## 5 Results and Discussion
### Chemical Characterization (CC) of RA, FA, A1, A2 and A3 Eluate Samples
Regarding acceptability as solid waste in landfills, the raw materials (FA and RA) and concrete mixes (A1, A2 and A3) cannot be considered inert because the TDS concentration values measured in the respective eluate samples (Table 7) are higher than the limit value of 4000 mg/kg defined in the EC Decision 2003/33/EC [24] for inert waste. However, based solely on TDS measurements, they can be considered as non-hazardous waste (Table 6) since the TDS values are lower than the limit values (\(60\times 10^{3}\) mg/kg) defined for non-hazardous waste in the same EC Decision [24]. Regarding the concentration of sulphate (SO4\({}^{2}\)), a sulphur (VI) compound, FA showed the highest value among all the materials (4400 mg/kg), much higher than the limit values defined in the EC Decision 2003/33/EC [24]. Therefore, in terms of the sulphate concentration, it is advisable to use FA in concrete because the solidification process caused by the hydration of this binder is effective on retaining heavy metals, namely sulphate. The sulphate concentration in the remaining samples was close to 30 mg/kg, much lower than the limit values [24]. On what concerns the metallic parameters analysed in the eluate samples (Table 7), of the studied elements, Cd, Hg, Ni, and Pb belong to the list of priority substances in the field of water policy and Cd and Hg are considered priority hazardous substances [48]. From this set of metals evaluated in the RA sample, only Ba presented a concentration value above the limit of quantification (4.0 mg/kg) (Table 7). RA can be considered a non-hazardous waste because the concentrations of all the elements in the respective eluate (Table 7) were lower than the limit values established for non-hazardous waste the EC Decision 2003/33/EC [24] (Table 6). On the contrary, in the FA eluate sample, Ba (4.6 mg/kg), Cr (2.5 mg/kg), Mo (10 mg/kg) and Se (4.0 mg/kg) were detected at concentrations above the quantification limits (Table 7). The concentration values of metals leached from the FA in this study were close to the average values obtained by [PERSON] et al. [45] (determined based on 23 different samples of FA). In the present work, among these metal concentrations, only Se is above the limit value (0.5 mg/kg) defined for non-hazardous waste in EC Decision 2003/33/EC [24] and thus FA is classified as potentially hazardous for the environment (Table 6 and [21]). For the eluate samples from A1, A2, and A3, only Ba, Cr, and Mo showed concentrations above the minimum detection limit (Table 7). A1 eluate presented a Ba concentration (4.0 mg/kg) lower than that of FA (4.6 mg/kg); the remaining metal parameters showed concentration values lower than the detection limit (Table 7). For the A2 sample, the Ba concentration (9.0 mg/kg) was approximately twice the one recorded in the FA sample, while the concentrations of Cr and Mo were 80% and 94% lower in A2 compared with FA, respectively (Table 7). A similar trend was observed for the A3 sample (Table 7). It is worthy of note that the concentration of Se for all three concrete samples was zero or below the detection limit (Table 7). Importantly, the results show that all the metals concentrations in the A1, A2 and A3 eluates were lower than the limit values defined for non-hazardous waste in the landfill EC Decision [24]. Overall, based on the results obtained from the CC, the samples RA, A1, A2, and A3 were classified as non-dangerous and the sample FA as potentially hazardous regarding possible contamination of aquatic environments associated to possible leaching of metals from waste disposal in landfills (Table 6).