Geopolymers, synthesized through alkaline activation of aluminosilicates, have emerged as a sustainable alternative for traditional ordinary Portland cement. In spite of the satisfactory mechanical performance and sustainability-related benefits, the large scale acceptance of geopolymers in the construction industry is still limited due to poor understanding of the composition-property relationships. Molecular simulation is a powerful tool to develop such relationships, provided that the adopted molecular structure represents the experimental data effectively. Toward this end, this paper presents a new molecular structure of sodium aluminosilicate hydrate geopolymer gels, inspired from the traditional calcium silicate hydrates gel. In contrast to the existing model—where water is uniformly distributed in the structure—we present a layered-but-disordered structure. This new structure incorporates water in the interlayer space of the aluminosilicate network. The structural features of the new proposed molecular structure are evaluated in terms of both short- and medium-range order features such as pair distribution functions, bond angle distributions, and structure factor. The structural features of the newly proposed molecular structure with interlayer water show better correlation with the experimental observations as compared to the existing traditional structure signifying an increased plausibility of the proposed structure. The proposed structure can be adopted as a starting point toward the realistic multiscale simulation-based design and development of geopolymers.

This paper evaluates the fracture toughness of sodium aluminosilicate hydrate (N-A-S-H) gel formed through alkaline activation of fly ash via molecular dynamics (MD) simulations. The short- and medium-range order of the constructed N-A-S-H structures shows good correlation with the experimental observations, signifying the viability of the N-A-S-H structures. The simulated fracture toughness values of N-A-S-H (0.4–0.45 MPa m^0.5) appear to be of the same order as the available experimental values for fly ash-based geopolymer mortars and concretes. These results suggest the efficacy of the MD simulation toward obtaining a realistic fracture toughness of N-A-S-H, which is otherwise very challenging to obtain experimentally, and no direct experimental fracture toughness values are yet available. To further assess the fracture behavior of N-A-S-H, the number of chemical bonds formed/broken during elongation and their relative sensitivity to crack growth are evaluated. Overall, the fracture toughness of N-A-S-H presented in this paper paves the way for a multiscale simulation-based design of tougher geopolymers.