## 3 Results
### Environmental Performance  
Based on the results already presented, namely, the mechanical performance and the activator content used, paste P2B was determined to be the most adequate to study the potential of the technique to fix the contaminants present in the MT. This study was based on the use of two different methodologies (_LTT1_ and _LTT2_) to obtain an eluate solution from the solid pastes. Three different materials were submitted to the _LTT1_ test, namely, the original MT without any addition, the activated MT (P0), and the MT+FA paste designated P2B. Only the latter was submitted to _LTT2_.  
The leachable concentrations obtained during test _LTT1_ were compared with the threshold values presented in Council Decision 2003/33/EC for inert waste, non-hazardous waste, and hazardous waste (Figure 12). In general, activation of the original MT (paste P0) produced a decrease in the leached concentration, the exceptions being As, which showed a significant increase after the AA reactions, and chlorides and sulphates. The eluate collected from paste P2B improved upon the performance showed by the P0 paste, although elements Cr and Pb presented slightly higher solubility, above even the values presented by the original MT. Nevertheless, the addition of FA resulted in increased  
Figure 11: SEM micrographs and EDX analysis of pastes Pref (**a**), P1A (**b**), P2B (**c**), and P3B (**d**) (EDX data summarised in Table 5).  
performance and complied with the most severe limits imposed by the European standard (_inert_ waste disposal).  
The solubility of paste P2B was evaluated for up to 28 days (readings taken after 3, 7, 14, and 28 days) by submerging a monolith-type specimen in deionised water (_LTT2_ test). The leaching solution was not refreshed after each sample collection, meaning that the leachate ions were able to accumulate. The decision of not refreshing the solution was made to enable the detection of a potential leaching asymptotic level, as described by [PERSON] et al. [51]. The results are presented in Figure 13, together with the threshold values for non-hazardous waste.  
The pH of the solution decreased with time (Table 6), which could be a consequence of the consumption of OH\({}^{-}\) ions during the dissolution of Si and Al species [52], reinforced by the reduced amorphous calcium content [53]. [PERSON] et al. [16] obtained a pH reduction throughout 360 days, from 12.5 to 11.2, of copper-zinc tailings stabilised with sodium-silicate-activated slag, which was attributed to the formation of acid via the oxidation of pyrite. Although in this case the pH reduction occurred in different curing conditions (20 \({}^{\circ}\)C and 85% RH), a similar origin (py  
Figure 12: Element concentrations in the leaching solution (test _LTT1_) obtained from paste P2B and in comparison with the acceptance limits defined in 2003/33/EC.  
Figure 13: Concentration evolution in the solution where the paste P2B was submerged (_LTT2_), compared with the waste acceptance limits (2003/33/EC) for non-hazardous waste.  
The leaching LTT2 test results were compared with the soluble SO3 values measured directly from the water collected in the pond and from a sample of MT that was submerged in water for 28 days upon arrival to the laboratory (Figure 14). The value obtained after 28 days is comprehensively lower that the value obtained from the pond, while the value obtained in the LTT2 test is even lower (approximately 45%) than the 28-day solubility, confirming the effectiveness of the stabilisation in terms of environmental performance. The figure also presents the conductivity measured in the same samples. In this case, the value measured in the water from the LTT2 test was significantly increased relative even to the pond water. This is most likely explained by the addition of fly ash, which generated a conductivity value in the submersion water higher than that in the water from the MT.