Sorel cement

Sorel cement (also known as magnesia cement) is a non-hydraulic cement first produced by Frenchman Stanislas Sorel in 1867. Sorel cement (also known as magnesia cement) is a non-hydraulic cement first produced by Frenchman Stanislas Sorel in 1867. 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. The name 'Sorel cement' is also used for zinc oxychloride cements, also discovered by Sorel, which 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 3Mg(OH)2·MgCl2·8H2O and 5Mg(OH)2·MgCl2·8H2O, respectively; or, equivalently, Mg2(OH)3Cl·4H2O and Mg3(OH)5Cl·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 CO2 from the air to form magnesium chlorocarbonates. 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. The pore solution in wet Sorel cement is alkaline (pH 8.5 to 9.5), but significantly less so than that of Portland cement (pH 12 to 13). 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.