Calcium silicate hydrate (C-S-H) gel, the important composition of the cement hydrate, contributes the most to the mechanical properties and durability of the cement-based materials. Due to the complex compositional and morphological nature of the gel, the intrinsic structure of the C-S-H gel has not been comprehensively understood yet.

In this thesis, a new C-S-H model at nano-scale has been developed to reveal the structure, dynamics, mechanical and transport properties of the C-S-H gel. First, by using reactive force field, the model was constructed by meeting the requirements of the experimental findings on the silicate chain skeleton, the local structure of the calcium oxygen octahedrons as well as the hydroxyl distribution. The model developed in this way contained both...[ Read more ]

Calcium silicate hydrate (C-S-H) gel, the important composition of the cement hydrate, contributes the most to the mechanical properties and durability of the cement-based materials. Due to the complex compositional and morphological nature of the gel, the intrinsic structure of the C-S-H gel has not been comprehensively understood yet.

In this thesis, a new C-S-H model at nano-scale has been developed to reveal the structure, dynamics, mechanical and transport properties of the C-S-H gel. First, by using reactive force field, the model was constructed by meeting the requirements of the experimental findings on the silicate chain skeleton, the local structure of the calcium oxygen octahedrons as well as the hydroxyl distribution. The model developed in this way contained both Ca-O and Si-O bonds as well as OH bond. It had a Ca/Si ratio of 1.69. Virtual uniaxial tension test was performed on the model to characterize the mechanical properties of the layered structure. It demonstrated the anisotropic nature of the C-S-H gel: The calcium silicate sheet, connected by the stable ionic-covalent Ca-O and Si-O bond, was hard to stretch broken, while the interlayer region distributed with unstable H-bonds network was easy to fracture.

Secondly, the effect of water and calcium atoms on the model structure has been studied under various conditions. The penetration of water molecules transforms the C-S-H gel from amorphous to layered structure by silicate de-polymerization. The mechanical tests associated with structural analysis reveal that the structural water molecules can weaken the stiffness and the cohesive force greatly by replacing the ionic-covalent bond to unstable H-bond. Meanwhile, the calcium atoms help to de-polymerize the long silicate chains and reduce the mean silicate chain length, which weakens the mechanical contribution from the Si-O backbone. Furthermore, the two factors can interplay each other. With the increase of Ca/Si ratio, the broken silicate chains become more defective with large amount non-bridging oxygen atoms, which further accelerate the water adsorption in the C-S-H gel.

In addition to mechanical strength of C-S-H gel, the transport properties of the C-S-H gel, responsible for the durability of cement-based material, has been investigated by simulating the reactivity, structure and dynamics of water molecules confined in the calcium-silicate-hydrate (C-S-H) nano-pore. Due to the high reactive C-S-H surface, hydrolytic reaction happens in the solid-liquid interfacial zone and the partial surface adsorbed water molecules transform to the Si-OH and Ca-OH groups. The defectives silicate chains and solvated Ca_w atoms near the surface contribute to the glassy nature of surface water: large packing density, pronounced orientation preference, and distorted organization. The stable H-bonds connected with Ca-OH and Si-OH groups can restrict the mobility of the surface water molecules. With increasing distance from the channel, the structural and dynamical behavior of the water molecules vary and gradually translate into bulk water properties at distances of 10~15 Å from the liquid-solid interface.

Finally, to understand the influence of mineral admixture to the cement hydrate, the C-S-H model has been modified by substituting Ca atoms with Al atoms. Aluminate species plays an essential role in bridging the defective silicate chains and transforming the C-S-H gel to the branch network structure, which enhances the mechanical properties significantly.

The main contributions of the thesis are listed in the following two respects:

1. A new cement model was developed to increase the understanding of structure, dynamics, mechanics and reaction mechanisms of the C-S-H gel at molecular level. It is efficient and transferable to predict the properties of cement subject to different environmental attraction and tensile loadings.

2. Molecular dynamics on the cement hydrate at nano-scale provides good guidance on how to modify the molecular structure of C-S-H gel by selecting the proper Ca/Si ratio, introducing the mineral admixtures and controlling the humidity conditions. It potentially improves the concrete design.