Atomistic Simulations of The Structure of Calcium Silicate Hydrates: Interlayer Positions, Water Content And A General Structural Brick Model

Portland cement has been in use by human civilization for over a century. Yet, we do not fully understand its hydration reaction mechanisms due to the complexity of the system and its continued reaction over time making it hard to study it experimentally. Additionally, it contributes to about 10 % of the world carbon dioxide emission. In order to make cement more sustainable, supplementary cementitious materials (SCMs) are substituted for PC. There are many undesirable effects of some SCMs on the early age hydration and later age properties limiting a wider application. One of the key factors hindering the educated designing of new sustainable cements is the lack of knowledge of the atomistic structure of the main hydrate phase of PC, Calcium Silicate Hydrate (C-S-H) and its interaction with SCMs. C-S-H has a variable stoichiometry, nano sized morphology, variable water content and a complex layered structure. Although C-S-H has been studied for decades, the atomic structure of this nano crystalline phase is not clearly known or agreed upon and remains an open question. The proposed structures of C-S-H are mainly based on 14 Å tobermorite, a natural mineral. In this thesis a new bottom up approach has been developed to precisely define and create defects or chemical building blocks¿ in a reduced unit cell of 14 Å tobermorite called the Brick Model. In this model, a string of characters can represent the multitude of structural features in C-S-H with precise control over its arrangement. The model can efficiently capture the structural features of C-S-H and can propose a probable full scale atomic structure of C-S-H including surfaces. We discovered a bridging¿ calcium position using Dynamic Nuclear Polarization Nuclear Magnetic Resonance (DNP NMR) experiments combined with density functional theory (DFT) calculations on structures generated using the brick model. With molecular dynamics simulations (MD), we looked at the intricate interlayer structure of C-S-H. To further understand the complex interlayer structure of C-S-H and the interaction between the defects within the bulk material, the brick model was used to generate a variety of distinct defects and calculate their enthalpies. Our results indicate that the presence of silanols in C-S-H are energetically unfavourable and a lower water content results in a more ordered or crystalline interlayer region. We discovered a second new enthalpically stable position for the interlayer calcium and identified two types of water based on their ¿crystallinity¿ or how strongly the water molecules are coordinated to calcium ions in the interlayer. The results on the defects¿ interactions indicate that the structural units of C-S-H with different interlayer volumes, due to the presence of different types of defects interact and create strains in the C-S-H sheet structure. However, the calculated interaction energy is not prohibitive of their existence. With these findings explanations for the lack of long range order or nano-crystallinity of C-S-H have been proposed which give us valuable insights into key aspects of C-S-H such as growth mechanisms and ageing. Thus the current results can be expected to facilitate the engineering of the C-S-H mesostructure and eventually leading to more sustainable and futuristic cements with less carbon footprint and energy demand.