Characterization and utilization potential of class F fly ashes

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2013
Acar, İlker
In this thesis, characterization of two class F fly ashes (FA) from Çatalağzı and Sugözü thermal power plants were carried out and their utilization potentials in three different fields were examined. Characterization of sintered samples and determination of their utilization potentials in ceramic industry is the first research area in this thesis. For this purpose, the class F fly ash samples were first pressed into cylindrical specimen without the addition of any organic binders or inorganic additives, and then sintered to form ceramic materials. Effects of sintering temperature and time on sintering characteristics were investigated. In the experiments, the cylindrical specimens were first preheated to 300oC for 1 h to remove moisture and any other gases. The specimens were then fired at the temperatures of 1000oC, 1050oC, 1100oC and 1150oC for the sintering times of 0.5, 1.0, 1.5 and 2.0 hours. Heating rate of 10oC/min was kept constant throughout the experiments. Quality of sintered samples was evaluated in terms of ceramic specifications such as density, water absorption, porosity, shrinkage and splitting tensile strength. In addition, mineralogical and microstructural changes during sintering were determined with X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. According to literature data, better microstructure, the highest density and strength with the lowest porosity, water absorption and shrinkage values are the indications of the optimum sintering conditions. Based on these specifications, Sugözü fly ash gave better results compared to Çatalağzı fly ash, and the optimum conditions were achieved at the sintering temperature of 1150oC for the sintering time of 1.5 hours for both samples. Pozzolanic reactivity of the fly ashes and their utilization potentials in civil engineering applications were also examined in detail during this study. For this purpose, Çatalağzı (CFA) and Sugözü (SFA) fly ashes were first subjected to a specific hydraulic classification process developed at CAER (University of Kentucky, Center for Applied Energy Research) to recover ultrafine fly ash particles. The overflow products with average particle sizes of 5.2 μm for CFA and 4.4 μm for SFA were separated from the respective as-received samples with average particle sizes of 39 μm and 21 μm. After the classification stage, the pozzolanic activities of these ultrafine fly ash fractions (UFA) and as-received samples were examined by preparing a number of mortar (mixture of Portland cement (PC), FA or UFA as partial cement replacement, sand and water) and paste (mixture of PC, FA or UFA as partial cement replacement and water) specimens. Control samples containing only PC were also prepared and tested through the experiments for the comparison of the results. In the mortar experiments, three different PC replacement ratios by FA and UFA (10%, 20% and 30%) were used to examine the effects of FA and UFA samples on the fresh and hardened mortar properties such as water requirement, compressive strength, drying shrinkage and water expansion. These mortar tests indicated that ultrafine fractions of Çatalağzı (CUFA) and Sugözü (SUFA) fly ashes provided more than 10% reduction in water demand compared to the control sample for 30% PC replacement. The mortar cubes containing CUFA and SUFA samples exhibited also higher strength development rates after 14 days compared to the ones with as-received samples and PC only. At the end of the curing age of 112 days, both CUFA and SUFA provided more than 40% increase in compressive strength compared to the control sample for the PC replacement ratios higher than 20%. As a comparison, SUFA gave better results than CUFA in both water demand and compressive strength tests. The mortar bars prepared with the both FA and UFA samples exhibited very low shrinkage and expansion values. These values decreased generally with increasing PC replacement ratio especially after 14 days. In the paste experiments, thermogravimetric analyses (TGA) of the paste specimens prepared by using only with 20% PC replacement were carried out to determine pozzolanic reactivity of the samples. The difference between the remaining Ca(OH)2 (portlandite) contents in the paste specimens containing the fly ashes and the reference PC paste was used as a measure of pozzolanic reactivity. After 112 days, 68.56% and 62.68% Ca(OH)2 content of PC only pastes were obtained with the pastes containing CUFA and SUFA samples, respectively, corresponding to 11% and 13% more Ca(OH)2 consumptions in reference to the respective as-received samples. X-ray diffraction (XRD) analyses were also performed for comparison of main portlandite peak intensities in the paste specimens containing FA or UFA with those in the PC only paste during cement hydration. According to these XRD analyses, portlandite content in PC/UFA pastes decreased significantly after 14 days compared to the PC only paste. All of these tests and analyses showed that a highly reactive lower cost pozzolan with very fine particle size and higher surface area compared to regular fly ash pozzolans can be produced from both Çatalağzı and Sugözü fly ashes using a relatively simple hydraulic classification technology. Cenosphere recovery potentials from Çatalağzı and Sugözü fly ashes were also studied in this thesis. Determination of cenosphere content was done under optical microscope by particle counting on the basis of point and area. Based on the point-counting data, CFA and SFA samples originally contain 11.30% and 4.50% cenospheres, respectively. Variations of cenosphere contents in the fly ash samples were examined by using float-sink, screening and air classification tests. The results pointed out that cenosphere contents decreased with decreasing size and increasing density for both samples. According to the float-sink tests, Çatalağzı fly ash has much more floating products and more cenospheres than Sugözü fly ash for the same density interval. Based on the air classification results, cenospheres were concentrated in the underflow products, and cenosphere contents increased with increasing air pressure and decreasing motor speed for both samples. The most efficient cenosphere separation technique among the examined methods was screening. Cenosphere contents of CFA and SFA increased to 21.65% and 11.83%, respectively by only using simple screening through 38 μm.

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Citation Formats
İ. Acar, “Characterization and utilization potential of class F fly ashes,” Ph.D. - Doctoral Program, Middle East Technical University, 2013.