The cement is a mixture of magnesium oxide (burnt magnesia) with magnesium chloride with the approximate chemical formula Mg4Cl2(OH)6(H2O)8, corresponding to a weight ratio of 2.5–3.5 parts MgO to one part MgCl2.
Composition and structureEdit
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 3Mg(OH)
2O and 5Mg(OH)
2O, respectively; or, equivalently, Mg
2O and Mg
Phase 5 crystallizes mainly as long needles which are actually rolled-up sheets. These interlocking needles give the cement its strength.
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. It also achieves high strength in a shorter time.
Other differences between magnesium-based cements and portland cement include water permeability, preservation of plant and animal substances, and corrosion of metals. These differences make different construction applications suitable.
Prolonged exposure of Sorel cement to water leaches out the soluble MgCl
2, leaving hydrated brucite Mg(OH)
2as the binding phase, which without absorption of carbon dioxide, can result in loss of strength.
Fillers and reinforcementEdit
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. However, the low alkalinity makes it more compatible with glass fiber reinforcement. It is also better than Portland cement as a binder for wood composites, since its setting is not affected by the lignin and other wood chemicals.
Magnesium oxychloride cement is used to make floor tiles and industrial flooring, in fire protection, wall insulation panels, and as a binder for grinding wheels. Due to its resemblance to marble, it is also used for artificial stones, artificial ivory (e.g. for billiard balls) and other similar purposes.
Sorel cement is usually prepared by mixing finely divided MgO powder with a concentrated solution of MgCl
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 MgO particles and/or MgCl
2 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 MgO:MgCl
2 of 13:1 (instead of the stoichometric 5:1).
Magnesite and dolomite are both abundant raw materials however their manufacture into cements has been limited in the west due to limited markets. Its widespread acceptance and use for building board in China, has made this country the dominant supplier of raw materials. In the west, magnesium based cements are relatively expensive but only because of the relatively high cost of the magnesium oxide raw material  compared to portland-cement based concrete outside Asia.
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