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

Ellina Bernard, Barbara Lothenbach, Daniel Rentsch
2020 Materials & design  
Magnesium alumino silicate hydrate is the main product of the MgO-Al 2 O 3 -SiO 2 -NaNO 3 -H 2 O system at low Al content (8-16 wt.%) • Precipitation of hydrotalcite decreases the incorporation of aluminium in the magnesium alumino silicate hydrate • Possible formation of zeolitic-like precursor (N-A-S-H) gels in low content • Small uptake of sodium in the diffusive layer of the solids occurs at this pH (~8-10) Keywords: Magnesia silicate cement Alkali magnesia alumino silicate cement Phase
more » ... mblage Magnesium silicate hydrates M-S-H Magnesium alumino silicate hydrates M-A-S-H Hydrotalcite 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 NaNO 3~1 00 mmol/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), 29 Si MAS NMR data showed that M-A-S-H phases formed, with a similar structure as M-S-H. 27 Al and 23 Na MAS NMR data showed that only little sodium was sorbed, while aluminium was possibly incorporated in both tetrahedral and octahedral sites of M-A-S-H. We found evidence that the presence of sodium nitrate led to the formation of hydrotalcite-like phase probably containing NO 3 − 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).
doi:10.1016/j.matdes.2020.109391 fatcat:cnanjk6j35akljirrwc2agbkfu