## 3 Results and Discussion
### Mineralogy and Microstructure of Cu\({}^{2+}\)-Bearing Geopolymers  
XRD and SEM analyses of the precipitates from the Cu:GP and Cu:GP\({}_{\text{PH}}\) series, obtained after the Cu\({}^{2+}\) immobilization experiments, identified Cu\({}^{2+}\)-substituted GP, in addition to unreacted quartz and anatase and three types of newly formed Cu\({}^{2+}\)-bearing minerals (Figure 3). All Cu-GP materials collected after ion exchange and leaching tests were also studied by XRD, but they did not reveal mineralogical changes and are therefore not shown. The external SSA values of all reacted GPs varied between 1.98 and 2.35 m\({}^{2}\)/g, which is within the analytical uncertainty of the BET measurements.  
The copper nitrate hydroxide gerhardtite (Cu\({}_{2}\)(NO\({}_{3}\))(OH)\({}_{3}\)) precipitated in varying proportions under the acidic conditions (pH 4.1 to 5.7) of the Cu:GP series (Figure 3a), confirming that this mineral is the most stable polymorph at ambient temperature of the copper hydroxyl nitrate family, which include gerhardtite, rouaite and likasite [56]. The formation of well-crystallized gerhardtite (FWHM: 0.13-0.18\({}^{\circ}\) 20) particles with an average size of 0.5 to 1.0 \(\upmu\)m and a plate-like morphology on the GP grains and within the pore space (Figure 3b) can be explained by its lower solubility compared to the copper(II)  
Figure 3: Mineralogy and microstructure of Cu-GPs obtained from the Cu\({}^{2+}\) immobilization experiments of the Cu:GP series (left panel) and Cu:GP\({}_{\text{PH}}\) series (right panel). (**a**,**c**) XRD patterns of all precipitates; (**b**,**d**) SE images of precipitates from experiments Cu:GP-12 and Cu:GP\({}_{\text{PH}}\)-12, respectively.  
nirate trihydrate salt, used for the preparation of the Cu\({}^{2+}\) solutions and a related high supersaturation of gerhardtite with respect to the acidic solutions of the Cu:GP series [57]. However, a considerable amount of Cu\({}^{2+}\) was also incorporated in the GP matrix, as evident from the CuO/Al\({}_{2}\)O\({}_{3}\) ratio of 0.3 \(\pm\) 0.1 in experiment Cu:GP-1:25, where no gerhardtite formed. All other GP matrices had a CuO/Al\({}_{2}\)O\({}_{3}\) ratio of 0.2 \(\pm\) 0.1. The amount of Cu\({}^{2+}\), which was bound to gerhardtite, increased at elevated initial Cu:GP ratio (Figure 3a). Thus, the low solubility of gerhardtite most likely reduced the uptake capacity of Cu\({}^{2+}\) by GP.  
The copper(II) (hydr)oxide minerals sperriniite (Cu(OH)\({}_{2}\)) and tenorite (CuO) formed under the highly alkaline conditions (pH 11.1 to 12.4) of the Cu:GP\({}_{\mathrm{PH}}\) series (Figure 3c). This finding is supported by experimental evidence of copper(II) (hydr)oxides formation controlling the Cu\({}^{2+}\) concentrations in aqueous solutions containing hydroxide (OH\({}^{\ast}\)) and nitrate (NO\({}_{3}\)\({}^{-}\)) or sulphate (SO\({}_{4}\)\({}^{2-}\)) as sole anions [58; 59]. Geochemical equilibrium modeling further indicated that CuO and Cu(OH)\({}_{2}\) phases can limit the metal ion mobility in cement and geopolymer matrices under alkaline conditions [60]. The formation of plate-like sper-tinite (\(\sim\)0.1-0.2 \(\upmu\)m in largest dimension) and of blocky tenorite (\(\sim\)0.2-0.5 \(\upmu\)m in size) was controlled mainly by the pH treatment applied to this experimental set (Figure 3d). Thus, a large fraction of Cu\({}^{2+}\) added to the Cu:GP\({}_{\mathrm{PH}}\) series was immobilized by barely soluble copper(II) (hydr)oxides. However, a portion of Cu\({}^{2+}\) was chemically bound to GP, as evidenced by the CuO/Al\({}_{2}\)O\({}_{3}\) ratio of 0.20 \(\pm\) 0.1 in experiment Cu:GP\({}_{\mathrm{PH}}\)-1:25, where the amount of semi-crystallized tenorite (FWHM: 0.77-0.89\({}^{\circ}\) 20) and poorly crystallized sperriniite (FWHM: 1.2-1.6\({}^{\circ}\) 20) formed is negligible (Figure 3c). All other GPs had a CuO/Al\({}_{2}\)O\({}_{3}\) ratio of 0.15 \(\pm\) 0.5, signifying profound Cu\({}^{2+}\) incorporation in the GP structure.