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Valorization of mining and metallurgical wastes through alkali activation

Karmali Vasiliki

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Summary

The present PhD thesis aims to study in depth the alkali activation potential of metallurgical and mining wastes, including various types of metallurgical slags and laterite leaching residues, for the production of alkali activated materials (AAMs) with desirable chemical and mineralogical composition, and beneficial properties. AAMs were produced under the optimum synthesis conditions using activating solution consisting of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The effect of various parameters, namely H2O/Na2O and other molar ratios present in the activating solution and the reactive paste, particle size of raw materials, curing temperature and ageing period on the main properties of the produced AAMs, including compressive strength, porosity, water absorption and density was also evaluated. The durability and structural integrity of selected AAMs were investigated after firing at temperatures between 200 and 1000 oC for 4 or 6 h, immersion in distilled water and acidic solutions (1 M HCl and 1 M H2SO4), and subjection to freeze-thaw cycles for a period of 7-30 days. Several analytical techniques, namely X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy and Scanning electron microscopy (SEM-EDS) were used for the identification of the morphology and structure of the raw materials and the produced AAMs. Regarding alkali activation of a Polish ferronickel slag (PS), it is underlined that the AAMs produced acquired compressive strength that exceeds 65 MPa, while after firing at 400 oC, the compressive strength increased to 115 MPa, and this indicates that they can be used as fire-resistant materials. On the other hand, the AAMs produced from fayalitic slags resulted before and after plasma treatment, i.e. FS and FSP, acquired lower compressive strength values, namely 45 MPa and 27 MPa, respectively, while mixing them with 50 wt% ferronickel slag (LS) resulted in higher compressive strength, namely 64 MPa (FS50LS50 AAM) and 46 MPa (FSP50LS50 AAM), respectively.An innovative aspect of this PhD thesis is the production of AAMs from laterite leaching residues, since until the start of the study no references were available in literature on this topic. This by-product has very low inherent alkali activation potential. The experimental results proved that when the leaching residues were mixed with metakaolin or slags they were successfully alkali activated and under the optimum synthesis conditions the produced specimens acquired compressive strength values ranging between 26 MPa and 51 MPa.The results of this PhD thesis prove that the valorization or co-valorization of metallurgical wastes is a viable alternative for the production of AAMs with beneficial properties that can be used as binders or in several construction applications, thus improving the sustainability of this industrial sector following the principles of industrial symbiosis and circular economy.

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