From Saltwiki

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

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
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


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.


Gypsum is one of the most common salts causing the deterioration of inorganic porous building materials. It is present on most all 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

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

Solubility properties

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

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.


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

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

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

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


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

On an object

Under the polarising microscope

Under the Scanning Electron Microscope (SEM)



[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. 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. 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. 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, 415Link 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. 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. 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. 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, 157-165Link 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, 238-249Link 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, , 90-99Link to Google Scholar
[Zehnder.etal:2009]Zehnder, K.; Schoch, O. (2009): Efflorescence of mirabilite, epsomite and gypsum traced by automated monitoring on-site. Journal of Cultural Heritage, 10 (3), 319-330, Url, 10.1016/j.culher.2008.10.009Link to Google Scholar