Friedel's salt

Friedel's salt
IUPAC name Calcium chloroaluminate
Other names Friedel's salt Calcium aluminium chlorohydrate Calcium aluminium chlorohydroxide Calcium aluminium oxychloride
style="background: #F8EABA; text-align: center;" colspan="2" | Properties
Molecular formula	Ca2Al(OH)6(Cl, OH) · 2 H2O
Appearance	White solid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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^– ↔ O^2– + 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 + SO₄^2-

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. Georges Friedel also synthesised Calcium aluminate (1903) in the framework of his work on the Macles theory (twin crystals). This point requires further verification.

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^-, SeO₃^2-, SeO₄^2-

See also

• Aluminium chlorohydrate
• Cement
  • Sorel cement, a mixture of general formula: Mg₄Cl₂(OH)₆
  • Stanislas Sorel, a french engineer who made a new form of cement from a combination of magnesium oxide and magnesium chloride
• Concrete
  • Salt-concrete, also known as salzbeton
• Chloride
• Layered double hydroxides
• Tricalcium aluminate
• Friedel-Crafts reaction

• Friedel family, a rich lineage of French scientists:
  • Charles Friedel (1832–1899), French chemist known for the Friedel-Crafts reaction
  • Georges Friedel (1865–1933), here above mentioned, French crystallographer and mineralogist; son of Charles
  • Edmond Friedel (1895-1972) (1895–1972), French Polytechnician and mining engineer, founder of BRGM, the French geological survey; son of Georges
  • Jacques Friedel, (1921-), French physicist; son of Edmond, see the French site for Jacques Friedel

References

• Chatterji, S. (1995). "On the applicability of Fick's second law to chloride ion migration through portland cement concrete". Cement and Concrete Research. 25 (2): 299–303. doi:10.1016/0008-8846(95)00013-5. 

External links

• Friedel's salt, Ca₂Al(OH)₆ (Cl, OH) · 2H₂O: Its solid solutions and their role in chloride binding
• Glossary of cement concrete and SEM terminologyde:Friedelsches Salz