Supersaturation

In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a liquid. A supersaturated solution is in a metastable state; it may be brought to equilibrium by forcing the excess of solute to separate from the solution. The term can also be applied to a mixture of gases. History Early studies of the phenomenon were conducted with sodium sulfate, also known as Glauber's Salt because, unusually, the solubility of this salt in water may decrease with increasing temperature. Early studies have been summarised by Tomlinson. It was shown that the crystallization of a supersaturated solution does not simply come from its agitation, (the previous belief) but from solid matter entering and acting as a "starting" site for crystals to form, now called "seeds". Expanding upon this, Gay-Lussac brought attention

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

Influence of bicarbonate ions on the deterioration of mortar bars in sulfate solutions

Wolfgang Kunther, Barbara Lothenbach, Karen Scrivener

This work investigates the influence of bicarbonate ions on the deterioration of cementitious material exposed to sulfate ions. Mortars based on a CEM I and on a CEM III/B cement were investigated. Experimental investigations were compared to thermodynamic modeling and phase characterization to understand the differences in deterioration. The presence of bicarbonate ions significantly reduced the expansion of the CEM I mortars. Thermodynamic modeling showed that at high concentrations of bicarbonate ettringite and gypsum become unstable. Microstructural characterization combined with information from thermodynamic modeling suggests that conditions of high supersaturation with respect to ettringite are unlikely in the samples exposed in solutions containing bicarbonate. Consequently, expansive forces are not generated by the crystallization pressure of ettringite. There was little expansion of the CEM III/B sample even in the sodium sulfate solution. In the bicarbonate solution this mortar showed a highly leached zone at the surface in which calcite was observed. (c) 2012 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2013

Investigation of Sulfate Attack by Experimental and Thermodynamic Means

Wolfgang Kunther

EPFL2012

Perturbation of the low-pressure diamond growth process by carbon supersaturation and radiofrequencey [i.e. radiofrequency] plasma

Investigation of Sulfate Attack by Experimental and Thermodynamic Means

Influence of bicarbonate ions on the deterioration of mortar bars in sulfate solutions

Perturbation of the low-pressure diamond growth process by carbon supersaturation and radiofrequencey [i.e. radiofrequency] plasma

Investigation of Sulfate Attack by Experimental and Thermodynamic Means

Influence of bicarbonate ions on the deterioration of mortar bars in sulfate solutions