## 3 Results and Discussion
### Physical Properties of Cenospheres  
As seen in Figure 2, the fresh fly ash exhibits reddish brown color. The fly ash sample used in this study contained mostly the small-sized particles; approximate weight fraction of \(\sim\)74% for the particle size finer than 50 \(\upmu\)m. Fineness of the fly ash sample was less than 34% of the material retained on the 45-\(\upmu\)m sieve, which is the criterion for natural pozzolanic fly ash following the ASTM C618 [46]. Bulk density of the fly ash sample is 1.3 g/cm\({}^{3}\), apparent density varied in the range of 2.4-2.9 g/cm\({}^{3}\). Figure 2: The bulk cenospheres were collected by sink-float method, using a fly ash-to-water ratio of 1:10. Shell thicknesses of those cenospheres in each size range were depicted. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Method** & **Condition** & **Cenospheres Recovery (\%) 1** \\ \hline \multirow{3}{*}{Sink-float method} & Manual stirring & 1.43 \\  & Ultrasonication & 1.52 \\ \cline{1-1}  & Size sieving & 1.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Data comparing the recovery yield of cenospheres collected from different experimental conditions, particularly using fly ash-water ratio of 1:10 (10 wt. %). The Mae Moh cenospheres separated from fly ash is sand-like texture, tiny granules. The collected cenospheres exhibits brownish grey color (Figure 2). Density of the bulk cenospheres obtained from different separation conditions are in the range between 0.95 to 1.03 g/cm\({}^{3}\). These low densities indicate that the floating part collected is lightweight and cenospheric. The relatively high density could possibly be obtained for a prolonged soaking period. This is due to the larger amount of aggregates forming on the cenosphere surface in the form of calcium carbonate, hence determining the cenosphere recovery yield [28; 49]. The calcium aggregates covering the surface of cenospheres can be seen in the morphological study of cenospheres. The bulk cenospheres are typically of a broad size distribution, as seen in the literature [13; 16; 50; 51]. The cenosphere samples after being sieved into four series of size (<45, 45-105, 106-250, >250 \(\upmu\)m) have different colors varying from brownish to dark gray to black as the size increases (see Figure 2). Density, average particle size of the cenospheres in each size fractions, and their weight fractions are shown in Table 4. The distribution of cenospheres is seen across various size fractions. Density of the starting bulk cenospheres before sieving is 1.03 g/cm\({}^{3}\). After being classified in each size range, the density and the average particle size of the cenospheres were found to vary from 0.76 to 1.93 g/cm\({}^{3}\) (see Table 4). For the size ranges between 45-100 and > 250 \(\upmu\)m, it is apparent that the density increased with the particle size. While the density of the small fraction (<45 \(\upmu\)m) sits outside this trend. The high density of this fraction (1.40 g/cm\({}^{3}\)) is attributed to the significant contribution of large amount of solid mass associated with the more densely packing of those small particles per a volume unit leading to such high density. The average particle size obtained in this study was found to vary from 25.9 to 229.2 \(\upmu\)m (Table 4), as all the bulk cenospheres floated and formed a layer covering water surface. The particle size of predominantly silicate-based, hollow cenospheres lighter than water can vary from 5 up to 500 \(\upmu\)m [37]. In Figure 2, the SEM images of the cenospheres shell thickness of different sizes are illustrated in comparison. It is clearly seen that the larger particle sizes tend to have a thick wall. The shell thickness of the cenospheres are in the range of \(\sim\)0.7 to 21.6 \(\upmu\)m, and the shell thickness was plotted as a function of particle diameter as demonstrated in Figure 3a. A large variation in shell thickness is seen with larger particle size.