Sodium sulfate

Authors: Hans-Jürgen Schwarz , Michael Steiger, Tim Müller, Amelie Stahlbuhk
back to Sulfate

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Abstract

In this article the different phases of sodium sulfate and their properties will be presented.

Phases and hydrate phases

There are four different phases of sodium sulfate whereof only two are stable. The others are metastable but were also detected.
Thenardite Na₂SO₄
Sodium sulfate phase III Na₂SO₄ metastable
Sodium sulfate heptahydrate Na₂SO₄•7H₂O metastable
Mirabilite Na₂SO₄•10H₂O

Occurrence

Both thenardite and mirabilite occur as natural minerals. In nature sodium sulfate occurs in mineral waters in the form of double salts, as deposits of former salt lakes. The hydrated sodium sulfate was first described by Glauber in 1658 where he called it "sal mirabilis". Mirabilite is also known as "Glauber's salt" in honor of its discoverer.

Origin and formation of thenardite / mirabilite in monuments

When sodium ions in conjunction with other anions enter porous inorganic building materials, sodium sulfate may be formed by reaction with sulfate contributed by other sources, for example air contaminated with sulfur oxide gases. Portland cement contains a certain amount of sodium or potassium sulfate. In Germany, the standardization institute (DIN) allows a content of up to 0.5 % soluble alkalis. This means that 100 kg of Portland cement containing only 0.1% soluble Na₂O can form 520g of Mirabilite when reacting with sulfate [calculation by Arnold/Zehnder 1991]. Sodium ions can also enter into monuments from various cleaning materials and, in older restoration products, such as water glass. Ground water, and even surface water, are also a possible source of Na⁺-ions as well as sulfate ions. De-icing road salt may contain a large amount of soluble sodium chloride. Finally, in coastal areas, sea water is a significant source of NaCl.

Solubility

Figure 1: Solubilities in the system Na₂SO₄-H₂O as a function of temperature. The molalitym as [n(Na₂SO₄•xH₂O)•kg(H₂O)^-1] is plotted versus the temperature. The equilibria of different phases are distinguished by different colours. Dashed lines mark metastable equilibria. According to [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
.

The phases of sodium sulfate are easily soluble in water (figure 1), so they have a high mobility in porous materials as well. The solubility of the phases depends on the temperature.
The decahydrate mirabilite is stable below 32.4 °C. Above this temperature the anhydrous thenardite is the stable crystalline phase, which in turn is metastable below this transition temperature. In the case of a temperature drop in a solution that is saturated with respect to thenardite, high supersaturations with respect to mirabilite and with that its precipitation are possible, which involves a certain damage potential.

Table 1: Solubilities in mol/kg of different phases of sodium sulfate at 20 °C [according to [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
].

phase	solubility [mol/kg] at 20°C
thenardite	3.706
sodium sulfate phase III	4.428
sodium sulfate heptahydrate	3.143
mirabilite	1.353

Hygroscopicity

The deliquescence behaviour of the different sodium sulfate phases is shown in figure 2 as a function of temperature. The equilibrium humidities of the thenardite/mirabilite conversion are also represented.
The temperature dependencce for thenardite and for mirabilite are contradictory, as the deliquescence humidity of mirabilite decreases with increasing temperature whereas those of thenardite increases and it is influenced to a lesser extend by temperature changes.

Figure 2: Deliquescnece behaviour in the system Na₂SO₄-H₂O as a funcction of temperature. The water activity a_w is plotted versus the temperature. Deliquescence humidities of the different phases are labled by different colours. Dashed lines indicate metastable equilibria. Equilibrium humidities for the thenaridte/mirabilite converison are also shown. according to [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
.

Table 2: Deliquescence and equilibrium humidities at 20°C [according to [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
].

considered phase transition	deliquescence or equilibrium humidity at 20 °C
sodium sulfate phase III-solution	82.9 %
thenardite-solution	86.6 %
sodium sulfate heptahydrate-solution	89.1 %
mirabilite-solution	95.6 %
thenardite-mirabilite	76.4 %

Hydration behavior

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Conversion of mirabilite (?) into thenardite

The Na₂SO₄ – H₂O system:

The only stable forms of sodium sulfate are the decahydrate (mirabilite) and the anhydrite (thenardite). The generation of mirabilite by recrystallization of the salt from an aqueous supersaturated solution occurs at 32.4°C. In particular, the transition from thenardite to mirabilite and the incorporation of 10 water molecules in the crystal lattice causes a volume expansion of 320%. This transition happens at a relatively low temperature (32-35°C), the damage caused by this salt is highly dependent on the temperature and thus on the environment. This temperature range is given as a guide, because this transition could happen for example at 25°C at 80% relative humidity, or even at 0°C at 60.7% relative humidity [information from Gmelin]. Due to this strong dependence on the environmental parameters, an estimate of the damage caused on buildings by crystallization and hydration of sodium sulfate are very difficult to obtain.

Pictures of salt and salt damage

Under the polarizing microscope

• Sodium sulfate crystals between to glass plates
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  plain polarized light

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  crossed polarisers, red I

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  plain polarized light

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  crossed polarisers

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  crossed polarisers, red I

• 

  plain polarized light

• 

  crossed polarisers, red I

• 

  plain polarized light

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  plain polarized light

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  crossed polarisers, red I

• Sodium sulfate crystals, crystallised out of a water extraction of a real sample
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  plain polarized light

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  plain polarized light

Literature

[Arnold.etal:1991]	Arnold, Andreas; Zehnder, Konrad (1991): Monitoring Wall Paintings Affected by soluble Salts. In: Cather, Sharon (eds.): The Conservation of Wall Paintings: Proceedings of a symposium organized by the Coutrauld Institut of Art and the Getty Conservation Institute, London, July 13-16, The Getty Conservation Institute, 103-136.
[Saidov:2012]	Saidov, Tamerlan Adamovich (2012): Sodium sulfate heptahydrate in weathering phenomena of porous materials. dissertation, Technische Universiteit Eindhoven, Url
[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 ()
[Steiger.etal:2008]	Steiger, Michael; Asmussen, Sönke (2008): Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress. In: Geochimica et Cosmochimica Acta, 72 (17), 4291-4306, Url
[Vogt.etal:1993]	Vogt, R.; Goretzki, Lothar (1993): Der Einfluss hygroskopischer Salze auf die Gleichgewichtsfeuchte und Trocknung anorganischer Baustoffe, unveröffentlichter Bericht.
[Winkler.etal:1970]	Winkler, Erhard M.; Wilhelm, E.J. (1970): Saltburst by Hydration Pressure in Architectural Stone in Urban Atmosphere. In: Geological Society of America, Bulletin, 81 (), 567-572

Further literature

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[Amirthalingam:1977]	Amirthalingam, V.; Karkhanaavala, M. D.; Rao, U. R. K. (1977): Topotaxic phase change in Na2SO4. In: Acta Cryst., A33 (), 522-523
[Bayh.etal:1966]	Bayh, W.; Haussuehl, Siegfried (1966): Elastische und mechanische Eigenschaften von Na2SO4 (Thenardit). In: Acta Crystallogr, 20 (6), 931-932
[Brown.etal:2000]	Brown, P. W.; Badger, S. (2000): The distributions of bound sulfates and chlorides in concrete subjected to mixed NaCl, MgSO4, Na2SO4 attack. In: Cem. Concr. Res., 30 (10), 1535-1542
[Brown.etal:2001]	Brown, P. W.; Badger, S. (2001): Reply to the discussion by William G. Hime and Stella L. Marusin of the paper "The distribution of bound sulfates and chlorides in concrete to mixed NaCl, MgSO4, Na2SO4 attack". In: Cem. Concr. Res., 31 (7), 1117-1118
[DeClercq.etal:2012]	Clercq, Hilde; Jovanović, Maja; Linnow, Kirsten; Steiger, Michael (2012): Performance of limestones laden with mixed salt solutions of Na2SO4–NaNO3 and Na2SO4–K2SO4. In: Environmental Earth Sciences, (), 1-11, Url, 10.1007/s12665-012-2017-0
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[Lindstroem.etal:2015]	Nadine Lindström; Nicole Heitmann; Kirsten Linnow; Michael Steiger (2015): Crystallization behavior of NaNO3–Na2SO4 salt mixtures in sandstone and comparison to single salt behavior. In: Applied Geochemistry, 63 (), 116 - 132, Url, https://doi.org/10.1016/j.apgeochem.2015.07.007
[Lindstroen.etal:2016]	Nadine Lindström; Tanya Talreja; Kirsten Linnow; Amelie Stahlbuhk; Michael Steiger (2016): Crystallization behavior of Na2SO4–MgSO4 salt mixtures in sandstone and comparison to single salt behavior. In: Applied Geochemistry, 69 (), 50 - 70, Url, https://doi.org/10.1016/j.apgeochem.2016.04.005
[Linnow.etal:2012]	Linnow, Kirsten; Steiger, Michael; Lemster, Christine; Clercq, Hilde; Jovanović, Maja (2012): In situ Raman observation of the crystallization in NaNO3–Na2SO4–H2O solution droplets. In: Environmental Earth Sciences, (), 1-12, Url, 10.1007/s12665-012-1997-0
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Water glass is the common name for a liquid solution of alkaline silicates, also known as Sodium silicate Na2SiO3 or liquid glass.

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

the relative humidity, above which a material, e.g. a salt, absorbes humidity from the air and dissolves.