Effect of aluminate and carbonate in magnesia silicate cement

The effect of alumina and carbonates on magnesium silicate hydrate (M-S-H) cements was investigated at high Mg/Si ratios (1.5-2.5), constant Na2CO3 concentration (17 g/L) and Al/Si ratio of either 0, 1 or 3.2. Either MgO or MgO/hydromagnesite (50/50 wt %) was used, as well as silica fume, metakaolin or amorphous Al2O3. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), 29Si and 27Al MAS NMR data showed that M-(A-)S-H phases formed in all cases and hydrotalcite in presence of aluminum. Little hydromagnesite had reacted in the presence of aluminum. The 23Na MAS NMR data showed that only little sodium was sorbed; the thermogravimetric analysis coupled with infrared exhaust gas analysis and 13 C MAS NMR data showed that carbonates were incorporated in the hydrotalcite and traces of carbonate were sorbed. Thermodynamic modelling carried out with an updated Mg-phases database was in relatively good agreement with the phase assemblage observed if the reactivity of the starting materials was considered.

Automated summary

The effect of alumina and carbonates on magnesium silicate hydrate (M-S-H) cements was studied under high Mg/Si ratios, constant Na2CO3 concentration, and varying Al/Si ratios. The results showed the formation of M-(A-)S-H phases and hydrotalcite in the presence of aluminum, but little reaction with hydromagnesite. Thermodynamic modeling, considering the reactivity of the starting materials, demonstrated relatively good agreement with the observed phase assemblage.