Molecular dynamics simulation of NASH and its composites

Abstract

Geopolymer, as an environmentally friendly substitute material for ordinary Portland cement (OPC), possesses attractive engineering properties. In the past decades, efforts have been devoted to improving geopolymer’s sustainability and performances, investigating the reaction mechanism, and exploring its practical applications. However, due to its complicated amorphous constituents, the understanding of geopolymer gel structure is still insufficient. A comprehensive understanding of it at the molecular level is needed to provide insights into the nature of geopolymerisation reaction and further suggestions for material design. Thus, this thesis utilizes molecular dynamics (MD) simulation to investigate the sodium-alumino-silicate-hydrate (NASH) gel and its composites from a theoretical and computational point of view.
First of all, the geopolymerisation reaction has been mimicked based on a realistic bottom-up modelling method. By employing seven types of experimentally characterized oligomers, the reactive MD simulation reveals the poly-condensation reaction and the formation of 3D cross-linked network structure of NASH gels. The effects of Si/Al ratio and oligomers’ configuration in the system on the reaction degree, structure complexity, skeleton bond angle distribution, and water mobility are evaluated. In the MD simulation, the skeleton chains breakage and hydrolysis reaction during the uniaxial tensile test are observed. More broken Al-O than Si-O bonds are found, indicating a weaker Al-O bond strength. Apart from the structure-property relationship, the interactions between NASH gels and other inorganic or organic materials are investigated based on the developed NASH gel models.
Nano-silica modification has been an effective approach to improve the mechanical property and durability of geopolymers. This thesis provides an insight into the interaction between amorphous nano-silica and NASH gel at the atomic level by MD simulation. Geopolymerisation reaction between these two parts in the composite is observed from the density distribution function and RDF results, with an interfacial transition zone (ITZ) of an average thickness of ~10 Å. Adding amorphous silica increases the reaction degree, bridge oxygen percentage, and enhances the stability of NASH gel. Mechanical strengths in various phases are found in the order of ITZ > NASH-s > neat NASH gel. The tensile strength of NASH gels is consequently improved by 10~40%, particularly for those with low Si/Al ratios. In addition, the enhancement of NASH gels by nano-silica is confirmed at the molecular level.
Moreover, NASH/polyethene (PE) composite models have been built with the developed NASH gel models to simulate the interface between NASH gel and PE fibre. It reveals the fibre-reinforced mechanism in the matrix at the molecular level. Based on the structure analysis, energy and dynamics characterization, and pull-out simulation, the adhesion mechanism and bonding performances have been explored. The adhesive bonding between NASH gel and PE mainly depends on the interfacial interaction (i.e. atomic pair interaction and the van der Waals force (vdW)) and mechanical interlocking (i.e. derived from the surface roughness). The weak H-bond is found between PE and NASH, which provides the dominant Coulombic interaction. A larger Si/Al ratio increases the bonding by mainly enhancing the vdW interaction energy. The optimum pull-out shear bonding strength of 0.43 GPa is achieved at a Si/Al ratio of 2.5. In addition, increasing the internal moisture in NASH gels increases adhesion properties, by providing more H-bonds at the interface. At the Si/Al ratio of 2.5, the bonding strength of a dry structure is calculated as only 70% of that of a full water one.
In conclusion, this thesis proposes a reliable reactive modelling approach for NASH gels. It also provides insights into structural information and the interaction mechanism of geopolymers and its composites at the microscopic level, which is crucial for further materials study and design.

Date of Award	Mar 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Hainam Do (Supervisor), Bo Li (Supervisor) & Alvaro Garcia Hernandez (Supervisor)