The objective of this project is to link the chemical composition and molecular structure of calcium silicate hydrate (C-S-H) to the viscoelastic mechanical properties of the material. C-S-H is the main binding hydration product of portland cement that governs properties of concrete. If alumina is available during the production of C-S-H, typically from alumino-silicate supplementary cementitious materials, C-S-H is able to incorporate aluminum as a guest ion, producing a calcium aluminosilicate hydrate (C-A-S-H). Alumina incorporation is seen to alter its molecular structure by increasing the tetrahedral chain length, a change that is seen to promote crosslinking between C-S-H layers. By applying a multiscale static and dynamic mechanical testing that include a novel nanomechanical characterization technique of dynamic nanoindentation, the main hypothesis that will be tested during this project is to determine whether C-A-S-H is less viscoelastic than C-S-H due to this increased polymerization. To relate chemical composition and molecular structure with viscoelastic properties, phase-pure C-S-H and C-A-S-H will be synthesized in the lab with varying composition and characterized using advanced techniques such as nuclear magnetic resonance (NMR) for local atomic environment, X-ray diffraction (XRD) for detection of crystalline phases, and X-ray fluorescence (XRF) for bulk chemical composition. Furthermore, a combination of NMR characterization before and after mechanical testing will indicate if there are any changes in molecular structure during viscoelastic deformation, knowledge that will be transformative.
If the hypothesis that alumina substitution in C-S-H reduces creep is proved right, this research will lead to a new method of controlling the viscoelasticity of concrete that can ultimately lead to longer life of concrete structures. Concrete is the most widely used construction material in the world and production of it is associated with approximately 5% of the global CO2 emissions. Thus, any increase in the longevity of concrete structure will mean reduction in the amount of concrete produced and associated greenhouse gas emission. In addition, the main source of alumina in C-S-H is usually fly ash, a waste material that also reduces the portland cement content in concrete and thereby reduces its greenhouse gas emission. The research is well integrated with educational plans, including (a) training graduate student in advanced chemo-mechanical characterization of materials at multiscale, (b) incorporating research findings into graduate level classes, and (c) generating enthusiasm for engineering and sustainable construction among talented middle school students.
