DESTABILISATION HAUTE TEMPERATURE DE POUDRES GRANITIQUES : EVOLUTION DES PROPRIETES MINERALOGIQUES ET PHYSIQUES

Abstract : This work has two main objectives : 1- studying melting reactions of powders of weathered granites during heat-treatment, 2- developing their use in traditional ceramic industries as the minerals contained in these granites (quartz, feldspars, clays) correspond to the main raw materials used in the manufacture of ceramic products. A- Statistical relationships between the basic properties of weathered granites and the properties of traditional ceramics Thirty powders obtained from more or less altered granites were compacted, dried, and fired at 1050°C and 1175°C during 3h30 ; these conditions are similar to those used in the ceramics industry. Chemical and physical measurements were conducted on the raw materials and on various processed powders after compaction, drying and heating. Data were treated using Principal Component Analysis (P.C.A.) to determine the main parameters that control the properties of end-products. This statistical study proved that : 1- iron, feldspar and mica contents influences the colour and/or mechanical properties of potsherds ; 2- experimental conditions used in the elaboration of green proofs influence porosity before and after firing ; 3- the presence of clay minerals promotes the cohesion of green proofs. This study clearly confirms that weathered granites can be used as raw materials for producing traditional ceramics. B- Breakdown of granitic minerals during heat-treatment. Applications on quartz-feldspars system and on muscovite. This work details the textural transformations of granitic powders that depend on mineralogical transformations induced by heat-treatment. Pressed powders were heated at 1175°C during 5, 10, 40 minutes and 3, 24, 68 hours under atmospheric pressure, then quenched in air. Transformation kinetics are controlled by different microscopic methods (petrographical microscope, S.E.M., T.E.M.), electron microprobe, X-ray diffraction, image analysis and Infra-Red spectroscopy. Except quartz, granitic minerals are decomposed in the first 3 hours of heating. Feldspars melting leads to a strong decrease of porosity inside and between particle aggregates. Trapped pores remain spherical in the homogeneous silicate melt, due to the trapping of water vapour. Muscovite disappears after 5 minutes of heating. Reaction products in pseudomorphed muscovite are needle-shaped and are identified as Al-Si with a spinel structure that will be transformed later to mullite. The silicate melt resulting from muscovite breakdown develops upon textural ripening of minerals. The orientation of needles is controlled by the crystallography of the initial muscovite. In contrast with what was observed for quartzo-feldspathic matrix, muscovite breakdown leads to the appearance of pores that enclose small water vapour amounts resulting from phyllosilicate dehydroxylation. This porosity increases with heating time. Initially, pore shape is controlled by the crystallography of the initial muscovite whereas for long run durations, the pores become spherical. Changes in pore shape and pore volume can be assigned to a decrease of the rigidity of breakdown products allowed by textural ripening of needles. However, as the granitic materials studied have a low muscovite content, porosity induced by muscovite breakdown remains marginal with regard to the total porosity of potsherds.
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Applied geology. Institut National Polytechnique de Lorraine - INPL, 2002. French


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Submitted on : Monday, September 12, 2005 - 11:42:52 AM
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Karine DEVINEAU. DESTABILISATION HAUTE TEMPERATURE DE POUDRES GRANITIQUES : EVOLUTION DES PROPRIETES MINERALOGIQUES ET PHYSIQUES. Applied geology. Institut National Polytechnique de Lorraine - INPL, 2002. French. <tel-00010105>

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