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The molecular structure, dynamics and mechanical properties of sustainable geopolymer material at elevated temperature: a molecular dynamics study

Abstract

Sodium aluminosilicate hydrate (NASH) gel is the primary adhesive constituent in environmentally friendly geopolymer. In this study, to understand thermal behavior of the material, molecular dynamics is utilized to investigate the molecular structure, dynamic property, and mechanical behavior of NASH gel subjected to temperature elevating from 300 to 1500 K. The aluminosilicate skeleton in NASH gel provides plenty of oxygen sites to accept the H-bond from the invaded water molecules. Around 18.2% of water molecules are decomposed and produce silicate and aluminate hydroxyls. About 87% of hydroxyls are associated with aluminate skeleton, which weakens Al-O bonds and disturbs O-Al-O angle and local structure transforming from aluminate tetrahedron to pentahedron and octahedron. With increasing temperature, both Al-O-Si and Si-O-Si bonds are stretched to be broken and the network structure of NASH gel is gradually transformed into branch and chain structure. Furthermore, the self-diffusivity of water molecules and sodium dramatically increases with the elevation of temperature, because the decrease in connectivity of aluminosilicate network reduces chemical and geometric restriction on water and ions in NASH gel under higher temperature. The high temperature also contributes to around 63% of water molecules further dissociating and hydroxyl groups forming, meanwhile proton exchange between water molecules and aluminosilicate network frequently happens. In addition, uniaxial tensile test is utilized to study the mechanical behavior of the NASH gel at different temperature. During the tensile test, aluminosilicate network is depolymerized into branch or chain structure and plays critical role in resisting the tensile loading. In this process, the breakage of aluminosilicate skeleton is accompanied with hydrolytic reaction that further deteriorates the structure. Due to the reduction of chemical bond stability at elevated temperature, both tensile strength and stiffness of NASH gel are weakened significantly. However, ductility of the NASH gel is improved because of the higher extent of structural arrangement at yield stage and partly due to less water attacking. Hopefully, the study can provide valuable molecular insights on the design of alkali activated material with high sustainability and durability.

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Publication details

The article was received on 30 May 2018, accepted on 13 Jul 2018 and first published on 14 Jul 2018


Article type: Paper
DOI: 10.1039/C8CP03411G
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    The molecular structure, dynamics and mechanical properties of sustainable geopolymer material at elevated temperature: a molecular dynamics study

    D. Hou, Y. Zhang, T. Yang, J. Zhang, H. Pei, J. Zhang, J. Jiang and T. Li, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP03411G

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