## 6 Concluding Remarks  
About 240 kt/year and 474 kt/year of IBA are produced in 4 WtE plants in Portugal and 12 in Spain, respectively. IBA comprises a broad particle size distribution: finest particles include most soluble salts and potentially leachable heavy metal(loid)s, while the coarser parts mainly contain ceramics and glass. Nevertheless, natural weathering (2-3 months in the open) significantly reduces the leaching potential. IBA is considered non-hazardous in the Iberian countries. The utilization rate and criteria diverge in Portugal and Spain, as well as within these countries. Around 10% of Portuguese IBA and 41% in Spanish IBA are still landfilled. Around 15 kt (6% of IBA) and 40 kt (9% of IBA) of metals have been recycled in Portugal and Spain, respectively. The remaining Portuguese IBA (84%) is either temporarily stored (11%), used in landfills for soil substitution in intermediate and final covers, construction of paths, accesses, and platforms (41%), or in civil engineering work and road construction (48%). The remaining Spanish IBA (50%) is mainly used as secondary material in civil engineering or road construction (77%), while only 4% is used in the conditioning of landfills and their surroundings and 19% is temporarily stored, awaiting future reuse. Both countries regulate the use of mineral IBA outside landfills, but differently. In Portugal, the National Civil Engineering Laboratory issues an individual permit to one processing plant (Valorsul). There is no specific legislation for IBA. The material (40%) may be used in earthmoving works (landfills and pavement beds), layers/sub-base layers of roads, and landscape recovery of quarries or open-pit mines as filling material. In Spain, the use of IBA outside landfills (38%) is only regulated in Catalonia and Cantabria, following different criteria but based on legislation specific to IBA. In Catalonia, IBA is permitted, e.g., in road subbase, leveling of terrain and embankments, filling, and restoration of degradable areas from extractive activities. In Cantabria, IBA can only be used as a secondary aggregate in road construction or concrete formulation. Natural resources (e.g., aggregates) have been increasingly consumed, and the use of IBA can contribute to a real circular economy. The development of well-defined \"end-of-waste\" criteria for IBA could be very relevant in EU countries, and Iberian countries in particular, favoring reuse while guaranteeing environmental protection. The \"mirror entry\" classification (LoW) of IBA also creates difficulties and divergences in reuse. The assessment of the HP 14 \"ecotoxic\" is the main challenge in the EU (including Iberia). A standard method for this assessment is essential, and according to the literature, after natural weathering of IBA, it is likely to be non-hazardous. In the future, alternative applications will be increasingly studied, while ensuring environmental protection. Iberian countries would benefit if clear rules were proposed at the EU level. Data collection, B.S.B., M.J.Q., A.M.-A. and J.M.C. ; supervision, J.L.P., R.C.M., M.J.Q. and J.M.C. ; writing--original draft, B.S.B., A.M.-A. and J.M.C. ; writing--review and editing, B.S.B., A.M.-A., J.L.P., R.C.M., M.J.Q. and J.M.C. All authors have read and agreed to the published version of the manuscript. This research was funded by FCT/MCTES through UIDB/00102/2020, UIDP/50017/2020 + UIDB/50017/2020, and SFRH/BD/147920/2019. Thanks are due to FCT/MCTES for their financial support to CIEPQPF (UIDB/ 00102/2020) and CESAM (UIDP/50017/2020 + UIDB/50017/2020) through national funds. B.S.B. is the recipient of an individual research grant by the FCT/MCTES with the reference SFRH/BD/ 147920/2019, through national funds and ESF (European Social Fund). J.M.C. and A.M.-A. are also grateful to the Catalan government for the DIOPMA (2017 SGR 118) quality accreditation awarded to their research group. The Portuguese WtE plants (Valorsul, LIPOR, ARM, and Teramb) are acknowledged for providing the data we requested.