Sodium sulfate

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Authors: Hans-Jürgen Schwarz , Michael Steiger, Tim Müller, Amelie Stahlbuhk
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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
Link to Google Scholar
.


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
Link to Google Scholar
].
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 aw 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
Link to Google Scholar
.



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
Link to Google Scholar
].
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.


The importance of the heptahydrate in the damage process

see [Saidov:2012]Title: Sodium sulfate heptahydrate in weathering phenomena of porous materials
Author: Saidov, Tamerlan
Link to Google Scholar


Pictures of salt and salt damage

Under the polarizing microscope


Weblinks

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Literature

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