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Article

Utilization of Waste Clay–Diatomite in the Production of Durable Mullite-Based Insulating Materials

by
Svetlana Ilić
1,*,
Jelena Maletaškić
1,
Željko Skoko
2,
Marija M. Vuksanović
1,
Željko Radovanović
3,
Ivica Ristović
4 and
Aleksandra Šaponjić
1,*
1
Vinča Institute of Nuclear Sciences, National Institute of the Republic of Serbia, University of Belgrade, Mike Petrovića Alasa 12-14, 11000 Belgrade, Serbia
2
Department of Physics, Faculty of Science, University of Zagreb, Bijenička 32, 10000 Zagreb, Croatia
3
Innovation Centre of the Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Serbia
4
Faculty of Mining and Geology, University of Belgrade, Đušina 7, 11000 Belgrade, Serbia
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(13), 7512; https://doi.org/10.3390/app15137512
Submission received: 13 June 2025 / Revised: 28 June 2025 / Accepted: 2 July 2025 / Published: 4 July 2025
(This article belongs to the Special Issue Development, Characterization, Application and Recycling of Novel Construction Materials, 2nd Edition)
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Abstract

Featured Application

Due to their low thermal conductivity, mullite-based materials obtained from mining waste might be successfully used as insulating materials, while designing and texturing their microstructure can further enhance their insulating properties.

Abstract

Microstructural, mechanical and qualitative phase identification of durable mullite-based ceramics obtained by utilization of waste clay–diatomite has been studied. Mullite-based ceramics were fabricated using waste clay–diatomite from the Baroševac open-cast coal mine, Kolubara (Serbia). The raw material consists mainly of SiO2 (70.5 wt%) and a moderately high content of Al2O3 (13.8 wt%). In order to achieve the stoichiometric mullite composition (3Al2O3-2SiO2), the raw material was mixed with an appropriate amount of Al(NO3)3·9H2O. After preparing the precursor powder, the green compacts were sintered at 1300, 1400 and 1500 °C for 2 h. During the process, rod-shaped mullite grains were formed, measuring approximately 5 µm in length and a diameter of 500 nm (aspect ratio 10:1). The microstructure of the sample sintered at 1500 °C resulted in a well-developed, porous, nest-like morphology. According to the X-ray diffraction analysis, the sample at 1400 °C consisted of mullite, cristobalite and corundum phases, while the sample sintered at 1500 °C contained mullite (63.24 wt%) and an amorphous phase that reached 36.7 wt%. Both samples exhibited exceptional compressive strength—up to 188 MPa at 1400 °C. However, the decrease in compressive strength to 136 MPa at 1500 °C is attributed to changes in the phase composition, the disappearance of the corundum phase and alterations in the microstructure. This occurred despite an increase in bulk density to 2.36 g/cm3 (approximately 82% of theoretical density) and a complete reduction in open porosity. The residual glassy phase (36.7 wt% at 1500 °C) is probably the key factor influencing the mechanical properties at room temperature in these ceramics produced from waste clay–diatomite. However, the excellent mechanical stability of the samples sintered at 1400 and 1500 °C, achieved without binders or additives and using mined diatomaceous earth, supports further research into mullite-based insulating materials. Mullite-based materials obtained from mining waste might be successfully used in the field of energy-efficient refractory materials and thermal insulators. for high-temperature applications

1. Introduction

Guided by economic and ecological approaches, many scientists and professionals are increasingly utilizing industrial, construction and mining waste in accordance with the Sustainable Development Goals (SDGs), which are implemented through the concept of the circular economy. Since the circular economy of materials has a long-term impact on minimizing negative environmental impacts such as air, soil and groundwater pollution, the utilization of natural and waste materials is increasingly being used in the production of lightweight concretes (LWCs) as lightweight aggregates [1,2]. In recent years, significant attention has also been directed toward geopolymer concretes, which are considered a sustainable and environmentally friendly alternative to conventional Portland cement in traditional cementitious systems. Recent comprehensive studies have emphasized the potential of these environmentally friendly building materials, which are produced from extracted soils and sediments such as clays, mining waste, slags, etc. In particular, these mining wastes have proven to be effective precursors for geopolymer binders [3,4,5]. Given their promising properties, such materials could be used in the construction of highways, tunnels and embankments, enhancing roadway performance of roads, including improved fire and frost resistance [6,7,8].
Another valuable application of industrial waste and natural materials, including mining waste such as kaolin, diatomite, bauxite and other minerals that accumulate in landfills, is in the production of highly durable refractories. As already mentioned, the use of such materials for the production of high-performance technical ceramics remains a challenge. It requires innovative strategies for the management of sustainable post-mining landscapes and balanced societal decisions to promote a green economy—one that provides equitable solutions for both people and the planet. At the same time, the proper design of ceramic materials is essential to achieve the appropriate technological properties (i.e., thermal, mechanical and chemical) and performance (e.g., highly resistant mullite refractories) for various industries such as iron, steel, cement, glass, petrochemicals, etc. Solving the problem of industrial, construction and mining waste not only mitigates air and soil pollution but also enables economic savings through the production of highly durable mullite refractories [9,10,11]. One of the most common industrial waste materials used for the production of mullite refractories is coal fly ash, which remains one of the most significant environmental problems in today’s society [12,13]. The utilization of coal fly ash, which is typically located at uncontrolled landfills, supports the implementation of a circular economy, thereby mitigating growing pollution and depletion of natural resources. In addition, natural and mining-derived materials such as kaolin, diatomaceous earth and bauxite are also used extensively in the production of lightweight bricks, panels and precast elements. These materials significantly reduce structural loads and enable the design and construction of more efficient high-rise buildings and structures. In addition, precast elements can be designed to fulfill specific project requirements, including acoustic performance, fire resistance and thermal insulation [14,15,16,17].
The applicability of natural materials and waste/by-products in the production of mullite-based refractories has already been confirmed. Highly vitreous bodies offer specific advantages in processing [18]. First, the high viscosity of the glassy phase (high content of SiO2) allows the ionic diffusion of nucleating agents essential for the formation of mullite. This promotes the growth of mullite within the network and forms a loose framework of randomly arranged needle-like structures. This characteristic microstructure prevents deformation at high temperatures (shape retention) and ensures excellent long-term dimensional stability at temperatures above 1500 °C. However, a potential drawback arises when excessive amounts of the glassy phase accumulate at triple points, which can negatively affect mechanical performance. Engineering the microstructure, for example, by introducing closed spherical pores, can reduce thermal conductivity while improving mechanical performance and high compressive strength due to controlled porosity [19]. In addition, natural and waste-derived materials often contain alkali, alkaline earth and transition metal oxides (such as FeO, Fe2O3, TiO2, MnO, MgO, CaO, Na2O, K2O and P2O5), as well as rare earth elements, all of which significantly influence the properties of the final product. These components promote anisotropic grain growth and thus increase the strength and toughness of the resulting interlocked microstructure. This microstructure typically consists of intertwined orthorhombic acicular mullite grains, needles or whiskers with an aspect ratio of over 30 [14,19,20]. Furthermore, the high content of glassy phases in naturally occurring hydroxy-aluminosilicates (kaolinite, halloysite, pyrophyllite, sericite), alumosilicates (kyanite, sillimanite), mineral sources (albite, mining waste, kaolinite) and processing conditions (e.g., precipitation of aluminum oxide on kaolinite) is a significant factor; precipitation of aluminum oxide on the surface of waste sand particles) and the temperature of mullite formation (mullitization) could be reduced to the range of 1200–1400 °C [21,22].
This study aims to determine the suitability of waste clay–diatomite powder from mine tailings as a precursor, coupled with the Al(NO3)3·9H2O, in order to obtain durable mullite-based ceramics that could be applied as thermal insulators and/or refractory materials. The qualitative phase identification, microstructure, pore size distribution, porosity percentage content, bulk density, relative density, open porosity, linear shrinkage, and compressive strength of the resulting mullite ceramics were investigated. Following the simple preparation of precursor powders and earlier investigations [23] where mullite compacts were pressed at 2 MPa and sintered at 1300, 1400, and 1500 °C for 2 h [24], further research has been undertaken. In the present study, the prepared powder was pressed into green compacts at 50 MPa and sintered at three different temperatures (1300, 1400 and 1500 °C) for 2 h.

2. Materials and Methods

The starting diatomite was sourced from the surface coal mine Baroševac, Lindfield D, Kolubara (Serbia), and obtained via the air flotation method, commonly used in mining. Its chemical composition (Table 1) was determined using standard silicate rock analysis, as described in a previous study [23]. A flowchart of the synthesis process is shown in Figure 1.
These sintering temperatures were chosen to achieve a balance between phase composition and mechanical performance.
The XRD (X-ray diffraction) patterns of the heat-treated compacts were scanned with a Rigaku Ultima IV diffractometer using Cu Kα radiation at 40 kV and 40 mA in the 2θ range between 10° and 80° with a step of 0.02° and a scan rate of 10°/min. Phase analysis was performed with PDXL2 software (version 2.0.3.0) [25] using reference diffraction patterns from the International Centre for Diffraction Data (ICDD) database [26], version 2012. The average crystallite size (D) and the internal lattice strain (Δd/d) of the sintered samples were estimated from the Williamson–Hall diagrams [27]. In addition to the quantification of the crystalline phase (Table 2), the X-ray diffraction patterns were subjected to a semi-quantitative evaluation of the amorphous phase using the EVA v.7.1 software package (Bruker AXS). After an automatic polynomial background fit, the program separates the broad diffuse scattering (“amorphous halo”) from the Bragg peaks. The ratio between the integrated halo intensity and the total diffracted intensity is then used to estimate the amorphous fraction so that no external standard is required.
The apparent density and open porosity of sintered mullite samples were determined using the Archimedes method. Linear shrinkage was calculated by measuring the diameter of the samples before and after heat treatment.
A Tescan Mira 3 XMU FESEM (Field Emission Scanning Electron Microscopy, TESCAN, Kohoutovice, Czech Republic) was used for the microstructural characterization of the sintered mullite samples. Prior to FE-SEM analysis, the powders were coated with Au using a Polaron SC502 sputter coater. EDS analysis was performed using an INCAx-act LN2-free analytical silicon drift characteristic X-ray detector with PentaFET® Precision and the AZtec 4.3 software package (Oxford Instruments, Abingdon, Oxfordshire, UK) connected to a TESCAN Mira 3 XMU using a generator voltage (20.0 kV) and a working distance of 15 mm.
The images obtained from the FESEM were used to examine the porosity of the material. The assessment of the porosity of the material as well as the distribution of the pore diameters was performed using the Image Pro Plus software 6.0 (Media Cybernetics, Rockville, MD, USA).
The mechanical stability of sintered cylindrical mullite ceramics, i.e., with a diameter of 7.4–8.6 mm and a height of 15.4–17.4 mm, was assessed by compression testing in a universal testing machine, Instron 1185, using the Test Instrument Explorer software. The cylindrical specimens were compressed for 1 min at a test speed of 1 mm until they broke. The compressive strength was determined by dividing the maximum force by the cross-sectional area of the test. At least four replicates were tested for each sample.

3. Results

Starting from diatomite, the formation of the mullite phase has occurred at a relatively low sintering temperature, which is primarily due to the presence of small amounts of alkali and alkaline earth oxides, as well as Fe2O3. These components promote viscous flow sintering and facilitate mullitization reaction by reducing the viscosity of the molten glassy phase. As shown in Table 1, the diatomaceous earth used in this study consists mainly of SiO2 and Al2O3, with the missing Al2O3 content being supplemented by aluminum nitrate [24]. The moderately high Al2O3 content in diatomite originates from kaolinite and muscovite phases, while SiO2 appears as quartz and silicate glass (evidenced by a broad hump around 2θ of 25° in the XRD pattern, Figure 2a). Quartz particles surrounded by a liquid, glassy phase during heating are more readily transformed into cristobalite, as previously reported [23]. Other studies have also confirmed that the mullitization temperature is lowered due to eutectic reactions promoted by these oxides [28,29,30]. Kaolinite (Al4[Si4O10](OH)8), serves as a binder for other compacted components in glass-ceramics made from clay, the initial mixture of which usually contains clays, fluxes and fillers. Meanwhile, quartz remains stable at processing temperatures and plays a role as a filler by reducing shrinkage and deformation. The presence of muscovite (mica), which contains potassium (K), promotes the formation of a liquid phase during firing and enables the crystallization of elongated mullite grains at lower temperatures [31,32]. In addition, K2O together with CaO and Fe2O3 contributes to grain growth, so their presence in the starting material is beneficial for mullite formation [33].

3.1. The Structural (XRD) Analysis

According to the results of the XRD analysis (Figure 2 and Table 2), the primary phases detected at 1300 °C and 1400 °C are mullite (3Al2O3·2SiO2, ICDD NO. 01-088-2049, M), cristobalite (SiO2, ICDD PDF No. 01-071-6248, Cr), corundum (α-Al2O3, ICDD PDF No. 00-046-1212, C) and quartz (SiO2, ICDD PDF No.01-089-8936, Q). The presence of a liquid phase at higher temperatures (above 1300 °C) facilitates mullite formation by reaction with Al2O3. At temperatures above 1400 °C, the increased intensity of the mullite peaks indicates a higher mullite content and improved crystallinity. After sintering at 1500 °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° 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 °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 °C, decreases gradually at 1400 °C, and disappears completely at 1500 °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 Chakraborty [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 °C and 1500 °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 °C, 29.8 wt.% at 1400 °C, and 36.7 wt.% at 1500 °C, as 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.

3.2. The Microstructural (FE-SEM/EDS) and Mechanical Analysis

The development of the characteristic mullite microstructure is shown in Figure 4. At 1300 °C, the sample consists mainly of round, equiaxed grains distributed in a glassy matrix (Figure 4a,d). During sintering at 1400 °C, grow larger and begin to coalesce, resulting in a denser and more compact matrix (Figure 4b,e). At 1500 °C, a well-developed microstructure of defined, interlocked mullite rods emerges (Figure 4c,f). These rod-shaped mullite grains are generally around 5 µm 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 °C and 1400 °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/cm3), cristobalite (2.35 g/cm3), quartz (2.65 g/cm3) and corundum (4.00 g/cm3). A theoretical density of 2.4 g/cm3 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 °C were determined, with the sample at 1500 °C having the highest amorphous phase content (36.7 wt%). As shown in Table 3, the relative density increases sharply from 1300 to 1400 °C and gradually from 1400 to 1500 °C, finally reaching 82% of the theoretical density (TD) for the 1500 °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 strength of the mullite bodies reaches a peak value of 188 MPa at 1400 °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 °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]. The pores represent the dark part of the image. Pore size measurements obtained using the Image-Pro Plus image analysis software are presented in Figure 5, where the distinct separation and distribution of the pores within the material can be clearly observed.
A series of FE-SEM images with a magnification of 1.00 kx (field of view: 190 μm) were used to evaluate the pore diameter distribution in the samples sintered at different temperatures, as shown in Figure 6.
The differences in the mullite obtained at different sintering temperatures are also evident in the size and distribution of the pores, as shown in the microstructures shown in Figure 6. The pore sizes were measured using image analysis, and the corresponding histograms of the pore size distribution (Dmean) are shown for each sample. In Figure 6a, the mullite sample sintered at 1300 °C has the highest frequency of pores with average diameters between 1 and 1.5 µm. In contrast, for the samples sintered at 1400 °C and 1500 °C, the majority of pore diameters are in the 1–2 µm range. The FE-SEM analysis reveals that with increasing sintering temperature, the average pore diameter increases, although the overall porosity decreases, as shown in Figure 7.
The porosity of the sample sintered at 1400 °C decreased by 21.3% compared to the sample sintered at 1300 °C. For the sample sintered at 1500 °C, the reduction in total porosity is even more significant, amounting to 58.4% (Figure 7).

3.3. EDS Analysis

Energy dispersive X-ray spectroscopy (EDS) was performed over wide surface areas of the specimens to determine the elemental composition of the mullite samples sintered at 1300, 1400 and 1500 °C for 2 h (Figure 7, Figure 8, Figure 9 and Figure 10). The EDS elemental maps confirmed the presence of key phases previously identified by X-ray diffraction. In particular, silicon, aluminum and oxygen were found to be constituents of the crystalline phases—mullite, corundum, quartz and cristobalite. Notably, segregation of the SiO2 phase into isolated glassy areas was observed in the sample sintered at 1300 °C (Figure 6), a phenomenon that was not observed in the samples sintered at 1400 °C and 1500 °C (Figure 9 and Figure 10). Other elements detected, such as Al, K, Fe and Ca, exhibited a uniform spatial distribution across all samples.

4. Discussion

The utilization of natural materials—in particular, mining waste such as kaolin, diatomite, 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. Mullite 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 °C.
In this study, diatomaceous earth from the Kolubara open-face coal mine (Serbia), specifically from the Baroševac (Lindfield D) in Serbia, was used as the starting material. The chemical composition of the diatomite revealed a notably high Al2O3 content of 13.78 wt%, which is atypical for diatomaceous earth, as it usually consists mainly of SiO2 with a minimal alumina content. The XRD analysis confirmed the presence of clay minerals, 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 mullite monoliths were successfully produced.
The structural characterization of the sintered mullite samples confirmed that at the highest processing temperature of 1500 °C, a mechanically stable, single-phase mullite 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 °C, the compressive strength decreased to 136 MPa. At 1500 °C, the mullite 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 °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 °C had a bulk density of 2.36 g/cm3 (82% theoretical density) and contained 63.24 wt.% crystalline mullite 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 °C to 29.8 wt.% at 1400 °C and up to 36.7 wt.% at 1500 °C. While EDS mapping performed over areas of ~200 μm did not allow a clear distinction between crystalline and amorphous regions, especially in the sample sintered at 1500 °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. While the glass phase promotes densification, it can simultaneously weaken the supporting crystalline mullite network, emphasizing the importance of a balanced ratio between glassy and crystalline components in glassy ceramic systems.
The results presented in this study have focused future research on thermal conductivity and thermal shock resistance, which are of crucial importance for refractory materials. Preliminary results on thermal conductivity have shown that this material is promising for further investigation with a thermal conductivity of approximately 0.24 W/(m·K) (unpublished results). These values are significantly better than previously published results [45,46]. However, the design of a sintered mullite matrix composite (magnesia-stabilized zirconia/WO3), known as hollow mullite grains [47], lowered the thermal conductivity to 0.4 W/(m·K) at room temperature and 900 °C. However, this material also has excellent properties, including thermal shock resistance, as evidenced by a decrease in Young’s modulus of only 0.5% after 10 cycles [46]. The detected phases with low (zircon, ZrSiO4) or even negative value (aluminum tungstate, Al2(WO4)3) of thermal expansion coefficient [48] reduced the thermal expansion coefficient and improved the thermal shock resistance of the resulting material.
The reduced thermal conductivity observed in the sintered mullite materials can be attributed to several microstructural and compositional features. Specifically, the loosely packed mullite grains, along with the presence of pores and intergranular cavities, contribute to the suppression of thermal conduction by interrupting the continuous thermal paths. Furthermore, these structural voids hinder convective heat transfer by restricting air movement within the closed porosity. In addition to porosity, grain boundaries, triple junctions and the intergranular glassy phase also have a significant influence on heat transfer. These disordered regions, particularly the amorphous phase, act as effective phonon scattering centers, restricting phonon propagation and significantly reducing the overall thermal conductivity of the ceramic [49]. Given the promising thermal behavior observed, future research will focus on a more detailed compositional and structural characterization. This will include Rietveld refinement of the mullite phase, quantification of the amorphous and crystalline components, analysis of macroporosity and systematic evaluation of thermal shock resistance and thermal conductivity. These efforts are aimed at further optimizing the performance of waste-derived mullite ceramics for high-temperature applications, particularly in the field of energy-efficient refractory materials and thermal insulators.

5. Conclusions

In this study, sintered mullite ceramics were successfully produced from diatomaceous earth and aluminum nitrate at temperatures ranging from 1300 to 1500 °C for 2 h. The resulting mullite monoliths showed remarkable mechanical stability over the entire temperature range, with the sample sintered at 1400 °C achieving the highest compressive strength of 188 MPa. Interestingly, although the sample sintered at 1500 °C exhibited well-formed rod-shaped mullite grains (~5 µm 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 µm at 1300–1400 °C to ~1 µm at 1500 °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·K)) suggest that these mullite 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.

Author Contributions

Conceptualization, S.I. and A.Š.; methodology, S.I., M.M.V. and A.Š.; software, J.M., Ž.S. and M.M.V.; validation, I.R. and A.Š.; formal analysis, Ž.S., Ž.R. and J.M.; investigation, S.I., A.Š. and M.M.V.; resources, S.I., M.M.V. and A.Š.; data curation, S.I., M.M.V., Ž.S. and A.Š.; writing—original draft preparation, S.I., M.M.V. and A.Š.; writing—review and editing, J.M., Ž.R., Ž.S. and I.R.; visualization, S.I., M.M.V. and Ž.S.; supervision, A.Š.; project administration, S.I. and A.Š.; funding acquisition, A.Š., M.M.V., J.M. and I.R. All authors have read and agreed to the published version of the manuscript.

Funding

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).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

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.

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Figure 1. Flowchart of the synthesis process.
Figure 1. Flowchart of the synthesis process.
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Figure 2. XRD patterns of mullite samples sintered at 1300, 1400, and 1500 °C for 2 h (a) and the inset of splitting of diffraction peaks at around 2θ = 26° (b).
Figure 2. XRD patterns of mullite samples sintered at 1300, 1400, and 1500 °C for 2 h (a) and the inset of splitting of diffraction peaks at around 2θ = 26° (b).
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Applsci 15 07512 g003aApplsci 15 07512 g003b
Figure 3. X-ray diffraction (XRD) patterns of the samples heat-treated at 1300 °C (a), 1400 °C (b), and 1500 °C (c). The broad hump between ~15° and 35° 2θ indicates the presence of an amorphous component, while the shaded area represents the fitted background and amorphous halo, used for semi-quantitative estimation of the amorphous phase content.
Figure 3. X-ray diffraction (XRD) patterns of the samples heat-treated at 1300 °C (a), 1400 °C (b), and 1500 °C (c). The broad hump between ~15° and 35° 2θ indicates the presence of an amorphous component, while the shaded area represents the fitted background and amorphous halo, used for semi-quantitative estimation of the amorphous phase content.
Applsci 15 07512 g003aApplsci 15 07512 g003b
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Figure 4. FE-SEM images of the mullite samples sintered at 1300 °C (a,d), 1400 °C (b,e), and 1500 °C (c,f).
Figure 4. FE-SEM images of the mullite samples sintered at 1300 °C (a,d), 1400 °C (b,e), and 1500 °C (c,f).
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Figure 5. Extracting pores in an image using Image Pro Plus.
Figure 5. Extracting pores in an image using Image Pro Plus.
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Figure 6. FE-SEM micrographs of the mullite samples sintered at (a) 1300 °C: (b) 1400 °C, and (c) 1500 °C with appropriate histograms of pore size diameter distribution.
Figure 6. FE-SEM micrographs of the mullite samples sintered at (a) 1300 °C: (b) 1400 °C, and (c) 1500 °C with appropriate histograms of pore size diameter distribution.
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Figure 7. Porosity percentage content of mullite samples sintered at 1300, 1400, and 1500 °C for 2 h.
Figure 7. Porosity percentage content of mullite samples sintered at 1300, 1400, and 1500 °C for 2 h.
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Figure 8. EDS spectrum of the mullite sample sintered at 1300 °C.
Figure 8. EDS spectrum of the mullite sample sintered at 1300 °C.
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Figure 9. EDS spectrum of the mullite sample sintered at 1400 °C.
Figure 9. EDS spectrum of the mullite sample sintered at 1400 °C.
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Figure 10. EDS spectrum of the mullite sample sintered at 1500 °C.
Figure 10. EDS spectrum of the mullite sample sintered at 1500 °C.
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Table 1. Chemical composition of starting material waste clay–diatomite.
Table 1. Chemical composition of starting material waste clay–diatomite.
OxidesSiO2Al2O3CaOMgOFe2O3K2ONa2OTiO2Loss on Ignition (1000 °C)
wt.%70.4813.783.010.742.890.890.130.347.70
Table 2. Volume fraction of phases, mullite lattice parameters (a,b,c), crystallite size, and lattice strain of mullite samples sintered at 1300, 1400, and 1500 °C for 2 h.
Table 2. Volume fraction of phases, mullite lattice parameters (a,b,c), crystallite size, and lattice strain of mullite samples sintered at 1300, 1400, and 1500 °C for 2 h.
Sintering Temperature (°C)Volume
Fraction (vol.%)		Mullite Lattice
Parameters (Å)		Crystallite
Size
(nm)		Lattice
Strain
(%)
130053.42 (M)
10.76 (Cr)
4.01 (C)
6.09 (Q)		7.5550(2) a
7.7053(4) b
2.8918(2) c		68 ± 0.20.001115
140058.13 (M)
7.87 (Cr)
1.47 (C)
2.67 (Q)		7.5488(3) a
7.7028(3) b
2.8916(6) c		70 ± 0.40.000958
150063.24 (M)
0.06 (Q)		7.5391(2) a
7.6914(1) b
2.8885(4) c		72 ± 0.30.000801
Table 3. Bulk density, relative density, open porosity, linear shrinkage, and compressive strength of mullite bodies sintered at 1300, 1400, and 1500 °C.
Table 3. Bulk density, relative density, open porosity, linear shrinkage, and compressive strength of mullite bodies sintered at 1300, 1400, and 1500 °C.
Sintering
Temperature (°C)		130014001500
Bulk density (g/cm3)1.692.202.36
Relative density (TD%)587682
Open porosity (%)37160
Linear shrinkage (%)14.121.120.5
Compressive strength (MPa)52.80188.67136.35
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Ilić, S.; Maletaškić, J.; Skoko, Ž.; Vuksanović, M.M.; Radovanović, Ž.; Ristović, I.; Šaponjić, A. Utilization of Waste Clay–Diatomite in the Production of Durable Mullite-Based Insulating Materials. Appl. Sci. 2025, 15, 7512. https://doi.org/10.3390/app15137512

AMA Style

Ilić S, Maletaškić J, Skoko Ž, Vuksanović MM, Radovanović Ž, Ristović I, Šaponjić A. Utilization of Waste Clay–Diatomite in the Production of Durable Mullite-Based Insulating Materials. Applied Sciences. 2025; 15(13):7512. https://doi.org/10.3390/app15137512

Chicago/Turabian Style

Ilić, Svetlana, Jelena Maletaškić, Željko Skoko, Marija M. Vuksanović, Željko Radovanović, Ivica Ristović, and Aleksandra Šaponjić. 2025. "Utilization of Waste Clay–Diatomite in the Production of Durable Mullite-Based Insulating Materials" Applied Sciences 15, no. 13: 7512. https://doi.org/10.3390/app15137512

APA Style

Ilić, S., Maletaškić, J., Skoko, Ž., Vuksanović, M. M., Radovanović, Ž., Ristović, I., & Šaponjić, A. (2025). Utilization of Waste Clay–Diatomite in the Production of Durable Mullite-Based Insulating Materials. Applied Sciences, 15(13), 7512. https://doi.org/10.3390/app15137512

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