## 3 Results and Discussion
### Chemical Composition
The SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio was plotted against Al\({}_{2}\)O\({}_{3}\); see Figure 7b. The increased Al\({}_{2}\)O\({}_{3}\) content seemed not to affect the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio as the particle size increased. Correlating well with the data in Figure 7a, the SiO\({}_{2}\) content increased in a certain extent along with the increase in the particle size. As comparing with the starting fly ash, the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratios of cengospheres have lower values (2.11, 2.13, and 2.19 for the respective size fractions of \(<\)45, 45-105, and 106-250 \(\upmu\)m) than fly ash (2.32). This observation is in a good agreement with those reported in the literature [13; 28; 68]. Except the size \(>\)250 \(\upmu\)m, the value is slightly higher (2.39) than fly ash. Our previous study also found that, using acetone-water mixture as the separation medium, the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratios of the collected cengospheres were approximately in the close range (2.20-2.23) when using the fly ash/medium ratios from 1:2.5 to 1:10 [28]. This clearly confirms that cengospheres originated from the same pulverized coal combustion and the combustion process condition have the same major chemical composition. Considering Figure 7b, the increase of SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio of the larger particles (\(\blacktriangle\)) comparing to the smaller particles (\(\blacktriangleright\), \(\blacksquare\)) is probably due to a phenomenon observed in many studies; the SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) contents reduced with increasing the particle size, while the extent of Al\({}_{2}\)O\({}_{3}\) content reduction was measured in a very low amount with respect to the SiO\({}_{2}\) content [13; 34; 50]. The small particles (\(<\)45-250 \(\upmu\)m, \(\blacktriangleright\), \(\blacktriangleright\), \(\blacksquare\)) having lower SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio (\(\sim\)2.1-2.2) have a single-ring shell structure, whereas the larger cengospheres particles (\(>\)250 \(\upmu\)m, \(\blacktriangle\)) having higher SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio (\(\sim\)2.3) have a porous wall shell structure, more likely in a network shell formed structure. An observation was reported on the relation between the SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio and the shell structure for the cengospheres separated from Class F fly ash. A high SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio (2.57) was found favorably to form small particles (\(<\)25 \(\upmu\)m) with a spherical shape and the single-ring shell structure, while the relatively larger cengosphere particles (63-150 \(\upmu\)m) with a network structure have a relatively lower SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio (1.64-1.8) [16]. To this point of view, it is interesting to note herein that there is a significant variation in chemical composition and cengospheres yield across the power stations and fly ash sources. The chemistry in cengospheres is strongly governed by the process of ash cengospheres formation in the furnace. The chemical composition of the molten droplets associated with the temperature in the furnace considerably determined the particle size of cengospheres [16; 69]. The formation of large particle size was attributed to a result of an increased Al content that increases viscosity of the molten droplets, in turn stabilizing them during the expansion, rather than exploding into small fragments [18]. Besides, the higher SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio was suggested as an index parameter for a lower ash sintering temperature, favoring a ring structure [16; 70]. Cenospheres from combustion of coal of various origins were found in different structures and compositions.