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 of retaining them to a certain extent in its crystallographical structure.
Composition
Friedel's salt is a
layered double hydroxide (LDH) of general formula:
:
or more explicitly for a positively-charged LDH mineral:
:
or by directly incorporating water molecules into the Ca,Al hydroxide layer:
:
where
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
and
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water ...
anions occupy the interlayer to compensate the excess of positive charges.
In the
cement chemist notation
Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals.
Abbreviations of oxides
The main ...
(CCN), considering that
:
and doubling all the stoichiometry, it could also be written in CCN as follows:
:
A simplified chemical composition with only Cl
– in the interlayer, and without OH
–, as:
:
can be also written in cement chemist notation as:
:
Friedel's salt is formed in cements initially rich in
tri-calcium aluminate (C
3A). Free-chloride ions directly bind with the AFm hydrates (C
4AH
13 and its derivatives) to form Friedel's salt.
Importance of chloride binding in AFm phases
Friedel's salt plays a main role in the binding and retention of chloride anions in cement and concrete. However, Friedel's salt remains a poorly understood phase in the CaO–Al
2O
3–CaCl
2–H
2O system. A sufficient understanding of the Friedel's salt system is essential to correctly model the reactive transport of chloride ions in reinforced concrete structures affected by chloride attack and steel reinforcement corrosion. It is also important to assess the long-term stability of salt-saturated Portland cement-based grouts to be used in engineering structures exposed to seawater or concentrated brine as it is the case for radioactive waste disposal in deep salt formations.
Another reason to study AFm phases and the Friedel's salt system is their tendency to bind, trap and to immobilise toxic anions, such as
129I
−, , and , in cementitious materials. Their characterization is important to conceive anion getters and to assess the retention capacity of cementitious buffer and concrete barriers used for radioactive waste disposal.
Chloride sorption and anion exchange in AFm phases
Friedel's salt could be first tentatively represented as an
AFm phase
An AFm phase is an "alumina, ferric oxide, monosubstituted" phase, or aluminate ferrite monosubstituted, or , mono, in cement chemist notation
Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily b ...
in which two chloride ions would have simply replaced one sulfate ion. This conceptual representation based on the intuition of a simple stoichiometric exchange is very convenient to remind but such a simple mechanism likely does not directly occur and must be considered with caution:
:
Indeed, the reality appears to be more complex than such a simple stoechiometric exchange between chloride and sulfate ions in the AFm crystal structure. In fact, it seems that chloride ions are electrostatically sorbed onto the positively charged
2Al(OH)6 · 2H2O">a2Al(OH)6 · 2H2Osup>+ layer of AFm hydrate, or could also exchange with hydroxide ions (OH
–) also present in the interlayer. So, the simple and "apparent" exchange reaction first presented here above for the sake of ease does not correspond to the reality and is an oversimplified representation.
Similarly, Kuzel’s salt could seem to be formed when only 1 Cl
– ion exchanges with in AFm (half substitution of sulfate ions):
:
However, Mesbah et al. (2011) have identified two different types of interlayers in the crystallographic structure they have determined and it precludes the common anion exchange reaction presented here above as stated by the authors themselves in their conclusions:
Kuzel's salt is a two-stage layered compound with two distinct interlayers, which are alternatively filled by chloride anions only (for one kind of interlayer) and by sulfate anions and water molecules (for the other kind of interlayer). Kuzel's salt structure is composed of the perfect intercalation of the Friedel's salt structure and the monosulfoaluminate structure (the two end-members of the studied bi-anionic AFm compound). The structural properties of Kuzel's salt explain the absence of extended chloride to sulfate or sulfate to chloride substitution.
The staging feature of Kuzel's salt certainly explains the difficulties to substitute chloride and sulfate: the modification in one kind of interlayer involves a modification in the other kind of interlayer in order to preserve the electroneutrality of the compound. The two-stage feature of Kuzel's salt implies that each interlayer should be mono-anionic.
So, if the global chemical composition of Friedel's salt and Kuzel's salt corresponds well respectively with the stoichiometry of a complete substitution, or a half substitution, of sulfate ions by chloride ions in the crystal structure of AFm, it does not tell directly anything on the exact mechanism of anion substitution in this complicated system. Only detailed and well controlled chloride sorption, or anion exchange, experiments with a complete analysis of all the dissolved species present in aqueous solution (also including OH
–, Na
+ and Ca
2+ ions) can decipher the system.
Discovery
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 Georges Friedel (19 July 1865 – 11 December 1933) was a French mineralogist and crystallographer.
Life
Georges was the son of the chemist Charles Friedel. Georges' grandfather was Louis Georges Duvernoy who held the chair in comparative anat ...
, mineralogist and crystallographer, son of the famous French chemist
Charles Friedel
Charles Friedel (; 12 March 1832 – 20 April 1899) was a French chemist and mineralogist.
Life
A native of Strasbourg, France, he was a student of Louis Pasteur at the Sorbonne. In 1876, he became a professor of chemistry and mineralogy at ...
.
Georges Friedel also synthesised calcium aluminate (1903) in the framework of his work on the
macles theory (twin crystals). This point requires further verification.
[Biography of Georges Friedel by F. Greandjean on annales.org., in French.](_blank)
/ref>
Formation
* Relation with Tricalcium aluminate.
* Incorporation of chloride.
* Solid solutions.
See also
* AFm phase
An AFm phase is an "alumina, ferric oxide, monosubstituted" phase, or aluminate ferrite monosubstituted, or , mono, in cement chemist notation
Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily b ...
* Aluminium chlorohydrate
* Cement
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement m ...
** Sorel cement
Sorel cement (also known as magnesia cement or magnesium oxychloride) is a non-hydraulic cement first produced by the French chemist Stanislas Sorel in 1867.Sorel Stanislas (1867).Sur un nouveau ciment magnésien. ''Comptes Rendus Hebdomadaires de ...
, a mixture of general formula: Mg4Cl2(OH)6
** Stanislas Sorel
Stanislas Sorel (born 1803, Putanges, France; died 18 March 1871, Paris) was a French civil engineer, inventor, and chemist, raised the son of a poor clock-maker.Beach, Alfred Ely (1871). Stanislas Sorel. Sci. Am, 25, 151.
A poorly known aspect ...
, a French engineer who made a new form of cement from a combination of magnesium oxide
Magnesium oxide ( Mg O), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg2+ ions and O2− ...
and magnesium chloride
Magnesium chloride is the family of inorganic compounds with the formula , where x can range from 0 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in nat ...
* Concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
** Salt-concrete
Salt-concrete (or ''salzbeton'') is a building material that is used to reduce the water inflow in mining shafts in salt mines. It is composed of 16% cement, 39% halite, 16% limestone powder, 14% water and 15% sand.
History
Salt-concrete was used ...
, also known as ''salzbeton''
* Chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...
* Layered double hydroxides
* Tricalcium aluminate
* Friedel-Crafts reaction
* Friedel family Four French scientists with the same Friedel family name are in direct lineage, Charles, Georges, Edmond and Jacques:
* Charles Friedel (1832–1899), French chemist known for the Friedel-Crafts reaction
* Georges Friedel (1865–1933), French cr ...
, a rich lineage of French scientists:
** Charles Friedel
Charles Friedel (; 12 March 1832 – 20 April 1899) was a French chemist and mineralogist.
Life
A native of Strasbourg, France, he was a student of Louis Pasteur at the Sorbonne. In 1876, he became a professor of chemistry and mineralogy at ...
(1832–1899), French chemist known for the Friedel-Crafts reaction
** Georges Friedel Georges Friedel (19 July 1865 – 11 December 1933) was a French mineralogist and crystallographer.
Life
Georges was the son of the chemist Charles Friedel. Georges' grandfather was Louis Georges Duvernoy who held the chair in comparative anat ...
(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
Jacques Friedel ForMemRS (; 11 February 1921 – 27 August 2014) was a French physicist and material scientist.
Education
Friedel attended the Cours Hattemer, a private school. He studied at the École Polytechnique from 1944 to 1946, and the ...
, (1921-2014), French physicist; son of Edmond, see the French site for Jacques Friedel
Jacques Friedel ForMemRS (; 11 February 1921 – 27 August 2014) was a French physicist and material scientist.
Education
Friedel attended the Cours Hattemer, a private school. He studied at the École Polytechnique from 1944 to 1946, and the ...
References
Further reading
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External links
Friedel's salt, Ca2Al(OH)6 (Cl, OH) · 2H2O: Its solid solutions and their role in chloride binding
{{DEFAULTSORT:Friedel's Salt
Aluminates
Calcium compounds
Cement
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Crystallography
Hydrates
Hydroxides
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