# Utilization of Waste Clay-Diatomite in the Production of Durable Mullite-Based Insulating Materials
## 5 Conclusions  
In this study, sintered multite ceramics were successfully produced from diatomaceous earth and aluminum nitrate at temperatures ranging from 1300 to 1500 \({}^{\circ}\)C for 2 h. The resulting multite monoliths showed remarkable mechanical stability over the entire temperature range, with the sample sintered at 1400 \({}^{\circ}\)C achieving the highest compressive strength of 188 MPa. Interestingly, although the sample sintered at 1500 \({}^{\circ}\)C exhibited well-formed rod-shaped multite grains (~5 um in length and ~500 nm in diameter) and increased density (82% of theoretical density), its compressive strength decreased to 136 MPa. This decrease is due to the increased content of the intergranular glassy phase, which promotes densification but impairs the load-bearing crystalline framework.  
The microstructural analysis revealed a clear trend of pore refinement with increasing sintering temperature. The pore size decreased from ~1.5 um at 1300-1400 \({}^{\circ}\)C to ~1 um at 1500 \({}^{\circ}\)C, accompanied by a reduction in total porosity from 10% to 4%, as estimated from FESEM images. However, the presence of a few large, irregularly shaped pores indicates that further porosity characterization is required using techniques such as mercury intrusion porosimetry or helium pycnometry.  
Preliminary measurements of thermal conductivity (~0.24 W/(m\(\cdot\)K)) suggest that these multite ceramics possess excellent insulating properties. Coupled with their mechanical robustness and the sustainable utilization of industrial by-products, such as mine tailings from the Kolubara coal basin, these materials are very promising for high-temperature insulation and refractory applications. Future work will focus on thermomechanical characterization, including detailed porosity profiling, thermal shock resistance, and evaluation of long-term performance under service conditions.  
Conceptualization, S.I. and A.S.; methodology, S.I, M.M.V. and A.S.; software, J.M., Z.S. and M.M.V.; validation, I.R. and A.S.; formal analysis, Z.S., Z.R. and J.M.; investigation, S.I, A.S. and M.M.V.; resources, S.I, M.M.V. and A.S.; data curation, S.I, M.M.V., Z.S. and A.S.; writing--original draft preparation, S.I, M.M.V. and A.S.; writing--review and editing, J.M., Z.R., Z.S. and I.R.; visualization, S.I, M.M.V. and Z.S.; supervision, A.S.; project administration, S.I. and A.S.; funding acquisition, A.S., M.M.V., J.M. and I.R. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Contract Nos. 451-03-136/2025-03/200017, 451-03-136/2025-03/200287, and 451-03-136/2025-03/200126) and gratefully acknowledges the support of project CeNIKS co-financed by the Croatian Government and the European Union through the European Regional Development Fund--Competitiveness and Cohesion Operational Programme (Grant No. KK.01.1.1.02.0013).  
Not applicable.  
The data presented in this study are available on request from the corresponding author or co-authors. The data are not publicly available.  
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.