Sorel cement: properties and utilization

Sorel cement (also called “magnesium oxychloride cement”) is a non-hydraulic cement, produced by a mixture of magnesium oxide (magnesia) with magnesium chloride (often in the form MgCl2·6H2O), with a weight ratio of 2.5-3.5 parts MgO to 1 part MgCl2. Its recipe and properties are known since 1867. If compared to the more common Portland cement, discovered in the same period, the Sorel cement shows higher compressive strength, better resilience and capacity of bonding fillers (e.g., gravel, sand, expanded clays, glass fibres or even wood particles). However, when the Sorel cement is exposed to water, for a prolonged period, tends to dissolve. In addition, the presence of Cl ions makes Sorel cement incompatible with steel reinforcement, promoting metal corrosion phenomena. These two limitations represent the main criticalities of the Sorel cement, and are responsible for the success of the Portland cement against the Sorel one. However, the new route toward building materials produced with less energetic protocols is leading to a revaluation of the Sorel cement. The clinker manufacturing process is, in fact, highly energetic (Tmax 1450-1500°C), whereas the highest temperature usually necessary to produce Sorel cement is that required to promote the dissociation of MgCO3 to MgO+CO2, which nominally does not exceed 650°C. However, the production of highly reactive magnesia requires higher temperatures. At present, Sorel cement is used to make industrial flooring, fire retardant materials, or insulation boards. In addition, magnesium oxychloride cement is one of material used to fabricate the engineered barriers for deep geological repositories of high-level nuclear waste in salt-rock formations. The aim of this presentation is to report our preliminary experimental findings on the crystal-chemistry of lab-made Sorel cement and on the behaviour of its crystalline components at non-ambient conditions, in order to provide their compositional, thermal and compressional parameters. The crystalline phases usually found in Sorel cement consist of complex magnesium oxychlorides (mainly, but no exclusively, the so-called “phase 3” 3Mg(OH)2·MgCl2·8H2O and “phase 5” 5Mg(OH)2·MgCl2·8H2O) and brucite, sometimes coupled with unreacted periclase or magnesium chlorocarbonates. The fractions of the crystalline components depend on the initial cement formulation, but even by setting time and other variables (e.g., the reaction with CO2). A multi-methodological approach has been used, including in-situ X-ray diffraction experiments at non-ambient conditions at the ID15b beamline, ESRF, Grenoble (France).