## 4 Discussion
After 28 days, the solubility increase was even more dramatic and, in this case, occurred also with the Pb and Zn ions. This process is probably governed by diffusion [54], although a possible failure of the physical encapsulating structure (i.e., local breakage that exposed some previously protected ions) should not be ignored [55]. The As leached from the original MT was below the thresholds proposed for inert, non-hazardous, and hazardous waste. However, after activation (P0), the increase in As solubility was very significant, surpassing not only the _inert_ limit but also the _non-hazardous_ limit. The difficulty in fixating As ions in aluminosilicate matrixes is well known [54,56], although this behaviour tends to improve with time [38,57]. It is related with the oxidation of the As ion, capable of assuming different oxyanoin forms, which possess very high solubility in alkaline environments but, nevertheless, are not bound to the final aluminosilicate matrix [57]. Therefore, it is possible that the increased alkalinity, as a result of the addition of the activator, favoured the release of As ions. This increased arsenic solubility was reverted again for paste P2B, suggesting that the addition of a more effective precursor to the original MT (fly ash) resulted in the production of a higher volume of AA gel, which was then capable of containing the As ions more effectively than paste P0. It is somewhat worrying that the _LTT1_ test, which is strongly based on standard EN 12457-4, was not able to detect the solubility levels of this particularly threatening element, revealed only by the _LTT2_ test, although it is reasonable to assume that the _LTT2_ conditions might be too extreme and, thus, hardly representative of the real field environment. Regarding the immobilization of the Cr, Cu, Pb, and Zn species, with all of them showing leachate values below the normalized limits, [PERSON] et al. [50] sustained that geopolymers can immobilise lead ions through a combination of physical encapsulation and chemical bonding. [PERSON] et al. [55] observed a similar combination when studying the geopolymer immobilisation of Cr, Cd, and Pb, and concluded that the predominant mechanism depends on the solubility level of the heavy metal ions, i.e., highly soluble salts are more spread throughout the matrix, while less soluble salts form higher-volume nuclei which are then involved by the gel matrix. However, [PERSON] et al. [58] showed that the inclusion of such metals like Cr and Pb in cementitious gel may decline in the presence of chloride ions (due to their interaction and subsequent formation of soluble chlorides), which would then increase their leaching. Studies regarding the immobilisation of Cr in alkali-activated fly ash systems have shown that this ion has a negative effect on the activation mechanism and, subsequently, on the hardening process, and is thus not very efficiently contained in the geopolymeric matrix [55; 59]. Nevertheless, the maximum Cr solubility values found in the present work are well under the _non-hazardous_ waste limit established in the 2003/33/EC normative. The above-mentioned papers also concluded that Pb is very effectively immobilised in aluminosilicate gel, which is in accordance with the present data, even if a substantial increase was obtained between 14 and 28 days. The leaching potential of some inorganic ions (Ca, Fe, K, Mg, and Na) not listed in the acceptance criteria defined in the normative 2003/33/EC are presented in Table 7. Decreasing release rates were registered for the Ca, Mg, and Na species as a consequence of their fixation, while the Fe and K species showed increasing solubility, most likely due to the oxidation of pyrite. It is noteworthy that the test was \"static\" (water was not changed after sampling), which allowed the saturation of the leachant and subsequent ion precipitation.