MgO-based binders have been widely studied for decades. Recently, the MgO-SiO2-H2O system was developed as a novel construction material, however, its reaction mechanism remains unclear. This paper investigated the reaction products and kinetics of MgO/silica fume (SF) pastes with MgO calcinated at different temperatures. The results indicate that MgO presented larger grain size after calcination at higher temperature. Mg(OH)(2) and magnesium silicate hydrate (M-S-H) gel were formed when using MgO calcined at 850, 950, and 1050 degrees C. However, only M-S-H gel was formed when using MgO calcined at 1450 degrees C. The reaction kinetics of MgO could be described using = 1 - e(-k*t). The reaction rate of MgO increased with decreasing calcination temperature, increasing SF dosage, and the addition of sodium hexametaphosphate. Only M-S-H gel was formed when the reaction rate of MgO was below the demarcation line (about 0.250 x 10(-6) s(-1)), and the corresponding demarcation area was around 14 days.