## 3 Results and Discussion
### Phase Composition  
Figure 8a,b showed X-ray diffraction patterns for the fly ash sample and the bulk cenospheres, respectively. The XRD has resulted in the occurrence of both amorphous and crystalline phases, identifying fly ash and cenospheres a multicomponent system of minerals and phases. In Figure 8a, the phase composition found for Mae Moh fly ash sample includes anhydrite (CaSO\({}_{4}\)), multite (3 Al\({}_{2}\)O\({}_{3}\):2 SiO\({}_{2}\)), quartz (SiO\({}_{2}\)), Lime (CaO), magnetite (Fe\({}_{3}\)O\({}_{4}\)), potassium magnesium silicate (K\({}_{2}\)MgSi\({}_{5}\)O\({}_{12}\)), portlandite (Ca(OH)\({}_{2}\)), merwinite (Ca\({}_{3}\)Mg(SiO\({}_{4}\))\({}_{2}\)), and srebrodolskite Ca\({}_{2}\)((Fe1.559A10.441)O\({}_{5}\)). Fly ash consists basically of aluminosilicate glass, minor mineral matter with high silicate abundance and dominant oxide tendency, and char; the major minerals and phases are mullite, quartz, char, kaolinite-metakaolinite, and hematite, while the minor minerals are cristobalite, plagioclase, K-feldspar, anhydrite, melilite, and corundum [49; 68; 71]. Height of the amorphous halo observed at \(\sim\)18\({}^{\circ}\)-38\({}^{\circ}\) (in Figure 8a) reflects the information for the non-crystalline or poorly crystalized phases. The inorganic amorphous matter was found due to the partial dehydroxylation and destruction of clay minerals, thus forming such poorly crystallized matter from the liberated organically bound elements during the coal combustion [68]. The phase composition of the bulk Mae Moh cenospheres was found to include calcite (CaCO\({}_{3}\)), multite, anhydrite, quartz, magnetite, and ettrtingite (Ca\({}_{6}\)Al\({}_{2}\)(SO\({}_{4}\))\({}_{3}\)(OH)\({}_{12}\):26H\({}_{2}\)O) (see Figure 8b). The cenospheres are more abundant in mineral matter, including multite and calcite, than the bulk fly ash [68]. The cenosphere shell consists of the majority of glass phase (70-90 wt.%), multite, and quartz, whereas the minor minerals of cristobalite, potassium feldspar, plagioclase, hydromica, gypsum, and iron oxides were considered as impurities [16; 37; 51; 54]. The phase analysis has revealed that multite, anhydrite, and quartz identified in the fly ash sample and the bulk cenospheres were found to have the same lattice parameters. The lattice parameters of those phases are, respectively, as follows: \(a\) = 7.5840 A, \(b\) = 7.6930 A, and \(c\) = 2.8900 A (orthorhombic); \(a\) = 6.9933 A, \(b\) = 7.0017 A, and \(c\) = 26.2411 A (orthorhombic); and \(a\) = 4.9650 A and \(c\) = 5.4240 A (hexagonal). The obtained lattice parameters of multite are in the range of known multite compositions found in the concentration interval 57-92 mol% Al\({}_{2}\)O\({}_{3}\)[72], particularly close to the composition of a multite solid solution containing \(\sim\)64 mol% Al\({}_{2}\)O\({}_{3}\) in the nonmagnetic cenospheres with the crystallite size of \(\sim\)200 nm [14]. The quartz component identified in the fly ash and cenospheres is the characteristic of the original coal, with such lattice parameters mentioned above indicating a peculiarities associated with the defect structure of quartz with aluminum ions probably embedded in the lattice due to dissolution of quartz in the molten glass phase during the cenosphere formation [14; 42]. Besides, a noticeable difference observed is magnetite; the magnetite structure of the bulk cenospheres collected in this study is orthorhombic (\(a\) = 5.9340 A, \(b\) = 5.9250 A, and \(c\) = 16.7520 A), whereas that of fly ash is cubic (\(a\) = 8.3778 A). The formation of the ash cenospheres involves fundamentally the chemical oxidation reactions, the resulting gas generation, and particle expansion [69]. Within the Fe-S-O molten droplet precursor, the reactions consume sulfur dioxide gas while solidifying and forming the ash cenospheres. As such, the cenospheres contain sulfur in the shell and magnetite formed as the dominant final phase of iron oxide.