Effect of free CaO content on the phase compositions of magnesium oxychloride cement

Abstract

The free CaO (FC) in light-burnt MgO due to the impurity of magnesite ores cannot be avoided, and causes the declination of properties of magnesium oxychloride cement (MOC). In order to clearly explain the declination mechanism of MOC properties due to FC, the effect of FC contents on the formation of phase compositions and the compressive strength of MOC pastes were studied in detail. The findings revealed that the addition of FC sharply increased the pH value of MOC pastes, and changed the final compositions of the hardened MOC pastes. Specifically, FC contents more than 4% could cause the formation of phase 5 by the phase 3 transformation at MgO/MgCl₂/H₂O molar ratio of 3:1:11, and FC contents more than 2% could lead to the formation of phase Mg(OH)₂ by phase 5 transformation at MgO/MgCl₂/H₂O molar ratio of 5:1:13. Consequently, the transformation of phase compositions caused the remarkable declination of compressive strength of MOC pastes. The content of FC should be given attention in the MgO/MgCl₂/H₂O molar ratio, and the permissible contents of FC should be limited to less than 4% and 2% relevant to the MgO/MgCl₂/H₂O molar ratios at 3:1:11 and 5:1:13, respectively.