Abstract
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 SO3 from the vitreous phase.
Highlights
Siliceous fly ash is a waste material created during combustion of coal in pulverized coal boilers, accumulated in the dust removal zone [1]
Rietveld quantitative X-ray diffraction analysis confirmed that both fly ashes studied consist mainly of glassy phase, which may constitute more than 80% of the powder mass
The presence of this vitreous phase indicates that a liquid phase is formed during sintering, which enhances the process and causes the shrinkage shown by the HMTA microscope studies
Summary
Siliceous fly ash is a waste material created during combustion of coal in pulverized coal boilers, accumulated in the dust removal zone [1]. It is used in the building materials industry as a raw material for the production of Portland clinker [2], as an addition to cement [3, 4], mortar [5] and concrete [6,7,8], as well as sintered ceramic products [9, 10]. Fly ash was researched as a starting material for the production of high-degree-of-consolidation and low-porosity ceramics [11, 12] This type of material is sintered with a large proportion of the liquid phase at well-defined temperature intervals to avoid thermal softening and/or bloating. Part of the emitted gases (CO2) is the result of oxidation of unburned carbonaceous materials and thermal
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