## 3 Results and Discussion
### Measurements of Main Physical Properties of Products  
#### 3.2.1 Firing Shrinkage Parameter  
During the firing process the produced pellets are naturally shrinking, which is considered as an undesirable property for refractory applications, whereas the addition of hematite aims to minimize this shrinkage. Figure 2 shows the obtained results for each product from the raw examined samples W5, W6, W7, and W8 and the respective pre-treated ones when fired as with the addition of 0, 5 and 10 wt.% hematite for the different thermal treatments applied at 1300 \({}^{\circ}\)C for 30 min and 120 min, and at 1600 \({}^{\circ}\)C for 60 min. The examined (four) samples present differences concerning the results of the firing shrinkage of products due to their initially different mineralogical content (see Table S1, Supplementary Material). As shown in Figure 2, the optimum results for each sample examined are:  
* Regardless of the thermal pre-treatment (e.g., for sample W5), the firing with hematite addition was found to have a low shrinkage effect in the case of the firing of sample W5. Figure 1: Optical microscope selected images (magnification: \(\times\)20) of W5 sample: (**a**) raw/initial material, (**b**) with the addition of 5% wt.% hematite after firing at 1300 \({}^{\circ}\)C for 30 min; (**c**) with the addition of 5% wt.% hematite, but after firing at 1300 \({}^{\circ}\)C for 120 min; (**d**) thermally pre-treated at 850 \({}^{\circ}\)C for 30 min with the addition afterwards of 5% wt. hematite and after firing at 1300 \({}^{\circ}\)C for 120 min; (**e**) with 10% wt.% hematite addition after firing at 1300 \({}^{\circ}\)C for 120 min; and (**f**) with 5% wt.% hematite after firing at 1600 \({}^{\circ}\)C for 60 min. Both the pre-treatment and hematite addition slightly increased the firing shrinkage of the products compared to the firing of the raw material, which is an undesirable outcome for the product. * With regard to the samples W6 and W7 which both contained magnesite, which decomposes and reacts with hematite to form magnesioferrite, it therefore directly also affected the firing shrinkage [15]. The results showed that the firing shrinkage of products at 1300 \({}^{\circ}\)C, containing 5 or 10 wt.% hematite and pre-treated thermally at 850 \({}^{\circ}\)C for 30 min were almost equal to the firing shrinkages of the products without the addition of hematite. * However, different results were found for the sample W8, which contains a higher percentage of pyroxenes, leading to lower shrinkage of products with the addition of hematite which were thermally treated at 1300 \({}^{\circ}\)C for 30 min. Nevertheless, the results show the better sintering of the product obtained from the W8 sample after the application of thermal treatment at 1300 \({}^{\circ}\)C for 120 min, hence presenting the lowest firing shrinkage. * When the thermal treatment is applied at 1600 \({}^{\circ}\)C for 60 min, the optimum results for the firing shrinkage parameter were detected for the product of the W8 sample containing 5 wt.% Fe\({}_{2}\)O\({}_{3}\) and for the thermally pre-treated W5, W6 and W7 samples. Figure 3 shows the comparison between the firing shrinkage results among the products of the fired samples regarding the addition of 5 or 10 wt.% hematite, in order to evaluate the optimum raw material. It is obvious that for the thermal treatment at 1300 \({}^{\circ}\)C the sample W8 presents the lowest firing shrinkage. However, when the thermal treatment at 1600 \({}^{\circ}\)C is applied, then the firing shrinkage is doubled for this sample, whereas the minimum firing shrinkage (9.5%) is shown for the sample W6 PT with 5 wt.% hematite addition. At this point is should be underlined that firing shrinkage values greater than 10%  
Figure 2: The firing shrinkage values (FS%) of examined samples (thermally pre-treated or not), with or without the addition of hematite: (**a**) W5; (**b**) W6; (**c**) W7; (**d**) W8.