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Title Structure and dynamics of water in ordinary Portland and low CO2 cements
Abstract Cement is the single most used material in the world. Despite many centuries of intensive usage and an ever-increasing worldwide demand, many fundamental physico-chemical questions regarding its structure at the nanoscale level remain elusive. During the process of cement setting, different hydrates nucleate and grow, of which calcium silicate hydrate (C-S-H) is the most important binding phase. C-S-H is indeed the key compound controlling the final cement properties, such as strength and durability. Although a large deal of research effort has been devoted to the study of C-S-H, its poorly crystalline character has made it inherently difficult to characterize its structural and dynamical properties, and the properties of water adsorbed at its interfaces. Moreover, the presence of aluminate phases both in ordinary Portland cement and in new low-CO2 emissions cement formulations results in C-A-S-H, an Al-bearing C-S-H. Understanding water organization in the C-S-H phase is not only important for the setting behavior, but also because water plays a key role in the dissolution-recrystallization and carbonation processes which are the main causes of the loss of cement strength. Moreover, water diffusion in C-S-H nanopores is important to understand the exchange of ions in pollutant removal or in the context of nuclear waste storage. In this Ph.D. project, the structure and dynamics of water in the different pores of C-(A)-S-H were studied using a combination of neutron and X-ray scattering techniques with laboratory-based methods such as water sorption isotherms (WSI), infrared spectroscopy and thermogravimetric analyses. The results of the scattering techniques were analyzed with molecular dynamics (MD) simulations. In particular, the structure of water was studied using neutron diffraction with isotopic substitution (NDIS), a method that allowed probing the local ordering of water molecules adsorbed at C-(A)-S-H / water interfaces. The NDIS results coupled with MD simulations showed that the main mechanism for water adsorption is the coordination of water by hydrophilic calcium ions, which induce a strong hydrogen-bond network with other water molecules and with surface oxygen atoms. The C-S-H surface combines wet and dry regions, with wet areas that were dominated by the presence of hydrophilic calcium ions with strong hydration shells, whereas dry areas exposed only silica chains and silanol hydroxyls. The results of the X-ray scattering and MD simulations revealed that water exerts an influence on the ordering of C-S-H, with the more hydrated C-S-H structure showing a larger degree of mesoscale ordering. The dynamics of water were studied using inelastic incoherent neutron scattering (IINS), a method that probes inter- and intra-molecular vibrations of matter, yielding a vibrational density of states of water in the C-(A)-S-H phase. The experimental data were interpreted with the help of the calculated power spectra from the MD models. The multilayer surface adsorbed water showed a characteristic intensity in the 300-550 cm-1 region, which turned into ice-like features upon capillary condensation at higher RH values. The so-called “fingerprint” of confined water was observed at lower energies for drier C-(A)- S-H samples. In good accordance with the WSI results, water in C-(A)-S-H samples at higher Ca/Si ratios was found to be more structured and less bulk-water like, due to an increased number of hydrophilic cites created by calcium ions. The results from this thesis will contribute to the understanding of the properties of interfacial water at C-(A)-S-H, a key phase to understand cement mechanical properties and durability.
Organisation ISIS , ISIS-TOSCA , STFC
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Language English (EN)
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Thesis PhD, Universite Grenoble Alpes, 2021. https://www.ill.e…hese_Zhakiyeva_Z.pdf 2021