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.
