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Caractérisation microstructurale et hydrique des pâtes de ciment et des bétons ordinaires et à très hautes performances

Abstract : Results obtained for several cement pastes and concretes in the field of microstructure, shrinkage and moisture properties are presented. In particular, very high performance materials have been studied (mixes with low water-cement ratio and which contain silica fume and superplasticizer). A variety of experimental techniques were used. Thus, classical pore investigation methods (mercury intrusion, nitrogen adsorption, ...) were used to determine the textural parameters. Yet, these methods often appeared incomplete or inadequate to provide the exact pore network characteristics of hardened cement pastes or concretes, in particular for very high performance materials. The difficult accessibility to the thin hydrates porosity (C-S-H gel) is mainly responsible for these methods insufficiencies. Moreover, the necessary drying and outgassing, prior to the measure, make the interpretation of the results particularly hard. So, a more pertinent type of experiment has also been carried out: water vapour desorption and adsorption tests. From these experiments, isotherms, it means "Water Content versus Relative Humidity" curves, were obtained, at equilibrium state, for a constant temperature (T = 23 °C), and for every desorption/adsorption path. The analysis of these curves doesn't only give textural parameters (porosity, B.E.T. specific surface area, pore size distribution,...) in the mesopore range, not accessible by some other methods, but also allows the study of the water/solid surface interaction (thickness of the liquid film adsorbed on the solid surface, energy state of pore water, hysteresis, ...). Moisture transfer kinetics are very slow in such a microporous medium and they are function of the specimen thickness; moisture equilibrium state was established after several months with our few millimeters specimens. Diffusivity coefficients were calculated from weight loss (or gain) curves versus time, for each relative humidity step. During these tests, hygrometrical strains were also measured, in order to evaluate desiccation shrinkage in function of relative humidity, and to compare it to autogenous shrinkage. Moisture characterization has been completed with the determination of water content profiles on drying specimens. By using different investigation methods, we have got a complete description of hardened cement pastes and concretes pore structure, from the C-S-H gel pores to macropores scale. We were able to point out the similarities and the discrepancies, about microstructure and moisture properties, between the different materials and to quantify the influence of mix parameters like for instance water-cement ratio or silica fume content.
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Véronique Baroghel-Bouny. Caractérisation microstructurale et hydrique des pâtes de ciment et des bétons ordinaires et à très hautes performances. Mécanique [physics.med-ph]. Ecole Nationale des Ponts et Chaussées, 1994. Français. ⟨tel-00523299⟩

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