Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Dr. Ram Mohan

Abstract

Concrete is the most used material for construction, and it is the most produced man-made product in the world. Concrete makes the contemporary architecture of the world conceivable and brings significant development to communities. Nevertheless, there are many downsides to the use of concrete such as the large amount of energy and water used in its production, large carbon footprint, shrinkage and expansion that can cause cracking and other failure mechanisms, and the required use of large amounts of material to ensure stability of concrete structures. These drawbacks create the need for engineering and tailoring the properties of concrete. An ideal concrete material should be environmentally friendly, sustainable, strong, safe, with little need for maintenance or replacement, low cost and easily accessible to all communities. Concrete is a hierarchical complex random composite material on many length scales. The main constituent of concrete is cement paste. Cement binds all other phases and contributes significantly to concrete strength. Cement paste microstructure contains unhydrated and hydrated solid components, and voids. The main phase of unhydrated cement is tricalcium silicate (C3S) and the main phases of hydrated cement are calcium silicate hydrate (CSH) and calcium hydroxide (CH). To successfully engineer an ideal concrete, the cement paste, which is the main component, must be studied in detail and all components and interactions must be fully understood. But there are still several unknowns such as the kinetics behind hydration of cement clinker, the control of rheology related to the crystal morphology, the molecular structure of the main hydrated phase CSH, and the interactions between different phases. The present research focused on the development of a method to model cement paste and predict its mechanical behavior at nanoscale as a layered composite of hydrated and unhydrated phases, using reactive molecular dynamics. The main purpose of these models was to reveal the 2 molecular interactions of cement paste phases and their effect on localized mechanical performance. The proposed methodology can be extended to the analysis of three or more cement paste composite phases and to study the inclusion of nanoparticles on the material behavior and hydration. Hydrated C3S was used as a simplified material and two experimental methods (x-ray diffraction and scanning electron microscopy) were used to identify, quantify, and map the crystalline and amorphous phases on matured hydrated C3S. The data obtained from the experimental methods was used to construct models at molecular level of two layered phases of cement paste. The molecular interactions were defined by a ReaxFF, a reactive energy field, and the dynamics of the systems, mechanical properties, and the response to different loading conditions were studied. Finally, experimental characterization based on nanoindentation was used to calculate the elastic modulus and hardness of individual phases of hydrated C3S and compare these calculations to the properties calculated by present molecular dynamics modeling. In particular, in this work constitutive material models of cement paste in response to shock wave conditions were developed. The Grüneisen parameter of cement paste was calculated and proposed for the first time. Also, equations of state of individual phases of cement paste under hydrostatic compression were established and the mechanical response of the materials can be used for studying the anisotropic behavior of these phases. The mechanical properties calculated from nanoindentation were in good agreement with the hydrostatic compression models. The hydration of C3S was further analyzed with reactive molecular dynamics, and models of layered composites of cement paste phases were proposed. Additionally, constructive modeling methods and processes investigated in the present work are effective and extendable to other hierarchical material systems with appropriate material chemistry configurations and energy definitions.

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