Search results for "alumina"
showing 10 items of 154 documents
Experimental study of Si–Al substitution in calcium-silicate-hydrate (C-S-H) prepared under equilibrium conditions.
2009
International audience; C-A-S-H of varying Al/Si and Ca/(Al+Si) ratios have been prepared introducing C-S-H (Ca/Si=0.66 and 0.95) at different weight concentrations in a solution coming from the hydration of tricalcium aluminate (Ca3Al2O6) in water. XRD and EDX (TEM) analyses show that using this typical synthesise procedure, pure C-A-S-H is obtained only for calcium hydroxide concentrations below 4.5 mmol L−1. Otherwise, calcium carboaluminate or strätlingite is also present beside C-A-S-H. The tobermorite-like structure is maintained for C-A-S-H. A kinetic study has shown that the formation of C-A-S-H is a fast reaction, typically less than a few hours. The Ca/(Al+Si) ratio of C-A-S-H mat…
Mechanisms of cement hydration
2011
Abstract The current state of knowledge of cement hydration mechanisms is reviewed, including the origin of the period of slow reaction in alite and cement, the nature of the acceleration period, the role of calcium sulfate in modifying the reaction rate of tricalcium aluminate, the interactions of silicates and aluminates, and the kinetics of the deceleration period. In addition, several remaining controversies or gaps in understanding are identified, such as the nature and influence on kinetics of an early surface hydrate, the mechanistic origin of the beginning of the acceleration period, the manner in which microscopic growth processes lead to the characteristic morphologies of hydratio…
Saturated Solutions of Anhydrous Phases in the System Lime-Silica-Water: Example of beta-C2S
1990
Saturated solutions rerely form when the anhydrous constituents of aluminous and portland cement are stirred in water or in lime solutions of increasing concentrations. Apart from monocalcium aluminate, concentration of ions in solution cannot exceed maximum supersaturation with respect to the hydrate most likely to precipitate. The present work shows such a behavior for β-C2S suspended in water and in lime solutions at low concentration. In more concentrated lime solutions, a short lifetime saturation state with respect to β-C2S seems to be reached.
SHORT-TERM PROCESSES OF RADIONUCLIDE IMMOBILIZATION IN CEMENT - A CHEMICAL APPROACH
1992
Abstract The ions released in solution by the constituents of cement (principally silicate, aluminate, OH and Ca ions) can combine with the anions and cations from nuclear wastes present in the mixing water to give very insoluble compounds that can fix these ions in the concrete matrix. In order to understand some of the particular physico-chemical processes involved in cement hydration in the presence of analogue elements, tricalcium silicate (C 3 S) was used instead of cement, which is too complex a mixture. It was found that the salt of a chemical analogue, a lanthanide salt, showed some accelerating effects when present in dilute amounts but resulted in large accelerating effects on QS …
Hydration of tricalcium aluminate in the presence of various amounts of calcium sulphite hemihydrate : Conductivity tests.
2006
Abstract Hydration of calcium aluminate C3A (3CaO·Al2O3) in the presence of calcium sulphite hemihydrate (CaSO3·0.5H2O), with the molar ratio of substrates close to 1, produces the C3A·CaSO3·11H2O calcium monosulphite aluminate phase. Small amounts of calcium sulphite added to calcium aluminate (the ratio of CaSO3·0.5H2O / C3A equalling 0 : 1) change the rate of C3A hydration and influence the whole reaction. Reaction processes for various ratios of the C3A–CaSO3·0.5H2O mixture were examined in pure distilled water with a considerable amount of liquid W / S = 38–50 (constant W / C3A). Processes in the liquid phase were monitored with conductivity equipment, and the XRD analysis was used to …
Mechanisms of RNA loading into exosomes
2015
AbstractUpon fusion of multivesicular bodies (MVBs) with the plasma membrane, intraluminal vesicles (ILVs) are released into the extracellular space as exosomes. Since the lipid composition of the exosomal membrane resembles that of raft microdomains, the inward budding process involves the raft-like region of the MVB limiting membrane. Although published research suggests that cellular RNAs may be selectively sorted into exosomes, the molecular mechanisms remain elusive. In this review, we suggest that there is a continuous interaction of cellular RNAs with the outer (cytoplasmic) surface of MVBs and that the selection for incorporation of these RNAs into ILVs is based on their affinity to…
EXAFS study of ceria–lanthana-based TWC promoters prepared by sol–gel routes
2003
Extended X-ray absorption fine structure (EXAFS) experiments at the Ce K- and La K-edges were performed on ceria–lanthana–alumina three-way catalysts promoters prepared by sol–gel routes, in order to investigate the effect of lanthanum doping on the ceria structure. The formation of Ce1−xLaxO2−x/2 solid solution, already observed by X-ray diffraction, was confirmed by EXAFS analysis, while no experimental evidence of a Ce–Al interaction was found. In presence of cerium and aluminum, lanthanum is involved in the formation of solid solution with CeO2 and of La–Al compounds. When the La:Al molar ratio is sufficiently high, the growth of a tridimensionally ordered LaAlO3 perovskite compound is …
Electrosynthesis of Ce–Co Mixed Oxide Nanotubes with High Aspect Ratio and Tunable Composition
2008
Cerium oxide and cobalt oxides have attracted the interest of several researchers due to their potential application in several technological fields electrochromism, lithium batteries, catalysis, etc.. Ceria has been used as a promoter in the so-called “three-way catalyst” for the control of toxic emission from automobile exhaust. The promotion consists of the enhancement of the noble metal dispersion, as well as stabilization of the supporting medium toward thermal sintering. 1,2 A direct catalytic effect of CeO2 in chemical processes such as water–gas shift reaction or NOx decomposition has been also evidenced. 3,4
The structure of pumice: An XPS and27Al MAS NMR study
1992
In order to investigate the surface structure of pumice, an amorphous aluminosilicate, samples of pumice and of standards of silica and alumina have been studied by x-ray photoelectron spectroscopy (XPS). The effect of a mild acid leaching was also considered. The analysis of the Si 2p, Al 2p and O 1s photoelectron peaks and the Si KLL and O KLL Auger peaks indicated that pumice is formed by tetrahedral silica with some units being replaced by aluminate, AlO2−. The acid treatment decreases the alkali and the aluminate ions. X-ray photoelectron spectra of the valence band of pumices and of standards of Al2O3 and SiO2 agree with the core-level results. To support the conclusions from the XPS …
Interlamellar Reactions of Tetracalcium Aluminate Hydrates with Water and Organic Compounds
1967
Tetracalcium aluminate hydrates are the first example of layer-structured crystals containing neutral sheets, which are highly capable of interlamellar adsorption of water and neutral organic compounds. In this respect tetracalcium aluminate hydrates present new aspects of the phenomenon of swelling, and bring about the challenge of comparison with the frequently examined clay-organic compounds. This report is concerned with the probable monolayer structure of tetracalcium aluminate hydrate which forms five hydration stages. A summary concerning configuration and properties of adsorption complexes with approximately 500 selected organic substances follows. As far as these substances are hom…