Ice VII from aqueous salt solutions: From a glass to a crystal with broken H-bonds

It has been known for decades that certain aqueous salt solutions of LiCl and LiBr readily form glasses when cooled to below approximate to 160 K. This fact has recently been exploited to produce a "salty" high-pressure ice form: When the glass is compressed at low temperatures to pressures higher than 4 GPa and subsequently warmed, it crystallizes into ice VII with the ionic species trapped inside the ice lattice. Here we report the extreme limit of salt incorporation into ice VII, using high pressure neutron diffraction and molecular dynamics simulations. We show that high-pressure crystallisation of aqueous solutions of LiCl center dot RH2O and LiBr center dot RH2O with R = 5.6 leads to solids with strongly expanded volume, a destruction of the hydrogen-bond network with an isotropic distribution of water-dipole moments, as well as a crystal-to-amorphous transition on decompression. This highly unusual behaviour constitutes an interesting pathway from a glass to a crystal where translational periodicity is restored but the rotational degrees of freedom remaining completely random.

Translational and Rotational Diffusion in Water in the Gigapascal Range

Livia Eleonora Bove Kado

First measurements of the self-dynamics of liquid water in the GPa range are reported. The GPa range has here become accessible through a new setup for the Paris-Edinburgh press specially conceived for quasielastic neutron scattering studies. A direct measurement of both the translational and rotational diffusion coefficients of water along the 400 K isotherm up to 3 GPa, corresponding to the melting point of ice VII, is provided and compared with molecular dynamics simulations. The translational diffusion is observed to strongly decrease with pressure, though its variation slows down for pressures higher than 1 GPa and decouples from that of the shear viscosity. The rotational diffusion turns out to be insensitive to pressure. Through comparison with structural data and molecular dynamics simulations, we show that this is a consequence of the rigidity of the first neighbors shell and of the invariance of the number of hydrogen bonds of a water molecule under high pressure. These results show the inadequacy of the Stokes-Einstein-Debye equations to predict the self-diffusive behavior of water at high temperature and high pressure, and challenge the usual description of hot dense water behaving as a simple liquid.

American Physical Society2013

Investigation of Sulfate Attack by Experimental and Thermodynamic Means

Wolfgang Kunther

This work investigates sulfate attack in complex sulfate environments by exposing different binder types to various sulfate solutions and comparing predicted phase and volume changes with experimental data. The most important aspects of this work can be grouped in three topics: The comparison of the predicted volume increase with the experimentally observed length changes. This part of the work shows that volume increase cannot be linked directly to the observed expansions. Additionally, the volume increase model does not provide an explanation for the driving force for the space filling to overcome mechanical constraints. A more plausible explanation of expansion lies in the theory of crystallization pressure, in which crystals forming from a supersaturated solution seem to be a more applicable explanation for the deterioration differences. However, this is difficult to verify directly as it is impossible to measure the solution concentration or individual crystals exert pressure on their surroundings. It is observed that expansion occurs in systems where thermodynamic modelling predicts the co-existence of ettringite with gypsum. In such a case, if monosulfate and gypsum are both present locally, the solution can be highly supersaturated with respect to ettringite within the exposed materials. An increase of sulfate contents in the C-S-H phase has been observed for these cases, indicating increased sulfate concentrations in the pore solution while providing the necessary confinement for the crystals in intimate phase mixtures to exceed pressure. The presence of bicarbonate ions in the sulfate solution reduces the expansion significantly for the CEM I and CEM III/B binders, and reduced the sulfate uptake in the phase assemblage. The CEM III/B mortars showed a highly leached zone at the surface in which also calcite was observed, this is attributed to the destabilisation of ettringite in the presence of high concentrations of bicarbonate ions. The microstructural characterization combined with the information from thermodynamic modelling suggests that conditions of high supersaturation with respect to ettringite are unlikely to occur in samples exposed to solutions containing bicarbonate ions. Degradation of mortars exposed to complex magnesium containing sulfate solutions showed that the presence of sodium, potassium and calcium in a magnesium solution reduce the deterioration symptoms significantly for the CEM I and CEM III/B mortars. The experimental observations suggest that sulfate attack in natural environments is not only less severe, due to reduced sulfate concentrations in the field, but are likely to be affected by the presence of bicarbonate anions and the common occurrence of different cations. Both of these aspects reduce the deterioration significantly and can be assumed to occur in most natural waters.

EPFL2012

Investigation of Sulfate Attack by Experimental and Thermodynamic Means

Wolfgang Kunther

EPFL2012