Friedel's salt

Friedel 's salt is a mineral compound which is formed in the concrete during the penetration of chloride containing solutions. It is produced by reaction of chloride ions with the calcium contained in the concrete and has the composition CaO · Al2O3 · CaCl2 3 · 10 H2O. It is considered the most famous and probably most stable chloride-containing phase of the hardened cement paste. The compound is named after its discoverer, the French mineralogist Georges Friedel.

Friedel 's salt is produced in concrete by the reaction of mono-sulphate (3 CaO · Al2O3 · CaSO4 · 12 H2O) upon the occurrence of chloride-containing solutions, such as de-icing salt. The chloride ion displaces this the sulfate ion from the salt.

In the presence of carbon dioxide chloride is displaced from the salt to form a carbonate. Friedel 's salt is not stable in the presence of sulphate ions, it reacts to form ettringite.