An atomistic characterization of the interplay between composition, structure and mechanical properties of amorphous geopolymer binders

The amorphous geopolymer binder-phase is the primary adhesive constituent within a geopolymer and consists of completely polymerized glassy networks of (SiO₄) and (AlO₄)^- tetrahedra as well as interstitial charge-balancing alkali cations. In this context, the mechanical properties of the geopolymer binder-phase were examined as a function of the underlying silicon to aluminum ratio using molecular dynamics (MD) simulations. Detailed structural analysis reveals that the presence of edge-sharing (AlO₄)^- tetrahedra, nanoscale voids as well as non-bridging oxygen and penta coordinated aluminum atoms significantly impact the ensuing elastic moduli, ultimate tensile strength and the nature of failure of the geopolymer binder-phase. In particular, the simulations indicate that there is an optimal silicon to aluminum ratio (~ 2-3) that results in enhanced mechanical properties. This study provides, for the first time, valuable insight into the structural mechanisms that are responsible for the strength and mechanical properties of the geopolymer binder-phase.