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Titel |
Dolomite Dissolution in Alkaline Cementious Media |
VerfasserIn |
Florian Mittermayr, Dietmar Klammer, Stephan Köhler, Martin Dietzel |
Konferenz |
EGU General Assembly 2010
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 12 (2010) |
Datensatznummer |
250044765
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Zusammenfassung |
Chemical alteration of concrete has gained much attention over the past years as many cases
of deterioration due to sulphate attack, thaumasite formation (TSA) or alkali silica reactions
(ASR) have been reported in various constructions (Schmidt et al, 2009). Much
less is known about the so called alkali carbonate reaction (ACR). It is believed
that dolomite aggregates can react with the alkalis from the cement, dissolve and
form calcite and brucite (Katayama, 2004). Due to very low solubility of dolomite
in alkaline solutions this reaction seems doubtful. In this study we are trying to
gain new insides about the conditions that can lead to the dissolution of dolomite
in concrete. Therefore we investigated concrete samples from Austrian tunnels
that show partially dissolved dolomite aggregates. Petrological analysis such as
microprobe, SEM and Raman spectroscopy as well as a hydrochemical analysis
of interstitial solutions and ground water and modelling with PhreeqC (Parkhurst
and Appelo, 1999) are carried out. In addition a series of batch experiments is set
up. Modelling approaches by PhreeqC show a thermodynamically possibility in
the alkaline range when additional Ca2+ in solution causes dolomite to become
more and more undersaturated as calcite gets supersaturated. Interacting ground
water is enriched in Ca2+and saturated with respect to gypsum as marine evaporites
are found in situ rocks. Furthermore it is more likely that Portlandite (Ca(OH)2)
plays a more important role than Na and K in the cement. Portlandite acts as an
additional Ca2+ source and is much more abundant than the alkalies. Some interstitial
solutions are dominated mainly by Na+ and SO42- and reach concentrations up to
30 g/l TDS. It is believed that solutions can even reach thenardite saturation as
efflorescences are found on the tunnel walls. In consequence dolomite solubility
increases with increasing ionic strength. pH > 11 further accelerate the process
of dedolomitization by the removal of Mg2+ as brucite (Mg(OH)2) precipitates.
Brucite and calcite were detected insitu by Raman spectroscopy surrounding partially
dissolved dolomite grains. Dolomite dissolution under alkaline condition is a dynamic
process of dissolution and precipitation stimulated by high Ca2+ content, high
ionic strength, low temperature and high pH with the consequence of low Mg2+
concentration.
References:
Katayama, T., 2004. How to identify carbonate rock reactions in concrete. Materials
Characterization 53, 85-104.
Parkhurst, D. L., Appelo, C. A. J. 1999. User’s guide to PHREEQC. U.S. Geol. Sur.:
312.
Schmidt, T., Lothenbach, B., Romer, M., Neuenschwander, J., Scrivener K., 2009.
Physical and microstructural aspects of sulfate attack on ordinary and limestone blended
Portland cements. Cement and Concrete Research 39, 1111-1121. |
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