Influence of sodium nitrate on the phases formed in the MgO-Al2O3-SiO2-H2O system

The incorporation of sodium and aluminium in magnesium silicate hydrate phases (M-S-H), possible binding phases in magnesium silicate cement, was investigated.Magnesium alumino silicate hydrate containing sodium (M-A-S-H N samples) were synthesized in batch experiments with NaNO3 ~ 100mmol/L at Mg/Si ratios equal to 0.8 and 1.2, and Al/Si ratios of 0, 0.1 and 0.2, and equilibrated at 20 and 50 °C. Thermogravimetric analysis (TGA), X-rays diffraction (XRD), 29SiMAS NMRdata showed thatM-A-S-H phases formed, with a similar structure as M-S-H. 27Al and 23Na MAS NMR data showed that only little sodiumwas sorbed,while aluminiumwas possibly incorporated in both tetrahedral and octahedral sites ofM-A-S-H.We found evidence that the presence of sodium nitrate led to the formation of hydrotalcite-like phase probably containing NO3
− and possibly to the trace formation of hydrated alumino-silicate containing sodium: N-A-S-H gel. These minor phases limited the aluminium uptake by M-S-H at higher Al contents (Al/Si= 0.2).