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Search Results: 1 - 10 of 626 matches for " hydratable alumina "
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Hidrata??o do óxido de magnésio em presen?a de alumina hidratável
Salom?o, R.;Villas-B?as, M. C. O.;Pandolfelli, V. C.;
Ceramica , 2010, DOI: 10.1590/S0366-69132010000300015
Abstract: the substitution of calcium aluminate cement by hydratable aluminas in mgo-containing refractory castables can afford several benefits for these materials mechanical properties and refractoriness. nevertheless, the way that hydratable aluminas affects mgo hydration and its consequences were not yet systemically explored. in the present work, four sources of magnesia and alumina were combined at different ratios and hydrated simultaneously. they were investigated by x-ray diffraction, thermogravimetry and apparent volumetric expansion measurements. it was found that the magnesia/alumina ratio and the differences of reactivity of each raw material can greatly affect both hydration processes, generating different hydrated compounds. it was also verified that this combination can be suitably used as powerful mgo anti-hydration technique.
Cinética de hidrata??o de ligantes à base de alumina hidratável ou aluminato de cálcio
Oliveira, I. R.;Garcia, J. R.;Pandolfelli, V. C.;
Ceramica , 2007, DOI: 10.1590/S0366-69132007000100004
Abstract: the dispersion of refractory castables matrix presents a great influence on their rheological behavior, which defines the most appropriate methods for placing these materials. the growing demand for automatically transported refractory castables has promoted the use of pumpable castables, usually specified as self flow compositions. nevertheless, castables with higher fluidity present longer workability, leading to extended demoulding times. because the strength development is intimately linked to the hydration process of calcium aluminate cement or hydratable alumina, it needs to be controlled in order to reach the minimum time for demoulding, contributing to reducing overall costs. the control of cement hydration depends on the knowledge of the variables that determine the kinetics of the involved reactions. in this context, the objective of this work was to evaluate the influence of the sources of the hydraulic binder, the temperature and the presence of matrix or inorganic additives on the hydration process, carried out through temperature measurements and oscillatory rheological tests, as a function of the time.
Agentes ligantes e seus efeitos em concretos refratários alumina-magnésia
Braulio, M. A. L.;Milanez, D. H.;Sako, E. Y.;Bittencourt, L. R. M.;Pandolfelli, V. C.;
Ceramica , 2010, DOI: 10.1590/S0366-69132010000400003
Abstract: the usual binder of alumina-magnesia castables is the calcium aluminate cement. due to in-situ spinel (mgal2o4) formation at high temperatures, these materials present an expansive behavior. when bonded with cement, further reactions (ca2 and ca6 formation) also contribute to the overall expansion. changing the most common cement used (~ 30 wt.% cao) for other containing less calcia (~ 20 wt.% cao) or for hydratable alumina are suitable alternatives for controlling the castables' volumetric stability. nevertheless, the binder replacement may affect castables properties, such as cold mechanical strength, mgo hydration degree and properties during and after sintering. therefore, the objective of the present paper is to analyze the effects of these binders on the alumina-magnesia castables processing. as a result, the binder systems can be used as a tool for designing the alumina-magnesia castables expansion, increasing the flexibility on the selection of steel ladles linings.
Concretos refratários preparados com alumina hidratável: efeito dos dispersantes
Oliveira, I. R.;Pandolfelli, V. C.;
Ceramica , 2009, DOI: 10.1590/S0366-69132009000100004
Abstract: a reactive alumina able of forming hydrated phases in water has been used as an alternative hydraulic binder in refractory castables. however, it has been observed that the drying of these materials is usually slower comparing to cement containing compositions. due to reduction of the permeability, this aspect increases the difficult of the vapor migration and can promote water vapor pressurization inside the structure and, eventually, explosion of refractories. additives usually used in refractory castables, in order to promote matrix dispersion, are shown to affect the hydratable alumina hydration mechanism. the dispersion of binder particles presents a main role in the development of hydratable phases but the coating of the particles surfaces by the additive inhibits the hydration. comb-like chain additives were indicated in order to match dispersion and hydration of castables containing hydratable alumina as a binder.
Agentes ligantes para concretos refratários: sílica coloidal e alumina hidratável
Ismael, M. R.;Salom?o, R.;Polito, L. A.;Pandolfelli, V. C.;
Ceramica , 2007, DOI: 10.1590/S0366-69132007000200005
Abstract: colloidal silica and hydratable alumina can be pointed out as important substitutes to the calcium aluminate cements as calcium-free binders for refractory castables. based on the advantages and drawbacks of these binders, it is reasonable to consider that their combination could maximize the advantages and reduce the negative aspects observed when they are individually used. the high permeability level generated by the colloidal silica would reduce the risks of explosive spalling of hydratable alumina containing castables during first heat-up. additionally, the total porosity of the colloidal silica containing systems could be decreased due to the hydration of the ha. in the present work, colloidal silica and hydratable alumina were combined in a new calcium-free binding system. their rheological behavior, permeability level, apparent porosity, mechanical strength and drying rate profile were compared with the performance of each binder individually.
Hidrata??o de ligantes na presen?a de matriz e aditivos
Oliveira, I. R.;Pandolfelli, V. C.;
Ceramica , 2007, DOI: 10.1590/S0366-69132007000300005
Abstract: hydraulic binders such as hydratable alumina and calcium aluminate cement present a major influence on the rheological properties and mechanical strength development of castables, defining their workability and demoulding times, respectively. such behavior is related to the hydration process and its control depends on the knowledge of the variables that determine the kinetics of the involved reactions. the matrix addition to cement influences the hydration behaviour, as well as, the length of the induction period, the composition of the phases and of the hydration products. additives such as the retarders can inhibit the process of cement dissolution, by the build up of insoluble barriers around the cement particles, and enhance the formation of more soluble hydrates. they can also behave as accelerators inducing the formation of less soluble hydrates and speeding up the beginning of the hydrate nucleation. in this context, the objective of this work was to evaluate the influence of different kind of matrix and additives on the hydration process of different sources of the hydraulic binder.
Reologia de concretos refratários na presen?a de diferentes tipos de aditivo e ligante hidráulico
Oliveira, I. R.;Pandolfelli, V. C.;
Ceramica , 2007, DOI: 10.1590/S0366-69132007000300008
Abstract: chemical additives may be added to the castables for different purposes, as for example, to disperse matrix fine particles through electrostatic/electrosteric mechanisms (dispersants) and to control the hydration process of cement particles (retarders/accelerators). the state of dispersion of the castables matrix particles presents a direct influence in their rheological behavior, defyning the techniques that can be used for their installation. the growing demand for automatically transported refractory castables has stimulated the use of pumpable castables due to its fast and high quality placing, without losses and with reduced costs. however, in order to explore the technological advantages of this process it is necessary that the rheological behavior and setting characteristics of castables are in tune with the process. in this context, the objective of this work was simultaneously evaluate the influence of the different additives on the matrix particles dispersion and the hydration process of different hydraulic binder, through fluidity and setting time measurements of refractory castables.
Ligantes hidráulicos e a hidrata??o de óxido de magnésio em concretos refratários
Salom?o, R.;Bittencourt, L. R. M.;Pandolfelli, V. C.;
Ceramica , 2010, DOI: 10.1590/S0366-69132010000300008
Abstract: the presence of calcium aluminate cement and hydratable alumina can significantly modify the magnesia sinter hydration behavior in aqueous suspensions. as a consequence of these studies, the present paper investigates how these binders content could affect magnesia hydration in refractory castables using hydration-dehydration tests, ph, ave, mechanical strength and porosity measurements. besides this, because for these materials the aspects related to mechanical strength, porosity and refractoriness also must be taken into account, binder-free, magnesia-free and magnesia-and-binder-free samples were also tested as references. it was found out that the deleterious effects of magnesia hydration can be greatly minimized by the binder content.
Electrochemical Self-Assembly of Nanoporous Alumina Templates Title  [PDF]
Muna Moussa Abbas
Advances in Materials Physics and Chemistry (AMPC) , 2013, DOI: 10.4236/ampc.2013.35035
Abstract: Porous alumina was fabricated electrochemically through anodic oxidation of aluminum by means of such a self-organized method. Anodic aluminium oxide (AAO) template with nanopores was grown by two-steps anodization processes from a high purity aluminium foil. The anodization process was carried out in a phosphoric acid electrolyte at ambient temperature with a different duration of anodization. The analysis observation by Atomic Force Microscopy (AFM) showed that nanopore size increased with anodization time. The nanopore sizes of porous alumina were (16.04, 26.19 and 37.39 nm) for (1, 2 and 3 hour) respectively.
Production of γ-Al2O3 from Kaolin  [PDF]
Seyed Ali Hosseini, Aligholi Niaei, Dariush Salari
Open Journal of Physical Chemistry (OJPC) , 2011, DOI: 10.4236/ojpc.2011.12004
Abstract: The paper reports a process for synthesis of γ-alumina from kaolin. Kaolin was transformed to meta-kaolin by calcination at 800oC for 2h. γ-alumina powder was synthesized through extracting alumina from meta- kaolin via H2SO4 and meta-kaolin reactions and consequently precipitation in ethanol, which led to form the aluminum sulfate. The precipitated aluminum sulfate was dried and calcined at 900 oC for 2h, which resulted the formation of γ-alumina. The structure of γ-alumina was confirmed by XRD and FTIR and the mean particles size of γ-alumina was determined by SEM to be 0.5 - 0.9 µm. The study revealed the kaolin could be promising material for preparation of γ-alumina.
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