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Magnesium based cement: 1 part MgCl2 for 3 or 5 parts of Mg(OH)2
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. In fact, in 1855, before working with magnesium compounds, Stanislas Sorel first developed a two component cement by mixing zinc oxide powder with a solution of zinc chloride. In a few minutes he obtained a dense material harder than limestone. Only a decade later, Sorel replaced zinc with magnesium in his formula and also obtained a cement with similar favorable properties. This new type of cement was stronger and more elastic than Portland cement, and therefore exhibited a more resilient behavior when submitted to shocks. The material could be easily molded like plaster when freshly prepared, or machined on a lathe after setting and hardening. It was very hard, could be easily bound to many different types of materials (good adhesive properties), and colored with pigments. Therefore, it was used to make mosaics and to mimic marble. After mixing with cotton crushed in powder, it was also used as a surrogate material for ivory to fabricate billiard balls resistant to shock. Sorel cement is a mixture of magnesium oxide (burnt magnesia) with magnesium chloride with the approximate chemical formula Mg4Cl2(OH)6(H2O)8, or MgCl2·3Mg(OH)2·8H2O, corresponding to a weight ratio of 2.5–3.5 parts MgO to one part MgCl2. Quite surprisingly, much more recently, another chemist, Charles A. Sorrell (1977, 1980) – whose family name sounds quite similar to that of Stanislas Sorel – also studied the topic and published works on the same family of oxychloride compounds based on zinc and magnesium, just as Sorel had done about 100 years before. The zinc oxychloride cement is prepared from zinc oxide and zinc chloride instead of the magnesium compounds.

The set cement consists chiefly of a mixture of magnesium oxychlorides and magnesium hydroxide in varying proportions, depending on the initial cement formulation, setting time, and other variables. The main stable oxychlorides at ambient temperature are the so called "phase 3" and "phase 5", whose formulas can be written as 3··8H2O and 5··8H2O, respectively; or, equivalently, ·4H2O and ·4H2O. Phase 5 crystallizes mainly as long needles which are actually rolled up sheets. These interlocking needles give the cement its strength. In the long term the oxychlorides absorb and react with carbon dioxide from the air to form magnesium chlorocarbonates.

These compounds are the primary components of matured Sorel cement, first prepared in 1867 by Stanislas Sorel. Phase 3 was properly isolated and described by Robinson and Waggaman (1909), and phase 5 was identified by Lukens (1932).

Sorel cement can withstand 10,000–12,000 psi (69–83 MPa) of compressive force whereas standard Portland cement can typically only withstand 7,000–8,000 psi (48–55 MPa). It also achieves high strength in a shorter time. Sorel cement has a remarkable capacity to bond with, and contain, other materials. It also exhibits some elasticity, an interesting property increasing its capacity to resist shocks (better mechanical resilience), particularly useful for billiard balls. The pore solution in wet Sorel cement is slightly alkaline (pH 8.5 to 9.5), but significantly less so than that of Portland cement (hyperalkaline conditions: pH 12.5 to 13.5). These differences make different construction applications suitable. Prolonged exposure of Sorel cement to water leaches out the soluble , leaving hydrated brucite as the binding phase, which without absorption of , can result in loss of strength.

In use, Sorel cement is usually combined with filler materials such as gravel, sand, marble flour, asbestos, wood particles and expanded clays. Sorel cement is incompatible with steel reinforcement because the presence of chloride ions in the pore solution and the low alkalinity (pH < 9) of the cement promote steel corrosion (pitting corrosion). Phase 5 of the magnesium oxychloride could be a useful complement, or replacement, for MgO (periclase) presently used as a getter in the WIPP disposal chambers to limit the solubility of minor actinides carbonate complexes, while establishing moderately alkaline conditions (pH: 8.5–9.5) still compatible with the undisturbed geochemical conditions initially prevailing in situ in the salt formations. The much more soluble calcium oxide and hydroxide (portlandite) are not authorized in WIPP (New Mexico) because they would impose a too high pH (12.5). As is the second most abundant cation present in sea water after , and that magnesium compounds are less soluble than those of calcium, magnesium based buffer materials and Sorel cement are considered more appropriate backfil materials for radioactive waste disposal in deep salt formations than common calcium based cements (Portland cement and their derivatives). Moreover, as magnesium hydroxychloride is also a possible pH buffer in marine evaporite brines, Sorel cement is expected to less disturb initial in situ conditions prevailing in deep salts formations.

Sorel cement is usually prepared by mixing finely divided powder with a concentrated solution of
In theory, the ingredients should be combined in the molar proportions of phase 5, which has the best mechanical properties. However, the chemical reactions that create the oxychlorides may not run to completion, leaving unreacted particles and/or in pore solution. While the former act as an inert filler, leftover chloride is undesirable since it promotes corrosion of steel in contact with the cement. Excess water may also be necessary to achieve a workable consistency. Therefore, in practice the proportions of magnesium oxide and water in the initial mix are higher than those in pure phase 5. In one study, the best mechanical properties were obtained with a molar ratio : of 13:1 (instead of the stoichiometry 5:1).

Periclase (MgO) and magnesite are not abundant raw materials, so their manufacture into Sorel cement is expensive and limited to specialized niche applications requiring modest materials quantities. China is the dominant supplier of raw materials for the production of magnesium oxide and derivatives. Magnesium based "green cements" derived from the more abundant dolomite deposits (dolomite), but also containing calcium carbonate, have to be not confused with the original Sorel cement, as this latter does not contain calcium oxide. Indeed, Sorel cement is a pure magnesium oxychloride.