Effect of Sintering Temperatures on Crystalline Phases

AFS5 was chosen to study the effect of sintering temperatures on precipitated phases and resulting physical properties, because it represents the mean composition of all the examined samples. The studied sintering temperatures were 1050, 1100, and 1150 ◦C, all for one hour. Figure 4 shows that, after sintering at 1050 ◦C, β-wollastonite (CaSiO3) became a primary phase with its characteristic lines at 3.33, 2.967, and 2.51 Å. Low quartz (SiO2) was the second phase, with its distinctive lines at 4.24, 3.33, and 2.46 Å, and gehlenite (Ca2Al2SiO7) was the third phase, with its characteristic lines at 3.74, 3.17, and 2.84 Å. When the temperature was raised to 1100 ◦C, the gehlenite lines decreased, evident in the mainline 2.84 Å. In addition, β-wollastonite began to convert into parawollastonite. This transformation is demonstrated by the increasing intensity of the line at 2.96 Å and the decreasing intensity of the β-wollastonite line at 3.33 Å. Upon raising the temperature to 1150 ◦C, β-wollastonite transformed into parawollastonite, which is evident from the decreased intensity of its line at 3.33 Å and increased intensity of the parawollastonite line at approximately 2.97 Å. Gehlenite wholly disappeared, as its distinctive line at 2.84 Å vanished. This increase in sintering temperatures promoted the formation of the fayalite (Fe2SiO4) phase. These results agree with Edrees et al. [42], who showed that β-wollastonite is an unstable phase at lower temperatures and turns into parawollastonite (stable phase) at higher temperatures. Ismail et al. [43] found that wollastonite has the property of a polymorphic structure, whether as β-wollastonite (unstable) or parawollastonite (stable). β-wollastonite turns into an independent parawollastonite at high temperatures. Qin et al. [44] also showed that gehlenite was formed with wollastonite and disappeared at high temperatures (1125 ◦C). The effect of the CaO/SiO2 ratio on the phase transformation of anorthite, based on steel slag and fly ash, was studied by Tabit et al. [41]. They deduced that the gehlenite phase was precipitated at 1100 ◦C.

**Figure 4.** X-ray diffraction patterns of AFS5 samples after sintering at different temperatures for one hour.
