*3.5. Solidifying Mechanism Analysis*

#### 3.5.1. XRD

The XRD pattern of the fly ash used for the test is shown in Figure 3. It can be seen that the main body of the fly ash is amorphous, and its crystalline substances are quartz and mullite, which are the main substances containing silicon and aluminum. The characteristic peaks of quartz appear at 22.53◦, 50.71◦, and 54.69◦; and that of mullite at 17.15◦, 27.33◦, 32.67◦, and 34.82◦. The diffraction peaks in a "bun" shape in the 2θ angle range of 15◦ to 30◦ indicate the presence of vitreous body in the fly ash, from which the activity of the fly ash is mainly derived.

**Figure 3.** XRD pattern of fly ash.

The variations in the mineral phase of the fly ash-based geopolymer and its solidified body are shown in Figure 4. Comparing Figure 4 with Figure 3, the crystalline phases in the fly ash-based geopolymer are mainly mullite and quartz, with a small amount of calcium silicate present at 39.54◦, and the crystalline diffraction peaks are somewhat reduced compared to that in the fly ash XRD pattern, demonstrating that the crystalline substances in the fly ash react to form amorphous substances during the alkali excitation process. Additionally, in the curve of the pure fly ash-based geopolymer, "bun"-shaped dispersion peaks appear in the interval 18◦ to 35 ◦, which are slightly shifted to the right compared to the fly ash curve, indicating that the dissolution of the vitreous body in the fly ash produces amorphous gel-like substance in the fly ash-based geopolymer; the absence of new phases and the disappearance of old phases mean that some of the crystalline particles in the fly ash are dissolved, and some are encapsulated by the gel material in the geopolymer, which corresponds to the SEM results.

**Figure 4.** XRD pattern of the fly ash-based geopolymer and its solidified body.

Compared to the curve in Figure 3, the overall curve in Figure 4 is flatter, which means a relatively high degree of reaction, and a more adequate polymerization of the silica-aluminous material in the raw material with the alkali activator, resulting in a good strength of the geopolymer. Diffraction peaks of crystalline minerals associated with Pb2+ are not found on the curve of the solidified body doped with Pb2+, and no new phases appear, which means that Pb2+ is not involved in the overall internal chemistry of the geopolymer and the Pb3SiO5 phase suggested by Palomo et al. [20] is not detected. It can be assumed that Pb2+ may have been bonded to the internal structural skeleton of the fly ash-based geopolymer, or the solidification of heavy metal ions by the geopolymer solidified body is a physical encapsulation.

### 3.5.2. SEM

Figure 5a shows a micrograph of the fly ash-based geopolymer in the early stage of the reaction. At this stage, the fly ash particles have not fully reacted; half of them are dissolved, while the other half appears partially encapsulated on the surface.

Figure 5b shows a micrograph of the fly ash-based geopolymer at a later stage of the reaction. It can be observed that the fly ash, after polymerization with the alkali activator, produces a very dense geopolymer gel (silica-aluminate gel) with a uniformly dense and amorphous outer surface; the particles in the fly ash are completely dissolved or encapsulated within the formed geopolymer gel; the beads are no longer visible compared to the pre-reaction period, but traces of their internal concavity left by the dissolution reaction can be found. Overall, the internal morphology of the fly ash-based geopolymer is dominated by a dense amorphous structure, which means that the reaction of the materials in this test is sufficient and the polymerization reaction of internal depolymerization and polycondensation is complete. As a result, the fly ash-based geopolymer obtained from the reaction possesses good physicochemical properties.

(**a**) Early stage (**b**) Later stage

**Figure 5.** SEM of the fly ash-based geopolymer in the early and later stages of the reaction.

Figure 6 shows the electron micrographs of the fly ash-based geopolymer during the reaction. Compared to the early stage of the reaction in Figure 5a, the wrapping condition in Figure 6a is more complete, and the progress of the reaction can be clearly observed. Figure 6b is a partial magnification, showing that the gel in the geopolymer slowly grows and extends from the top of the particle downwards until it completely wraps around the geopolymer, forming a kind of core-shell structure. These indicate that a portion of the fly ash particles is encapsulated by the gel into a core-shell structure, except for a portion being dissolved by the alkaline solution. The outer shell of this structure extends until it completely encapsulates the geopolymer and then forms a integral whole with its surrounding dense gel-like substances, which also has a beneficial effect on the mechanical properties of the geopolymer.

(**a**) Before magnification (**b**) After magnification

**Figure 6.** SEM of the fly ash-based geopolymer during the reaction.

Figure 7a shows the state during the solidifying Pb2+ reaction of the fly ash-based polymer, and the fully reacted state is given in Figure 7b. Compared to Figure 7a, a more complete and dense whole is formed in the latter figure, resulting in a good performing solidified body of the fly ash-based geopolymer.

(**a**) During reaction (**b**) After reaction

**Figure 7.** SEM of Pb2+ solidified body of the fly ash-based geopolymer.

#### 3.5.3. FT-IR

The infrared spectrum of the fly ash-based geopolymer and its solidified body is shown in Figure 8.

**Figure 8.** Infrared spectrum of the fly ash-based geopolymer and its solidified body.

According to the figure, the geopolymer exhibits O-H stretching vibration and O-H bending vibration at 3462.5 cm−<sup>1</sup> and 1638.7 cm−1, respectively; while the solidified body exhibits O-H stretching vibration and O-H bending vibration at 3471.3.4 cm−<sup>1</sup> and 1642.9 cm<sup>−</sup>1, respectively, which is surmised to be due to the presence of bound water within the geopolymer and the solidified body. The absorption peaks at 1087.3 cm−<sup>1</sup> of the geopolymer and 1025.6 cm−<sup>1</sup> of the solidified body in the infrared spectrogram correspond to the stretching vibration of Si-O-T (T = Si or Al). The introduction of Pb2+ has an effect on the stretching vibrations of Si-O-T around 1087.3 cm−<sup>1</sup> in the geopolymer, indicating that it is bonded into the backbone of the geopolymer. The absorption peaks near 564.8 cm−<sup>1</sup> of the geopolymer and 553.2 cm−<sup>1</sup> of the solidified body correspond to the stretching vibration of Al-O-Si. The bending vibration of Si-O or Al-O of the geopolymer and its solidified body is located near 461.5 cm−<sup>1</sup> and 455.9 cm<sup>−</sup>1, respectively. The peak at 1374.5 cm−<sup>1</sup> occurs only in the solidified body, which is analyzed as being due to the nitrate

contained in the heavy metal salts when Pb2+ is introduced. Overall, the infrared spectrum of the two samples is generally consistent, with no significant differences in the positions of the individual absorption peaks, indicating that the main structure of the solidified body is a complex three-dimensional mesh structure formed by the polymerization of silica-oxygen and aluminum-oxygen tetrahedra, although there are some differences in its physicochemical properties after the introduction of Pb2+.

#### **4. Conclusions**

In this study, the fly ash-based geopolymer was prepared to solidify Pb2+; the mechanical properties and leaching concentration of fly ash-based geopolymer were studied; the effects of different concentrations of Pb2+ on the compressive strength and solidifying effect of the solidified body were analyzed, and were compared with the cement solidified body; the reaction process, structural morphology, and solidifying mechanism were studied by XRD, FT-IR and SEM. The main conclusions are as follows:

(1) The compressive strength of the solidified body decreases compared to that of the pure geopolymer; the higher the concentration of Pb2+ added, the greater the decrease in strength. The solidifying performance of the fly ash-based geopolymer solidified body in a neutral environment is better than that in an acidic environment. Taking into account the compressive strength and leaching concentration, the optimum solidifying concentration of Pb2+ in the fly ash-based geopolymer is 2.0%.

(2) The fly ash-based geopolymer solidified body performs better than the cement solidified body, in terms of both mechanical properties (the compressive strength) and solidifying properties (leaching concentration). At Pb2+ concentration of 2%, the compressive strength of the geopolymer solidified body can still reach around 40 MPa and its leaching concentration is below the limit, whereas the compressive strength of the cement solidified body is only 21.25 MPa, and the leaching concentration is already as high as 8.29 mg·L<sup>−</sup>1.

(3) The analysis of the mineral composition, microscopic morphology, and chemical structure of the fly ash-based geopolymer and its solidified body by XRD, SEM and FTIR reveals that the fly ash-based geopolymer forms a complete whole inside with a very dense structure, and possesses good physicochemical properties. The solidifying of Pb2+ by the geopolymer is a combination of both chemical bonding and physical encapsulation.

**Author Contributions:** Data curation and investigation, R.T.; writing—original draft preparation, F.L.; writing—review and editing, X.Y.; supervision, B.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is supported by the National Natural Science Foundation of China [51878116 and 51902270]; Liaoning Province Key Project of Research and Development Plan [2020JH2/10100016]; Dalian Science and Technology Innovation Fund Project [2020JJ26SN060]; the Youth Innovation Team of Shaanxi Universities.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are not to be shared.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

