*3.1. Phase Transition Analysis*

The XRD patterns were evaluated using Highscore software v5.1, while the identification of the peaks was conducted using the PANalytical 2021 database. The peak search was realized considering only those with a minimum significance of 5.00, and a peak base with Gonio of 2.00, through the minimum second derivative method.

The XRD analysis of the CA samples (Figure 3) confirms the presence of multiple phases, such as quartz (96-900-9667), calcite (96-900-0967), anorthite (96-900-0362) and hematite-proto (96-900-2163). The detected hematite-proto contains Fe, H and O. Moreover, the XRD pattern of the metakaolin showed a high content of a typical amorphous structure. Compared to the CA pattern, the one specific to MK showed multiple peaks with high intensity. The most significant peak is positioned close to 25◦ (2θ) and corresponds to kaolinite (96-900-9231), the second peak, positioned close to 12.5◦ (2θ) corresponds to the same phase, while the peak close to 26.6◦ (2θ) corresponds to quartz (96-900-9667). Close to 35◦ (2θ) multiple peaks confirm the presence of muscovite (96-901-4938), while chloritoid (96-900-5444) was detected with the most significant peak close to 20◦ (2θ). The detected kaolinite contains Al, Si and O, the detected quartz contains Si and O, the detected muscovite contains K, Al, Si and O and the detected chloritoid contains Al, Fe, Mg, Si and O. As can be seen from Figure 3, in the case of MK, almost all of the identified peaks confirm the presence of kaolinite, quartz, muscovite and chloritoid.

**Figure 3.** The XRD pattern of the raw materials used in this study.

In the case of MT, the XRD pattern confirmed the presence of multiple phases rich in Fe or Si. Accordingly, quartz (96-901-5023), hematite (96-901-5965), pyrite (96-900-0595), calcium cyclo-hexaaluminate (96-100-0040) and hydrazinium copper sulfide (96-430-7636) were detected.

Due to the reaction between the activator and the aluminosilicate source, the dissolution of aluminates and silicates occurred, resulting in disordered and amorphous silicophosphate (Si–P), aluminophosphate (Al–P) or silico-alumino-phosphate (Si–Al–P) gels. However, the differences between the XRD spectra of the raw materials and the spectra of the acid-activated geopolymers are low because the Si–P, Al–P and Si–Al–P phases are typically amorphous (Figure 4). The existence of quartz and kaolinite at the same position was evident in previous studies [24,25]. Moreover, it can be observed that, due to the geopolymerisation reaction, multiple peaks disappeared, such as the peak around 22◦ (2θ) and the one around 29.5◦ (2θ). In addition, the intensity of the peaks decreased significantly. However, one new peak can be observed around 40.5◦ (2θ).

The calcite disappearance from the CA after the reaction with the phosphoric acid, i.e., the peak around 28◦ (2θ), disappeared due to geopolymerisation. This could be attributed to brushite formation or to the following chemical reaction (Equation (1)):

$$\text{3Ca}^{2+} + \text{2PO}\_4^{3-} + \text{H}\_2\text{O} = \text{3CaO} - \text{P}\_2\text{O}\_5 \text{ - H}\_2\text{O (C-P-H gel)}\tag{1}$$

In a phosphate acid environment, the Al oxides will be dissolute after the Ca compounds; therefore, the C–P–H gel will be formed before Al-P [10]. Therefore, by comparing the samples with MK against those without MK, it can be stated that the samples with higher Ca content will exhibit a lower setting time, due to the solubility differences between the divalent metals and the trivalent one.

**Figure 4.** The XRD pattern of the acid-activated geopolymers without mine tailings.

The broad peak at 27◦ (2θ) indicates the formation of the berlinite phase (AlPO4) which has a similar XRD pattern to quartz and will confer high mechanical properties to the final geopolymer [10,26]. Moreover, by comparing the XRD pattern of the MK with that of MK–geo, it can be observed that all of the peaks between at 26◦ (2θ) and 35◦ (2θ) appeared due to the reaction between the acid and the raw material. Furthermore, the intensity of all the peaks specific to kaolinite have been reduced.

In addition, as observed in [27,28], multiple Al–O–P phases, such as phosphotridymite, phosphocristobalite, aluminium phosphate or aluminium phosphate hydroxide appear, but they overlap with the patterns of other phases. However, a hydrated form of aluminium phosphate confirmed as metavariscite was detected [29].

In the samples with mine tailings (Figure 5), the characteristic diffraction of anhydrite (CaSO4) disappeared, which indicates that the structures of CaSO4 were dissolute due to the phosphoric acid presence. Furthermore, the resulting Ca contributes to the formation of brushite or amorphous calcium phosphate [30] and as crystalline ettringite in Aft and Afm monosulfate [31,32]. In addition, the peak around 12◦ (2θ), which corresponds to hydrazinium copper sulfide, increased significantly in the MTCA geopolymer.

**Figure 5.** The XRD pattern of the acid-activated geopolymers with mine tailings.
