Synthesis and morphology of slag-based alkali-activated materials

Large volumes of slags, with varying mineralogical and geochemical characteristics, are produced annually by the ferrous and nonferrous metallurgical industry. Most of them are considered as potential secondary resources and may be valorized, following the principles of circular economy, for the recovery of critical elements, including energy transition metals. Apart from the economic benefits, valorization of slags may contribute to the competitiveness of the metal industry and the conservation of natural resources. Valorization also results in minimization of environmental impacts on soils and waters in slag disposal areas, due to the prevention of solubilization and subsequent migration of potentially hazardous elements. The present study utilizes ferronickel (FeNi) slag for the production of alkali-activated materials. The effects of the main experimental parameters, including sodium hydroxide (NaOH) molarity, silica/sodium oxide (SiO2 /Na2O) molar ratio in the activating solution, precuring and curing time, curing temperature and aging period on several properties of the final products, are assessed. Background Fig. 1 Evolution of compressive strength of produced AAMs versus NaOH molarity as a function of curing temperature (20, 40 and 80 °C). Precuring and curing times = 24 hours. Error bars indicate the standard deviation of three measurements. Alkali activation may be used for the valorization of several industrial wastes, including sludges, process tailings and slags. The produced materials are partially amorphous, have beneficial properties and may be used as construction elements or as binders to partially substitute Portland cement. They are termed alkali-activated materials (AAMs), geopolymers or inorganic polymers (IPs). Alkali activation involves the reaction of (calcium ferro-)aluminosilicate precursors with strong alkaline solutions for the production of a reactive paste. This paste after curing for a few hours or days at relatively low temperatures of 20 to 90 °C and aging for seven to 28 days enables the production of specimens with desirable physicochemical and mechanical properties. Depending on the byproduct (waste) type, the mineralogy and, more specifically, the composition of the amorphous phase (for example, Si/Al ratio or (Ca+Fe)/Si ratio), the grain size and the type of alkali activating solution, the synthesis conditions need to be optimized to produce the desired AAMs. Methodology The slag (LS) used in this study is a byproduct of FeNi production from Greek laterites. Its chemical composition in the form of oxides, as well as the particle size of the ground slag, as indicated by the d90 and d50 percentile values, are shown in Table 1. Fig. 2 Back-scattered electron images of cross-sections of selected AAMs produced after curing at (a) 20 °C and (b) 80 °C.The operating conditions were NaOH molarity 8 M, SiO2/Na2O molar ratio in solution 1, precuring and curing times 24 hours and aging period seven days. EDS spectra show in several spot locations the presence of oxides and the formation of reaction products (Fo: forsterite, D: diaspore, Fa: fayalite, An: anorthite). NaOH and sodium silicate (Na2SiO3) were used as activating solutions. For the synthesis of AAMs, LS was mixed slowly for 10 min with the activating solution and the produced homogeneous paste was cast in plastic cubic molds, with an edge of 5 cm. The molds were left at room temperature for 24 to 96 hours for precuring to enable initiation of alkali activation reactions and sufficient hardening of the obtained paste. The specimens were then placed in an oven for curing at 40 and 80 °C for 24 hours. Aging of all specimens was carried out at room temperature for seven, 28 or 96 days. Table 2 shows the experimental details of the tests carried out. Several analytical characterization techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), were applied to study the characteristics of the raw material and the produced AAMs. The main parameters determined to assess the quality of the final products were compressive strength, density, porosity and water absorption. Results and discussion Figure 1 presents the evolution of the compressive strength of the produced AAMs versus NaOH molarity (6 to 10 M) as a function of curing temperature (20, 40 and 80 °C). It is seen that increasing the curing temperature from 20 to 80 °C substantially improves the compressive strength of the produced AMMs for all molarities tested. The optimum NaOH molarity is 8 M, while the maximum compressive strength obtained after curing at 80 °C for an aging period of seven days is 84 MPa. SEM back-scattered electron (BSE) images of selected AAMs are shown in Fig. 2. More specifically, SEM/EDS microscopic examination of AAMs produced after curing at 20 °C revealed the presence of several microcracks (smaller than 10 μm) in the polymeric matrix as well as a heterogeneous and porous structure with large unreacted/undissolved slag particles (larger than 20 μm) exhibiting sharp edges (Fig. 2a). On the other hand, the microstructure of the LS-based AAMs produced after curing at 80 °C exhibits a glassy and smooth surface with no visible cracks and defects (Fig. 2b). Conclusion Ferronickel slags can be successfully alkali activated. The maximum compressive strength of the produced AAMs exceeds 80 MPa. The valorization of metallurgical slags for the production of AAMs, apart from the obvious environmental benefits, is also in line with various United Nations Sustainable Development Goals (SDGs), including SDGs 11, 12 and 13. Funding The study received funding in the frame of ENICON project, “Sustainable processing of Europe’s low grade sulphidic and lateritic Ni/Co ores and tailings into battery grade metals,” https://enicon-horizon.eu/, from the European Union’s Framework Programme for Research and Innovation Horizon Europe under Grant Agreement No. 101058124. References A list of all references is available in the full paper.

Synthesis and morphology of slag-based alkali-activated materials

Authored by: Konstantinos Komnitsas, Arne Peys, Stylianos Tampouris, Vasiliki Karmali, Georgios Bartzas, Dimitra Vathi and Anna Kritikaki

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