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The object of this study is to investigate the thermal properties of fly ashes from the last, farthest dedusting zone in terms of their use as ceramic masses additives. Siliceous fly ash is valuable additive to ceramic mass, which not only reduces its plasticity, but also actively affects sintering process and shapes the properties of the final material. The finest fly ash fractions are potentially useful flux materials in ceramics; however, a significant limitation in their use is due to thermal expansion/ bloating occurring during high-temperature sintering. The bloating mechanism of fly ashes was investigated in relationship to their chemical composition with the use of DTA/TG/EGA analysis as well as high-temperature microscope. Chemical and phase compositions were studied by X-ray fluorescence and X-ray diffraction. Based on the results obtained, it can be concluded. The results indicate that bloating mechanism is caused by the co-occurrence of two phenomena accompanying sintering: appearance of high amount of liquid phase and simultaneous gas release from sintered material. The dominant mechanism is the simultaneous release of sulfur (IV) oxide and oxygen as a result of the redox reaction of removing SO 3 from the vitreous phase.Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.