*3.6. XRD Analysis*

The use of X-ray diffraction is a necessary tool for determining the different crystalline phases contained in a soil sample. Figure 6 displays the XRD pattern of the three soils (S1, S2, and S3) and Olive Pomace Biomass Slag (OPBS) samples. The soils were dominated by silicon oxide (SiO2), with no other crystalline phase detected. Except for Soil 02, which displayed a minor peak of CaCO<sup>3</sup> as validated by TGA curves and XRF, which had a high proportion of CaO oxide (2,56%) when compared to Soils 1 and 3. The strongest quartz

peaks were found between 20.71◦ and 26.45◦ for both the soil and the OPBS samples. XRD analysis of OPBS, on the other hand, revealed that the most prevalent crystalline phase was calcite (CaCO3) and quartz (SiO2), along with other minerals such as dolomite (CaMg, (CO3)2), fairchildite (K2Ca (CO3)2), and free lime (CaO). The results of XRD confirm that higher percentages of SiO2, CaO, and K2O detected by XRF analysis were the main elemental composition of the crystalline phases of OPBS. In sandy soils, significant weathering occurs at depth resulting in mineralogy where quartz is the dominant mineral in the sand and silt fraction and forms a considerable proportion of the clay fraction. It is noted that despite the prevalence of silica in the soil samples, other minerals were also detected by XRF in FF at a considerable percentage, e.g., Al2O and Fe2O3; however, the XRD patterns revealed only the quartz (SiO2) which is mainly due to the absence of clayey minerals such montmorillonite, bentonite, kaolinite, etc. [67]. As a result, the application of OPBS as a soil additive might influence the weathering of Si. Matichenkov et al. [68] reported that soil properties such as P, Al, heavy-metal behavior, and adsorption capacity are governed by soil silicon compounds. quartz peaks were found between 20.71° and 26.45° for both the soil and the OPBS samples. XRD analysis of OPBS, on the other hand, revealed that the most prevalent crystalline phase was calcite (CaCO3) and quartz (SiO2), along with other minerals such as dolomite (CaMg, (CO3)2), fairchildite (K2Ca (CO3)2), and free lime (CaO). The results of XRD confirm that higher percentages of SiO2, CaO, and K2O detected by XRF analysis were the main elemental composition of the crystalline phases of OPBS. In sandy soils, significant weathering occurs at depth resulting in mineralogy where quartz is the dominant mineral in the sand and silt fraction and forms a considerable proportion of the clay fraction. It is noted that despite the prevalence of silica in the soil samples, other minerals were also detected by XRF in FF at a considerable percentage, e.g., Al2O and Fe2O3; however, the XRD patterns revealed only the quartz (SiO2) which is mainly due to the absence of clayey minerals such montmorillonite, bentonite, kaolinite, etc. [67]. As a result, the application of OPBS as a soil additive might influence the weathering of Si. Matichenkov et al. [68] reported that soil properties such as P, Al, heavy-metal behavior, and adsorption capacity are governed by soil silicon compounds.

(Note: F.F = Fine-soil fraction (<63 µm); C.F = complete-soil fraction; OPBS = Olive Pomace Biomass

The use of X-ray diffraction is a necessary tool for determining the different crystalline phases contained in a soil sample. Figure 6 displays the XRD pattern of the three soils (S1, S2, and S3) and Olive Pomace Biomass Slag (OPBS) samples. The soils were dominated by silicon oxide (SiO2), with no other crystalline phase detected. Except for Soil 02, which displayed a minor peak of CaCO3 as validated by TGA curves and XRF, which had a high proportion of CaO oxide (2,56%) when compared to Soils 1 and 3. The strongest

*C* **2023**, *9*, x FOR PEER REVIEW 15 of 22

F.F 3 60.64 12.51 10.74 0.3 1.3 1.01 0.51 1.3 0.81 0.37 0.13 89.62 C.F 3 75.70 7.23 5.52 0.19 0.42 0.54 0.63 1.01 0.48 0.32 0.09 92.13 OPBS 26.29 3.26 1.07 0.05 3.52 22.38 0.56 17.63 0.12 2.75 1.41 79.04

Slag).

*3.6. XRD Analysis* 

**Figure 6.** XRD analysis of the three soils, S1, S2, and S3, and OPBS: 1 = (SiO2), 2 = (CaCO3), 3 = (CaMg, (CO3)2) 4 = (K2Ca (CO3)2), 5 = (CaO). **Figure 6.** XRD analysis of the three soils, S1, S2, and S3, and OPBS: 1 = (SiO<sup>2</sup> ), 2 = (CaCO<sup>3</sup> ), 3 = (CaMg, (CO3)<sup>2</sup> ) 4 = (K2Ca (CO<sup>3</sup> )2 ), 5 = (CaO).
