UC Berkeley

Calcium silicate hydrate (C-S-H), the predominant product of cement hydration and the principal binding phase in concrete, is fundamental to the material’s strength and durability. The compositional and structural characteristics of C-S-H have garnered significant attention due to its complex and pivotal role in concrete performance. With the advent of advanced computational science and high-efficiency computing technologies, sophisticated nanoscale simulations of C-S-H have become possible. These simulations offer profound insights into the nanostructural behavior of C-S-H and elucidate the underlying mechanisms that contribute to its performance. Numerous empirical models, developed using advanced characterization techniques such as X-ray Diffraction (XRD), Nuclear Magnetic Resonance (NMR), Small-Angle X-ray Scattering (SAXS), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM), have provided in-depth insights into the multiscale characteristics of C-S-H. These empirical models are critical foundations for constructing computational models, including Molecular Dynamics (MD) and coarse-grain models. However, due to the high computational cost, most atomistic models are limited to the nanoscale, often with dimensions smaller than 1 nm. The extensively studied structural and compositional characteristics of C-S-H are predominantly focused on average properties within ensemble systems at the microscale, creating a disparity between empirical parameters and computational inputs, and highlighting a significant challenge in accurately bridging these scales. This dissertation begins with an investigation of the hydration kinetics of alite. Furthermore, Multimodal transmission electron microscopy (TEM) is employed to extract comprehensive chemical and structural information, such as porosity and crystallinity. Our findings reveal that rapid nucleation of C-S-H occurs at the surface of grains within minutes, followed by subsequent C-S-H growth. The development of C-S-H fibrils can be categorized into two distinct stages: needle elongation and texture densification. Through quantitative analysis, the growth rate of C-S-H needles is estimated to be 7 nm/min, while a decrease is observed in the intrinsic porosity of C-S-H from 7.9% to 3.1% concurrent with the thickening of C-S-H lamella. Electron diffraction analysis further demonstrates the homogenization of C-S-H throughout hydration, involving structural compaction and silicate polymerization. Our work provides valuable insights into the origins of C-S-H nucleation and growth, thereby enhancing our understanding of hydration mechanisms at a fundamental level. Furthermore, electron energy loss spectroscopy (EELS) is utilized to study the local structure of C-S-H at an unprecedented spatial resolution of 5 nm. The chemical environments of silicon (Si) and calcium (Ca) elements, thickness, and dielectric properties are scrutinized. Statistical analysis of over 10,000 data points reveals significant heterogeneity in the silicate chemical environment, including different polymerization degrees and tetrahedral distortions. In contrast, the local Ca environment exhibits more homogeneity with a coordination number ranging from 7 to 9, indicating a weak octahedral-like symmetry for C-S-H. Additionally, our findings show that the local thickness of C-S-H predominantly hovers around ~15 nm, consisting of 13-14 layers, as validated through electron tomography. This work provides insights into the local structural features of C-S-H from the single particle perspective, facilitating the future development of more realistic C-S-H models. Finally, a full atomistic C-S-H colloid model is developed based on the nanoscale characterization results and applied to investigate the physical and mechanical properties at different gel water concentrations. It was discovered that water confined in nanometer gel pores decreases the mechanical properties of C-S-H due to its hydrolysis effect. A “ductile to brittle” transition is observed in the C-S-H colloid model with increasing gel water content, evolving from transgranular failure to intergranular fracture. The gel water confined in nanometer-sized pores displays glassy characteristics and acts as a lubricant, facilitating structural distortions during deformation. The topology effect of the hydrogen bond network on the evolution of the viscoelasticity of C-S-H at different humidities is elucidated. This work sheds light on the role of water in C-S-H colloids from a nanostructure perspective, contributing to a deeper understanding and further advancements in the field.