## 3 Results and Discussion
### Morphologies of Cenospheres
The quantity  
Figure 5: SEM images of cenospheres collected by sink-float method, using a fly ash-to-water ratio of 1:10: (**a,b**) broken particles of cenospheres, (**c,d**) small aggregates on cenosphere surface, (**e**) iron-rich particle, and (**f**) cenosphere particle covered by large aggregates. of spheres and spheroids varies within the limit of 10-80% vol. in fly ash. In this study, plerospheres were also observed (see the circle in Figure 4h). Cenospheres can be categorized into non-magnetic and magnetic particles. Depending upon physical characteristics, morphology, surface smoothness, shell thickness, and interior structure, the non-magnetic cenospheres can be transparent, and dark color. While the magnetic cenospheres are generally the particles with a spotty, porous, non-uniform shell, and dark spheres with white inclusions leading to opaque [18]. The high content of the spotty, non-uniform surface spheres is a result of the increase in Fe\({}_{2}\)O\({}_{3}\) content in the range from 4 to 21 wt.% in the magnetic particle type with porous shells [14]. The iron oxide-rich phased particles were found in the magnetically separated fractions of Class F fly ash [55]. The white and grey is a result of carbonate and sulphate crystallization on the cenosphere surface. The yellow, orange to red, brown to black colors in cenospheres were found due to the occurrence of varying amount of iron and probably other trace colorants, such as Cr, Co, Cu, Mn within the glass network [13]. The iron sphere, Figure 5e, is the particle consisting of iron oxide mixed with amorphous aluminosilicate. This type of particle was found mixing with the collected cenosphere fraction. The SEM images present distinct diversity in morphologies, heterogeneous regions of iron oxides on the outer surface of the spheres. Through surface condensation or intermixing with the aluminosilicate, the iron oxide forms a variety of textures on the particle surface, interior of the particles, and even almost the entire particle covering with the iron oxide [52]. The iron ferrosphere structures of skeletal-dendritic, foamy porous globules, homogeneous blocklike, and platelike could be found on the surface of fly ash with magnetic properties and contain Fe\({}_{2}\)O\({}_{3}\) content in a range of 20-88 wt.% [56]. Figure 5c,d,f is the pronounced effect of ash soaking period on the formation of calcite crystals on the cenosphere surface that leads to low quality of cenospheres to be obtained. Soaking the ash for a long period, the surface of the collected cenospheres was found covered with calcium carbonate crystallization consequently leading to different particle sizes obtained [13; 28]. Figure 5f is the SEM image of cenosphere obtained from the eight-day soaking condition. Such a surface occurrence was described to be considerably due to a result of 'later' crystallization in the cenospheres separation process forming precipitates on the shell. Those precipitates, such as calcite, portlandite, dolomite and gypsum [13], are mainly a result of the dissolution of lime, periclase, and gypsum-anhydrite from the ash spheres during the wet separation, and after drying, they can transform into the more stable form by absorbing carbon dioxide from the air [18].