Effect of Carbonates on the Formation of Magnesium Silicate Hydrates (M-S-H) and Magnesium Alumino Silicate Hydrates (M-A-S-H)

The effect of carbonates on the formation of magnesium silicate hydrate phases (M-S-H) and magnesium alumina-silicate hydrate phases (M-A-S-H) was investigated in paste experiments at a molar ratio Mg/Si = 1.5, an Al/Si ratio of either 0 or 1 and with and without addition of 1.7 wt. % Na2CO3. M-S-H pastes were synthesised from silica fume and MgO, while M-A-S-H pastes were synthesised from metakaolin and MgO. Thermogravimetric analyses, X-ray diffraction, and 29Si MAS NMR data showed that M-S-H and M-A-S-H phases formed much faster in the presence of sodium carbonate, which destabilised brucite. Furthermore, hydrotalcite formed in the presence of alumina. No evidence of the formation of Mg-carbonate phases in crystalline or amorphous form was observed. The MgO-metakaolin binder reached the highest compressive strength while the MgO-silica fume binder presented a slightly more than the half of the strength. By the addition of sodium carbonate compressive strengths could be improved for both systems.