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Conceptual model for concrete long time degradation in a deep nuclear waste repository
RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Sveriges tekniska forskningsinstitut, CBI - Cement- och betonginstitutet.
1996 (English)Report (Refereed)
Abstract [en]

Cement-based materials are fundamentally unstable in a long time perspective. With time the concrete will change properties both as a consequence of recrystallisation and chemical interaction with the environment.The hydration products are stabilized by the high pH within the concrete. Thus, when the cement paste has equilibrated with the environment new phases will form. Not only reactions inside the cement paste are of interest in this context. It is also important to consider reactions with the aggregate phase and in particular reactions with the silica component in the aggregate phase due to the high pH from alkali hydroxides and subsequent reactions of the alkali silica hydrate phases. This has been a special subject of research at CBI. The rate of the chemical reactions in concrete in general is governed by a combination of dissolution mechanisms, diffusion and permeability/porosity. The main controlling factor is the dissolution of calcium hydroxide (portlandite) which buffers and controls the pH in the cement paste. Another important factor is the permeability of the concrete. During the first period the permeability will decrease as a consequence of continued hydration and recrystallization of the cement paste. _x000D_ _x000D_ One of the main difficulties with a conceptual model for concrete degradation is the change in the geochemical environment with time.The degradation process must be treated as a sequence of different degradation mechanisms. During the first period the concrete will alter as a result of contact with atmospheric gases especially carbon dioxide which will carbonate the surface. Later the degradation will mainly be governed by the composition of the groundwater with which it will try to equilibrate. Thus the composition of both the groundwater and the concrete will change. Considering the groundwater chemical conditions at repository depths (500 m), it is possible that with time the groundwater will change composition from normal to saline. This may in fact be an advantage because the solubility of the cement paste components decreases. However, the concrete itself will influence the groundwater composition and create an aureole with increased pH around it. Most of the components in both the fresh and saline water will not be harmful to concrete. One of the problems may be the chlorine anions, as this anion may substitute for sulphate in some of the cement phases. This will not degrade the concrete but the sulphates in the cement may be released to the groundwater. The end product of the concrete, after leaching and after the pH buffer capacity has been lost, will be a mix of metastable calcium silicate hydrates, zeolite and clay minerals. _x000D_

Place, publisher, year, edition, pages
Cement och Betong Institutet , 1996. , p. 105
Series
CBI rapport, ISSN 0346-8240 ; 1996:2
Keywords [en]
Nuclear waste repository, Concrete degradation, Alkali-silica reactions, Sulphate attack, Chloride diffusion, Carbonation, Cement chemistry, Groundwater chemistry
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:ri:diva-2958OAI: oai:DiVA.org:ri-2958DiVA, id: diva2:960563
Available from: 2016-09-07 Created: 2016-09-07 Last updated: 2021-01-26Bibliographically approved

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Trägårdh, Jan

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