## 3 Results and Discussion
### Chemical Resistance of Cu\({}^{2+}\)-Bearing Geopolymer
#### 3.4.2 Acid Leaching  
The resistance of the Cu-GPs to acid leaching (pH 4.5) is shown in Figure 5. The Al\({}^{3+}\), Fe\({}^{3+}\), Na\({}^{+}\), Si(OH)\({}_{4}\) and Cu\({}^{2+}\) concentrations remained always \(\leq\)100 mg/L and \(\leq\)8 mg/L in the leachate solutions of the Cu:GP and Cu:GP\({}_{\rm pH}\) series, which proves the high acid resistance of all the Cu-GPs. The K\({}^{+}\) concentration ranged between 30 and 145 mg/L, equivalent to a dissolution quota of \(\sim\)1-2 wt.%, if all K\({}^{+}\) is assigned to K\({}_{2}\)CO\({}_{3}\) impurities present in the GP (Figure 1b). This K\({}_{2}\)CO\({}_{3}\) dissolution caused a slight increase in solution pH, from 5.1 to 5.6, in all experiments. Nevertheless, the BET-SSA and the crystallinity degree (FWHM\({}_{\rm gerhardtitle}\): 0.13-0.18\({}^{\circ}\) 20; FWHM\({}_{\rm nonrite}\): 0.75-0.91\({}^{\circ}\) 20; FWHM\({}_{\rm serpentinite}\): 1.1-1.5\({}^{\circ}\) 20) of all leached GPs remained unchanged within the analytical uncertainty of the BET and XRD measurements. Figure 4: Compilation of results obtained from the ion exchange experiments. (**a**,**d**) Cross-plots of the Cu\({}^{2+}\) and Na\({}^{+}\) concentrations showing stoichiometric ion exchange in the Cu:GP series and no ion exchange in the Cu:GP series. Note the different scale in (**a**,**d**); (**b**,**e**) Fraction of Cu\({}^{2+}\) liberated into the solutions upon ion exchange in the Cu:GP and Cu:GP\({}_{\rm pH}\) series. Note the small amounts of ‘mobile’ K\({}^{+}\) released back to the solutions and the different scale in (**b**,**e**); (**c**,**f**) The SE images of the Na\({}^{+}\)-exchanged samples Cu:GP-1:2 and Cu:GP\({}_{\rm pH}\)-1:2 reveal no microstructural changes after the ion exchange reaction. Ghd—gerhardtitle; \(\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{ \rm{\rm{ \rm{ \rm{ \rm{ \rm{                }}}}}}}}}}}}}}}\) 12 \\). **a**,**b**,**e**) Fraction of Cu\({}^{2+}\) liberated into the solutions upon ion exchange in the Cu:GP and Cu:GP\({}_{\rm pH}\) series. Note the small amounts of ‘mobile’ K\({}^{+}\) released back to the solutions and the different scale in (**b**,**e**); (**c**,**f**) The SE images of the Na\({}^{+}\)-exchanged samples Cu:GP-1:2 and Cu:GP\({}_{\rm pH}\)-1:2 reveal no microstructural changes after the ion exchange reaction. Ghd—gerhardtitle; \(\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{\rm{ \rm{\rm{\rm{ \rm{ \rm{ \rm{         }}}}}}}}}}}}}}}}}\)\). The fraction of Cu\({}^{2+}\) released back to the leachate solutions varied between 0.24 and 0.68% for the materials of the Cu:GP series (Figure 5a), which indicates that the Cu-GPs, including the co-precipitated gerhardtite, have a high resistance to leaching. This finding is consistent with the experimentally determined solubility minimum of gerhardtite under slightly acidic to near-neutral conditions (pH \(\sim\)5.5 to 7.5) and a low solubility product (K\({}_{\text{gerhardtite}}\) = \(\sim\)\(10^{-16};25\)\({}^{\circ}\)C) of this barely soluble mineral [61]. Accordingly, SE images taken from the leached Cu-GPs show only minor microstructural changes, such as a small increase of fine pores that were formerly filled by gerhardtite (Figure 5b). SEM-EDX analyses revealed no chemical alteration of the K-A-S-H matrices (i.e., CuO/Al\({}_{2}\)O\({}_{3}\) ratio = 0.3 \(\pm\) 0.2) upon leaching. The amount of Cu\({}^{2+}\) measured in the leachate solutions ranged from 0.23 to 0.65% for the materials of the Cu:GP\({}_{\text{pH}}\) series and increased slightly with increasing initial Cu:GP\({}_{\text{pH}}\) ratio (Figure 5c), which suggests that a partial dissolution of CuO and Cu(OH)\({}_{2}\) phases accounted for the Cu\({}^{2+}\) release. This assertion is supported by the lower stability of these minerals under acidic conditions and their higher solubility in pure water (K\({}_{\text{tenorite}}\) = \(10^{-8.6}\); K\({}_{\text{serpentinite}}\) = \(10^{-113};25\)\({}^{\circ}\)C) compared to gerhardtite [58; 62; 63; 64].