Effect of carbonates on the formation of magnesium silicate hydrates

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 Na 2 CO 3 for 100 g of binder). Thermogravimetric analysis, X-ray diffraction, and 29 Si MAS NMR data showed that M-S-H phases formed much faster in the presence of carbonates, which destabilised brucite. 23 Na MAS NMR data, cation exchange capacity measurements, thermogravimetric analysis coupled with infrared exhaust gas analysis, and 13 C 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.