Ceramics-Silikáty 66, (4) 498 - 507 (2022)

Magnesium hydroxide was used as a precursor to magnesium oxide (MgO). Magnesium oxide cement (MOC) was calcined at different temperatures to obtain MgO with different activities in the presence of 0%, 3% and 5% boric acid (H₃BO₃). Cement hydration, which is an exothermic reaction, determines the phase composition and pore structure. In addition, the effect of boron, a common impurity occurring in Salt Lake, on the MOC compressive strength was explored. H₃BO₃ has a negative effect on the MOC strength, and slows the MOC condensation and hardening. However, the presence of H₃BO₃ impurities does not change the hydration phase composition of MOC, because the specific surface area of MgO increases after calcination at 700 ̊C. As a result, the grain size decreases and the internal defects increase, which affects the phase number and morphology (5·1·8 phase) and, thereby, determines the compressive strength of the MOC. In addition, after calcination at 1000 ̊C, the adjacent MgO is continuously sintered. The large amount of rough magnesium borate generated adheres to the surface of MgO, which inhibits the hydration reaction of the MgO and the formation of the 5.1.8 phase and, thus, reduces the MOC compressive strength.