Effect of silica fume additions on the hydration behaviour of calcium aluminates

In the recent refractory castables technology, the calcium aluminate cement is being replaced in increasing proportion by very fine matrix components, such as silica fume and colloidal alumina. More efficient particle packing and the resulting lower water requirements have been the proposed explanation for the improved green density and strength, without sacrificing the castable workability. However, the aluminates are still responsible for the hydraulic setting of the castable. The hydration mechanisms of single calcium aluminates, and mixtures thereof, synthesised by combustion reaction of the appropriate nitrate-urea mixtures, were recently investigated. The present work describes the effect of silica time additions on the hydration behaviour of pure single calcium aluminates, also produced by combustion reaction. Results on the hydration mechanisms, the microstructure development and the resulting physical and/or mechanical properties of the hydrated samples, as a tiction of the water/cement ratio and the silica content, me presented and discussed.JNICT-Portugal - PALOP/24

Interpreting rheology and electrical conductivity: it all boils down to which particle size

Hypothesis: The electrical charges that develop on the surface of the ceramic particles upon contact with water, due to the interaction with ions in solution, result in a liquid-solid interface, which utterly modifies the properties of individual particles and the way they interact with each other to form a structure. This work explores a new approach to the relationships between structure and stability of suspensions. Experiments: For this purpose, suspensions with a constant 0.35 volume fraction of α-alumina particles, neither spherical nor smooth, and controlled ionic strength (0–90 mM KCl) were prepared and characterized in terms of flow behaviour, electrical conductivity and particle’s electrokinetic mobility. Findings: Electrical conductivity (132 µS/cm < conductivity < 5730 µS/cm) and rheology measurements (10−2 Pa s < viscosity < 104 Pa s) were found to complement each other to produce a more accurate picture of the suspension’s structure. Deviations of experimental data from well-accepted behavioural models were elucidated when the surface area equivalent particle size was used. With the electrical double layer thickness obtained from electrical conductivity measurements, this enabled the interpretation of the relationship between the suspension’s viscosity and the particles electrical conductivity, which provides a criterion for the stability of concentrated colloidal suspensions.publishe

Refractories education in Portugal

Industrial activity in Portugal has been grouped in two broad sectors, the structural ceramics (roof tiles, bricks, sewer pipes and refractories) and the non-structural ceramics (wall and floor tiles, sanitary ware, tableware and decorative ware). In spite of reasonable raw materials sources (refractory clays and dolomite), Portugal is mostly a refractories user. The major steel plants (BF and EAF) account for most of the refractories consumption in the country. Official statistics report 7 refractory plants in activity at the beginning of the decade, producing 30x10 3 tons/year, valued at one third of the imported refractories. Although professional training in ceramics is carried out from school level (plant workers and pottery) up to University level (Ceramics engineers), specific teaching on refractories is done only at graduate level. A small part of this is subsidiary, included in the curricula of the various Metallurgy university degrees

The influence of solid type and content on the rheological behaviour of coagulated slurries and their bearing on the consolidation by pressure filtration

The present work is part of a model study aimed at upgrading the technique of creating porosity within a ceramic body via the incorporation of organic particles, used in traditional ceramics, exploring at the same time the colloidal processing from coagulated slurries. The method described by Lange et al., based on the manipulation of short-range repulsive (lubricating) hydration force and long-range attractive van der Waals force, was used to pack to a high density a bimodal mixture of submicron ceramic particles (matrix) and larger organic particles (inclusions) during consolidation by pressure filtration of dispersed suspensions coagulated by added electrolyte. The results reported show how the rheological behaviour of dispersed ceramic slurries changes through coagulation by added electrolyte, the role played by added organic particles and how they affect the coagulation state, bearing on the conceptual relation between interparticle potential and the observed rheological behaviour. (C) 1996 Elsevier Science Limited.JNICT-Portugal - Ph.D. grant Ciência/BD/2123/9

Microstructure, permeability and mechanical behaviour of ceramic foams

This work is part of a model study aimed at upgrading the technique of creating porosity via the incorporation of organic particles used in traditional ceramics, exploring at the same time colloidal processing from coagulated slurries. The method described in another work, based on the manipulation of short-range repulsive (lubricating) hydration force and long-range attractive van der Waals force, was used to pack to a high density a bimodal mixture of submicron ceramic particles (matrix) and much larger organic particles (inclusions) during consolidation by pressure filtration of dispersed suspensions coagulated by added electrolyte. Investigations carried out to produce strong porous ceramic bodies, with a tailored pore structure, are described. The liquid-like rheology of the saturated consolidated body produced from the coagulated suspension explains the characteristic features observed after burn-up of the organic inclusions and sintering. The porous ceramic body reflects its processing history, which can be observed in its microstructure, mechanical behavior and gas permeability.Ph.D. grant - JNICT-PortugalPost-doctoral fellowship - CNPq-Brazi

Using phase diagrams to deal with moisture expansion

Consensus has it that the post-fabrication moisture expansion of fired clay-based materials is governed by the presence of amorphous and/or vitreous phases. In this work, very simple calculations using the lever rule and the phase diagram of the Al2O3-SiO2-K2O (A-S-K) system, were used to show that the experimentally observed dependencies between moisture expansion and the A/S and K/A weight ratios in clay-based compositions can be correlated with the amount of liquid phase formed during the initial melting of the composition, which remains in the fired body as a vitreous phase. This correlation might, if judiciously used, help ceramists in the choice of additives and/or processing conditions, while avoiding changes in the firing temperatures. Also, the results obtained throw a sharper light in the dual and, in this respect, antagonistic role played by the liquid phase during firing. (C) 2003 Elsevier Ltd and Techna S.r.l. All rights reserved.CNPq-Brazil (Invited Scientist fellowship

Preliminary assessment of Mica as a high-temperature fiber coating for SiC composites

The potencial of mica as a fiber coating for SiC fibers in a ceramic matrix composite has been explored. It has been shown that thermochemical effects are involved that degrade a thin layer of the coating upon consolidation. However, beyond the reaction layer, the debonding properties of mica are retained.US DARPA N.º 8-482490-25601-3INVOTANCalouste Gulbenkian Foundatio

Calcium aluminate cement in castable alumina: from hydrate bonding to the in situ formation of calcium hexaluminate

Calcium hexaluminate (CA6) is an intrinsically densification-resistant material, therefore, its porous structures are key materials for applications as high-temperature thermal insulators. This article reports on the combination of calcined alumina and calcium aluminate cement (CAC) in castable aqueous suspensions for the in situ production of porous CA6. The CAC content (10–34 vol%) and the curing conditions ensure structural integrity prior to sintering and maximize the development of hydrated phases. Changes in physical properties, crystalline phases, and microstructure were investigated after isothermal treatments (120–1500 °C), and three sequential porogenic events were observed. The hydration of CAC preserved the water-derived pores (up to 120 °C), and the dehydroxylation of CAC hydrates (250–700 °C) generated inter-particles pores. Moreover, the in situ expansive formation of CA2 and CA6 (900–1500 °C) hindered densification and generated intra-particle pores. Such events differed from those observed with other CaO sources, and resulted in significantly higher pores content and lower thermal conductivity.publishe

Combustion synthesis of aluminium titanate

Initial interest in aluminium titanate was due to its low thermal expansion coefficient and high thermal shock resistance, but further research was soon discouraged following the discovery of the expansion anisotropy and the instability of the compound over a specific range of temperatures. The development of a suitable active precursor powder could provide a possible solution to the fabrication difficulties (microcracking and decomposition). The scarce available thermodynamic data for the formation of Al2TiO5 from its constituent oxides indicate that the reaction is endothermic and only possible at high temperature because of the titanate being entropy stabilised The present work describes a straightforward combustion synthesis technique to prepare submicron Al2TiO5 powders, using the corresponding metal precursors-urea mixtures, at low temperature and short reaction times. A thermodynamic interpretation of the reaction is provided and the characteristics of the powder produced, like morphology, specific surface area and grain size, are discussed. The thermal behaviour of the combustion powder is compared with that of Al2TiO5 produced via the conventional ceramic solid state route. (C) 1998 Elsevier Science Limited All rights reserved.CNPq-Brazil - post-doctoral grant RHAE 360.324196-

Thermal behaviour of aluminium titanate combustion powders with MgO additions

Submicron Al2TiO5 powders produced by combustion synthesis might provide a possible solution to the known material's fabrication difficulties (microcracking and decomposition). The synthesis technique is very straightforward and the peculiar morphological characteristics of these powders suggest the possibility of producing an in-situ reinforced microstructure with improved mechanical properties. In this work, a systematic sintering/densification study carried out with pure and MgO doped Al2TiO5 powders, prepared by combustion from the corresponding metal precursor-urea mixtures, is described (MgO is a common additive used in reaction-sintered titanate ceramics to counteract the compound thermal instability). The MgO was introduced in the synthesis step and incorporated as a solid solution. The results obtained show the effects of heating rate, sintering temperature and added MgO on the thermal instability, sintering behaviour and microstructure development.FAPESP, Brazil - project 98/10017-