Friedel's salt

Friedel's salt
Names

IUPAC name Calcium chloroaluminate
Other names Friedel's salt Calcium aluminium chlorohydrate Calcium aluminium chlorohydroxide Calcium aluminium oxychloride
Properties
Chemical formula	Ca₂Al(OH)₆(Cl, OH) · 2 H₂O
Appearance	White solid
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Y verify (what is: Y/N?)
Infobox references

Friedel's salt is an anion exchanger mineral belonging to the family of the layered double hydroxides (LDHs). It has affinity for anions as chloride and iodide and is capable to retain them to a certain extent in its crystallographical structure.

Composition

Friedel's salt general formula is:

Ca₂Al(OH)₆(Cl, OH) · 2 H₂O.

In the cement chemist notation, considering that

2 OH⁻ ↔ O²⁻ + H₂O,

and doubling all the stoichiometry, it can also be written as follows:

3CaO·Al₂O₃·CaCl₂ · 10 H₂O

Friedel's salt can also be considered as an AF_m phase in which chloride ions have replaced sulfate ions and is formed in cements initially rich in tri-calcium aluminate (C₃A).

2 Cl⁻ + 3CaO·Al₂O₃·CaSO₄ · 10 H₂O → 3CaO·Al₂O₃·CaCl₂ · 10 H₂O + SO₄²⁻

It plays a main role in the retention of chloride anions in cement and concrete. However, Friedel's salt remains a poorly understood phase in the CaO-Al₂O₃-CaCl₂-H₂O system, and is critical for the stability of salt-saturated Portland cement-based grouts.

Discovery

Nowadays, Friedel's salt discovery is relatively difficult to trace back from the recent literature, simply because it is an ancient finding of a poorly known and non-natural product. It has been synthesised and identified in 1897 by Georges Friedel, mineralogist and crystallographer, son of the famous French chemist Charles Friedel.^[1] Georges Friedel also synthesised Calcium aluminate (1903) in the framework of his work on the Macles theory (twin crystals). This point requires further verification.^[2]

Formation

Relation with Tricalcium aluminate.
Incorporation of chloride.
Solid solutions.

Role in cement

Importance for the reactive transport of chloride in cement in relation with corrosion of steel reinforcement.

Anion getter

Trap toxic anions in cement such as, e.g., ¹²⁹I⁻, SeO₃²⁻, SeO₄²⁻

References

↑ Friedel, Georges (1897). "Sur un chloro-aluminate de calcium hydrate´ se maclant par compression". Bulletin de la Société française de minéralogie et de cristallographie 19: 122–136.
↑ Biography of Georges Friedel by F. Greandjean on annales.org., in French.

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Pitt, J. M.; M. C. Schluter; D. Y. Lee; W. Dubberke (1987). Sulfate impurities from deicing salt and durability of Portland cement mortar. Transportation Research Board.

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Suryavanshi, A. K.; J. D. Scantlebury; S. B. Lyon (1996). "Mechanism of Friedel's salt formation in cements rich in tri-calcium aluminate". Cement and Concrete Research 26 (5): 717–727. doi:10.1016/S0008-8846(96)85009-5.
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