Characterization of reaction products and reaction process of MgO–SiO2–H2O system at room temperature

• M–S–H gels were found in MgO/SF pastes cured at room temperature.
• Mg/Si ratio of M–S–H gels lay in the range of 0.67–1.0.
• Mg(OH)2 rapidly formed in first 3 days, then M–S–H gels were generated afterward.
• The pH value of pore solution played a key role on the formation of M–S–H gels.
• Reaction process of MgO–SiO2–H2O system at room temperature was summarized.

In order to clarify the reaction process of the MgO–SiO2–H2O system, the heat evolution, pore solution chemistry and reaction products of reactive MgO/silica fume (SF) pastes cured at room temperature were characterized. Based on the thermodynamic calculations and the experimental results, the final products tended to be M–S–H gels (magnesium silicate hydrates) rather than Mg(OH)2, and the Mg/Si ratio of M–S–H gels was in the range of 0.67–1.00. The pH value was increased with the dissolution of MgO and a large amount of Mg(OH)2 generated in the first 3 days. M–S–H gels were generated gradually afterward until SF or Mg(OH)2 were consumed completely. It should be noted that the pH value of pore solution played a key role on the formation of M–S–H gels. A proper pH value of pore solution (about 10.5) led to a high concentration of Mg2+ (0.18 mmol/L), eventually resulting in a high formation rate of M–S–H gels.