Search

Le present travail est une seconde etape faisant partie de notre travail de recherche sur l'influence des parametres de composition sur le comportement des betons autoplacants (BAP) a l'etat frais et durci. Il concerne l'etude de l'effet des rapports eau sur ciment et fines sur ciment sur le comportement du BAP a l'etat durci. Plusieurs essais ont ete realises tels que l'etalement, la boite en L, la stabilite au tamis, le ressuage, les resistances mecaniques, le retrait libre (retrait de sechage et retrait endogene) et une etude microstructurale (caracterisation mineralogique, distributions porometriques) afin de comprendre les roles joues par les differents constituants susceptibles d'entrer dans la formulation d'un BAP a base de materiaux locaux et chercher a mieux cerner les mecanismes qui regissaient les comportements vis-a-vis du retrait. Nous avons mis en evidence le role du reseau poreux et de la microstructure des hydrates. Les resultats obtenus offrent de belles perspectives pour optimiser les BAP en Algerie. Notre etude a permis de developper une variete de formulations de betons autoplacants repondant aux criteres rheologiques (bonne deformabilite, moins de ressuage, absence de segregation, meilleures performances mecaniques). Mots-cles: betons autoplacants, fines calcaires, porosimetre, retrait de sechage, retrait endogene, resistances mecaniques. This paper describes the second phase of our research project on the influence of the composition parameters on the behaviour of unset and hardened self-compacting concrete (SCC). It studies the impact of the water/cement ratio and the fores/cement ratio on the behaviour of hardened SCC. Several tests were carried out, such as surface settlement, L box, sieving, bleeding, mechanical strengths, free shrinkage (drying shrinkage and autogeneous shrinkage) and a microstructure study (mineralogical characterization, pore space distribution), to better understand the roles of the various possible SCC components made with local materials and to identify the mechanisms governing shrinkage. We highlighted the role of the pore network and the microstructure of the hydrates. Results show good SCC optimization potential in Algeria. Our study helped develop several SCC formulations meeting the rheological criteria (good deformability, less bleeding, no segregation, better mechanical performance). Key words: self-compacting concrete, limestone fines, porosimeter, drying shrinkage, autogeneous shrinkage, mechanical strengths., 1. Introduction Les betons autoplacants (BAP), developpes depuis une vingtaine d'annees, sont encore a l'heure actuelle qualifies de << nouveaux betons >>, car leur utilisation reste modeste bien qu'ils possedent [...]

Guanacoite with high Mg content from the type locality (El Guanaco mine, Chile) was investigated to elucidate the crystal-chemical characteristics of this arsenate mineral. In this study, WDS X-ray intensity maps revealed that guanacoite single crystals from the investigated specimen show distinct and characteristic compositional zoning, which varies cyclically along [100], which is in the direction of the elongation of the prismatic crystals (Witzke et al. 2006); the boundaries separating different compositional regions lie perpendicular to the prism faces. The crystal structure of a selected fragment [a = 5.459(2), b = 16.808(9), c = 6.917(3) [Angstrom], [beta] = 100.44(1)[degrees], V = 624.1(5) [[Angstrom].sup.3], [P2.sub.1]/c, and Z = 2], was solved using direct methods and refined to an R index of 3.09% for 1385 observed [[I.sub.o] > 4[sigma]([I.sub.o])] reflections measured at 103 K using MoK[alpha] X-radiation. The chemical formula based on the refinement is [([Cu.sub.0.85][Mg.sub.0.15]).sub.2] [Mg.sub.2]([Mg.sub.0.83][CU.sub.0.17])[(OH).sub.4][([H.sub.2]O).sub.4] [(As[O.sub.4]).sub.2]. The structure model obtained is in good agreement with the previous structure determination, although the occupancies of the three M sites (M = Mg or Cu) are distinctly different, i.e., the sites are considerably richer in Mg, especially M1 and M3. These structural results suggest that guanacoite's chemical formula should be enlarged from the Cu-rich composition [Cu.sub.2] [Mg.sub.2](Mg,Cu)[(OH).sub.4][([H.sub.2]O).sub.4][(As[O.sub.4]).sub.2] to [(Cu,Mg).sub.2][Mg.sub.2](Mg,Cu)[(OH).sub.4][([H.sub.2]O).sub.4] [(As[O.sub.4]).sub.2] to include the Mg-rich members (Mg up to 3.395 apfu). It appears that the chemical variability of guanacoite is mainly due to Mg-Cu substitution on the M1 and M2 sites. Keywords: Guanacoite, copper magnesium arsenate hydrate, crystal structure, chemical analysis, Jahn-Teller distortion

Numerical simulations and geologic studies suggest that high thermal anomalies beneath, within, and above thermally insulating layers of buried hydrated salts and gas hydrates could have triggered and sustained hydrologic processes on Mars, producing or modifying chaotic terrains, debris flows, gullies, and ice-creep features. These simulations and geologic examples suggest that thick hydrate deposits may sustain such geothermal anomalies, shallow groundwater tables, and hydrogeologic activity for eons. The proposed mechanism may operate and be self-reinforcing even in today's cold Martian climate without elevated heat flux. Keywords: Mars, heat flow, hydrous sulfates, clathrate hydrates, Aram Chaos. doi: 10.1130/G23783A.1

The features of the clathrate hydrate equilibria in mesoporous media are discussed in terms of a conceptual model, with the aim of resolving current inconsistencies concerning experimental and interpretative methods employed in studies of such systems. It is concluded that the Gibbs-Thomson (or Kelvin) relationship used to model clathrate inhibition in porous media must be modified correctly to reflect the hysteries between growth and dissociation.

The kinetics of methane hydrate formation prepared from polycrystalline deuterated ice are investigated. In-situ time-of-flight neutron powder diffraction is used for this purpose.