## 3 Results and Discussion
### Chemical Composition  
Analyzed by energy-dispersive spectroscopy (EDX), the cenospheres have an elemental composition similar to that of the starting fly ash. It was found that the elemental contents in the collected cenospheres include major and minor concentrations of Si, Al, Fe, Ca, S, Mg, Na, K, Ti, N. These elements were found on the fly ash and cenosphere particles from the spot analysis (Spot 1 in Figure 4a,g and Figure 5d,e). Analyzing the precipitates forming on the particle surface in Figure 5d, the EDX result of Spot 2 indicated that the precipitate mainly contained calcium concentration, \(19.9\pm 1.30\) wt.% Ca, comparatively larger than \(2.2\pm 0.4\) wt.% Ca of Spot 1 analyzed for the flat surface of cenosphere, whereas the larger size of precipitates forming aggregate crystals covering the cenosphere particle was found to contain even higher Ca content. The analyzed EDX result of Spot 1 in Figure 5f was \(38.3\pm 1.10\) wt.% Ca, \(13.8\pm 1.50\) wt.% C, and \(47.9\pm 0.9\) wt.% O. This composition is apparently in a close range to the theoretical molar mass percentage ratio of calcium carbonate, 40.04 wt.% Ca, 12.00 wt.% C, and 47.96 wt.% O [28; 57; 58]. For the iron sphere (Figure 5e), the enrichment of Fe content on the skeletal-dendritic structure (Spot 2) was much more pronounced, \(\sim\)51-57 wt.%, compared to the Fe composition of the aluminosilicate surface of the ash particles, \(\sim\)1-10 wt.%. Upon the environmental aspect, it is noteworthy to mention that heavy metals and metalloids could present in fly ash, analyzed by its water solution. The pH values of water or aqueous solutions of fly ash are mostly neutral to highly alkaline with pH from 7-11, and the ions mainly containing in fly ash solution are of Ca and S, and leachable trace elements possibly detected include As, Co, Cr, Mo, Ni, Se, Ti, V, and Zn [13; 59; 60; 61]. The water solutions generated from Mee Moh fly ash in this study were found to be in high pH, at 12-13. Investigation of the heavy metals and trace elements in Mee Moh fly ash from lignite coal has been reported in the literature [2; 3; 62; 63]. Mae Moh fly ash was found to contain a detectable concentration of many elements, including (ppm) Mn \(>>\) Ar, Zn \(>\) Cu, Cr, Ni, Pb, Mo, Co, Sb \(>\) Hg and \(>>\) Cd. These elements were leached out from fly ash under acidic conditions and various pH values. The variation of the trace elements is dependent on the combustion of different coals. Nonetheless, in fly ash from a single power plant, the concentration of the trace elements was found to vary on a daily basis of measurement [64]. The heavy metals and their fraction in fly ash are important concern for coal ash disposal and reuse options in commercial products. In addition to the heavy metals and trace elements, fly ash was also found to contain radioactive contamination, such as \({}^{235}\)U, \({}^{226}\)Ra, \({}^{238}\)U, \({}^{232}\)Th, and \({}^{40}\)K [65; 66; 67]. Acceptable radioactivity level is a key environmental factor for safe utilization of power plant wastes. Generally, most fly ashes have the radioactivity levels similar to that of natural materials. The high values in fly ash could be reduced to an acceptable limit in order to allow its utilization in mass construction. The chemical composition of fly ash and nanospheres in the oxide forms was characterized by X-ray fluorescence (XRF) and shown in Table 5. The sum of SiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\) in the fresh fly ash sample is 60.49%, which is in the range of 50%-70%, confirming its Class C fly ash according to ASTM C618 [46]. In general, nanospheres from different power plants have different bulk chemistry [14; 16; 18]. Nonetheless, SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\) are found as the predominant oxide composition of cenospheres. The XRF has revealed that the composition of all cenosphere fractions are mainly alumino-siliceous (rich in Si and Al).