Realistic atomic structure of fly ash-based geopolymer gels: Insights from molecular dynamics simulations

Abstract

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 the adopted molecular structure represents the experimental data effectively. Towards 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 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 shows 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 towards realistic multiscale simulation-based design and development of geopolymers.

Time resolved alkali silicate decondensation by sodium hydroxide solution

“…ReaxFF allow bond formation/breaking due to a multi-functional scheme in which the charge, coordination number and other parameters influencing on the bond-energy are recalculated every steps. These parameters has been extensively tested for silica gels, zeolites and other silicate structures [11,[61][62][63][64] and give good agreement compared to experiments. In the S.I., the potential is tested against the g(r) of Si-O distances, the angular distribution distribution of water and the water dipole distribution.…”

Fracture toughness of sodium aluminosilicate hydrate (NASH) gels: Insights from molecular dynamics simulations

“…The MD simulation is carried out using an open-source code LAMMPS 33 . The inter-atomic interactions are modelled by using a reactive forcefield (ReaxFF) 23 along with the charge equilibration method. A tolerance of 10 -6 kcal/mol is adopted during the charge equilibration.…”

“…performed in an open software code LAMMPS 33 . For inter-atomic interactions, ReaxFF potential 23 is used along with the charge equilibration method 40 . In this study, a tolerance of 10 -6 kcal/mol is adopted during the charge equilibration.…”

“…Highly heterogeneous nature of such geopolymer binders makes it challenging to extract such intrinsic fracture response of the main binding phase. Further, although fly ash-based geopolymers have been researched extensively [17][18][19] over the past four decades, the molecular structure of N-A-S-H has recently been proposed [20][21][22][23][24][25] . As such, a fundamental investigation using atomistic simulations, to study the fracture response of N-A-S-H remains an open problem.…”

“…The obtained glassy structure is hydrated by performing Grand Canonical Monte Carlo (GCMC) simulation 26 . The inter-atomic interactions are modeled using a reactive force field (ReaxFF) 27 , which is parametrized for elements such as Na, Al, Si, O, and H 23 . In order to evaluate the influence of water content on the fracture response of N-A-S-H, several molecular structures with varying water content are constructed.…”