Characterization of some natural and synthetic materials with silicate structures

Abstract: The present thesis deals with characterization of silicate structures with a determined morphology and structure such as ordered mesoporous silica and layered silicates. Mesoporous silica groups are amorphous solids exhibiting highly ordered pore structures with narrow pore size distributions and large surface areas. Porous materials are used in various applications such as in adsorption, separation, removal of pollution treatments, molds for templating, etc.Another interesting group of layered materials are crystal silicates with minerals of natural origin. The silicates have a structure that consists of staked layers in which planes of oxygen atoms coordinate to cations such as Si4+, Al3+, Mg2+, Fe3+ to form two dimensional sheets. The coordination of cations in adjacent sheets typically alternates between tetrahedral and octahedral. The properties and uses of the clays vary widely due to the differences in their structure and composition. Some important applications are paints, adsorption, intercalation, removal of pollutants from water and in ceramic industry.The thesis consists of two parts. In the first study characterization of synthesized and functionalized ordered mesoporous silica were performed. Mesoporous silica with a large surface area on which organic functional groups are grafted was used to synthesize cobalt nanoparticles. Investigation by SEM and TEM showed hexagonal particles, with a pore size about 10 nm. The functionalization of the silica was studied by FTIR and TG/DTA techniques and the obtained nanoparticles were characterized by XRD, TEM and EDX analysis. In the second study, an extended literature review on properties of clays is presented. Samples from three different clay deposits, Ivirgarzama (IC), Entre Rios (EC) and Uspha-Uspha (U) from Bolivia were characterized by different experimental techniques in order to assess their relevant features. The chemical and mineralogical analysis showed that the clays consist mainly of kaolinite and illite along with quartz in different amounts. Also, certain amounts of feldspar, iron and magnesium are present in the clays and with predominance in the EC clay. Thermal analysis (DSC/TG and dilatometer) and XRD were used to study the phase transformations and their microstructural evolution at sintering. The EC clay with a high alkali and iron content influenced both the onset of liquid formation and the onset of sintering. Mullite is a crystalline phase that strengthens the ceramics and it was formed in all the studied clays. Based on these results, the EC and U clays provide required characteristics that enable them for use in the fabrication of products with red tonality, especially bricks, roofing tiles and rustic floor tiles. The IC clay with relatively low iron content and with relatively good refractoriness can be used for production of firebricks and also for partially replacing kaolin and silica in white firing ceramics. Thus, the clays from Ivirgarzama, Entre Rios and Uspha-Uspha are promising raw materials and they should be considered as valuable resources for the production of building ceramics.