# Utilization of Waste Clay-Diatomite in the Production of Durable Mullite-Based Insulating Materials
## 2 Materials and Methods  
The starting diatomic was sourced from the surface coal mine Barosevac, 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. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Oxides** & **SiO2** & **Al2O3** & **CaO** & **MgO** & **Fe2O3** & **K2O** & **Na2O** & **TiO2** & **Loss on Ignition** \\ wt.\% & 70.48 & 13.78 & 3.01 & 0.74 & 2.89 & 0.89 & 0.13 & 0.34 & 7.70 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of starting material waste clay–diatomic. The XRD (X-ray diffraction) patterns of the heat-treated compacts were scanned with a Rigaku Ultima IV diffractometer using Cu K\(\alpha\) radiation at 40 kV and 40 mA in the 2\(\theta\) range between 10\({}^{\circ}\) and 80\({}^{\circ}\) with a step of 0.02\({}^{\circ}\) and a scan rate of 10\({}^{\circ}\)/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 (\(\Delta\)d/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. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sintering** & **Volume** & **Mullite Lattice** & **Crystallite** & **Lattice** \\
**Temperature** & **Fraction** & **Parameters** (Å) & **Size** & **Strain** \\
**(\({}^{\circ}\)C)** & **(vol.\%)** & **Parameter** (Å) & **(nm)** & **(\%)** \\ \hline \multirow{4}{*}{1300} & 53.42 (M) & 7.5550(2) \(\mathbf{a}\) & \multirow{4}{*}{68 \(\pm\) 0.2} & \multirow{4}{*}{0.001115} \\  & 4.01 (C) & 7.7053(4) \(\mathbf{b}\) & & \\  & 6.09 (Q) & 2.8918(2) \(\mathbf{c}\) & & \\ \cline{1-1} \cline{3-4}  & 58.13 (M) & 7.5488(3) \(\mathbf{a}\) & & \\ \cline{1-1} \cline{3-4}  & 7.87 (Cr) & 7.7028(3) \(\mathbf{b}\) & & \\ \cline{1-1} \cline{3-4}  & 1.47 (C) & 2.8916(6) \(\mathbf{c}\) & & \\ \cline{1-1} \cline{3-4}  & 2.67 (Q) & 7.5391(2) \(\mathbf{a}\) & & \\ \cline{1-1} \cline{3-4}  & 63.24 (M) & 7.6914(1) \(\mathbf{b}\) & 72 \(\pm\) 0.3 & \\ \cline{1-1} \cline{3-4}  & 0.06 (Q) & 2.8885(4) \(\mathbf{c}\) & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Volume fraction of phases, mulilte lattice parameters (_a,b,c_), crystallite size, and lattice strain of mulilte samples sintered at 1300, 1400, and 1500 \({}^{\circ}\)C for 2 h.  
Figure 1: Flowchart of the synthesis process. 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.