*3.2. Morphology Studies*

Figure 5 presents SEM photographs at different magnifications of the fractured samples AFS1–AFS8 after heat treatment at 1100 ◦C. It can be observed from the figure that all ceramic samples possessed porous structures, and there were wide variations in the pore size distribution (the pore diameter ranged from 4 to 80 μm) and shape. Circular and irregular pore outlines were noticed in all ceramic samples. Moreover, closed and interconnected pores are apparent in the photos. Porosity decreased and density increased with increases in arc furnace slag (AFS) weight, as is confirmed from the porosity percentage results (decreasing from 41% for AFS1 to 28% for AFS8), and the increasing density of wall pores (Figure 5). This can be attributed to the decrease in CaO and increase in MgO. As MgO molecules are smaller than CaO molecules, this caused a decrease in liquidus temperature, as reported in a previous study [45]. In addition, decreasing Al2O3 content caused a decrease in the liquidus temperature of the ceramic particles. When Al2O3 content increases, the amount of nonbridging oxygens (NBOs) decreases and consequently, Al–O–Si bonds form and raise viscosity values [46]. This decrease in liquidus temperature leads to more condensed ceramic samples with lower porosity. Moreover, columnar and tabular crystals, which are the characteristic structures of wollastonite crystals, were noticed.

**Figure 5.** SEM images at different magnifications of microstructures of the investigated samples after treatment at 1100 ◦C for 1 h.

Effect of Sintering Temperatures on Ceramic Morphology

The applied sintering temperatures for ceramic materials affect properties such as porosity, density, and microstructure. Accordingly, different sintering temperatures were applied on AFS5 to determine the optimum conditions for producing such ceramic. The sample was heat-treated at 1050 ◦C, 1100 ◦C, and 1150 ◦C at a constant time (Figure 6). As shown from the figure, an increase in sintering temperature from 1050 ◦C to 1100 ◦C led to a denser and more compact structure. This could be due to the formation of well-developed, dense, and long wollastonite crystals. Upon raising the sintering temperature to 1150 ◦C, a slight change in microstructure was noticed. This increase in temperature caused the appearance of new rod-shaped crystals with a minor increase in the porous structure and a loss of tiny pores. This might be because of the disappearance of the gehlenite phase and the formation of the fayalite phase, as well as due to the transformation of the β-wollastonite (triclinic) structure into the parawollastonite (monoclinic) structure.

**Figure 6.** SEM images at different magnifications of microstructures of AFS5 ceramic sample after treatment at 1050 ◦C, 1100 ◦C, and 1150 ◦C.
