*3.1. X-ray Diffraction Patterns*

X-ray diffraction graphs (Figure 3) of investigated ceramic samples AFS1–AFS9, after sintering at 1100 ◦C for one hour, showed all precipitated phases of the samples. The major crystalline phases formed were β-wollastonite (CaSiO3) (JCPDS No.29-372), parawollastonite (CaSiO3) (JCPDS No.27-88), gehlenite (Ca2Al2SiO7) (JSPDS No.35-755), and low quartz (SiO2) (JCPDS No.5-0490). Sample AFS1 contained β-wollastonite (CaSiO3) as the dominant phase with lines at 3.33, 2.96, 3.16, and 3.06 Å. Low quartz (SiO2), which has distinctive lines 4.23, 3.33, 2.29, and 1.82 Å, represented the second phase. Gehlenite (CaAl2SiO7) was the third phase with lines at 3.72, 2.85, 2.43, 2.04, and 1.75 Å. Samples AFS2–AFS4 showed a significant decrease in quartz phase content. In addition, it is noticed that there was a transformation of β-wollastonite into parawollastonite, as seen through reduction in line heights at 3.33 and 2.51 Å corresponding to β-wollastonite and increases in intensity for the main lines of parawollastonite, which are 2.96, 3.16, and 3.83 Å; thus, parawollastonite was the main phase, and gehlenite was considered the second phase with small amounts of low quartz. In sample AFS5, β-wollastonite and low quartz phases reappeared with lines at 3.33, 2.96, 3.16, and 3.06 Å for β-wollastonite and lines at 4.23 3.33, 2.29, and 1.82 Å for low quartz. Sample AFS6 showed the development of gehlenite phase, which became the main phase with increasing intensities of lines at 2.85, 3.72, 2.43, 2.04, and 1.75 Å. AFS6's pattern indicated the formation of β-wollastonite and quartz, while samples AFS7, AFS8, and AFS9 showed development of both low quartz and β-wollastonite phases and a noticeable decrease in the intensity of gehlenite lines at 3.72, 2.85, 2.43, 2.04, and 1.75 Å.

Through a deep glance at Table 2, it can be found that sample AFS1 contained a CaO/SiO2 ratio of approximately 0.68, which confirms the X-ray diffraction results. That accounted for β-wollastonite as a primary phase with low quartz and gehlenite as secondary phases. For samples AFS2–AFS4, the CaO/SiO2 ratio decreased from 0.63 to 0.50, corresponding with the transformation of β-wollastonite into parawollastonite. A decrease in the percentage of low quartz phase and an increase in the percentage of gehlenite were also noticed in samples AFS2, AFS3, and AFS4. The CaO/SiO2 ratio decreased to 0.41 in sample AFS5, which helped form β-wollastonite and gehlenite as the major phases and low quartz as secondary phase. For sample AFS6, the CaO/SiO2 ratio increased to 0.51; this led to the formation of gehlenite as the main phase with β-wollastonite and low quartz as secondary phases. Samples AFS7–AFS9 had nearly the same CaO/SiO2 ratio, approximately 0.52–0.53, which helped form β-wollastonite as the primary phase with low quartz as the second phase and gehlenite as the third phase. All the above results indicate that β-wollastonite (unstable phase) was formed more often than low quartz, with a small amount of gehlenite. The β-wollastonite phase turned into parawollastonite (stable phase) when the CaO/SiO2 ratio decreased. The parawollastonite phase was formed with greater amounts of gehlenite and smaller amounts of low quartz.

The obtained results indicate that a low CaO/SiO2 ratio enhances the formation of β-wollastonite and low quartz phases but hinders the development of gehlenite phases (samples AFS7–AFS9). At the same time, increasing the CaO/SiO2 ratio promotes the formation of parawollastonite and gehlenite phases but hinders the development of the low quartz phase (samples AFS2–AFS4).

Fan et al. [38] explained that by increasing the CaO/SiO2 ratio, the basicity increases, and consequently, the crystallization of the CaO-Al2O3-MgO-SiO2 system is improved [39,40]. Tabit et al. [41] found that parawollastonite and gehlenite were formed in samples containing higher calcium oxide percentages.

**Figure 3.** The major phases resulting from industrial wastes, determined via XRD after treatment at 1100 ◦C for one hour.
