Concrete is the most consumed manufactured commodity by mass in the world. The manufacturing of Portland cement, the binder of modern concrete, is responsible for 8-9% of global anthropogenic carbon dioxide (CO2) emission, and 2-3% of primary energy consumption. Study of green cement and concrete can provide efficient solutions to reduce the environmental burden of this commodity in the construction industry.
Most concrete is produced using calcium silicate hydrate (C-S-H)-based binder. Understanding the structure of C-S-H based phases is essential to designing a more sustainable cement and concrete. Dicalcium silicate (C2S) is a common clinker compound in both Portland cement and belite-ye’elimite-ferrite cement. The formation of C2S is less energy intensive and yields less CO2. C-S-H is the primary hydration product in hydrated C2S. Incorporation of aluminum-rich industrial by-product is a common method to reduce the environmental impact of concrete production, the principal binding phase of this blended cement-based concrete is calcium aluminosilicate hydrate (C-A-S-H). Sodium hydroxide and sodium silicate activated ground granulated blast-furnace slag are promising cementitious materials alternative to Portland cement due to their lower CO2 emission, the principal binding phase in these alkali-activated materials is sodium incorporated calcium aluminosilicate hydrate (C-N-A-S-H).
This thesis aims to understand different calcium (alumino)silicate hydrates at multi-scale. The characterization of C-S-H and its derivatives, C-A-S-H and sodium incorporated calcium (alumino) silicate hydrate (C-N-(A-)S-H), is reported. There are three main gaps in the current state-of-the art that need to be investigated: (1) β-C2S is the most common C2S polymorph in cement, yet the hydration of other polymorphs (e.g., α’H-C2S ) has rarely been studied, the molecular structure of C-S-H in hydrated α’H-C2S has not been reported; (2) the structure of synthetic C-S-H has extensively studied, however the influences of aluminum incorporation on the coordination environment of Ca and Si in C-A-S-H have not been well understood; (3) the influences of calcium to silicate molar ratio and aluminum induced cross-linking sites on the nanomechanical properties C-S-H have been understood. However, the molecular structure and influence of sodium incorporation of nanomechanical properties of C-N-(A-)S-H have not been resolved.
The studies described in this thesis uses synchrotron radiation-based X-ray techniques to provide new information of the above-mentioned questions. X-ray spectromicroscopy coupled with soft X-ray ptychographic imaging is used to compare the hydration of two polymorphs of dicalcium silicate (β-C2S and α’H-C2S) and to determine the structure of hydration product C-S-H at atomic to microscale. The coordination environments of Ca and Si of C-S-H in the hydrated C2S are studied. Wide-angle X-ray scattering, X-ray spectromicroscopy, and soft X-ray ptychographic imaging are utilized to investigate the atomic to micro-structure of C-S-H and C-A-S-H. The influences of Ca/Si and Al/Si ratios on the coordination chemistry and morphology of C-A-S-H are investigated. High-pressure X-ray diffraction is used to investigate the anisotropic mechanical properties of sodium incorporate C-S-H and C-A-S-H.
The experimental results indicated that C-S-H formed in hydrated β-C2S and α’H-C2S exhibited similar coordination symmetry of Ca. The silicate chains of C-S-H formed in hydrated α’H-C2S polymerize faster than those in hydrated β-C2S. The aluminum incorporation has no significant influences on the coordination symmetry of Ca of synthetic C-S-Hs, while aluminum incorporation increases their silicate polymerization. The morphology of synthetic C-(A-)S-Hs at the nanoscale is independent of the aluminum incorporation and their Ca/Si ratio at temperature of 7-50 °C. The foil of C-A-S-H is significantly coarser at synthesis temperature of 80 °C. Under hydrostatic pressure up to ~10 GPa, the sodium incorporated C-A-S-H exhibit significantly higher incompressibility along the c-axis than sodium incorporated C-S-H and alkali-free C-A-S-H along the c-axis. The experimental results in this thesis improve the understanding of the chemistry of C-S-H and its derivatives (C-A-S-H and C-N-(A-)S-H). The work is important for providing evidence to design high-performance C-S-H based concrete using a bottom-up approach.