# Utilization of Waste Clay-Diatomite in the Production of Durable Mullite-Based Insulating Materials
## 3 Results
### The Structural (XRD) Analysis  
According to the results of the XRD analysis (Figure 2 and Table 2), the primary phases detected at 1300 \({}^{\circ}\)C and 1400 \({}^{\circ}\)C are mullite (3 Al\({}_{2}\)O\({}_{3}\) 2 SiO\({}_{2}\), ICDD NO. 01-088-2049, M), cristobalite (SiO\({}_{2}\), ICDD PDF No. 01-071-6248, Cr), corundum (\(\alpha\)-Al\({}_{2}\)O\({}_{3}\), ICDD PDF No. 00-046-1212, C) and quartz (SiO\({}_{2}\), ICDD PDF No.01-089-8936, Q). The presence of a liquid phase at higher temperatures (above 1300 \({}^{\circ}\)C) facilitates mullite formation by reaction with Al\({}_{2}\)O\({}_{3}\). At temperatures above 1400 \({}^{\circ}\)C, the increased intensity of the mullite peaks indicates a higher mullite content and improved crystallinity. After sintering at 1500 \({}^{\circ}\)C for 2 h, the cristobalite and corundum peaks disappeared (Figure 2b, inset), confirming the formation of single-phase mullite at this temperature. The splitting of the diffraction peaks (120) and (210) at around 26\({}^{\circ}\) is a well-known indicator of the mullite transformation from a metastable tetragonal phase into a stable orthorhombic structure [34]. The clear separation of these peaks, which can be observed even at the lowest sintering temperature (1300 \({}^{\circ}\)C), confirms the onset of mullite formation at this temperature. However, the split diffraction peaks are accompanied by an intense corundum reflection (labeled as 2 in Figure 2b). This corundum peak is most pronounced at 1300 \({}^{\circ}\)C, decreases gradually at 1400 \({}^{\circ}\)C, and disappears completely at 1500 \({}^{\circ}\)C. This disappearance is attributed to the diffusive dissolution of corundum into the silica-rich glassy phase (such as soda-lime-silica glasses) [35], which simultaneously promotes further mullite formation. The volume fractions of the crystalline phases in the sintered samples as well as the mullite lattice parameters, crystallite sizes and lattice strains are listed in Table 2. With increasing sintering temperature, all lattice parameters of the mullite phase decreased, indicating improved crystallinity at higher temperatures. This observation is consistent with the findings of [PERSON] [36], according to which mullite, which initially crystallizes in silica-rich gels, appears as small, highly strained crystals. The reduction in lattice parameters reflects enhanced crystallinity rather than a change in chemical composition. In addition, the lattice strain, estimated from the slope of the Williamson-Hall diagrams, gradually decreases with temperature, indicating the relaxation of strain introduced during the initial phase of mullite formation. This strain typically arises from structural imperfections such as impurities, interstitial atoms or vacancies [37], so its reduction implies a decreasing defect concentration. Such behavior indicates a progressive rearrangement of atoms and improved structural coherence during sintering, which is consistent with the observed grain growth. Accordingly, the samples sintered at 1400 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C exhibit well-crystallized and structurally ordered mullite. In addition, the semi-quantitative analysis revealed a systematic increase in amorphous phase content with temperature: 25.8 wt.% at 1300 \({}^{\circ}\)C, 29.8 wt.% at 1400 \({}^{\circ}\)C, and 36.7 wt.% at 1500 \({}^{\circ}\)C, as  
Figure 2: XRD patterns of mullite samples sintered at 1300, 1400, and 1500 \({}^{\circ}\)C for 2 h (**a**) and the inset of splitting of diffraction peaks at around 20 = 26\({}^{\circ}\) (**b**). shown in Figure 3. This trend supports the disappearance of corundum and cristobalite reflections (Figure 2b) and suggests that their dissolution in the silica-rich melt contributes to the formation of a more continuous glassy matrix at higher temperatures. Figure 3: X-ray diffraction (XRD) patterns of the samples heat-treated at 1300 \({}^{\circ}\)C (**a**), 1400 \({}^{\circ}\)C (**b**), and 1500 \({}^{\circ}\)C (**c**).