Interactions between Amorphous Silica and Sodium Alumino-Silicate Hydrate Gels: Insight from Reactive Molecular Dynamics Simulation

Nano-modificationhas been an effective approach to improving themechanical properties and durability of geopolymer materials. Thisstudy investigates the interaction between amorphous nano-silica andsodium alumino-silicate hydrate (NASH) gel at the atomic level. Arealistic composite model is constructed to characterize the geopolymerizationreaction on the amorphous nano-silica substrate. The atomic structure,dynamic properties, and mechanical performances of NASH gels withSi/Al ratios from 1.5 to 3.0 are compared to investigate the roleof amorphous silica in geopolymers. The reaction between the two phasesis observed in the interfacial transition zone (ITZ), with a thicknessof around 10 & ANGS;. With the incorporation of amorphous silica, thereaction degree and complexity of the generated alumino-silicate skeletonstructure in NASH gel have been improved by approximately 10 and 6%,respectively. Consequently, the NASH gel modified by silica exhibitsa denser structure and a lower ion diffusion rate than the neat NASHgel, particularly at a low Si/Al ratio. Moreover, the simulation resultsindicate that the tensile strength of the ITZ is higher than thatof NASH with or without amorphous silica, while the tensile strengthof the silica-modified NASH gel is 10-40% higher than thatof the neat NASH gel. The NASH gel modified by silica can achievethe optimum uniaxial tensile strength of around 4.1 GPa at Si/Al ratiosof 2.5 and 3.0. Thus, this study has proved the reactivity of nano-silicain geopolymers and profiled its positive effects on NASH gel at theatomic level.