La kaolinite est l'un des minéraux argileux les plus abondants sur la croûte terrestre. Elle couvre de larges domaines d’applications en ingénierie. Cependant les études abordées sur cette argile restent assez modestes tant sur le plan expérimental que théorique. Cette thèse a pour but de mettre en lumière l’ensemble des propriétés fondamentales de la kaolinite, notamment structurales, mécaniques et thermodynamiques qui restent non encore étudiées à ce jour. Pour concrétiser cet objectif, nous avons employé la méthode de dynamique moléculaire basée sur un potentiel classique. Un bon accord a été trouvé entre la diffraction par rayon X obtenue et celle trouvée expérimentalement. Nous avons ensuite déterminé l’ensemble des propriétés mécaniques de cette argile. Dans le but de prédire le comportement de la kaolinite sous des contraintes géologiques importantes, nous avons considéré l’effet d’une pression hydrostatique sur cette argile. La variation des différentes quantités mécaniques sous pression a révélé l’existence d’une transition de phase structurale pour une pression d’environ 20 GPa. Ce résultat ouvre la voie pour concevoir, à partir de la kaolinite, d’autres types d’argiles avec de nouvelles propriétés. Le nouveau système d’argile, initialement issu de la kaolinite, pourra donc être utilisé dans diverses applications. Le second objectif de cette thèse concerne l’étude des propriétés thermodynamiques de la kaolinite à la pression atmosphérique, par un calcul de dynamique moléculaire basé sur un champ de force récemment développé. Parmi nos résultats les plus marquants, le calcul du point de surchauffage trouvé à 1572 K. La transformation du solide à l’état liquide a également été observée à partir des profils de densité ainsi qu’à partir des fonctions de distribution radiale. Enfin nous avons combiné les deux effets pression et température pour l’étude de la kaolinite. Unerelation a été proposée pour décrire la phase solide-liquide et déterminer la température de surchauffage de la kaolinite entre 0.1 MPa et 20GPa. Par ailleurs le double effet pression-température a permis d’établir un diagramme de phase pour la kaolinite qui aura pour rôle de donner une idée précise sur le comportement de cette argile dans des intervalles spécifiques de température et de pression. / Kaolinite is one of the most abundant clays minerals on the earth surface. It covers wide applications in engineering. However, a real lack of studies both experimental and theoretical exists for this clay. This thesis aims to shed light on fundamental properties of kaolinite, especially its structural, mechanical and thermodynamics properties, which are still unknown. To this end we have employed molecular dynamics method based on classical potential. Good agreement was found between the obtained X-ray diffraction and the experimental one. After this step we have determined the mechanical properties of this clay. In order to predict the behaviour of kaolinite under geological constraints, we have considered theeffect of hydrostatic pressure on the clay. The variation of different mechanical quantities reveals the existence of structural phase transition for a pressure of 20 GPa. This result opens the way for conceiving, from kaolinite, other kind of clays with different properties. The new clay system, initially issued from kaolinite, could be then used for further applications.The second objective of this thesis is the study of thermodynamics properties of kaolinite under atmospheric pressure, by means of molecular dynamics calculation based on a recently developed force field. Among our main results, the calculation of superheating point, which is found at 1572 K. The transition from solid to liquid structure has also been observed from the density profiles and radial distribution functions. Finally, we have combined both pressure and temperature effects for the study of kaolinite. A relation has been proposed to describe solid-liquid phase and to determine the superheating point between 0.1MPa and 20 GPa. Besides, the double effect pressure-temperature allowed us to establish a phase diagram for kaolinite which can give a precise idea on the behaviour of this clay in some specific range of temperature and pressure.

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There are lots of reactions in formation of mullite during sintering of kaolinite powder at high temperature. Primary mullite (M(I), 2Al2O3¡PSiO2) is the result of a solid state reaction of kaolinite. And secondary mullite (M(II) or M(III), 3Al2O3¡P2SiO2) is formed by vitrification after nucleation and grain growth. It has been investigated that the growth mechanism of secondary mullite and the texture of ceramics are affected by the process of green tape. Expect that the growth direction of crystal has been analyzed, the texture of kaolinite green tape formed by tape casting after sintered can not be reproduced. In order to realize the mechanism in formation of mullite we study different green tape process by vitrification and observe the microstructure of mullite after sintering.. The microstructure is analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Thesis (M.S.)--Virginia Polytechnic Institute. 1951. / Abstract. Includes bibliographical references (leaves 29-30). Also available via the Internet.

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A detailed investigation of the kinetics of dissolution of kaolinite clay in hydrochloric acid solution has been carried out. The rate of dissolution increases with calcination temperature, calcination time, and leach temperature. For example, almost 98% of the aluminum in the clay sample roasted at 540°C for 1 h can be dissolved in 20 min. The presence of fluoride ions in the leach solution significantly enhances aluminum dissolution. Changes in the pore structure of kaolinite clay due to calcination and hydrochloric acid leaching have been studied by a gas adsorption technique. The results obtained show that the surface area, macropore volume, and micropore volume of the leached residue increase with the extent of leaching up to a leach time of 1 h and thereafter decreases. Calcination at 500-750°C appears to destroy the structure of the clay but has no significant effect on the porous nature of the clay.

Call number: LD2668 .T4 1950 K58 / Master of Science

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