# Utilization of Waste Clay-Diatomite in the Production of Durable Mullite-Based Insulating Materials
## 4 Discussion  
The utilization of natural materials--in particular, mining waste such as kaolin, diatomic, bauxite, and other material residues from landfills--offers both environmental and economic advantages. The production of structural and functional ceramics from this waste contributes significantly to the conservation of natural resources and at the same time reduces environmental pollution. Moreover, the appropriate design of these ceramics ensures that high-performance standards are met without relying on cost-intensive production methods. Multite ceramics, especially in the form of refractory materials, are among the most promising candidates for use in various high-temperature industrial processes. As mentioned above, a wide range of natural and waste-derived materials have been successfully employed in their production, which have exceptional long-term dimensional stability at temperatures above 1500 \({}^{\circ}\)C.  
In this study, diatomaceous earth from the Kolubara open-face coal mine (Serbia), specifically from the Barosevac (Lindfield D) in Serbia, was used as the starting material. The chemical composition of the diatomic revealed a notably high Al\({}_{2}\)O\({}_{3}\) content of 13.78 wt%, which is atypical for diatomaceous earth, as it usually consists mainly of SiO\({}_{2}\) with a minimal alumina content. The XRD analysis confirmed the presence of clay minerals,  
Figure 10: EDS spectrum of the multite sample sintered at 1500 \({}^{\circ}\)C.  
Figure 9: EDS spectrum of the multite sample sintered at 1400 \({}^{\circ}\)C.  
especially kaolinite and muscovite, which confirms the high alumina content. In addition, morphological analysis using SEM identified the characteristic fossilized diatom frustules, confirming the diatomaceous origin of the material [23]. The presence of other oxides in the composition served as fluxing agents and promoted the diffusion of reactive species during thermal processing. Following powder preparation and subsequent fabrication steps, durable mulitte monolithts were successfully produced. The structural characterization of the sintered mulitte samples confirmed that at the highest processing temperature of 1500 \({}^{\circ}\)C, a mechanically stable, single-phase mulitte compact was achieved. Despite a slight increase in density, the elimination of open porosity, and a reduction in overall and surface porosity compared to the sample sintered at 1400 \({}^{\circ}\)C, the compressive strength decreased to 136 MPa. At 1500 \({}^{\circ}\)C, the mulitte grains exhibited a well-developed morphology, forming an interconnected network of thicker and longer rod-like structures compared to the finer tubular grains observed at 1400 \({}^{\circ}\)C. Although image analysis and porosity measurements revealed a very low total porosity (-4%) and no open pores, the decrease in compressive strength is likely due to the increased presence of a glassy phase. This hypothesis is supported by the nature of the aluminosilicate-based starting system, which contained numerous alkalis, alkaline earth, and transition metal oxides--components typical of soda-lime-silica-type glasses. The sample sintered at 1500 \({}^{\circ}\)C had a bulk density of 2.36 g/cm\({}^{3}\) (82% theoretical density) and contained 63.24 wt.% crystalline mulitte but retained a significant content of amorphous phase. The semi-quantitative analysis revealed a steady increase in amorphous phase content with sintering temperature, from 25.8 wt.% at 1300 \({}^{\circ}\)C to 29.8 wt.% at 1400 \({}^{\circ}\)C and up to 36.7 wt.% at 1500 \({}^{\circ}\)C. While EDS mapping performed over areas of \(\sim\)200 \(\upmu\)m did not allow a clear distinction between crystalline and amorphous regions, especially in the sample sintered at 1500 \({}^{\circ}\)C--the increased glassy content is evident. These results indicate that the intergranular glass phase plays a crucial role in governing the mechanical behavior of these ceramics produced from clay waste.