Effect of CaO on the reaction process of MgO-SiO2-H2O cement pastes

As a potential cementitious material for nuclear waste immobilization magnesium silicate hydrate (M-S-H) phase was prepared by MgO and silica fume at room temperature. Attributable to the slow hydrolysis rate of MgO, the strength of M-S-H paste is lower than Portland cement in 3 days. This work has demonstrated that CaO content has a direct influence on the chemical environment in pore solution and phases transformation in CaO-MgO-SiO2-H2O (C/M/S/H) system. Appropriate CaO content (with optimal Ca/Si molar ratio at approximately 0.02) can accelerate M-S-H gel formation in high W/C paste contributing to early age strength improvement. Ca/Si molar ratio should be controlled at a range between 0.02 and 0.1, because the formation of the calcium silicate hydrate (C-S-H) phase is much is easier, which results in the inhibition of the M-S-H formation.