Concrete is one of the most widely used engineering materials worldwide, significantly exceeding any other infrastructure material, such as steel and wood. At present, most of the feasible options for microstructural improvement of cement and concrete have been implemented. Thus, there is an urgent need to find scientific and technically-viable improvements by engineering the cementitious phases at the nanometer scale to increase concrete strength and durability. Calcium silicate hydrate (C-S-H) is the main hydration product of Portland cement and the principal binding agent in the cement paste; as such, C-S-H is the main contributor to the mechanical properties of concrete.

Monteiro lab has previously performed high-pressure x-ray diffraction (HP-XRD) experiments on calcium (alumino)silicate hydrate (C-(A-)S-H), a structurally similar material as C-S-H, using diamond anvil cell (DAC). The results from these studies confirm the hypothesis that fiber-like C-(A-)S-H nanoparticles will preferentially orient to the compression direction under uniaxial deformation, and no texture will form under hydrostatic stresses. However, owing to the lack of control of applied loads in the DAC, where each incremental loading (or unloading) is typically in the order of GPa, it is necessary to perform the experiments under more relevant and realistic conditions, i.e., with applied loads in the order of MPa, for studying mechanical properties of cement paste and concrete. To better control the loading pressures, this research uses deformation-DIA (D-DIA) multi-anvil apparatus and large-volume press (LVP) system at beamline 13-BM-D of Advanced Photon Source to conduct in situ shear deformation experiments and measure the orientation behaviors of C-S-H with x-ray diffractions.

The first goal of this dissertation aims to investigate the rearrangement of C-S-H nanoparticles under shear stresses, which has been postulated to be a potential root cause of concrete creep. Furthermore, the structure of C-S-H nanoparticles is modified with (3-aminopropyl)triethoxysilane (APTES) and polycarboxylate ethers (PCEs), and the goal is to examine the effects on the texture of these modified nanocomposites.

The x-ray diffraction results reveal the structural modifications of C-S-H via intercalations of different organic contents. The bulk modulus of these modified-CSH nanocomposites was computed using the Birch-Murnaghan equation of state. The results show that the intercalations of small organic molecules APTES produce more chemically stable and higher bulk modulus C-S-H nanocomposites than the intercalations of C-S-H with PCEs.

The results from the high-pressure x-ray diffraction experiments at beamline 13-BM-D show for the first time that C-S-H nanoparticles could start forming texture at deviatoric stress of around 120 MPa and could represent the energy barrier for the initiation process of the C-S-H nanoparticles’ orientations under shear loading. In a separate cyclic loading experiment, x-ray radiography images capture the delay in strain response of C-S-H under shear loading; the results may be the first direct measurements from the experiment that show the viscous nature of C-S-H, which is linked to concrete creep.

Finally, the results from the shear deformation experiments on the modified C-S-H structure demonstrate that CSH-APTES nanocomposites show more resistance to developing preferred orientations under deviatoric stresses than unmodified C-S-H samples. Higher deviatoric stresses are also required to drive the "initiation" process of forming textures for CSH-APTES nanocomposites compared to unmodified C-S-H nanoparticles.

The studies from these experiments open up a new chapter in understanding the cementitious phase at the nanometer scale. In particular, if rearrangement of C-S-H nanoparticles is the root cause of concrete creep, then the development of creep may be monitored by HP-XRD techniques. The results also show that the intercalations of small organic molecules into the layered structure of C-S-H could effectively design a more creep-resistant concrete. These structural modifications help us better understand C-S-H's structure-property relationships and provide a promising strategy for designing C-S-H to resist creep from the bottom-up approach.

Concrete infrastructure is not as durable as desired, although concrete is the most consumed engineered material in the world by mass. To achieve carbon neutrality and slow down global warming, the construction community is under increasing pressure to improve the sustainability and durability of next-generation high-performance concrete.

The desire to extend the life cycle of modern concrete infrastructure and reduce its lifespan carbon footprint has led to renewed interest in determining the reasons for the long-term durability of ancient Roman concrete. Besides, fiber-reinforced cementitious composites (FRCC), initially introduced in the 1990s, show the promise of controlling crack propagation, enhancing tensile ductility, and improving the structural durability of concrete infrastructure. The composition and microstructure of ancient Roman concrete and FRCC are the key factors of the long-term durability and ductility under mechanical loadings and environmental exposures.

Understanding the microstructural mechanism of ancient Roman concrete and FRCC’s high performances in the mechanical tests (e.g., uniaxial compression test, uniaxial tension test, and three-point bending test) and durability test (e.g., water absorption test, alkali-silica reaction test, and self-healing test) is essential to design the next-generation concrete. There are three main gaps in the current state-of-the-art that need to be investigated: (1) mechanisms for the long-term durability of ancient Roman concrete in water; (2) mechanisms for the crack resistance capacity of ancient Roman concrete under loading; (3) mechanisms for the ductility and durability of FRCC. However, the previous studies' lack of in-situ analytical methods and volumetric representation for concrete’s microstructure under different field conditions limits the ability to cross the gaps.

In this thesis, synchrotron X-ray computed microtomography (μCT) experiments were conducted to characterize the three-dimensional microstructure of ancient Roman concrete and FRCC to new information on the above-mentioned questions. Three sets of experiments were conducted to (1) characterize the microstructure and predict water permeability of ancient Roman concrete using μCT and neutron radiography; 2) characterize the microstructure evolution and ductile fracture pattern of ancient Roman concrete in the in-situ compression test and in-situ three-point bending test; and 3) characterize the microstructural evolutions of FRCC during the in-situ compression test, in-situ three-point bending test, and ex-situ durability tests.

The state-of-the-art machine-learning derived pipeline for automated μCT image analysis was developed to process the three-dimensional raw μCT image, classify different components (phases), characterize the morphological properties, and conduct the statistical analysis. The pipeline outperformed the traditional methods on multiple tasks, especially image segmentation, in the accuracy, speed, robustness, and reproductivity. In the computer vision analysis, preprocessed μCT images were segmented into different phases accurately using machine learning-based and deep learning-based image segmentation models. The advanced algorithms and prototype software quantified the structure, distribution, orientation, and connectivity of the pores, cracks, and fibers in segmented phase images. For in-situ experiments, advanced digital volume correlation (DVC) algorithms calculated displacement map, strain field, strain localization, crack initiation, and crack propagation based on the 3D tomograms of the reference state and the deformed state. Besides, the results of other experiments (e.g., neutron radiography images, scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP)) were combined in the data processing pipeline to investigate the sample microstructure systemically.

The experimental results indicated that the highly porous ancient Roman concrete samples have similar water capillary penetration coefficients to modern PC paste/concrete samples. The low pore connectivity decreased the permeability and contributed long-term durability of ancient Roman concrete. Ductile fracture patterns were observed once cracks were introduced in ancient Roman concrete. The optimal aggregate grading curves and isolated pore structure contributed to the crack resistance. For crack resistance mechanisms in in-situ tests, the crack initiation happened at the ITZ around aggregates, and micro-cracks were diffracted and furcated by aggregates, especially the relict lime clasts in ancient Roman concrete samples. The reinforcement in the ITZ of aggregates obstructed crack propagation and produced toughening. The furcation and multi-crack propagation during the stable fracture pattern consumed additional energy and contributed to the ductility of ancient Roman concrete.

Fibers play an important role in the ductility and durability of FRCC. Fibers reinforced the fracture planes in the FRCC through fiber debonding, bridging, bending, stretching, and fracturing under tensile load, which caused multi-crack propagation, reduced the average crack width and improved the toughness. During the uniaxial tensile test, the strain localization was observed around the connected pores with a volume exceeding 0.01 mm3, and multi-cracks propagated through these regions. Therefore, filling the pores within a specific size range using micro-scale fine aggregate may further improve the strength and ductility of the specimen. In durability tests, fibers controlled the average crack width and reduced the connectivity of the pore network, which contributed to the high durability of the FRCC. The work is important for providing mechanisms to design high-performance concrete at the microscale using a bottom-up approach.