# Utilization of Waste Clay-Diatomite in the Production of Durable Mullite-Based Insulating Materials
## 3 Results
### The Microstructural (FE-SEM/EDS) and Mechanical Analysis  
The development of the characteristic mullite microstructure is shown in Figure 4. At 1300 \({}^{\circ}\)C, the sample consists mainly of round, equiaxed grains distributed in a glassy matrix (Figure 4a,d). During sintering at 1400 \({}^{\circ}\)C, grow larger and begin to coalesce, resulting in a denser and more compact matrix (Figure 4b,e). At 1500 \({}^{\circ}\)C, a well-developed microstructure of defined, interlocked mullite rods emerges (Figure 4c,f). These rod-shaped mullite grains are generally around 5 \(\upmu\)m long and have a diameter of around 500 nm (Figure 4c). Closer inspection reveals that they are shaped as regular rectangular prisms with side lengths of about 300-400 nm (Figure 4f). In contrast, the grains formed at lower sintering temperatures (1300 \({}^{\circ}\)C and 1400 \({}^{\circ}\)C) are smaller and thinner, indicating slower grain growth. As the samples exhibited some porosity, the pore size and distribution were estimated using Image-Pro Plus software in accordance with previously published methodologies in similar studies [38; 39; 40]. The results presented in Table 3 show that the sintered mullite monoliths are dense, hard and structurally reliable materials. The density of the mullite bodies increases at higher sintering temperatures. The relative density was calculated on the basis of the theoretical densities of mullite (3.17 g/cm\({}^{3}\)), cristobalite (2.35 g/cm\({}^{3}\)), quartz (2.65 g/cm\({}^{3}\)) and corundum (4.00 g/cm\({}^{3}\)). A theoretical density of 2.4 g/cm\({}^{3}\) was taken from the literature for the amorphous silicate phase [41]. Using the rule of mixtures and the volume fractions obtained from the XRD data (Table 2), the theoretical densities of the samples sintered at 1300, 1400 and 1500 \({}^{\circ}\)C were determined, with the sample at 1500 \({}^{\circ}\)C having the highest amorphous phase content (36.7 wt%). As shown in Table 3, the relative density increases sharply from 1300 to 1400 \({}^{\circ}\)C and gradually from 1400 to 1500 \({}^{\circ}\)C, finally reaching 82% of the theoretical density (TD) for the 1500 \({}^{\circ}\)C sample, where the open porosity approaches zero. Linear shrinkage also increases with sintering temperature and stabilizes at around 20%, which is consistent with previously reported values [24]. The compressive  
Figure 4: FE-SEM images of the mullite samples sintered at 1300 \({}^{\circ}\)C (**a,d**), 1400 \({}^{\circ}\)C (**b,e**), and 1500 \({}^{\circ}\)C (**c,f**). strength of the mullite bodies reaches a peak value of 188 MPa at 1400 \({}^{\circ}\)C. This increase is due to the favorable phase composition, optimized microstructural properties (e.g., grain morphology and size), increased bulk density and the almost complete elimination of open porosity. At 1500 \({}^{\circ}\)C, the formation of a continuous glassy matrix promotes densification and eliminates open porosity but also reduces the integrity of the crystalline mullite framework. This microstructural shift probably explains the decrease in compressive strength to 136 MPa. The mechanical performance of these ceramics produced from waste is therefore strongly determined by the balance between the supporting crystalline mullite needles and the intergranular glassy phase. When porous materials are produced, their structural characteristics are typically assessed using key parameters such as void content, average pore diameter, and specific surface area, which together provide valuable insights into the material's porosity and performance [42,43]. Evaluation of these properties often relies on image analysis techniques. For example, the Image Pro Plus software facilitates this process by enabling precise extraction and distinction of the light and dark regions within microscopic images. This allows researchers to clearly identify the solid portions of the sample in contact with the surface and to quantify the empty spaces or pores distributed throughout the material [44].