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

• Published in: Materials & design 2021-01, Vol.198, p.109391, Article 109391
• Main Authors: Bernard, Ellina, Lothenbach, Barbara, Rentsch, Daniel
• Format: Article
• Language: English
• Subjects: Alkali magnesia alumino silicate cement; Hydrotalcite; Magnesia silicate cement; Magnesium alumino silicate hydrates M-A-S-H; Magnesium silicate hydrates M-S-H; Phase assemblage
• ISSN: 0264-1275; 1873-4197
• DOI: 10.1016/j.matdes.2020.109391

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 ~ 100 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), 29Si MAS NMR data showed that M-A-S-H phases formed, with a similar structure as M-S-H. 27Al 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 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). [Display omitted] •Magnesium alumino silicate hydrate is the main product of the MgO-Al2O3-SiO2-NaNO3-H2O 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)