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1.
Environmental and biological impact of fly ash and metakaolin-based alkali-activated foams obtained at 70°C and Fired at 1,000°C
Cristina Leonelli, Janez Turk, Giovanni Dal Poggetto, Michelina Catauro, Katja Traven, Alenka Mauko Pranjić, Vilma Ducman, 2022, original scientific article

Abstract: Alkali-activated foams (AAFs) are inorganic porous materials that can be obtained at temperatures well below 100° C with the use of inorganic wastes as aluminosilicate precursors. In this case, fly ash derived from a Slovenian power plant has been investigated. Despite the environmental benefits per se, due to saving of energy and virgin materials, when using waste materials, it is of extreme importance to also evaluate the potential leaching of heavy metal cations from the alkali-activated foams. This article presents an environmental study of a porous geopolymer derived from this particular fly ash, with respect to the leachability of potentially hazardous elements, its environmental toxicity as determined by biological testing, and the environmental impact of its production. In particular, attention was focused to investigate whether or not 1,000 °C-fired alkali- activated fly ash and metakaolin-based foams, cured at 70 °C, are environmentally friendlier options compared to unfired ones, and attempts to explain the rationale of the results were done. Eventually, the firing process at 1,000 ° C, apart from improving technical performance, could reinforce heavy metal cation entrapment within the aluminosilicate matrix. Since technical performance was also modified by addition of different types of activators (K-based or Na-based), as well as by partial replacement of fly ash with metakaolin, a life cycle assessment (LCA) analysis was performed to quantify the effect of these additions and processes (curing at 70 ° C and firing at 1,000 °C) in terms of global warming potential. Selected samples were also evaluated in terms of leaching of potentially deleterious elements as well as for the immobilization effect of firing. The leaching test indicated that none of the alkali-activated material is classified as hazardous, not even the as-received fly ash as component of new AAF. All of the alkali-activated foams do meet the requirements for an inertness. The highest impact on bacterial colonies was found in samples that did not undergo firing procedures, i.e., those that were cured at 70 °C, which induced the reduction of bacterial Enterococcus faecalis viability. The second family of bacteria tested, Escherichia coli, appeared more resistant to the alkaline environment (pH = 10–12) generated by the unfired AAMs. Cell viability recorded the lowest value for unfired alkali-activated materials produced from fly ash and K-based activators. Its reticulation is only partial, with the leachate solution appearing to be characterized with the most alkaline pH and with the highest ionic conductivity, i.e., highest number of soluble ions. By LCA, it has been shown that 1) changing K-based activators to Na-based activators increases environmental impact of the alkali-activated foams by 1%–4% in terms of most of the impact categories (taking into account the production stage). However, in terms of impact on abiotic depletion of elements and impact on ozone layer depletion, the increase is relatively more significant (11% and 18%, respectively); 2) replacing some parts of fly ash with metakaolin also results in relatively higher environmental footprint (increase of around 1%–4%, while the impact on abiotic depletion of elements increases by 14%); and finally, 3) firing at 1,000°C contributes significantly to the environmental footprint of alkali- activated foams. In such a case, the footprint increases by around one third, compared to the footprint of alkali-activated foams produced at 70 ° C. A combination of LCA and leaching/toxicity behavior analysis presents relevant combinations, which can provide information about long-term environmental impact of newly developed waste-based materials.
Keywords: alkali activated materials, geopolimers, leaching, LCA
Published in DiRROS: 20.06.2023; Views: 255; Downloads: 139
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2.
Sustainable alkali-activated slag binders based on alternative activators sourced from mineral wool and glass waste
Majda Pavlin, Katja Koenig, Jakob Koenig, Uroš Javornik, Vilma Ducman, 2022, original scientific article

Abstract: In the present study, four different locally available waste glass materials (bottle glass-BG, glass wool-GW, stone wool-SW and cathode-ray tube glass-CRTG) were treated with hot concentrated potassium hydroxide (KOH) in order to obtain alternative alkali activators (AAAs). We evaluated the suitability of the solutions obtained for use as AAAs in the production of AAMs. AAMs were prepared using electric arc furnace slag and selected AAAs with a higher content of dissolved Si. We evaluated the performance of the AAMs in comparison to that of slags activated with KOH or potassium-silicate (K-silicate). The compressive strength of the AAMs prepared with KOH-based AAAs were high when Si and Al were simultaneously abundant in the AAA (9.47 MPa when using the activator sourced from the CRTG), and low with the addition of KOH alone (1.97 MPa). The AAM produced using commercial K-silicate yielded the highest compressive strength (27.7 MPa). The porosity of the KOH-based AAM was lowest when an alternative BG-based activator was used (24.1%), when it was similar to that of the AAM prepared with a K-silicate. The BG-based activator had the highest silicon content (33.1 g/L), and NMR revealed that Si was present in the form of Q0, Q1 and Q2. The concentrations of toxic trace elements in the AAAs used for alkali activation of the slag were also determined, and leaching experiments were performed on the AAMs to evaluate the immobilisation potential of alkali-activated slag. In the SW AAAs the results show acceptable concentrations of trace and minor elements with respect to the regulations on waste disposal sites, while in the activators prepared from BG, CRTG and GW some elements exceeded the allowable limits (Pb, Ba, Sb, and As).
Keywords: alkali activated materials/geopolymers, alternative activators, NMR, leaching
Published in DiRROS: 08.06.2023; Views: 296; Downloads: 184
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3.
Alkali activation of metallurgical slags : reactivity, chemical behavior, and environmental assessment
Isabella Lancellotti, Federica Piccolo, Katja Traven, Mark Češnovar, Vilma Ducman, Cristina Leonelli, 2021, original scientific article

Abstract: Alkali-activated materials (AAMs) represent a promising alternative to conventional building materials and ceramics. Being produced in large amounts as aluminosilicate-rich secondary products, such as slags, they can be utilized for the formulation of AAMs. Slags are partially crystalline metallurgical residues produced during the high temperature separation of metallic and non-metallic materials in the steelmaking processes. In the present study, the electric arc furnace carbon or stainless steel slag (EAF) and secondary metallurgical slag such as ladle furnace basic slag (LS) were used as precursors in an alkali-activation process. EAF slag, with its amorphous fraction of about 56%, presented higher contents of soluble Si and Al species with respect to ladle slag R (35%). However, both are suitable to produce AAM. The leaching behavior shows that all the release values are below the regulation limit. All the bivalent ions (Ba, Cd, Cu, Ni, Pb, and Zn) are well immobilized in a geopolymeric matrix, while amphoteric elements, such as As and Cr, show a slight increase of release with respect to the corresponding slag in alkaline and aqueous environments. In particular, for Sb and As of AAM, release still remains below the regulation limits, while Mo presents an increase of leaching values that slightly exceeds the limit for landfill non-dangerous waste.
Keywords: slag, aluminosilicate materials, chemical reactivity, cold consolidation, alkali activation, leaching test, open access
Published in DiRROS: 22.05.2023; Views: 296; Downloads: 154
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4.
Impact of leaching on chloride ingress profiles in concrete
Alisa Machner, Marie Helene Bjørndal, Aljoša Šajna, Nikola Mikanovic, Klaartje De Weerdt, 2022, original scientific article

Abstract: To investigate the effect of leaching on chloride ingress profiles in concrete and mortar, we exposed concrete and mortar specimens for 90 and 180 days to two different exposure solutions: 3% NaCl, and 3% NaCl with KOH added to limit leaching. The solutions were replaced weekly. After exposure, we determined total chloride profiles to investigate the chloride ingress, and portlandite profiles to assess the extent of leaching. The results showed that leaching during exposure greatly affects the chloride ingress profiles in mortar and concrete. We found that leaching leads to considerably higher maximum total chloride content and deeper chloride penetration into the concrete than in the specimens where leaching was limited. We recommend therefore that leaching should be taken into account in standard laboratory testing and that more mechanistic service life models should be used to take into account the impact of leaching.
Keywords: chloride ingress, service life prediction, leaching, concrete, portlandite, open access
Published in DiRROS: 04.05.2023; Views: 262; Downloads: 163
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5.
Leaching and geochemical modelling of an electric arc furnace (EAF) and ladle slag heap
Mojca Loncnar, Ana Mladenovič, Vesna Zalar Serjun, Marija Zupančič, Hans A. van der Sloot, 2022, original scientific article

Abstract: Old metallurgical dumps across Europe represent a loss of valuable land and a potential threat to the environment, especially to groundwater (GW). The Javornik electric arc furnace (EAF) and ladle slag heap, situated in Slovenia, was investigated in this study. The environmental impact of the slag heap was evaluated by combining leaching characterization tests of landfill samples and geochemical modelling. It was shown that throughout the landfill the same minerals and sorptive phases control the leaching of elements of potential concern, despite variations in chemical composi- tion. Although carbonation of the disposed steel slags occurred (molar ratio CO3/(Ca+Mg) = 0.53) relative to fresh slag, it had a limited effect on the leaching behaviour of elements of potential concern. The leaching from the slag heaps had also a limited effect on the quality of the GW. A site-specific case, however, was that leachates from the slag heap were strongly diluted, since a rapid flow of GW fed from the nearby Sava River was observed in the landfill area. The sampling and testing approach applied provides a basis for assessing the long-term impact of release and is a good starting point for evaluating future management options, including beneficial uses for this type of slag.
Keywords: EAF slag, field verification, geochemical modelling, ladle slag, leaching, release prediction, steel slag heap
Published in DiRROS: 28.04.2023; Views: 327; Downloads: 173
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6.
Assessment of natural zeolite clinoptilolite for remediation of mercury-contaminated environment
Ivona Nuić, Mateja Gosar, Marin Ugrina, Marina Trgo, 2022, original scientific article

Abstract: The soil at ancient roasting sites in the surroundings of the Idrija mine (Slovenia) is highly contaminated with mercury. To assess the impact of mercury on groundwater by infiltration and find an eco-friendly remediation method, the leaching of mercury from the soil containing 1347 mg Hg/kg, followed by sorption of the total leached mercury on cost-effective natural zeolite (NZ) clinoptilolite, was performed. The leaching of soil in ultrapure water of pHo = 3.00–11.46 after 24 h resulted in the total leached mercury concentration in the range 0.33–17.43 µg/L. Much higher concentrations (136.9–488.0 µg/L) were determined after the first few hours of leaching and were high above the maximum permissible level in water for human consumption. The NZ showed very good sorption of the total leached mercury, with a maximum removal efficiency of 94.2%. The leaching of mercury in presence of the NZ resulted in a significant decrease of the total leached mercury (1.9–20.3 µg/L compared to 12.8–42.2 µg/L), with removal efficiencies up to 90.5%, indicating immobilization of mercury species. The NZ has a great potential for economically viable remediation of mercury-contaminated environment. However, efforts should be made in the further study of mercury leachability to reduce the mercury concentration in water to acceptable levels.
Keywords: Idrija mine, mercury-contaminated soil and water, leaching, natural zeolite clinoptilolite, remediation
Published in DiRROS: 23.06.2022; Views: 540; Downloads: 431
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