Characterization and preparation of lightweight silica based ceramics for building applications

Abstract: Bolivia is a country with considerable quantities of non-metallic resources. Some important deposits of these resources are located in the high and cold Altiplano as well as in the tropical areas. These deposits are not being exploited industrially but both areas have production of some ceramics in a handmade and empiric way. The present study deals with the characterization and production of ceramics by using available Bolivian raw materials such as clays and diatomaceous earth. The study also investigates how the performance of the raw materials could be improved by addition of agro-industrial residues produced locally, for example rice husks and ashes from Brazil nut shells and sugar cane bagasse. The aim of this study is to add values to these raw materials and residues for production of porous, lightweight ceramics with low thermal conductivity and high enough strength for building applications. Various characterization techniques were used to evaluate the quality of the raw materials and to characterize the obtained products, such as X-ray diffractometry, chemical analysis by ICP-SFMS, differential scanning calorimetry, thermogravimetry, dilatometry, mass spectrometry, SEM-EDS and N2-physisorption. Bulk density, open porosity, thermal conductivity and compressive strength properties were also evaluated on the sintered samples. The studied clays contain kaolinite, illite phases along with quartz and flux elements. Mullite is a crystalline phase that strengthens the ceramics and it was formed during heating. The clays differ in alkalis and Fe content and the conclusions are that clays with high amounts of alkalis and Fe can be used in the fabrication of dense products with low porosity and red tonality especially for bricks, roof- and floor tiles. The clay with relatively low Fe content can be used for the production of white ceramics such as sanitary ware and porcelain bodies. Diatomaceous earth raw material from Llica-Altiplano area was characterized and used to produce lightweight ceramics. The morphology of diatomaceous earth shows shapes of fossilized diatom shells with open micropores of less than 1µm. Diatomaceous earth consists of amorphous phase of SiO2 and some small amounts of impurities. The agro-industrial residues such as rice husk, Brazil nut shell ash and sugarcane bagasse ash were characterized using the techniques mentioned above. Rice husk is a source of silica where the large organic content gives porosity to the final product. The rice husk shows a total weight loss of 82.5 % due to the burnt off organic content up to 600 °C. Brazil nut shell ash (BNS ash) is a combustion residue rich in K and Ca which is an advantage as it can form low melting phases that bond the particles together. Sugar cane bagasse ash is another source of silica that when added to the clay will reduce shrinkage and improve the workability of the clay for example for extrusion processes. Lightweight ceramics were prepared by mixing diatomaceous earth with different amounts of BNS ash and sintering the samples attemperatures between 750-950 °C. The best combination of strength and thermal conductivity was achieved for a mixture of 10 wt% of BNS ash in the diatomaceous earth sintered at 850 °C. The achieved open porosity and density were 49 % and 1.06 g/cm3, respectively. Also this sample shows a thermal conductivity of 0.20 W/mK and compressive strength of 8.5 MPa, which is adequate strength that satisfies the regulations for building brick applications. Porous ceramics were prepared by mixing red clay, rice husk and BNS ash in different amounts and sintering at various temperatures between 950-1150 °C. The results show the beneficial effect of adding both BNS ash and rice husks to the clay based ceramic material as the resulting open porosity was as high as 40 % and the linear shrinkage only 1 % at 1150 °C for a sample with 20 % BNS ash and 10 % rice husks. The achieved thermal conductivity was 0.27 W/mK, bulk density 1.4 g/cm3 and compressive strength 7.6 MPa. Another study in this thesis deals with synthetic silica based ceramics. Mesoporous silica SBA-15 was synthesized and functionalized with organic functional groups for application as template for synthesizing nanoparticles.