## 3 Results and Discussion
### Chemical Composition
All the cenospheres fractions are a multicomponent system with SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) contents in the intervals \(\sim\)41-53 and \(\sim\)19-25 wt.%, respectively. The fractions contained a relatively lesser Fe\({}_{2}\)O\({}_{3}\) content of \(\sim\)7-13 wt.%. Whereas CaO content is in the range of 6 to 16 wt.%. The stack columns are illustrated in Figure 6 for a better view of comparison. The compositions (SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), Fe\({}_{2}\)O\({}_{3}\), and CaO) of the bulk cenospheres before being sieved seemed to be approximately in the average intervals of those compositions of cenospheres in each size range. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Item**} & \multicolumn{8}{c}{**Composition (Wt.\%)**} \\ \cline{2-9}  & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **CaO** & **SO\({}_{3}\)** & **K\({}_{2}\)O** & **TiO\({}_{2}\)** & **MnO** \\ \hline Fly ash & 32.12 \(\pm\) 0.29 & 13.82 \(\pm\) 0.19 & 14.55 \(\pm\) 0.27 & 24.49 \(\pm\) 0.05 & 12.03 \(\pm\) 0.26 & 2.31 \(\pm\) 0.02 & 0.55 \(\pm\) 0.02 & 0.55 \(\pm\) 0.01 \\ \hline Cenospheres & & & & & & & \\ \hline Bulk & 47.23 \(  
The physical characteristics in terms of color, size range, and shell structure of Mae Moh cenospheres are schematically presented to elucidate the formation structure of cenosphere shell in correlation with particle size and composition (see Figure 6a,b). The brownish-to-light gray colored smaller particles (size intervals \(<\)45 and 45-105 \(\upmu\)m) have a single-ring structure, whereas the grey-to-dark grey colored larger particles (size intervals 106-250 and \(>\)250 \(\upmu\)m) have a porous shell. These are peculiar to the narrow fraction of cenospheres from Mae Moh fly ash. It was reported that the skeleton of cenospheres is encapsulated by \(\sim\)90 wt.% of an amorphous glass phase, forming the structural stability of cenospheres over a wide range of temperature [13; 50]. The cenospheres obtained from the XRF imaging (Figure 6c) are transparent, glassy, and opaque, with size distribution obviously seen even though the cenospheres were sieved into a narrow size fraction of 106-250 \(\upmu\)m. This observation suggests that a narrow size fraction obtained from size sieving can yet remain a variation in particle size, from dense fine particles to large spheres. This is in agreement with the published report [16] that, for each narrow size fraction (e.g., 45-63; 63-67; 64-65; 63-67; 65-90; 90-106, 106-125, 125-150, 150-250, \(>\)250 \(\upmu\)m), the relatively larger sized cenospheres were found to mix with the smaller sized cenospheres, and dense fly ash was clearly seen through microscopic investigation. The proportions of larger size increased substantially with an increase in cenosphere size fraction, as the larger size are of network structure. Figure 7a, d show the pair dependence of SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)-Al\({}_{2}\)O\({}_{3}\) of the cenopheres in each size fraction. Figure 7a shows that the SiO\({}_{2}\) content in the cenospheres linearly depends on Al\({}_{2}\)O\({}_{3}\) content. Comparing large particle group (45-105, 106-250, and \(>\)250 \(\upmu\)m) to the small particle group (\(<\)45 \(\upmu\)m), the SiO\({}_{2}\) content increased with the increased Al\({}_{2}\)O\({}_{3}\) content. Among the large particle group, an opposite trend was observed; both SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) contents were found to slightly decrease with the increased particle size. Figure 6: Oxide compositions of the cenospheres plotted in stack columns: (**a**) bulk cenospheres and (**b**) cenospheres after being sieved into different size fractions. The cenospheres were collected by sink-float method, using a fly ash-to-water ratio of 1:10. The data were taken from Table 5. (**c**) X-ray fluorescence (XRF) image of the collected cenospheres of size interval 106–250-\(\upmu\)m, the arrow marked is the 150-\(\upmu\)m size.