Promotion of circular economy: steelwork dusts as secondary raw material in conventional mortars.

Among the actions proposed by the European Union for the implementation of Circular Economy is the use of waste as a secondary raw material (SRM). During the fusion of the scrap, a steel dust is generated, named electric arc furnace dust (EAFD). The EAFD is composed mainly of potentially leachable heavy metals and is classified as a "hazardous" waste. Worldwide, approximately 70% of EAFD is deposited in landfills, with a previous treatment through cement-based materials to prevent the metals' mobility. However, this action is not in accordance with the Circular Economy concept. The present investigation analyses the use of EAFD as SRM in conventional mortar production for its use as a construction material. Different substitution percentages (25, 50 and 100%) were used replacing the siliceous filler by EAFD. A preceding characterisation of the waste by X-ray fluorescence, X-ray diffraction, specific surface area, bulk density, electron microscopy and particle size distribution was performed. The investigation analysed the behaviour of conventional mortars by tests of workability, compressive strength, mineralogy, water absorption by capillarity, and leaching behaviour in granular and monolithic states. The results obtained indicate a slight improvement in mechanical behaviour with the incorporation of EAFD, the reason why its use as SRM in conventional mortars would benefit the construction industry and would encourage the Circular Economy. From an environmental point of view, the mechanisms of Pb fixation should be improved in a granular state.

A Double Barrier Technique with Hydrotalcites for Pb Immobilisation from Electric Arc Furnace Dust.

A new line of mortars incorporating hydrotalcites was developed. This research article shows the results of a study of a double barrier technique (DBT) for Pb immobilisation from electric arc furnace dust (EAFD) in mortars with the addition of three different hydrotalcites (H1, H2, and H3). Electric arc furnace dust (EAFD) is a hazardous waste due to its heavy metal leachability. The aim was to obtain a mortar in which, due to its chemical composition, heavy metal leaching satisfied environmental criteria. Previously, a physical and chemical characterisation of mortar material components was carried out. The leaching behaviour of Pb from EAFD in double barrier (DB) mortars with different hydrotalcites was analysed for compressive strength to determine treatment effectiveness. DB mortars could be considered monoliths because their compressive strengths were higher than 1 MPa but exhibited a decrease due to hydrotalcite incorporation. The mortar EAFD25_H2 (with ethylenediaminetetraacetate (EDTA) in the interlayer of the hydrotalcite) showed one minor reduction in compressive strength with respect to the reference mortar because formation of Portlandite was observed, which is a characteristic of cement hydration. The conventional immobilisation mortar (EAFD25) did not achieve Pb immobilisation. However, DB mortars with dimercaptosuccinate (DMSA) in the interlayer of the hydrotalcite reduced Pb release by ~50%, from 20.29 mg kg-1 (EAFD25) to 9.88 mg kg-1 (EAFD25_H3). In addition, EAFD25_H3 included the lowest hydrotalcite content, thereby improving the immobilisation ratio. The results of this study contribute to better Pb immobilisation, thus satisfying environmental criteria.