*3.3. Porosity and Density Studies*

The prepared samples are considered highly porous materials with very high helium porosity values (∅He) and shallow bulk density values (ρb) (Table 3 and Figure 7). The lowest density values were primarily assigned to the first four samples AFS1–AFS4 and the ninth sample AFS9, which consisted primarily of monoclinic wollastonite, CaSiO3, ρ<sup>g</sup> = 2.86–3.09 g/cm3, [47] and gehlenite, Ca2Al2SiO7, ρ<sup>g</sup> = 3.04 g/cm<sup>3</sup> [48], with some quartz content, SiO2, ρ<sup>g</sup> = 2.62–2.65 g/cm3 [49] (Table 3).


**Table 3.** Main resulting phases and values of porosity and density for the investigated ceramic samples after being treated at 1100 ◦C for 1 h.

Parawoll. = parawollastonite, β-woll. = β-wollastonite, Geh. = gehlenite, Qz-low = low quartz.

**Figure 7.** Plot showing the inverse proportional relationship between the bulk density (ρb) and the helium porosity (∅He) of the AFS samples, and the frequency bars and broken line distribution. Open circle represents sample AFS9, which was removed from processing due to its abnormal porosity value.

The inverse proportional relationship between bulk density and porosity (Figure 7) is due to the high dependence of pore volume and grain density on the principal components of the material. In general, the porosity values decreased from AFS1 to AFS8 (with an exception for sample AFS4). Sample AFS9 showed an increase in porosity value. Therefore, porosity decreased as arc furnace slag content increased from 10% to 80%, in samples AFS1 and AFS8, respectively (Table 2, Figure 7). ∅He can be estimated as a function of the bulk density and the arc furnace slag content, based on two mathematical models, as shown in Figure 7.

Effect of Sintering Temperatures on Porosity and Density Values

The implication of sintering temperatures on porosity was checked for sample AFS5, characterized by a median ratio of arc furnace slag to ceramic sludge (50% for each). Porosity decreased with increasing sintering temperature, which was due to the increasing crystallization grade and crystal size with increasing temperatures (Table 4).

**Table 4.** Effect of sintering temperatures on density, porosity and phases developed for sample AFS5.


Parawoll. = parawollastonite, β-woll. = β-wollastonite, Geh. = gehlenite, Fay. = Faylite, Qz-low = low quartz.

Table 4 exhibits the porosity and density results of sample AFS5 after sintering at different temperatures, along with the precipitated phases at each temperature. This table shows that porosity decreased upon increasing the sintering temperature from 1050 ◦C to 1100 ◦C. This was because of the increase in compactness of the well-sintered sample. However, when sintering temperature was increased to 1150 ◦C, a slight increase in porosity and decrease in density was found, which can be explained by the transformation of βwollastonite into parawollastonite as well as due to the complete vanishing of gehlenite. Although the fayalite phase with its high density value, 4.39 [50], was precipitated, the density of AFS5 sample decreased after sintering at 1150 ◦C. This may be due to its low content in the sample.
