## 3 Results and Discussion
### Structural Analysis  
As discussed on the physical properties of foam glass-ceramics earlier, the finding can be verified through microstructure analysis as shown in Figure 7. Figure 7 shows that the pore distribution closed pores of foam glass-ceramics at different content of ES. Based on Figure 7a, it shows that the small pores appeared with homogeneously distributed. The average size of the pore diameter was 682 \(\upmu\)m. Increasing the ES content to the 3 wt % gives a significant difference in the pore size as shown in Figure 7b. Therefore, the diameter of the pore distributed from 3 wt % ES was in the range of 870-1500 \(\upmu\)m. The pores revealed from 6 wt % ES as shown in Figure 7c was insignificant different from pore distribution between 3 wt % ES. However, the microstructure shows that there was an inhomogeneous pore distribution; indicate small pores which the size of diameter in the range of 250-350 \(\upmu\)m while for large pores in the range of 861-1200 \(\upmu\)m of diameter. This phenomenon occurs  
Figure 5: Stress–strain curve relative to the content of ES. Figure 6: Linear expansion–compressive strength relative to the content of ES. due to the high internal pressure inside the pores. Indeed, the high content of the foaming agent would increase the gas pressure and outweighs the strength of pore walls, thus making open pores in the surface [19]. Moreover, the previous research by using banana leaves as a foaming agent stated that the samples contain high banana leaves could cause the release of gases as a result break of the pore's wall and coalescence of pores occur [20]. Meanwhile, as an increasing amount of ES to 9 wt %, it can see the decrease in pores, 508-700 \(\upmu\)m of diameter as shown in Figure 7d due to the high viscosity of the system. The changing of viscosity influenced by the crystallization process inside the glass melt [21]. The previous research study by using coal fly ash and CaCO\({}_{3}\) revealed that pores are homogenized sized obtained with 6% of calcium carbonate [22]. Figure 8 shows the XRD pattern of 1 wt %, 3 wt %, 6 wt % and 9 wt % ES at 800 \({}^{\circ}\)C of temperature. There was a sharp peak of cristobalite (SiO\({}_{2}\), 96-900-8228) at 1 wt % ES because of the high content of SiO\({}_{2}\) while a little growth of wollastonite (CaSiO\({}_{3}\), 96-900-5778) with 33.26% of crystallinity. Percent of the crystallinity of the semicrystalline phase was determined and evaluated. The amorphous region exists as a broad hump in the range of 21-35\({}^{\circ}\) of 20 meanwhile the crystalline phase was formed underlying the amorphous phase. The observed peak of the crystalline phase was compared with the reference pattern by the Crystallography Open Database (COD). Therefore, the finding peak in this research was cristobalite (SiO\({}_{2}\), 96-900-8228) located at -22\({}^{\circ}\) of 20 and wollastonite (CaSiO\({}_{3}\), 96-900-5778) located 27\({}^{\circ}\), 29\({}^{\circ}\), 30\({}^{\circ}\) and 32\({}^{\circ}\) of 20. As increasing ES content to 3 wt %, 6 wt % and 9 wt %, the crystallinity was 37.89%, 38.19% and 66.80%, respectively. The high crystal content from 9 wt %, which contributes to the high viscosity of glass melt. Consequently, it would give an  
Figure 7: Microstructure of foam glass–ceramics. (a) 1 wt %; (b) 3 wt %; (c) 6 wt %; (d) 9 wt % ES. impact on the physical properties of foam-glass ceramics due to the insufficient foaming in glass melt [21]. Hence, 3 wt % of ES revealed an optimum phase of cristobalite and wollastonite. For 1 wt % of ES, there was an early stage of crystallization where the highest growth of cristobalite (SiO\({}_{2}\), 96-900-8228) because of the high content of SiO\({}_{2}\) and small peak intensity of wollastonite (CaSiO\({}_{3}\), 96-900-5778). Meanwhile, the intensity of cristobalite (SiO\({}_{2}\), 96-900-8228) slightly decreased and the peak of wollastonite (CaSiO\({}_{3}\), 96-900-5778) has appeared for the 3 wt % of ES.