Sodium sulfate

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

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]Author: Steiger, Michael; Asmussen, Sönke
Journal: Geochimica et Cosmochimica Acta
Number: 17
Pages: 4291-4306
Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Url: https://doi.org/10.1016/j.gca.2008.05.053
Volume: 72
Year: 2008
Key: sodium sulfate,
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]Author: Steiger, Michael; Asmussen, Sönke
Journal: Geochimica et Cosmochimica Acta
Number: 17
Pages: 4291-4306
Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Url: https://doi.org/10.1016/j.gca.2008.05.053
Volume: 72
Year: 2008
Key: sodium sulfate,
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]Author: Steiger, Michael; Asmussen, Sönke
Journal: Geochimica et Cosmochimica Acta
Number: 17
Pages: 4291-4306
Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Url: https://doi.org/10.1016/j.gca.2008.05.053
Volume: 72
Year: 2008
Key: sodium sulfate,
Link to Google Scholar.



Table 2: Deliquescence and equilibrium humidities at 20°C [according to [Steiger.etal:2008]Author: Steiger, Michael; Asmussen, Sönke
Journal: Geochimica et Cosmochimica Acta
Number: 17
Pages: 4291-4306
Title: Crystallization of sodium sulfate phases in porous materials: The phase diagram Na2SO4–H2O and the generation of stress
Url: https://doi.org/10.1016/j.gca.2008.05.053
Volume: 72
Year: 2008
Key: sodium sulfate,
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

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]Author: Saidov, Tamerlan Adamovich
Note: https://doi.org/10.6100/IR737583 , ISBN: 978-90-386-3268-1 Salt weathering is widely recognized as one of the most common mechanisms for deterioration of porous materials: monuments, sculptures and civil structures. One of the most damaging salts is sodium sulfate, which can have different crystalline modifications: thenardite (anhydrous), mirabilite (Na2SO4.10H2O), and heptahydrate (Na2SO4.7H2O), which is thermodynamically metastable. Na2SO4.7H2O has a well-defined supersolubility region limited by the so-called heptahydrate supersolubility line. To predict and prevent crystallization damage of porous materials it is necessary to know the salt phase that is responsible for damage as well as its nucleation and growth behavior. The crystallization of sodium sulfate can be induced by increasing the supersaturation either by drying or by cooling of a sample. In this study the supersaturation was measured non-destructively by Nuclear Magnetic Resonance (NMR). First, the crystallization was studied in bulk solutions. For this purpose an NMR setup was combined with time lapse digital microscopy, allowing simultaneous measurement of supersaturation in a droplet and visualization of the crystal growth. Two crystallization mechanisms were tested: diffusion controlled and adsorption controlled. The crystallization of heptahydrate was found to have so-called adsorption-controlled behavior. As a second step towards understanding sodium sulfate crystallization in porous materials, mineral powders were added to sodium sulfate solutions. This allowed studying the transition from in-bulk to in-pore crystallization of sodium sulfate. It was found that mineral powders act as additional nucleation centers, which accelerate the precipitation of crystalline phases from a solution, but do not have an effect on the crystalline phase that is growing. Next, the crystallization of sodium sulfate in porous materials was studied. The internal properties of the materials influence the dynamics of crystallization by providing a surface for nucleation. This is in correspondence with grain-boundary crystallization theory. It was found that the internal properties of porous materials do not influence the crystalline phase that is formed. In all measurements that were performed, the formation of sodium sulfate heptahydrate was observed with a reproducibility of 95%. No spontaneous crystallization of mirabilite directly from a solution was observed. Finally, the crystallization pressure was studied. To this end NMR measurements and optical length measuring techniques were combined. This allowed studying the crystalline phase being formed and the crystallization pressure caused by crystal formation during cooling and drying of the samples. It was found that a crystallization pressure capable to damage common porous materials can be expected from mirabilite. Series of weathering tests showed two ways for mirabilite formation: cooling of sodium sulfate solution to cryohydrates and rewetting of previously formed thenardite. Doctoral degree 30-10-2012; Department of Applied Physics; Supervisors: K. Kopinga and G.W. Scherer; Co-promotor: L. Pel
School: Technische Universiteit Eindhoven
Title: Sodium sulfate heptahydrate in weathering phenomena of porous materials
Type: dissertation
Url: https://pure.tue.nl/ws/portalfiles/portal/3710663/737583.pdf
Year: 2012
Key: sodiumsulfate, heptahydrate
Link to Google Scholar


Pictures of salt and salt damage

Under the polarizing microscope

  • Sodium sulfate crystals between to glass plates

  • plain polarized light

  • crossed polarisers, red I

  • plain polarized light

  • crossed polarisers

  • crossed polarisers, red I

  • plain polarized light

  • crossed polarisers, red I

  • plain polarized light

  • plain polarized light

  • crossed polarisers, red I


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

  • plain polarized light

  • plain polarized light

Weblinks


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