In this paper, molecular dynamics simulations are used to study the effect of molecular water and composition (Si/Al ratio) on the structure and mechanical properties of fully polymerized amorphous sodium aluminosilicate geopolymer binders. The X-ray pair distribution function for the simulated geopolymer binder phase showed good agreement with the experimentally determined structure in terms of bond lengths of the various atomic pairs. The elastic constants and ultimate tensile strength of the geopolymer binders were calculated as a function of water content and Si/Al ratio; while increasing the Si/Al ratio from one to three led to an increase in the respective values of the elastic stiffness and tensile strength, for a given Si/Al ratio, increasing the water content decreased the stiffness and strength of the binder phase. An atomic-scale analysis showed a direct correlation between water content and diffusion of alkali ions, resulting in the weakening of the AlO4 tetrahedral structure due to the migration of charge balancing alkali ions away from the tetrahedra, ultimately leading to failure. In the presence of water molecules, the diffusion behavior of alkali cations was found to be particularly anomalous, showing dynamic heterogeneity. This paper, for the first time, proves the efficacy of atomistic simulations for understanding the effect of water in geopolymer binders and can thus serve as a useful design tool for optimizing composition of geopolymers with improved mechanical properties. Published by AIP Publishing.