A molecular dynamics study of N-A-S-H gel with various Si/Al ratios

In this paper, the atomic structures of sodium aluminosilicate hydrate (N-A-S-H) gels with different Si/Al ratios are studied by molecular dynamics simulation. An N-A-S-H gel model was obtained from the polymerization of Si(OH)(4) and Al(OH)(3) monomers with the use of a reactive force field (ReaxFF). The simulated atomic structural features, such as the bond length, bond angle, and simulated X-ray diffraction pattern of the gel structure are in good accordance with the experimental results in the literature. Si-O-Al is found to be preferred over Si-O-Si in the N-A-S-H gel structure according to the amount of T-O-T bond angles and distribution of Si-4(mAl). Pentacoordinate Al is identified in all simulated N-A-S-H models. It provides strong support to current knowledge that pentacoordinate Al in geopolymer does not only come from raw material. Furthermore, the structural analysis results also show that N-A-S-H gel with lower Si/Al ratios has a more cross-linked and compacted structure.