Effect of carbonates on the formation of magnesium silicate hydrates

Abstract

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.

Research progress on magnesium silicate hydrate phases and future opportunities

“…5. Additionally, higher Mg/Si ratios of up to 1.5, as in serpentine group minerals, can be observed in presence of carbonates [45], which were not reached otherwise, not even after a 3-yearequilibration at 20°C, or 2-year-equilibration at 50°C [41].…”

“…5. It was recently observed that dissolved carbonates also speed up the formation of M-S-H [69] by increasing the concentration of Mg due to the formation of aqueous complexes [45], as detailed in Fig. 5.…”

“…As much as M-S-H has similarities to phyllosilicates, it is rather a precursor of them presenting a variable composition and a variable structure (mixture of T:O and T:O.T), i.e. a solidsolution with Mg/Si ratios ranging from 0.75 to 1.5 [40,45]. A schematic sketch for the structure of M-S-H is given in Fig.…”

“…The uptake of Mg on exchangeable surface site is very small due to the low Mg concentrations in solution at pH values above 10.5 [41]. However, the presence of carbonates in solution leads to the formation of aqueous Mg-carbonate complexes, which increase the Mg concentration as well as the Mg uptake at the exchangeable surface sites [45].…”

“…In this letter, the thermodynamic modelling was carried out using the Gibbs free energy minimization program GEMSelektor [76] combined with the GEMS-PSI thermodynamic database [77] and the cemdata2018 database [78]. These databases were completed with thermodynamic data from [40,46,62] to describe the Ca and Al uptake in M-S-H: (C-)M-(A)S-H, the uptake of different anions in hydrotalcite from [79], the possible formation of (hydrated) magnesium carbonates [45] and Mg-phosphates phases and complexes from [80]. The modelling were used to predict the stability domains of M-S-H and M-A-S-H in the absence and presence of carbonates.…”