Bicarbonate

Résumé

Le bicarbonate, ou hydrogénocarbonate, est un ion polyatomique dont la formule chimique est HCO3−. Le bicarbonate doit son préfixe (« bi- ») à son double caractère acido-basique (dit caractère amphotère) : il est à la fois acide et base, appartenant à deux couples acido-basiques différents. Il s'agit de la forme intermédiaire dans la double déprotonation de l'acide carbonique : ôter le premier proton (ion H+) de l'acide carbonique donne le bicarbonate ; ôter le second proton donnera la forme carbonate ionique. Le mot s'écrit au pluriel (bicarbonates) lorsqu'il s'agit des sels de l'ion bicarbonate : par exemple, le bicarbonate de sodium (souvent appelé « bicarbonate de soude » par abus de langage, ou « soda à pâte » et « petite vache » au Canada) de formule NaHCO3, qui est le sel de l'ion bicarbonate le plus répandu et le plus utilisé. Il contribue à la digestion par notre organisme. Propriétés Dans l'eau, le dioxyde de carbone, l'acide carbonique, l'ion bicarbonate et l'io

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https://fr.wikipedia.org/wiki/Bicarbonate

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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

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Biomimetic apatite coatings - Carbonate substitution and preferred growth orientation

Biomimetic apatite coatings were obtained by soaking chemically treated titanium in SBF with different HCO3- concentration. XRD, FTIR and Raman analyses were used to characterize phase composition and degree of carbonate substitution. The microstructure, elemental composition and preferred alignment of biomimetically precipitated crystallites were characterized by cross-sectional TEM analyses. According to XRD, the phase composition of precipitated coatings on chemically pre-treated titanium after exposure to SBF was identified as hydroxy carbonated apatite (HCA). A preferred c-axis orientation of the deposited crystals can be supposed due to the high relative peak intensities of the (0 0 2) diffraction line at 2 theta = 26 degrees compared to the 100% intensity peak of the (2 1 1) plane at 2 theta = 32 degrees. The crystallite size in direction of the c-axis of HCA decreased from 26 nm in SBF5 with a HCO3- concentration of 5 mmol/l to 19 nm in SBF27 with a HCO3- concentration of 27 mmol/l. Cross-sectional TEM analyses revealed that all distances correspond exactly to the hexagonal structure of hydroxyapatite. The HCO3- content in SBF also influences the composition of precipitated calcium phosphates. Biomimetic apatites were shown to have a general formula of Ca10-x-yMgy(HPO4)(x-z)(CO3)(z)(PO4)(6-x)(OH)(2-x-w)(CO3)(w/2). According to MR and Raman analyses, it can be supposed that as long as the HCO3- concentration in the testing solutions is below 20 mmol/l, only B-type HCA (0 < z < 1; w = 0) precipitates. At higher HCO3- concentration, it can be assumed that AB-type HCA (z = 1;0 < w < 1) is formed. (C) 2007 Elsevier B.V. All rights reserved.

2007

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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

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EPFL2012