Friedel's salt

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. 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⁻, SeO₃²⁻, SeO₄²⁻