# Effect of Fly Ash on the Properties of Ceramics Prepared from Steel Slag
## 3 Results and Discussion
### XRD and FTIR Analyses  
The XRD spectra of the sintered samples with different fly ash contents at the optimum sintering temperature are shown in Figure 3. The main crystal phases of sample 1 are quartz, diopside, and augite.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Sintering** & **Sintering** & **Water** & **Flexural** \\  & **Temperature**, \({}^{\circ}\)**C** & **Shrinkage, \%** & **Absorption, \%** & **Strength, MPa** \\ \hline
1 & 1130 & 8.18 & 0.26 & 62.2 \\
2 & 1130 & 8.28 & 0.48 & 52.89 \\
3 & 1135 & 7.96 & 0.04 & 51.12 \\
4 & 1145 & 8.42 & 0.03 & 43.37 \\
5 & 1160 & 7.94 & 0.26 & 35.58 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Optimum sintering temperature and physical properties at the optimum sintering temperature.  
Figure 2: Variation in shrinkage, water absorption, and flexural strength at different sintering temperatures: (**a**) shrinkage versus sintering temperature; (**b**) water absorption versus sintering temperature; (**c**) flexural strength versus sintering temperature.  
The diffraction peak of quartz decreases with an increase in fly ash content. For sample 4, the main crystal phases are quartz, diopside, augite, and anorthite. With an increase in the fly ash content to 20 wt %, the diffraction peaks of quartz in sample 5 are hardly detectable, which implies that there is either no quartz, or only a small amount. Diopside, augite, and anorthite constitute the main crystalline phases. The reason for this is that, with an increase in fly ash addition, the SiO\({}_{2}\) content in the batches decreases and the Al\({}_{2}\)O\({}_{3}\) content increases. During sintering, the liquid phase is produced, and Al\({}^{3+}\) is dissolved in the diopside by diffusion in the liquid phase [19], which increases the diffraction peak of augite. When the Al\({}_{2}\)O\({}_{3}\) content reaches a critical amount, anorthite is formed.  
To obtain more information on the effect of fly ash on phase transition, the FTIR spectra of the samples were measured in the range of 400 cm\({}^{-1}\) to 4000 cm\({}^{-1}\) at room temperature, as shown in Figure 4.  
The peak appearing at 1070 cm\({}^{-1}\) is attributed to the asymmetric stretching vibration of Si-O\({}^{-}\) (O\({}^{-}\): non-bridging oxygen) bands in the Q\({}^{3}\) (Q\({}^{2}\): a tetrahedron with n bridging oxygen) tetrahedral units of augite. The peak at 962 cm\({}^{-1}\) shifted to a lower frequency when fly ash was added. The transmittance bands occurring in the region of 400-550 cm\({}^{-1}\) are related to the bending vibration mode of Si-O-Si bridges [20]. The peak at 930 cm\({}^{-1}\) in sample 5 is due to the vibration of the Q\({}^{1}\) and Q\({}^{2}\) units.  
Figure 4: Fourier transform infrared (FTIR) spectra of samples 1–5 at optimum sintering temperature.  
Figure 3: XRD spectra of samples 1–5 at optimum sintering temperature.  
The peak at 665 cm\({}^{-1}\) is associated with the stretching vibrations of Si-O or Al-O bands of diopside or augite. In samples 4 and 5, the peak at around 530 cm\({}^{-1}\) is due to the increase in the fly ash content, which leads to an increase in the Al\({}_{2}\)O\({}_{3}\) content and the formation of anorthite, which results in the bending vibration of Si-O-Si and the stretching vibrations of Ca-O in anorthite [21]. The peak at 459 cm\({}^{-1}\) is assigned to the bending mode of Si-O-Si and O-Mg-O of diopside and the stretching vibration modes of the Ca-O band of anorthite [22]. The peak at 690 cm\({}^{-1}\) shifted toward lower frequencies, which was due to the addition of fly ash causing an increase in Al\({}_{2}\)O\({}_{3}\) content, and the Al\({}^{3+}\) ions replaced the Si\({}^{4+}\) ions in the tetrahedral structure of Si-O-Si to form a network structure of Si-O-Al [23]. In sample 5, the disappearance of the peak at 798 cm\({}^{-1}\) is related to the disappearance of the quartz phase. These results are consistent with the results of XRD analysis.