The effect of carbonates on the formation of magnesium silicate hydrate phases (M-S-H) was investigated in paste experiments. At molar ratio Mg/Si = 1.5, M-S-H pastes were synthesised from silica fume and MgO or MgO/hydromagnesite in a sodium carbonate-rich environment (dissolved 1.7 g of Na2CO3 for 100 g of binder). Thermogravimetric analysis, X-ray diffraction, and 29Si MAS NMR data showed that M-S-H phases formed much faster in the presence of carbonates, which destabilised brucite. 23Na MAS NMR data, cation exchange capacity measurements, thermogravimetric analysis coupled with infrared exhaust gas analysis, and 13C MAS NMR data showed that traces of sodium and carbonates were sorbed on M-S-H. However, no evidence of the formation of Mg-carbonate phases was observed in crystalline or amorphous form. Thermodynamic modelling carried out with an updated database for Mg-phases indicated that M-S-H and hydromagnesite should be stable, in contrast to the observed partial destabilisation of hydromagnesite.

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