Gideon Arthur Lyngdoh

Concrete is the most widely used construction material in the world for building infrastructures, bridges, and tunnels. However, cement production contributes to 5-7% of the global CO_2 emissions, and thus, there is a significant need for innovative sustainable alternatives without compromising on the mechanical properties. Most of the approaches to address these concerns so far rely on the trial and error-based experimental response. Hence, robust multiscale simulation-based design approaches are needed to be developed for fundamental understanding as well as the comprehensive design of these new sustainable innovative materials. However, performing the simulations with millions of degrees of freedom could be daunting and requires high computation time. Thus, building data-driven models by training through the experimental/simulated data can serve as an efficient alternative to stimulate the design and development of new materials compositions for desired performance requirements. To address the poor sustainability credential-related issue, this thesis first evaluates the performance of geopolymers which are synthesized through alkaline activation of aluminosilicates. Analogous to calcium silicate hydrates (C-S-H) gel in cement paste, sodium aluminosilicate hydrate (N-A-S-H) gel is the primary binding phase in geopolymers formed via alkaline activation of fly ash. In this thesis, a realistic molecular structure of N-A-S-H geopolymer gels, inspired by the traditional calcium silicate hydrates gel, is proposed using molecular dynamics (MD) simulation. In contrast to the existing N-A-S-H model—where water is uniformly distributed in the structure—a layered-but-disordered structure is presented where water molecules are incorporated in the aluminosilicate network's interlayer space. The developed structures are further validated using experimental observations. To address the durability performance of geopolymers, the dynamics of confined water and its interplay with alkali cations in disordered N-A-S-H gel are evaluated using reactive force field molecular dynamics. This is achieved by exploiting the evolution of mean squared displacements and the Van Hove correlation function. It is observed that the Si/Al ratio significantly influences the diffusion of confined water and sodium. Increased conversion of the Si-O-Na network to Si-O-H and Na-OH components with an increase in water content helps explain the alkali-leaching issue in fly ash-based geopolymers observed macroscopically. Moreover, the fracture properties of the disorder N-A-S-H gel are explored via molecular dynamics simulations. The simulated fracture toughness values of N-A-S-H are validated with the experimental results obtained using the nanoindentation technique, where the principle of conservation of energy is implemented to evaluate the fracture toughness from the load-penetration depth responses. Afterward, to address the

This paper investigates the dynamic compressive behavior of wollastonite fiber-reinforced cementitious mortars using multiscale numerical simulations. The rate dependent behavior of the multiphase heterogeneous systems is captured in a multiscale framework that implements continuum damage towards effective property prediction. The influence of wollastonite fiber content (% by mass) as cement replacement on the dynamic compressive strength and energy absorption capacity is thereafter elucidated. An average compressive strength gain of 40% is obtained for mortars with 10% wollastonite fiber content as cement replacement, as compared to the control mortar at a strain rate of 200/s. The rate dependent constitutive responses enable the computation of energy absorption, which serves as a comparative measure for elucidating the material resistance to impact loads. Approximately a 45% increase in the dynamic energy absorption capacity is observed for the mixture containing 10% wollastonite fibers, as compared to the control case. Overall, the study establishes wollastonite fibers as a sustainable and dynamic performance-enhanced alternative for partial cement replacement. Moreover, the multiscale numerical simulation approach for performance prediction can provide an efficient means for the materials designers and engineers to optimize the size and dosage of wollastonite fibers for desired mechanical performance under dynamic loading conditions.

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.

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.