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

Nano-modification has been an effective approach to improving the mechanical properties and durability of geopolymer materials. This study investigates the interaction between amorphous nano-silica and sodium alumino-silicate hydrate (NASH) gel at the atomic level. A realistic composite model is constructed to characterize the geopolymerization reaction on the amorphous nano-silica substrate. The atomic structure, dynamic properties, and mechanical performances of NASH gels with Si/Al ratios from 1.5 to 3.0 are compared to investigate the role of amorphous silica in geopolymers. The reaction between the two phases is observed in the interfacial transition zone (ITZ), with a thickness of around 10 Å. With the incorporation of amorphous silica, the reaction degree and complexity of the generated alumino-silicate skeleton structure in NASH gel have been improved by approximately 10 and 6%, respectively. Consequently, the NASH gel modified by silica exhibits a denser structure and a lower ion diffusion rate than the neat NASH gel, particularly at a low Si/Al ratio. Moreover, the simulation results indicate that the tensile strength of the ITZ is higher than that of NASH with or without amorphous silica, while the tensile strength of the silica-modified NASH gel is 10–40% higher than that of the neat NASH gel. The NASH gel modified by silica can achieve the optimum uniaxial tensile strength of around 4.1 GPa at Si/Al ratios of 2.5 and 3.0. Thus, this study has proved the reactivity of nano-silica in geopolymers and profiled its positive effects on NASH gel at the atomic level.