**3. Results and Discussion**

## *3.1. Temperature and Powder XRD Analysis*

The combustion process of the designed SHS reaction takes about 10 s after tungsten wire ignition. The center temperature of all samples in SHS reactions are measured and depicted in Figure 2. With the increment of soil wastes, the center temperatures of Cu-0, Cu-5, Cu-10, Cu-15, Cu-20, and Cu-25 samples decrease from 1679 to 1052 ◦C in Figure 2. The center temperature of the Cu-0 sample is the highest at 1679 ◦C, while the Cu-25 sample with the maximum soil content exhibits

the lowest temperature at 1052 ◦C. Apparently, the increase of soil wastes led to the decrease of SHS reaction temperature.

**Figure 2.** Real temperature curves of all samples during SHS reaction.

The XRD patterns in Figure 3 show that the specimens with soil waste (Cu-5 to Cu-25 samples) are composed of Cu, SiO2, and Gd2Ti2O7 (PDF No. 23-0259), while the sample without soil waste only contains Gd2Ti2O7 pyrochlore. From Cu-5 to Cu-25 specimens, the main phase of all samples is Gd2Ti2O7 pyrochlore, demonstrating that the increase of soil wastes does not change the phase composition. In Figure 3, the content of Cu in these SHS-ed samples increases with the increment of soil content, but Cu is hardly found in the Cu-0 sample. Because the temperature of Cu-0 reaction is the highest, the Cu melts and condenses into bulky grains during the high temperature reaction. With the decrease of reaction temperature, the size of copper particles decreases. Meanwhile, all SHS-ed samples were ground into powder for XRD testing, where the granulated Cu was sifted out directly. By contrast, the content change of SiO2 has no regular pattern, which may be affected by the heat insulator silica sand. However, unknown phases appear in the Cu-25 sample, which may be related with the large amount of simulated radioactive soil. Therefore, the Cu-20 specimen was selected for further analysis.

**Figure 3.** X-ray diffraction (XRD) patterns of all SHS-ed samples.

#### *3.2. Raman Analysis and Microstructure Characterization*

Raman spectroscopy was carried out to further analyze the crystal structure and internal bonds of pyrochlore. Raman spectroscopy is an important technique, especially in systems where oxygen displacement induces structure transformation, such as distinguishing fluorite from pyrochlore in pyrochlore ceramics [33]. Different from the A2B2O7 fluorite structure with only one F2g vibration mode, the pyrochlore structure contains six Raman modes (A1g, Eg, and 4F2g). Typical wavenumbers of pyrochlore phase at room temperature are 520 cm−<sup>1</sup> (A1g), 330 cm−<sup>1</sup> (Eg), and 200, 310, 450, 580 cm−<sup>1</sup> (4F2g) [33,34]. For Ti-pyrochlore, the most prominent characteristic of Raman spectra are the intensive band at 320 cm−<sup>1</sup> and the A1*<sup>g</sup>* band at 520 cm<sup>−</sup>1. The band around 320 cm−<sup>1</sup> includes Eg + F2g modes with very close frequency, which is mostly attributed to O–A–O bond vibration. The A1*<sup>g</sup>* band at 520 cm−<sup>1</sup> is believed to be related to A–O stretching [35,36].

The Raman spectra of Cu-0, Cu-10, and Cu-20 samples are shown in Figure 4. The six Raman active vibration modes (A1g, Eg and 4F2g) are explicitly assigned. In addition, the Si–O stretching vibration at 1100 cm−<sup>1</sup> and the Si–O–Si symmetric bending vibration near 700 cm−<sup>1</sup> are also included. The Raman spectra peaks of three specimens are similar except for some changes in strength, which means the pyrochlore structure of Gd2Ti2O7 remains unchanged. In particular, the characteristic F2g (200 cm−<sup>1</sup> and 455 cm<sup>−</sup>1) bands are well defined in the Cu-0 specimen. On the contrary, the vibration intensity of Eg + F2g modes (320 cm−1) and A1g mode (520 cm−1) increase significantly in the Cu-10 and Cu-20 samples. It is evident that this drastic change is due to the addition of simulated radioactive soil. On the basis of previous literatures [33–36], we preliminarily speculate that some ions in the simulated radioactive soil (possibly containing Ce) occupy the A and B sites of pyrochlore structure, resulting in steep changes of oxygen ions' environment and peak intensity of Raman spectra.

**Figure 4.** Raman spectra of the Cu-0, Cu-10, and Cu-20 samples.

As shown in Figure 5, the microstructure and elemental distribution of the compact Cu-20 specimen are exhibited in the SEM and elemental mapping images. It can be found that the pores mainly exist in the ceramic matrix rather than the copper phase. It may be argued that the melting point of copper (1083.4 ◦C) is lower than the combustion temperature of the Cu-20 sample. Therefore, gas can easily be discharged from the copper into the ceramic matrix. The Cu-20 sample consists of four phases, labeled as A, B, C, D in Figure 5a. According to Figure 5b–f and XRD analysis, we speculate that the A region is copper, the B region should be Gd2Ti2O7, the C region is SiO2, and the D region represents TiO2. The impurity TiO2 phase is produced from the raw materials of the reaction system and the original soil. However, no TiO2 exists in the previous XRD result of Cu-20. It is possible that the diffraction peaks of TiO2 are not obvious because of its low content.

**Figure 5.** (**a**) SEM image of Cu-20 specimen, and element mapping images of (**b**) O, (**c**) Si, (**d**) Ti, (**e**) Cu, (**f**) Gd.

The EDX elemental spotting analysis of the Cu-20 sample is presented in Figure 6. The EDX spotting image of "B" phase in Figure 6a is presented in Figure 6b. Combined with the XRD and EDX mapping results, the existence of Gd, Ti, Ce, and O in the EDX spotting spectra indicates that the "B" phase is Ce doped Gd2Ti2O7 pyrochlore phase. The average elemental quantities are acquired by taking at least five points of "A" area as listed in the inserted table of Figure 6b, which results in the chemical formulation of Gd1.96Ti1.94Ce0.09O7. Meanwhile, a small amount of Ce is also found in the soil phase according to Figure 6c, indicating that the simulated nuclide Ce of radioactive soil waste can exist in both the pyrochlore phase and soil phase. At the same time, most of the elements in original soil are retained in the soil phase. Figure 6d shows that only Ti and O are present in the D region, which confirms that the D phase is TiO2.

**Figure 6.** Energy dispersive X-ray spectroscopy (EDX) elemental spotting analysis: (**a**) Representative SEM image of the Cu-20 sample, (**b**) EDX spectrum and elemental composition of the labeled "B" area in (**a**), (**c**) EDX spectrum of region C in (**a**), (**d**) EDX spectrum of region D in (**a**).
