Mechanism of CaO in Light-Burned Magnesia on the Formation Process, Mechanical Properties, and Water and Chloride Resistance of the 517 Phase

With the continuous exploitation of high-quality magnesite ores, a large number of low-grade magnesite ores with high calcium contents were abandoned, thereby occupying the land, polluting the environment, and causing substantial resource waste. Therefore, resource utilization of low-grade magnesite ores is urgently needed. These low-grade magnesite ores can be used by calcination to prepare raw materials of magnesium oxysulfate (MOS) cement, but CaO could be produced during calcination because dolomite is a dominant mineral. Therefore, this paper mainly studied how CaO affects the formation process, mechanical properties, and water and chloride resistance of the pure 517 phase. Light-burned dolomite (LBD) was used to offer CaO, replacing a certain amount of light-burned magnesia (LBM) in this work. It was found that CaO will react with Mg2+ and SO42- to form an amorphous phase that remains stable when curing in air but unstable when immersing in water. When immersed in water, Ca2+ would continue to react with SO42- to precipitate gypsum, thus destroying the original structure and reducing the stability of the pure 517 phase. Therefore, in the preparation of MOS cement, attention should be given to the content of CaO, and modifiers should be added to obtain a low solubility calcium compound to reduce the negative effects of CaO on MOS cement.