1,720,982 research outputs found
Modeling of the interaction between chemical and mechanical behavior of ion exchange resins encapsulated into a cement-based matrix
No full textLes résines échangeuses d’ions (REI) sont communément utilisées dans l’industrie nucléaire pour purifier des effluents contaminés non-stockables. Les REI deviennent ainsi un déchet solide qu’il est possible de conditionner. L’une des méthodes de conditionnement consiste en un enrobage dans une matrice cimentaire. Ce procédé pose un certain nombre de questions quant à la stabilité dimensionnelle de l’enrobé. Le déchet, une fois enrobé, est en effet susceptible d’interagir chimiquement avec la matrice cimentaire ce qui peut, dans certains cas, entrainer son gonflement par le biais de pressions internes. C’est autour de cette problématique que ce travail de thèse a vu le jour avec pour objectif de développer une modélisation physico-chimique multi-échelle du composite afin d’en estimer le comportement mécanique macroscopique. Cette étude s’est exclusivement intéressée aux interactions chimie-mécaniques induites par des REI cationiques pouvant engendrer à long terme un comportement expansif de l’enrobé.Ion exchange resins (IER) are widely used in the nuclear industry to purge non directly storable infected effluents. IER then become a solid waste which could be stored as any classical nuclear waste. One way of conditioning consists in embedding it into a cement paste matrix. This process raises some concerns regarding the cohesiveness of the composite. Once embedded, the IER might indeed interact with the cement paste which would lead, in some cases, to the swelling of the composite. This thesis has been set up to address this potential issue, with the aim to develop a numerical tool able to predict the mechanical behavior of this kind of material. This work only focuses on the long term behavior and more specifically on the potential degradations of the cement paste /IER composite due to cationic IER
Étude multi-physique des matrices cimentaires bas carbone et bas pH exposées à des environnements contenant du magnésium
No full textL’IRSN est en charge d’évaluer la sûreté de la conception du projet de stockage en couche géologique profonde des déchets radioactifs (Cigéo). L’un des grands enjeux de sûreté de ce dossier concerne les dispositifs de fermeture. Les matériaux cimentaires utilisés dans les ouvrages de génie civil (stockage géologique souterrain de déchets radioactifs, barrages etc.) sont conçus pour résister à diverses agressions ou sollicitations pendant plusieurs dizaines voire centaines d'années. Cependant, en contact d’eau douce ou d’eau de mer, la lixiviation de la matrice cimentaire s’accompagne d’un enrichissement en magnésium conduisant à la formation de brucite (comblant la porosité et formant une couche protectrice) et/ou de silicate de magnésium hydratés (M-S-H) dont les propriétés sont peu étudiées. Avec l’émergence de nouveaux liants, la composition minéralogique des bétons hydratés change et les mécanismes d’altération en présence de magnésium sont différents de ceux rencontrés dans des ciments ordinaires riches en portlandite. La formation des M-S-H est favorisée vis-à-vis de celle de la brucite. Cette thèse a pour objectif : (i) de comprendre les mécanismes réactionnels de l’attaque magnésienne et de la formation des M-S-H au sein des matrices cimentaires à faible teneur en calcium, (ii) d’étudier l’influence de la formation des M-S-H sur les propriétés micro-structurelles et mécaniques et (iii) de proposer des données microstructurales et mécaniques des M-S-H intégrables dans des modèles chemo-mécaniques.Dans un premier temps, l’influence de la composition de la pâte cimentaire sur les phases formées lors de l’attaque magnésienne a été étudiée. La présence de portlandite implique la formation de brucite. Une hydratation non complète des liants à faible C/S, même en l’absence de portlandite, peut aussi provoquer la formation de brucite. Les M-S-H se forment au sein de la pâte et correspondent à un enrichissement en magnésium en profondeur. Dans un second temps, une caractérisation des M-S-H a été réalisée sur des pâtes de M-S-H afin d’acquérir des données sur les propriétés physiques et mécaniques intrinsèques aux M-S-H et ainsi pouvoir, via une modélisation chemo-mécanique à petite échelle, comprendre les mécanismes d’altération des propriétés sous ces attaques. Enfin, une étude multi-physique, à l’échelle mésoscopique, de l’altération de matrices à faible teneur en calcium par des eaux contenant du magnésium a été réalisée. Deux pâtes cimentaires à bas C/S (pâte modèle à base de silice colloïdale et pâte réelle à base de fumée de silice et de laitier – T3) ont été exposées à des solutions avec des concentrations en Mg différentes (5 et 50 mM). La caractérisation chimique et minéralogique a permis de mettre en évidence qu’une profonde décalcification a lieu en parallèle d’un fort enrichissement en magnésium de la pâte, correspondant à la formation de M-S-H. Les caractérisations micro-structurelles et mécaniques (par indentation), réalisées pour observer l’évolution le long de ce front de dégradation, ont montré une augmentation de la porosité et une forte perte de propriétés élastiques locales malgré la formation des M-S-H. Une homogénéisation mécanique, à partir des données déterminées précédemment sur les pâtes de M-S-H, confirment que les M-S-H issus de dégradation possèdent de faibles propriétés élastiques. L’augmentation de la concentration en Mg ne modifie pas la minéralogie et la chimie de la zone dégradée mais uniquement la profondeur dégradée. Les résultats sur la pâte réelle (T3) sont similaires à ceux de la pâte modèle, validant les résultats sur celle-ci. En complément, la pâte modèle a aussi été étudiée en lixiviation pure pour comparer l’impact de la lixiviation à celle de l’attaque magnésienne sur les liants bas C/S. Une étude via un code de transport réactif a été réalisée pour mieux comprendre les différences de cinétique et de phénoménologie. Elle confirme que la présence de magnésium accélère la dégradation.IRSN is in charge of assessing the safety of the design of the French deep geological repository project for radioactive waste (Cigéo). One of the major safety issues concerns the closure systems. The cementitious materials used in civil engineering structures (underground geological disposal of radioactive waste, dams, etc.) are designed to withstand various stresses and strains for several decades or even hundreds of years. However, in contact with soft or sea water, the leaching of the cement matrix is accompanied by an enrichment in magnesium leading to the formation of brucite (filling the porosity and forming a protective layer) and/or magnesium silicate hydrates (M-S-H), the properties of which are little studied. With the emergence of new binders, the mineralogical composition of hydrated concrete changes and the mechanisms of alteration in the presence of magnesium are different from those encountered in ordinary portlandite-rich cements. The formation of M-S-H is favoured over that of brucite. The objectives of this thesis are: (i) to understand the reaction mechanisms of magnesium attack and M-S-H formation in low-calcium cementitious matrices, (ii) to study the influence of M-S-H formation on microstructural and mechanical properties and (iii) to propose microstructural and mechanical data of M-S-H that can be integrated in chemo-mechanical models.Firstly, the influence of the composition of the cementitious paste on the phases formed during magnesium attack was studied. The presence of portlandite implies the formation of brucite. Incomplete hydration of low C/S binders, even in the absence of portlandite, can also lead to the formation of brucite. M-S-H is formed within the paste and corresponds to a deep magnesium enrichment. Secondly, a characterisation of M-S-H was carried out on M-S-H pastes in order to acquire data on the intrinsic physical and mechanical properties of M-S-H and thus to be able, via small-scale chemo-mechanical modelling, to understand the mechanisms of damaged properties under these attacks. Finally, a multi-physical study, at the mesoscopic scale, of the damaged properties of low-calcium matrices by waters containing magnesium was carried out. Two low C/S cementitious pastes (model paste based on colloidal silica and real paste based on silica fume and slag - T3) were exposed to solutions with different Mg concentrations (5 and 50 mM). The chemical and mineralogical characterisation showed that a deep decalcification takes place in parallel with a strong magnesium enrichment of the paste, corresponding to the formation of M-S-H. Micro-structural and mechanical characterisations (by indentation), carried out to observe the evolution along this degradation front, showed an increase in porosity and a strong loss of local elastic properties despite the formation of M-S-H. Mechanical homogenisation, based on previously determined M-S-H paste data, confirms that degraded M-S-H have low elastic properties. Increasing the Mg concentration does not change the mineralogy and chemistry of the degraded zone but only the degraded depth. The results on the real paste (T3) are similar to those on the model paste, validating the results on the latter. In addition, the model paste was also studied in pure leaching to compare the impact of leaching with that of magnesium attack on the low C/S binders. A study using a reactive transport code was carried out to better understand the differences in kinetics and phenomenology. It confirms that the presence of magnesium accelerates the degradation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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