## 4 Discussion  
The MT recovered from the Neves Corvo operation is a very interesting material in terms of density and mechanical behaviour, thus showing high potential for the construction industry. However, it bears a prohibitive environmental toll that needs to be significantly mitigated before it can be used. This motivation is also supported by the financial and environmental costs that are associated with the current landfilling of MT. Although the MT was successfully activated with a 12.5 molal sodium hydroxide solution, without the addition of a second (and more effective) precursor or a thermal pretreatment, it still presented a low UCS, and its metal ion immobilisation was less than ideal. The reaction products included sodium sulphate carbonate in the form of burkeite (Na6(CO3)(SO4)2). In order to improve both the mechanical and environmental performance, a second residue (FA) was added to the MT as a precursor. The results showed that an increase in the FA/MT ratio produced only a slight strength improvement, justifying the choice of an average FA/MT ratio (which favours the disposal of MT) for the environmental tests. The most influential variable on UCS was the activator's NaOH concentration, with the 15 molal paste showing the highest strength. However, the values \begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**Collection Period**} \\ \hline
**3 d** & **7 d** & **14 d** & **28 d** \\ \hline
11 50 & 11 03 & 10 72 & 10 08 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: pH of the solution at the time of sampling (_LTT2_). Figure 14: Comparison of the soluble SO3 and electrical conductivity measured in the water from the mine ponds, in the water where the original MT and FA were submerged for 28 days, and in the water where a P2B monolith was submerged for 28 days. obtained with concentrations below 15 molal are considered acceptable for several civil engineering applications (e.g., road base and sub-base; structural embankments; backfill material); thus, paste P2B (12.5 molal) was chosen for environmental testing. The FTIR spectra showed that S-O bonds present in the original (anhydride) MT were broken in three of the four pastes tested (including P2B), suggesting the release of sulphur ions during the dissolution stage of the reactions. X-ray diffractograms detected the presence of burkeite in the pastes, a new mineralogical phase which is essentially sodium sulphate, as a result of the combination of the sulphate and sodium ions. Based on the SEM/EDX data, direct activation of the MT (i.e., without FA) produced N-A-S-H gel, responsible for the strength development showed by this paste. The same gel was also developed in the FA/MT pastes. Since alkalis could neutralise the acidic environment generated by the oxidation, the lower the alkali content, the faster it became depleted during this neutralisation process; thus, a lower activation degree of the FA was achieved. Therefore, the dissolution process was taken further in the P2B paste than in any of the remaining pastes. Additional evidence of this behaviour was obtained when measuring the pH of the solution used for the _LTT2_ test, performed during 28 days on a P2B specimen (Table 6), since the pH reduction can be explained by the progressive oxidation of pyrite. A comparison between both types of leachate tests revealed some similarity in terms of aggressiveness, since the values registered with the _LTT1_ test (which takes approximately 24 h to develop) are, in general, similar to the values obtained after 3 days in the _LTT2_ test. The solubility of the Cr, Cu, Pb, and Zn ions did not change significantly between 3 and 14 days, suggesting a rapid surface removal of these species after only 3 days. The exception to this trend was As, which presented a relevant solubility increase between 3 and 14 days, up to a point where even the 2003/33/EC normative As threshold for _non-hazardous_ waste was exceeded.