Preparation technology and microstructural changes of low-activity magnesium oxide based on salt lake bischofite

In response to the high cost of magnesium phosphate cement (MPC), due to the high production energy consumption of raw material MgO (sintering at 1600 °C for 5 h), and the requirement of adding retarders during the preparation process, we proposed using common elements B, Li, Na, K, and Cl in salt lakes to promote the sintering performance of MgO. We subsequently studied the preparation technology of low-activity MgO obtained from the pyrolysis of bischofite in salt lakes and low-temperature sintering (≤ 1200 °C). The migration law of coexisting elements and low-temperature sintering mechanism of salt lake bischofite was revealed during pyrolysis and low-temperature sintering, expanding the utilization path of salt lake bischofite and promoting resource recycling. We simultaneously developed a preparation technology for low-activity MgO used as raw MPC material, avoiding the use of retarders and reducing the raw material costs in the preparation of MPC. The results showed that coexisting elements migration was present in four different types of processes during the sintering process. Octahedral particles with a preferred orientation and exposed MgO (111) crystal face were obtained. With increased sintering temperature and time, the content and particle size of MgO increased and the specific surface area and porosity decreased, which led to the prolonged neutralization reaction time, as well as increased sintering degree and decrease in hydration activity. The formation of the liquid phase by bischofite and its coexisting elements was the key to MgO sintered at 1000 °C.