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Abstract: Decalcification causes critical degradation of calcium silicate hydrate (C-S-H) – the principal binder in concrete. Contrary to structural change, the decalcification-induced change of C-S-H mechanical properties has not been sufficiently investigated. To fill this gap, here C-S-H was synthesized and decalcified using NH4NO3 solutions. It was then subjected to chemical analysis and high pressure X-ray diffraction study. Our results indicate that C-S-H decalcifies only in the interlayer which reduces its stiffness along the c-axis. The incompressibility and the transverse isotropy remain unchanged for the calcium silicate main layer, despite the vast change of silicate chain linkage. The intralayer Ca does not leach as long as C-S-H maintains the tobermorite-like structure. A ~30% reduction of bulk modulus is estimated for decalcified C-S-H. Our results add fundamental knowledge to the structure-property correlation of C-S-H subjected to decalcification.

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Abstract: Using synchrotron x-ray diffraction, we show that the long-accepted monoclinic structure of the “collapsed” high-pressure phases reported in seven lanthanide elements [Nd, Tb, Gd, Dy, Ho, Er, and (probably) Tm] is incorrect. In Tb, Gd, Dy, Ho, Er, and Tm we show that the collapsed phases have a 16-atom orthorhombic structure ( o F 16 ) not previously seen in the elements, whereas in Nd we show that it has an eight-atom orthorhombic structure ( o F 8 ) previously reported in several actinide elements. o F 16 and o F 8 are members of a new family of layered elemental structures, the discovery of which reveals that the high-pressure structural systematics of the lanthanides, actinides, and group-III elements (Sc and Y) are much more related than previously imagined. Electronic structure calculations of Tb, combined with quantum many-body corrections, confirm the experimental observation, and calculate that the collapsed orthorhombic phase is a ferromagnet, nearly degenerate with an antiferromagnetic state between 60 and 80 GPa. We find that the magnetic properties of Tb survive to the highest pressures obtained in our experiments (110 GPa). Further calculations of the collapsed phases of Gd and Dy, again using the correct crystal structure, show the former to be a type-A antiferromagnet, whereas the latter is ferromagnetic.

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Abstract: The effect of pressure on the room temperature solubility of hydrogen in Zircaloy-4 was examined using synchrotron X-ray diffraction on small ground flake samples in a diamond anvil cell at pressures up to 20.9 GPa. Different combinations of hydrogen level/state in the sample and of pressure transmitting medium were examined; in all three experiments, it could be concluded that pressure resulted in the dissolution of δ hydrides and that interstitial hydrogen seemingly retards the formation of ω Zr. A pressure of around 9 GPa was required to halve the hydride fraction. These results imply that the effect of pressure is thermodynamically analogous to that of increasing temperature, but that the effect is small. The results are consistent with the volume per Zr atom of the α, δ and ω phases, with the bulk moduli of α and δ, and with previous measurements of the hydrogen site molar volumes in the α and δ phases. The results are interpreted in terms of their implication for our understanding of the driving forces for hydride precipitation at crack tips, which are in a region of hydrostatic tensile stress on the order of 1.5 GPa.

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Abstract: This thesis largely focuses on the mechanistic analysis of the Assembly-Disassembly- Organisation-Reassembly (ADOR) process through a range of crystallographic techniques including powder X-ray diffraction and Pair Distribution Function (PDF) analysis and subsequent analysis using solid-state kinetics. Chapter 4 describes the development of a new standard protocol to using the ADOR process. The protocol describes the development of a procedure used for identifying the optimum conditions (time of reaction, temperature, acidity, etc.) for the ADOR process. In developing the protocol, Ge-containing UTL zeolites were subjected to hydrolysis conditions using both water and hydrochloric acid as media, which provides an understanding of the effects of temperature and pH on the Disassembly (D) and Organisation (O) steps of the process that define the potential products. Samples were analysed by powder X-ray diffraction to yield a time course for the reaction at each set of conditions. Chapter 5 continues work on the ADOR process and presents the first kinetic study on the two most prominent steps in the process; Disassembly and Organisation. By using solid- state kinetic models, Avrami-Erofeev and its linear equivalent Sharp-Hancock, the dependence on temperature and presence of liquid water was investigated and the activation energy of the rearrangement process quantified. Work on the rearrangement step aimed to understand where the silica species intercalates from and which material formed as the kinetic and thermodynamic product from the reaction. Chapter 6 describes a study into the Disassembly and Organisation steps of the ADOR process through in situ Pair Distribution Function (PDF) analysis. This hopes to shed light on the selectivity of the ADOR process in different media and the mechanism by which the double-four-ring (d4r) breakdown. On a different note, Chapter 7 describes the refinement of synthesis conditions used to prepare poly-crystalline CPO-27-M (MOF-74) with lower concentrations of base and at low temperature. Refinement of the synthesis of single crystal CPO-27-Mg, -Zn and UTSA-74 was undertaken and the necessary components to forming large single crystals understood.

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Abstract: The understanding of the gypsum (CaSO4∙2H2O) formation pathway from aqueous solutions has been the subject of intensive research in the last couple of years. This interest stems from the fact that gypsum appears to fall into a broader category of crystalline materials whose formation does not follow classical nucleation and growth theories. The pathways involve transitory precursor cluster species, yet the actual structural properties of such clusters are not very well understood. Here, we show how in situ high-energy X-ray diffraction (HEXD) experiments and molecular dynamics (MD) simulations can be combined to derive the structure of small CaSO4 clusters, which are precursors to crystalline gypsum. We fitted several plausible structures to the derived pair distribution functions (PDFs), and explored their dynamic properties using unbiased MD based on both rigid-ion and polarizable force-fields. Determination of the structure and (meta)stability of the primary species is important both from a fundamental and applied perspective; for example, this will allow for improved design of additives for greater control of the nucleation pathway.