The effect of water molecules on the structure, dynamics, and mechanical properties of sodium aluminosilicate hydrate (NASH) gel: A molecular dynamics study
Y Zhang and JL Zhang and JY Jiang and DS Hou and JR Zhang, CONSTRUCTION AND BUILDING MATERIALS, 193, 491-500 (2018).
Sodium aluminosilicate hydrate (NASH) gel is the primary adhesive constituent in geopolymer. In the nanostructure of NASH gel, structural water has a great impact on the structure and mechanical properties of NASH. In this study, molecular dynamics simulation is utilized to investigate the effect of water on the structure, dynamics, and mechanical properties of NASH gel. The result shows that water molecules in NASH gel are physically and chemically bound to Si-Al framework by the formation of H-bond as well as Si-OH, Al-OH and Na-OH. These hydrolytic reactions result in the elongation of Al-O bond and the structural transformation from tetrahedral aluminum to pentahedral aluminum. The structural weakening caused by hydrolysis for aluminate skeleton leads to an increase in the mobility of aluminum and a decrease in the stability of Al-O bond. However, the sodium in NASH structure becomes stabilized as a result of the hydrolysis of aluminate skeleton and the formation of Na-OH. In contrast, silicate skeleton exhibits hydrophobic nature and not sensitive to water. Mechanically, hydrolytic reaction weakens the tensile performance of NASH gel by two mechanisms, which is promoting the de-polymerization of Si-Al framework during the tensile process, and preventing the re-polymerization of broken framework. Meanwhile, the de-polymerization of Si-Al framework can further promote hydrolytic reaction during the tensile testing. (C) 2018 Elsevier Ltd. All rights reserved.
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