*3.3. Scanning Electron Microscope (SEM)*

Figure 5 shows SEM photographs of the glass–ceramics samples (G10 to G80) after heat treatment at 1000 ◦C for 2 h. It can be noticed from the figure that all samples are generally marked with well-crystallized oriented tubular crystals except sample G60, which is characterized by randomly oriented crystal bundles. Moreover, the crystal lengths are decreased from G10 to G80. These tubular crystals are identical to wollastonite crystals. Rounded crystals are noticed in the G80 sample, which is attributed to the formation of cordierite crystals, as confirmed by XRD analysis. The cordierite (Mg2Al4Si5O18) is nominally increased from G10 to G80 at the expense of the wollastonite (CaSiO3) phase in the cordierite–wollastonite system designed in this work. This increase in cordierite percentage increases the glass melt viscosity, as observed during the glass preparation process. This finding can be explained on the base of Al2O3 content, where it increased as

the cordierite percentage increased in the glass composition. Al2O3 is capable of forming bonds with silica by forming bridging oxygens and generates Al–O–Si bonds causing an increase in glass viscosity [31]; hence, the glass crystallization affinity can be likely decreased, and the glassy phase is usually detected as a matrix for the formed crystals. In this study, the glassy phase can be observed in the crystal interstices, specifically in the G60 sample.

**Figure 4.** X-ray diffraction patterns of G60 after heat-treatment at 950 ◦C for 2 h and 1000 ◦C for 2 h.

On the other hand, a selected glass sample was heat-treated at a temperature close to the nucleation temperature to study the effect of temperature heat treatment on the formed microstructure of the glass–ceramic. Sample G60 is selected because it is characterized by optimum properties measured in this study. Figure 6 presents G60 glass–ceramic heattreated at 700 ◦C for 3 h and 1000 ◦C for 1 h. It can be observed from the figure that fine-grained microstructure is noticed. While the sample, after being heat-treated at 700 ◦C for one h and then 1000 ◦C, has a coarse-grained microstructure.
