der Lösung (alternativ kann Probematerial mit Salzsäure versetzt werden, was ebenfalls zur Bildung von Kristallnadeln führt). Im Vergleich zu anderen Salzen, die ebenfalls nadelig rekristallisieren können wie z.B. [[Natrit|Natriumcarbonat]], weisen Gipsnadeln eine deutlich geringere Länge auf.<br>  
'''Laboratory examination:''' Gypsum is slightly water soluble, therefore gypsum-containing sample material only dissolves slightly, when mixed with distilled Water. In solution, gypsum- containing sample material recrystallizes by carefully concentrating the solvent. At first, single needles form, then increasingly needle- like gypsum aggregate in proximity of the seam of the solvent emerges. Alternatively, sample material can be dissolved in hydrochloric acid, which also leads to the formation of crystal needles. Compared to other salts that can recrystallize in needle-like shapes, e.g. [[natrite|sodium carbonate]], gypsum needles are clearly shorter.<br>


'''Brechungsindizes:''' &nbsp;&nbsp; n<sub>x</sub> = 1.521; n<sub>y</sub> =1.523; n<sub>z</sub> =1.530<br>'''Doppelbrechung''':&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Δ = 0.009<br>'''Kristallklass'''e:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; monoklin<br>  
'''Refraction indices:''' &nbsp;&nbsp;
n<sub>x</sub> = 1.521; n<sub>y</sub> =1.523; n<sub>z</sub> =1.530<br>
'''birefringence''':&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Δ = 0.009<br>
'''crystal class''':&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; monoclinic<br>


'''[[Polarized light microscopy|Polarized light microscopy examination:]]'''<br>
Apart from the typical acicular habit of gypsum crystals, (especially in recrystallized material) different morphological characteristics appear. These can be useful for identifying gypsum. Gypsum particles (in raw material samples) display shapes of rounded fragments and plate- like rhombohedra, clearly showing the inner cleavage planes. Furthermore, the occurrence of twinning shapes is typical for gypsum crystals, whether they are lath- shaped, tabular or lamellar. The assignment of refractive indices is carried out in accordance with the immersion method  using media with indices nD=1,518 und nD=1,53. Due to the often small- scale particles the examination using the Schoeder van der Kolk method is more significant and reliable than the Becke- Line test.
Gypsum crystals belong to the class of monoclinic crystals. Thus, they show, depending on the orientation of the single particle under the microscope, a parallel or respectively a symmetrical extinction, but mainly exhibit a characteristicly oblique axis position in the extinction position. On well developed crystal rhombi the oblique extinction can clearly be measured. Of all calcium sulfate crystals, gypsum has the lowest birefringence. Under crossed polarizers, gypsum has very low interference colors, lying within the gray to yellowish white range of the first order, (of course depending on the thickness of the particles).


'''[[Polarisationsmikroskopie|Polarisationsmikroskopische Untersuchung:]]'''<br>Außer dem typischen nadeligen Habitus von Gipskristallen (v.a. von rekristallisiertem Material) treten unterschiedliche morphologische Charakteristika auf, die bei der Identifikation von Gips hilfreich sind. Gipspartikel (in Rohprobematerial) zeigen sich häufig in Form von gerundeten Splittern und tafeligen Rhomboedern an denen deutliche, innere Spaltflächen ablesbar sind. Darüber hinaus ist das Auftreten von Zwillingsformen sowohl bei lattigen Partikeln wie auch Tafeln und Plättchen typisch für Gips. Die Zuweisung der Brechungsindizes erfolgt entsprechend der Immersionsmethode unter Verwendung von Medien mit den Indizes nD=1,518 und nD=1,53, wobei aufgrund der zumeist sehr kleinteiligen Partikel die Überprüfung des Schroeder van der Kolk- Schatten aussagekräftiger und sicherer ist, als der Becke-Linien Test.
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'''Possibility for mistakes:'''<br>
Gipskristalle gehören zur Klasse der monoklinen Kristalle, zeigen also je nach Ausrichtung des Einzelpartikels unter dem Mikroskop zum einen sowohl parallele bzw. symmetrische Auslöschung, weisen v.a. jedoch eine charakteristische schiefe Achsenstellung in der Auslöschungsposition auf. An gut ausgebildeten Kristallrhomben ist diese schiefe Auslöschung zumeist klar messbar. Von allen Calciumsulfaten ist Gips am geringsten doppelbrechend und erscheint bei gekreuzten Polarisatoren mit sehr niedrigen Interferenzfarben, die (natürlich in Abhängigkeit der vorliegenden Partikeldicke) im Bereich grau bis gelblich weiß der ersten Ordnung liegen.  
The Analysis methods mentioned above clearly identify gypsum, provided the following evaluation criteria are explicitly clarified.<br>


<br>'''Verwechslungsmöglichkeiten:'''<br>Gips ist im dargestellten Analyseverfahren eindeutig zuweisbar, sofern die folgenden Untersuchungskriterien eindeutig geklärt sind:<br>
*low water solubility
*characteristic needle- like morphology of the recrystallized particles
*all observable indices have a  n<sub>D</sub> –value from 1,518 and 1,530
*gypsum crystal show low interference colors
*gypsum crystals have an oblique extinction


*geringe Wasserlöslichkeit
*charakteristisch nadelige Morphologie bei rekristallisierten Partikeln
*alle beobachtbaren Indizes besitzen einen n<sub>D</sub> –Wert zwischen 1,518 und 1,530
*Gipskristalle besitzen eine geringe Doppelbrechung und niedrige Interferenzfarben
*Gipskristalle weisen eine schiefe Auslöschung auf


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{|border="2" cellspacing="0" cellpadding="4" width="60%" align="left" class="wikitable"
{|border="2" cellspacing="0" cellpadding="4" width="100%" align="left" class="wikitable"
|+''Table 1: Salt phases with a gypsum- like chemical and optical properties''                   
|+''Tabelle 1: Salzphasen mit gipsähnlichen chemischen und optischen Eigenschaften''                   
|-
|-
|bgcolor = "#F0F0F0"|'''Salzphase'''
|bgcolor = "#F0F0F0"|'''salt phase'''
|bgcolor = "#F0F0F0"|'''Unterscheidungsmerkmale'''
|bgcolor = "#F0F0F0"|'''differentiating features'''
|-
|-
|bgcolor = "#F7F7F7"|'''[[Syngenit]]''' K<sub>2</sub>Ca(SO<sub>4)</sub> • 2H<sub>2</sub>O  
|bgcolor = "#F7F7F7"|'''[[Syngenite]]''' K<sub>2</sub>Ca(SO<sub>4)</sub> • 2H<sub>2</sub>O  
|bgcolor = "#FFFFEO"|alle beobachtbaren Indizes &lt; 1,518  
|bgcolor = "#FFFFEO"|all observable indices; 1,518  
|-
|-
|bgcolor = "#F7F7F7"|'''[[Tachyhydrit]]''' CaMg<sub>2</sub>Cl<sub>6</sub> • 12H<sub>2</sub>O  
|bgcolor = "#F7F7F7"|'''[[Tachyhydrite]]''' CaMg<sub>2</sub>Cl<sub>6</sub> • 12H<sub>2</sub>O  
|bgcolor = "#FFFFEO"|zumeist ein beobachtbarer Index &lt; 1,518 / nur parallele und symmetrische Auslöschung
|bgcolor = "#FFFFEO"|mostly observable index &lt; 1,518 / only parallel and symmetrical extinction
|-
|-
|bgcolor = "#F7F7F7"|'''[[Hydromagnesit]]''' Mg<sub>5</sub>[OH(CO<sub>3</sub>)<sub>2</sub>]<sub>2</sub> • 4H<sub>2</sub>O
|bgcolor = "#F7F7F7"|'''[[Hydromagnesite]]''' Mg<sub>5</sub>[OH(CO<sub>3</sub>)<sub>2</sub>]<sub>2</sub> • 4H<sub>2</sub>O
|bgcolor = "#FFFFEO"|ein Index zumeist &gt; 1,53
|bgcolor = "#FFFFEO"|mostly one index &gt; 1,53
|}
|}
<br>
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<!--
<!--
== Röntgendiffraktometrie ==
== X- Ray diffraction ==


== Raman-Stektroskopie ==
== Raman-spectroscopy ==


== DTA / TG  ==
== DTA / TG  ==


== IR-Spektroskopie ==
== IR-spectroscopie ==


= Umgang mit Gipsschäden =
== Handling of damage caused by gypsum ==


-->
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Line 159: Line 157:
== Photos of gypsum crystals and deterioration pattern caused by gypsum==
== Photos of gypsum crystals and deterioration pattern caused by gypsum==


=== On a object  ===
=== On an object  ===


<gallery caption="" widths="200px" heights="150px" perrow="3">
<gallery caption="" widths="200px" heights="150px" perrow="3">
Line 182: Line 180:
Image:CaSO4 pol 400x 01.JPG|Calcium sulfate crystallized out of a solution in water on a glass slide
Image:CaSO4 pol 400x 01.JPG|Calcium sulfate crystallized out of a solution in water on a glass slide
Image:CaSO4+NaCl reale Probe 01.JPG|Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide
Image:CaSO4+NaCl reale Probe 01.JPG|Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide
Image:CaSO4+NaCl reale Probe 02.JPG|Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide
Image:CaSO4+NaCl reale Probe 02.JPG|Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide  
Image:HJS CaSO4 092503-1.jpg|Calcium sulfate, Gypsum crystallized out of a solution in water on a glass slide
Image:HJS CaSO4 092503-1.jpg|Calcium sulfate, Gypsum crystallized out of a solution in water on a glass slide
</gallery>


</gallery>
<br clear=all> 
== Under the Scanning Electron Microscope (SEM)== 
<gallery caption="In a SEM" widths="200px" heights="150px" perrow="3"> 
Image:SA100_1.jpg | SEM micrograph of gypsum crystals.
Image:SG2-2.jpeg  | SEM of gypsum crystals.
Image:SG2-3.jpg | SEM of gypsum crystals.
Image:SG3-SPC2.jpeg|EDX spectra of gypsum crystals. 
Image:SG3-3.jpeg  | SEM of gypsum crystals.
Image:SG3-4.jpeg | SEM of gypsum crystals.
Image:SG1-5.jpeg | SEM of gypsum crystals.
Image:SG1-SPC.jpeg| EDX spectra of gypsum crystals.
Image:Gypsum star.jpg|SEM of gypsum crystals grown from the hemihydrate.</gallery>  
<br clear=all>
<br clear=all>


== Under the Scanning Electron Microscope (SEM)  ==
<gallery caption="In a SEM" widths="200px" heights="150px" perrow="3">
Image:SA100_1.jpg | Gypsum crystals in a SEM
Image:SG2-2.jpeg |  Gypsum crystals in a SEM
Image:SG2-3.jpg |  Gypsum crystals in a SEM
Image:SG3-SPC2.jpeg |EDX spektra of gypsum crystals in a SEM
Image:SG3-3.jpeg |  Gypsum crystals in a SEM
Image:SG3-4.jpeg |  Gypsum crystals in a SEM
Image:SG1-5.jpeg | Gypsum crystals in a SEM
Image:SG1-SPC.jpeg | EDX spektra of gypsum crystals in a SEM
</gallery>
<br clear=all>


==  Weblinks ==


<references />


== Literatur  ==


<bibprint/>
==  Weblinks == 
<references />  


==  Weblinks ==
== Literature ==  
<biblist/>


<references />
More Literature :
<bibprint filter="year:2011,  author:%Badosa%"/>
<bibprint filter="year:2007, volume:52, author:%Charola%"/>
<bibprint filter="year:1901, volume:5, author:%Cameron%"/>
<bibprint filter="year:1989, volume:34, author:%Ahad%"/>
<bibprint filter="year:1991,  author:%Livingston%"/>
<bibprint filter="year:1997,  author:%Neumann%, author:%Juling%"/>
<bibprint filter="year:1994,  author:%Schlütter%"/>
<bibprint filter="year:2009,  author:%Zehnder%"/>


[[Category:Gipsum]][[Category:Sulphate]][[Category:Salt]][[Category:InProgress]][[Category:Sulfate]]
[[Category:Gipsum]][[Category:Sulphate]][[Category:Salt]][[Category:editing]][[Category:Sulfate]][[Category:Müller,Tim]][[Category:Mainusch,Nils]][[Category:Schwarz,Hans-Jürgen]][[Category:List]]

Latest revision as of 10:55, 3 May 2023

Authors: Hans-Jürgen Schwarz , Nils Mainusch, Tim Müller
English Translation by Sandra Leithäuser
back to Sulfate


Gypsum[1][2]
SA101 1.jpeg
Mineralogical name Gypsum
Chemical name Calcium sulfate dihydrate
Trivial name Alabaster, Satin Spar, Selenite,
Chemical formula CaSO4•2H2O
Other forms CaSO4 (Anhydrite)
CaSO4•0.5H2O (Bassanite)
Crystal system monoclinic
Crystal structure
Deliquescence humidity 20°C > 99% RH at 20°C
Solubility (g/l) at 20°C 2.14 g/l
Density (g/cm³) 2.31 g/cm³
Molar volume 74.69 cm3/mol
Molar weight 172.17g /mol
Transparency transparent to opaque
Cleavage perfect, clearly visible formation of fibre
Crystal habit flat, prismatic, needle- like crystal, granular, massive aggregate
Twinning very common
Phase transition
Chemical behavior hardly soluble in water
Comments
Crystal Optics
Refractive Indices α = 1.5207
β = 1.5230
γ = 1.5299
Birefringence Δ = 0.0092
Optical Orientation biaxial positive
Pleochroism colorless
Dispersion 58°
Used Literature
[Robie.etal:1978]Title: Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures
Author: Robie R.A., Hemingway B.S.; Fisher J.A.
Link to Google Scholar [Dana:1951]Title: Dana's System of Mineralogy
Author: Dana J.D.
Link to Google Scholar




Calcium sulfate and gypsum[edit]

Abstract[edit]

The article discusses the system CaSO4-H2O, in reference to gypsum. Gypsum is one of the most important salts in the deterioration of building materials and, particularly, wall-paintings. Objects exposed to exterior conditions where air pollution is present are the most prone to damage from gypsum. The appearance and mechanism of the damage, as well as the examination methods are described. Images, microphotographs and examples from practical experiences illustrate the subject.

Introduction[edit]

Gypsum is one of the most common salts causing deterioration of inorganic porous building materials. It is present on most exterior exposed surfaces and even in interior environments, under different shapes and inducing various deterioration patterns.

Gypsum, one of the most prevalent minerals, forms by precipitation from aqueous solution at temperatures under approximately 40°C. When the solution reaches higher temperatures (> 60°C) anhydrite precipitates out. Calcium sulfate and calcium sulfate dihydrate are often present in rocks. Hemihydrate does not occur naturally.

Gypsum occurs naturally in salt deposits and deserts, where "desert rose" crystals form in combination with quartz inclusions. In natural salt deposits, gypsum and anhydrite sometimes form a caprock, i.e., a massive layer of material covering the deposit. Synthetic gypsum is produced in coal-fired power plants, as a by-product of flue-gas desulfurization.

Origin and formation of gypsum on monuments[edit]

On monuments made from porous inorganic building materials, particularly in urban environment where anthropogenic air pollutants are present, such as sulfur oxides (SOx) that eventually convert to sulfuric acid in the presence of moisture, the reaction of these gases with any calcium carbonate present in limestone, sandstone, mortars, renders, results in the formation of gypsum. The reaction taking place can be simplified as follows:

CaCO3 + H2SO4 → CaSO4 + H2O + CO2

It is to be remembered that gypsum can be a major component of some mortars, plasters and even some building stones (e.g., selenite, used for the base of the Garrisenda Tower in Bologna, Italy) thus being an integral part of the monuments fabric.

Damage potential and weathering activity[edit]

Solubility properties[edit]

Figure1: Solubility of CaSO4 in water (diagram: Michael Steiger)

Gypsum belongs to the group of salts with a low solubility in aqueous solution and therefore is less mobile than the more soluble ones. However, when other ions are present, its solubility can be significantly increased. For example, when halite, NaCl, is present, the solubility of gypsum can be increased by a factor four depending on the concentration ratio of the two salts.

Figure 2:Solubility of gypsum compared with other salts (after [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia
Link to Google Scholar)

Hydration behavior[edit]

The system CaSO4 –H2O:
Calcium sulfate can appear in three different hydrate phases:

  • Anhydrite (CaSO4)- the above mentioned anhydrous form.
  • Bassanite (hemihydrate) (CaSO4•0.5H2O)- a metastable form.
  • Gypsum (CaSO4•2H2O)- calcium sulfate dihydrate.

Anhydrite exists in different varieties with different chemical properties, such as different solubilities in water, depending on the conditions of its formation. The same applies to bassanite, the hemihydrate.
The transition temperature in aqueous solution for the gypsum-bassanite (dihydrate to hemihydrate) is in the 40°C-66°C range. Under normal climatic conditions, on monuments, the precipitation of calcium sulfate from aqueous solution will therefore be predominantly gypsum. Anhydrite forms when the temperature in solution is higher than 40°-60°C. However, in parallel with this reaction, metastable hemihydrate can also precipitate being subsequently transformed into the more stable dihydrate form.
When heating the dihydrate (as a solid and dry) to approximately 50°C, the chemically combined water is lost leaving the hemihydrate. However, a complete transition to hemihydrate only takes place at temperatures of around 100°C. If the dihydrate is heated to 500-600°C, the anhydrous calcium sulfate is formed. At temperatures above 1000°C the thermal decomposition into calcium oxide and SO3 is effected.

Hygroscopicity[edit]

Figure 3: The modification of the solubility of gypsum in water, in the presence of halite, is shown in Figure 3. If the concentration of halite is approximately 140 g/l in aqueous solution, around 8 g gypsum is dissolved (Angaben nach [DAns:1933]Title: Die Lösungsgleichgewichte der Systeme der Salze ozeanischer Salzablagerungen
Author: d'Ans, J.
Link to Google Scholar)

The pure gypsum salt has no defined deliquescence point. If, in the presence of halite, the relative humidity levels exceed 90% RH, gypsum crystals may dissolve, due to the deliqusecent behavior of halite. A decrease of humidity levels to approximately 75% RH will result in the recrystallization of gypsum.

Crystallization pressure[edit]

At crystallization in aqueous solution, with the saturation at a ration of 2:1, gypsum produces a linear growth pressure of 28,2-33,4 N/mm2 within a temperature range of 0-50°C. In comparison with other damaging salts, these values lie in the middle range of a calculated scale of values reaching from 7,2 to 65,4 N/mm2 [according to [Winkler:1975]Title: Stone: Properties, Durability in Man´s Environment
Author: Winkler, Erhard M.
Link to Google Scholar].

Hydration pressure[edit]

Gypsum as a constituent of an object, can only release the water of crystallization (chemically combined water) at temperatures of approx. 50°C, i.e. it will normally not dehydrate. In contrast the enclosure of the water of crystallization is possible, if anhydrite or hemihydrate are present in a monument. Both processes are associated with a change in volume (of 31.9 % at the conversion from hemihydrate- gypsum) and the emergence of hydration pressure [values according to [Sperling.etal:1980]Title: Salt Weathering on Arid Environment, I. Theoretical ConsiderationsII. Laboratory Studies
Author: Sperling, C.H.B.and Cooke, R.U.
Link to Google Scholar]. In the instance of a conversion from hemihydrate- gypsum (keyword Gipstreiben) at temperatures ranging from 0-20°C and an RH or 80%, a hydration pressure of 114 –160 N/mm2 can result -an extremely high value [according to [Stark.etal:1996]Title: Bauschädliche Salze
Author: Stark, Jochen; Stürmer, Sylvia
Link to Google Scholar].

Conversion reaction[edit]

The hazardous character of gypsum to historic substance, is connected to the conversion reaction calcite- gypsum. The gypsum molecules formed by calcite hold a volume, that exceeds the volume of the original calcite molecule by about 100%. In this context a relevant damage factor is the modification of the water solubility. Calcite has a water solubility of approx. 0,014g/l (20°C) and is therefore more difficult to dissolve than gypsum. When a conversion to gypsum takes place the result is a more water sensitive system. N.B. The research by Snethlage and Wendler [Snethlage.etal:1998]Title: Steinzerfall und Steinkonservierung - neueste Ergebnisse der Münchner Forschungen
Author: Snethlage, Rolf; Wendler, Eberhard
Link to Google Scholar analyses the influence of gypsum on the linear hygroscopic expansion of certain sandstone materials. The damages and the change in swelling behavior of the material was explained through the influence of gypsum.

Analytical identification[edit]

Microscopy[edit]

Laboratory examination: Gypsum is slightly water soluble, therefore gypsum-containing sample material only dissolves slightly, when mixed with distilled Water. In solution, gypsum- containing sample material recrystallizes by carefully concentrating the solvent. At first, single needles form, then increasingly needle- like gypsum aggregate in proximity of the seam of the solvent emerges. Alternatively, sample material can be dissolved in hydrochloric acid, which also leads to the formation of crystal needles. Compared to other salts that can recrystallize in needle-like shapes, e.g. sodium carbonate, gypsum needles are clearly shorter.

Refraction indices:    nx = 1.521; ny =1.523; nz =1.530
birefringence:      Δ = 0.009
crystal class:            monoclinic

Polarized light microscopy examination:
Apart from the typical acicular habit of gypsum crystals, (especially in recrystallized material) different morphological characteristics appear. These can be useful for identifying gypsum. Gypsum particles (in raw material samples) display shapes of rounded fragments and plate- like rhombohedra, clearly showing the inner cleavage planes. Furthermore, the occurrence of twinning shapes is typical for gypsum crystals, whether they are lath- shaped, tabular or lamellar. The assignment of refractive indices is carried out in accordance with the immersion method using media with indices nD=1,518 und nD=1,53. Due to the often small- scale particles the examination using the Schoeder van der Kolk method is more significant and reliable than the Becke- Line test. Gypsum crystals belong to the class of monoclinic crystals. Thus, they show, depending on the orientation of the single particle under the microscope, a parallel or respectively a symmetrical extinction, but mainly exhibit a characteristicly oblique axis position in the extinction position. On well developed crystal rhombi the oblique extinction can clearly be measured. Of all calcium sulfate crystals, gypsum has the lowest birefringence. Under crossed polarizers, gypsum has very low interference colors, lying within the gray to yellowish white range of the first order, (of course depending on the thickness of the particles).


Possibility for mistakes:
The Analysis methods mentioned above clearly identify gypsum, provided the following evaluation criteria are explicitly clarified.

  • low water solubility
  • characteristic needle- like morphology of the recrystallized particles
  • all observable indices have a nD –value from 1,518 and 1,530
  • gypsum crystal show low interference colors
  • gypsum crystals have an oblique extinction


Table 1: Salt phases with a gypsum- like chemical and optical properties
salt phase differentiating features
Syngenite K2Ca(SO4) • 2H2O all observable indices; 1,518
Tachyhydrite CaMg2Cl6 • 12H2O mostly observable index < 1,518 / only parallel and symmetrical extinction
Hydromagnesite Mg5[OH(CO3)2]2 • 4H2O mostly one index > 1,53




Photos of gypsum crystals and deterioration pattern caused by gypsum[edit]

On an object[edit]

  • Salt damage on a brick, St. Jakobi Church, Perleberg

    Salt damage on a brick, St. Jakobi Church, Perleberg


Under the polarising microscope[edit]

  • In thin section of bricks
  • Brick with gypsum caused damage, St. Jakobi Church, Perleberg, Germany

    Brick with gypsum caused damage, St. Jakobi Church, Perleberg, Germany

  • Brick with gypsum caused damage, St. Jakobi Church, Perleberg, Germany

    Brick with gypsum caused damage, St. Jakobi Church, Perleberg, Germany

  • Gypsum crystals between plaster layers

    Gypsum crystals between plaster layers

  • Gypsum crystallized out of a solution in water on a glass slide
  • Calcium sulfate crystallized out of a solution in water on a glass slide

    Calcium sulfate crystallized out of a solution in water on a glass slide

  • Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide

    Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide

  • Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide

    Calcium sulfate with sodium chloride of a real sample, Gypsum crystallized out of a solution in water on a glass slide

  • Calcium sulfate, Gypsum crystallized out of a solution in water on a glass slide

    Calcium sulfate, Gypsum crystallized out of a solution in water on a glass slide


Under the Scanning Electron Microscope (SEM)[edit]

  • In a SEM
  • SEM micrograph of gypsum crystals.

    SEM micrograph of gypsum crystals.

  • SEM of gypsum crystals.

    SEM of gypsum crystals.

  • SEM of gypsum crystals.

    SEM of gypsum crystals.

  • EDX spectra of gypsum crystals.

    EDX spectra of gypsum crystals.

  • SEM of gypsum crystals.

    SEM of gypsum crystals.

  • SEM of gypsum crystals.

    SEM of gypsum crystals.

  • SEM of gypsum crystals.

    SEM of gypsum crystals.

  • EDX spectra of gypsum crystals.

    EDX spectra of gypsum crystals.

  • SEM of gypsum crystals grown from the hemihydrate.

    SEM of gypsum crystals grown from the hemihydrate.




Weblinks[edit]

  1. ↑ http://webmineral.com/data/Gypsum.shtml seen on 30.07.2010
  2. ↑ http://www.mindat.org/min-1784.html seen on 30.07.2010

Literature[edit]

[DAns:1933] d'Ans, J. (1933): Die Lösungsgleichgewichte der Systeme der Salze ozeanischer Salzablagerungen, Verlagsgesellschaft für Ackerbau, M.B.H. BerlinLink to Google Scholar
[Dana:1951]Dana E.S. (eds.) Dana J.D. (1951): Dana's System of Mineralogy, 7, Wiley & SonsLink to Google Scholar
[Robie.etal:1978]Robie R.A., Hemingway B.S.; Fisher J.A. (1978): Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar pressure and higher temperatures. In: U.S. Geol. Surv. Bull, 1452 ()Link to Google Scholar
[Snethlage.etal:1998]Snethlage, Rolf; Wendler, Eberhard (1998): Steinzerfall und Steinkonservierung - neueste Ergebnisse der Münchner Forschungen. In: Münchner Geologische Hefte, A 23, Festschrift zum 65. Geburtstag von Prof. Dr. Dietrich D. Klemm, (), 177-201Link to Google Scholar
[Sperling.etal:1980]Sperling, C.H.B.and Cooke, R.U. (1980): Salt Weathering on Arid Environment, I. Theoretical ConsiderationsII. Laboratory Studies. In: Papers in Geography, 8 ()Link to Google Scholar
[Stark.etal:1996] Stark, Jochen; Stürmer, Sylvia (1996): Bauschädliche Salze, Bauhaus-Univ. WeimarLink to Google Scholar
[Winkler:1975] Winkler, Erhard M. (1975): Stone: Properties, Durability in Man´s Environment, Springer Verlag, WienLink to Google Scholar

More Literature :

[Badosa.etal:2011]Badosa, S.; Beck, K.; Brunetaud, X.; Al-Mukhtar, M. (2011): The role of gypsum in the phenomenon of spalling of stones. In: Ionannou, Ioannis; Theodoridou, Magdalini (eds.): Proceedings of the Conference "Salt Weathering on Buildings and Stone Sculptures", Limassol, Cyprus, 19.-22. Oct. 2011, 415.Link to Google ScholarFulltext link
[Charola.etal:2007]Charola, A. Elena; Pühringer, Josef; Steiger, Michael (2007): Gypsum: a review of its role in the deterioration of building materials. In: Environmental Geology, 52 (2), 207-220, Url, 10.1007/s00254-006-0566-9Link to Google Scholar
[Cameron.etal:1901]Cameron; Seidell (1901): Solubility of gypsum in aqeous solutions of certain electrolytes. In: Journal of Physical Chemistry, 5 (), 643-655Link to Google Scholar
[LalGauri.etal:1989]Lal Gauri, K.; Chowdhury, Ahad N.; Kulshreshtha, Niraj P.; Punuru, Adinarayana R. (1989): The sulfation of marble and the treatment of gypsum crusts. In: Studies in Conservation, 34 (4), 201-6Link to Google Scholar
[Livingston:1991]Livingston, R. (1991): The use of gypsum mortar in historic buildings. In: Brebbia, C. A.; Dominguez, J.; Escrig, F. (eds.): Structural Repair and Maintenance of Historic Buildings II, Computational Mechanics Publications, 157-165.Link to Google Scholar
[Neumann.etal:1997]Neumann, Hans-Hermann; Lork, A.; Steiger, Michael; Juling, Herbert (1997): Decay patterns of weathered quarz sandstones: Evidence of gypsum induced structural changes. In: Sveinsdottir, E.L. (eds.): Proceedings 6th Euroseminar on microscopy applied to building materials, Iceland Building Research Institute, 238-249.Link to Google Scholar
[Schluetter.etal:1994]Schlütter, Frank; Juling, Herbert; Blaschke, Rochus (1994): Black skins and gypsum crystallization on terra-cotta material--microscopical investigations on samples of the Schwerin Castle. In: Fitz, Stephen (eds.): NATO-CCMS pilot study "Conservation of historic brick structures:" proceedings of the 7th expert meeting, Venice, 22-24 November 1993, , 90-99.Link to Google Scholar
[Zehnder.etal:2009]Zehnder, K.; Schoch, O. (2009): Efflorescence of mirabilite, epsomite and gypsum traced by automated monitoring on-site. In: Journal of Cultural Heritage, 10 (3), 319-330, Url, 10.1016/j.culher.2008.10.009Link to Google Scholar

relative humidity

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

Calciumsulfatehemihydrate exists in two modifications: α - and β-hemihydrate. β-hemihydrate is formed in dry media, when gypsum is heated over 40°C, while α –hemihydrates is formed in aqueous media when heated over 97.2°C.

A molecule is an electrically neutral group of at least two atoms.

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral

Scanning Electron Microscope

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