Characterization of Calcium (Alumino)Silicate Hydrate Phases with Synchrotron-Based Techniques

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./nMost 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)./nThis 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./nThe 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./nThe 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.

Advisor: Monteiro, Paulo J.M.