*3.5. Implication of Waste Composition on Pore Volume*

The incremental increase in CaO and Al2O3 fractions was accompanied by increasing porosity values, while MgO and Fe2O3 contents were inversely related to the porosity values. The impact of increasing SiO2 content acted somewhat inversely upon the porosity values (Table 3, Figure 12). Consequently, the decreasing sludge content from AFS1 to AFS9 was responsible for the decreasing pore volume of the studied samples (Tables 2 and 3).

**Figure 11.** Effect of sintering temperature (1050–1150 ◦C) of AFS5 on the measured dielectric constant and electric conductivity plotted versus the applied frequency.

**Figure 12.** Plotting the main oxide composition versus the incremental percentages of porosity.

#### *3.6. Implication of Waste Composition on Electric Properties*

The waste composition of prepared ceramic samples impacted their electric activities. Electric conductivity and capacitance were primarily affected by the arc furnace slag content in an incremental increase from the AFS1 to the AFS9 samples (10–90%, respectively). Based on their dielectric and electric parameters, the AFS samples could be clustered into two groups, AFS1–AFS4 and AFS5–AFS9. These groups may be differentiated by the higher porosity values for the first group (41.12–41.66%) compared to the second group (31.16–47.26%, Table 3). This could be attributed to higher contents of CaO (32.73–23.79%) and Al2O3 (13.42–13.61%) in the first group (AFS1–AFS4) than the second one (Table 2).

Likewise, based on the dielectric constant values, the arc furnace slag–sludge mixtures could be clustered into two sample groups; the first one (AFS5–AFS9) was characterized by higher *ε*' and *σ* and lower *M*" than the second (AFS1–AFS4) (Figures 8–10). This could be attributed to the relatively high contents of Fe2O3 and MgO in samples AFS5–AFS9 and the relatively low contents of CaO and Al2O3 in these samples (Table 2).

#### *3.7. Implications of Crystal Size and Pore Volume on Electric and Dielectric Properties*

The crystal size had a direct effect on the electrical activity of the charged materials. Larger crystal sizes result in reduced surface area and, therefore, less polarized charged surfaces. For samples AFS5–AFS9, the crystals seemed to be well developed, as shown in Figure 5, while for samples AFS1–AFS4, the crystals had a lower surface area.

By contrast, porosity (∅ in%) is a complementary component of the crystal's volume (equal to 100–∅ in%), i.e., it has indirect effects on the studied samples [34,52,53]. Porosity is considered an additional parameter that controls the electrical polarization and conductivity of samples. High porosity means lower grain volume and less polarized surfaces, and vice versa. Samples AFS5-AFS8 had porosity values lower than the remaining samples. In

addition, they were characterized by higher *ε*' and *σ* (measured at 4.0 MHz as a midway frequency value), which are inversely related to the porosity values (Figure 13).

**Figure 13.** Dielectric constant (*ε*') and electric conductivity (*σ*) versus helium porosity (∅He). Sample AFS9 (open circle) was removed from statistical processing.

#### *3.8. Industrial Applications*

The highly porous nature of the studied ceramic samples, as produced from both arc furnace slag and sludge waste mixtures, suits them for creation of building materials such as wall tiles. The relatively low electric conductivity, imaginary electric modulus, and dielectric constant values of the studied samples indicate their applications as poor to fair semiconductors (at 50 Hz and higher frequencies). The increasing iron oxide content with increasing arc furnace slag percentages (from 1.6% for AFS1 to 7.04% for AFS9) reduces the possibility of this application by increasing electric activity. In addition, the presence of certain impurities in these samples increases their ability to conduct electric charges on the order of μS (Khater et al., 2019, 2020, Dimitrijev, 2011) [35,36,56].

From an economic point of view, lightweight porous ceramic prepared from recycling hazardous waste materials tends to cost less than porcelain and is much lighter. It is often used for wall and ceiling installations. However, there are some significant restrictions with this material: it is not as strong as porcelain, so it does not make the best walking surface; it can be freezing underfoot in the winter; and heavy tile can be challenging to install. Therefore, this kind of ceramic is a good candidate for wall tile applications.

Porous ceramic wall tile bodies, whose microstructure and composition differ entirely from those of porcelain bodies (higher porosity and smaller glassy phase content), can be incredibly affordable and widely used worldwide. All these requirements are highly fulfilled in this work through the recycling of both electric arc furnace slag and ceramic sludge waste.

In this work, cleaning the environment from industrial waste byproducts was our first goal. The production of lightweight ceramic materials via a low energy consuming process to improve our environment and reduce the headache caused by global warming was our second target.
