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 to work 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 matter harder than limestone.
Only a decade later, Sorel replaced zinc by 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 matter could be easily molded as 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. So, 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 resisting to shock.
In the late 19th century, several attempts were made to determine the composition of the hardened Sorel's cement, but the results were not conclusive. Phase 3 was properly isolated and described by Robinson and Waggaman (1909), and phase 5 was identified by Lukens (1932).
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.
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). 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 retarded by the lignin and other wood chemicals.
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 MgCl2 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:MgCl2 of 13:1 (instead of the stoichiometry 5:1).
Periclase (MgO) and magnesite (MgCO3) 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 ((Ca,Mg)(CO3)2) deposits (dolostone), but also containing 50 wt. % calcium carbonate, have not to be confused with the original Sorel cement, as this later does not contain calcium oxide. Indeed, Sorel cement is a pure magnesium oxychloride.