Veronika Blahůšková

This study focused on mullite-based and forsterite-based ceramic bricks fired at 1000 °C from mixtures of fly ash (40 mass%) and kaolins or vermiculites (60 mass%). The structural, physical, and mechanical properties were characterized by X-ray powder diffraction, nitrogen physisorption, mercury porosimetry, thermogravimetry, and compressive strength. In the development of green-material-derived photocatalysts, we evaluated fly ash ceramic bricks based on kaolins and vermiculites, which deserve deeper research. Alkali potassium in the mixtures positively influenced the reduction of the firing temperature, shrinkage, small porosity, and high compressive strength of ceramic bricks. The crystallization of mullite in fly ash was observed on exotherm maxima from 813 to 1025 °C. Muscovite/illite admixture in kaolins precursor of mullite-based ceramics reduced the crystallization temperature of mullite by up to 70 °C. Vermiculite–hydrobiotite–phlogopite in mixed layers of a raw vermiculite precursor of forsterite-based ceramics controlled the formation of enstatite and forsterite in the temperature range from 736 ± 6 °C to 827 ± 6 °C. Mullite- and forsterite-based ceramic bricks were also investigated for photocatalytic hydrogen production. The photocatalytic generation of hydrogen in the presence of mullite-based ceramic bricks was positively correlated with the percentages of Fe₂O₃ in the lattice of mullites and in the presence of forsterite-based ceramics with the presence of diopside. Mullite-based ceramic produced the highest yield of hydrogen (320 µmol/g_cat after 4 h of irradiation) in the presence of mullite with the highest 10.4% substitution of Fe₂O₃ in the lattice. The forsterite-based ceramic produced the highest hydrogen yields (354 µmol/g_cat after 4 h of irradiation) over more active diopside than forsterite.

The effective utilization of fly ash (FA) as a raw material for ceramics production is performed on the FA-kaolin mixtures containing kaolins 10% by mass. The mixtures in comparison with FA and three raw kaolins were annealed to mullite ceramics at temperatures of 1000, 1100, 1200 and 1300 °C. The main aims were to contribute to the discussion on the effect of impurity of Na,K-feldspars in kaolins and Fe₂O₃ in FA on sintering procedure, porous ceramics properties and mullite structural properties. The phases were characterized using X-ray diffraction and thermogravimetry DTA/TGA methods. Mercury intrusion porosimetry was used for characterization of porosity of ceramic samples. Results evidenced the influence of feldspars in kaolins and Fe₂O₃ in FA on the sintering temperatures and properties of mullite ceramics. The fully FA-based ceramic sintered at 1100 °C exhibited post-sintering properties of bulk density 2.1 g/cm³; compressive strength 77.5 MPa; and porosity, 2% in comparison with the FA/kaolin-based ceramics properties of bulk density 2.2 g/cm³; compressive strength, 60–65 MPa; and porosity from 9.3 to 16.4% influenced by Na,K-feldspars. The best structural and mechanical characteristics were found for the FAK3 sample, supported by the high content of kaolinite and orthoclase in the kaolin K3 additive. The FAK3 annealed at 1100 °C exhibited good compressive strength of 87.6 MPa at a porosity of 10.6% and density of 2.24 g/cm³ and annealed at 1300 °C the compressive strength of 41.3 MPa at a porosity of 19.2% and density of 1.93 g/cm³.

Metakaolin (MK) prepared by the calcination of kaolin at 550 °C and fly ash (FA) from the combustion of black coal in a granulating boiler were used to prepare unfired ceramic bonding systems via the alkali activation process. A long-term stability of the mechanical properties of the prepared samples similar to the unfired ceramic systems was observed. The optimal metakaolin and fly ash ratio, the type of the activator (NaOH or water glass) and its concentration were evaluated after the hydration in: a) laboratory conditions; b) hydration box; and c) under the hydrothermal activation. Raw materials and the samples prepared by alkali activation process were characterized by XRD, XRF, TG/DTA, and FTIR methods. The mechanical properties of the prepared samples were tested using a compressive strength test after 2, 28 and 56 days of hydration. The compressive strengths of 16 and 24 MPa after 28 days of hydration were reached for FA samples activated with water glass. The alkali activation of MK was successful in the NaOH solution of the molar concentration above 5 M. The compressive strength values of metakaolin, activated hydrothermally and hydrated at laboratory conditions, reached 11.2 and 5.5 MPa, respectively, for 5 M activator of NaOH.

The effect of talc in the two mixtures with the representative sample of fly ash (Class F) was investigated at sintering temperatures of 1000, 1100, and 1200 °C. X-ray diffraction, thermal DTA/TGA, and mercury intrusion porosimetry analyses were applied to characterize the mineral phase transformation of talc and fly ash in cordierite ceramic. The influence of iron oxide on talc transformation to Fe-enstatite was verified by the simulated molecular models and calculated XRD patterns and the assumption of Fe-cordierite crystallization was confirmed. The fly ash mixtures with 10 mass% of talc in comparison with 30 mass% of talc at 1000 °C and 1100 °C showed higher linear shrinkage and lower porosity. At a temperature of 1200 °C, sintering expansion and larger pores in mullite and cordierite ceramics also containing sapphirine and osumilite demonstrated that magnesium in FA and Tc structure did not react with the other constituents to form crystalline cordierite. The ceramics produced in the present work using fly ash and talc have similar properties to the commercial ceramics produced at sintering temperatures higher than 1250 °C.

The main goal of this paper was to characterize and find a useful solution for the decomposition of tar deposits. For the experimental part, tar deposits, formed by polymerization and condensation reactions, were chosen from a storage tank for tars. At first the initial analyses of tar deposits (elemental, thermogravimetric, and calorimetric analyses) were performed. After the characterization, the tar deposits were extracted in the Soxhlet extractor by acetone, toluene, and quinolone and activated with potassium hydroxide. As the final step of this work, the sorption characterization on the 3Flex Surface Characterization Analyzer (Micromeritics) was performed. The specific surface area of the samples was evaluated using two methods - a single point measurement at p/p0=0.2 and BET method. Micropore and external surface areas were calculated based on a t-plot analysis (carbon black model)