Fracture toughness of fly ash-based geopolymer gels: Evaluations using nanoindentation experiment and molecular dynamics simulation

•NASH gel is formed through alkaline activation of fly-ash.•Fracture toughness of NASH gel is obtained from nanoindentation by energy approach.•Atomistic simulation of N-A-S-H fracture is performed by molecular dynamics.•Fracture toughness from MD simulation shows good correlation with experimental value. This paper presents the fracture toughness of sodium aluminosilicate hydrate (N-A-S-H) gel formed through alkaline activation of fly ash. While the fracture toughness of N-A-S-H is obtained experimentally from nanoindentation experiment implementing the principle of conservation of energy, the numerical investigation is performed via reactive force field molecular dynamics. A statistically significant number of indentations are performed on geopolymer paste yielding frequency distribution of Young’s modulus. Four distinct peaks are observed in the frequency distribution plot from which the peak corresponding to N-A-S-H was separated using statistical deconvolution technique. The young’s modulus of N-A-S-H, thus obtained from statistical deconvolution shows excellent match with the values reported in the literature, thus confirming successful identification of indentations corresponding to N-A-S-H. From the load-penetration depth responses of N-A-S-H, fracture toughness was obtained following the principle of conservation of energy. The experimental fracture toughness shows good correlation with the simulated fracture toughness of N-A-S-H, obtained from reactive force field molecular dynamics. The fracture toughness of N-A-S-H presented in this paper paves the way for multiscale simulation-based design of tougher geopolymer binders.