(80f) Accelerated Conversion of Magnesium Oxychloride to Chlorartinite Using Pressurized Carbon Dioxide and the Resulting Increased Water Resistance

Magnesium oxychloride cement (MOC) is a fire resistant structural material with broad applications in passive fire protection, especially for scenarios where high-strength or low carbon footprint materials are needed; however, despite many advantages for construction applications, MOC can show low water stability, where untreated MOC degrades over time to magnesium hydroxide when exposed to water, and thus has not found widespread adoption in US construction.

MOC water resistance can increase over time when exposed to air. Previous studies have found that chlorartinite formation, Mg₂(CO₃)(OH)Clâ–ª2H₂O, forms in the surface of this material as a result of exposure to CO₂. It has been suggested that chlorartinite increases the water stability due to the lower water solubility of this material, but this phenomenon has not been quantified before. This study has used quantitative X-ray diffraction data and scanning electron microscope images to examine and quantify the conversion of magnesium oxychloride to chlorartinite and the improved water stability of the cement. Samples were exposed to pressurized CO₂ to accelerate the formation of chlorartinite on the cement surface and chlorartinite formation was assessed. The extent of chlorartinite formation was shown to correlate with improved water stability. We hypothesize that a protective semi-insoluble chlorartinite layer forms on the surface of the magnesium oxychloride crystals and slows the degradation of the magnesium oxychloride when exposed to water. Untreated samples convert to magnesium hydroxide when subjected to an accelerated water resistance test (submersion in deionized water at 60 °C for 24 hours). Samples exposed to CO₂ show reduced MOC conversion. Chlorartinite was observed to convert to hydromagnesite, magnesium hydroxide and amorphous phase, but at a much slower rate.