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In order to increase the utilization rate of stainless steel slag, reduce storage needs, and mitigate environmental impacts, this study replaces a portion of limestone with varying amounts of stainless steel slag in the calcination of Portland cement clinker. The study primarily examines the influence of stainless steel slag on the phase composition, microstructure, compressive strength, and free calcium oxide (ƒ-CaO) content of Portland cement clinker. The results show the following: (1) Using stainless steel slag to calcine Portland cement clinker can lower the calcination temperature, reducing industrial production costs and energy consumption. (2) With an increase in the amount of stainless steel slag, the dicalcium silicate (C2S) and tricalcium silicate (C3S) phases in Portland cement clinker initially increase and then decrease; the C3S crystals gradually transform into continuous hexagonal plate-shaped distributions, while the tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) crystal structures become denser. When the stainless steel slag content is 15%, the dicalcium silicate and tricalcium silicate phases are at their peak; the C3S crystals are continuously distributed with a relatively dense structure, and C3A and C4AF crystals melt and sinter together, becoming distributed around C3S. (3) As stainless steel slag content increases, the compressive strength of Portland cement clinker at 3 days, 7 days, and 28 days increases and then decreases, while ƒ-CaO content decreases and then increases. When the stainless steel slag content is 15%, the compressive strength at 28 days is at its highest, 64.4 MPa, with the lowest ƒ-CaO content, 0.78%. The test results provide a basis for the utilization of stainless steel slag in the calcination of Portland cement clinker.

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The complete detoxification and resource utilization of stainless steel slag are still facing challenges. In this paper, the harmless treatment of industrial stainless steel slag was realized by using a high temperature modification-crystallization control approach, and the influence of binary basicity (B) on selective enrichment of Cr-containing spinel in the slag and corresponding detoxification effect were systematically explored. Results demonstrated that the enrichment degree of Cr and the amount of Cr-bearing spinel separately reached 98.62 wt% and 13.32 wt% when B= 1.2. The reason was that low alkalinity promoted the reaction of Cr in slag to form Cr-containing spinel, thus improving the enrichment of Cr in the spinel and reducing the occurrence probability of Cr in the matrix phase. Furthermore, the toxicity test of modified slag powder was proceeded according to Chinese standard HJ/T 299-2007 and American standard EPA (Toxicity Characteristic Leaching Procedure, TCLP), and the feasibility of using modified slags as admixture for producing cement was discussed. Even if TCLP was adopted, the Cr leaching concentration in modified slag powder with B= 1.2 was only 1.59 mg/L, which was far lower than the national limit of heavy metal leaching concentration (15 mg/L) of solid waste. Remarkably, when the powder was formed as cement, the Cr leaching concentration was even lower than ICP-OES detection line. Meanwhile, its mechanical property was better than that of cement prepared without powder, indicating the detoxified slag powder met the requirements of concrete admixture. This paper provides a new way with certain economic value for detoxification and large-scale utilization of stainless steel slag.

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This study presents an economical and efficient method to decolourise dye wastewater using industrial waste stainless steel slag (SSS). Titanium dioxide was immobilised on SSS by a precipitation-calcination method. Samples with different TiO2 loadings (prepared using either titanium isopropoxide precursor or commercial TiO2 nanoparticles) were used to decolourise an organic contaminant (methylene blue) under dark and UV conditions in aqueous solution, and their adsorption and photocatalytic performances were compared. Samples with 15 and 25 TiO2 wt% prepared by the precursor method had normalised photocatalytic efficiencies per gram close to that of bare TiO2; using an adsorption-photocatalysis process led to efficiencies 4.4 and 1.6 times higher than that of pure TiO2. The improvement in catalytic performance (greater for samples with less than 50% TiO2 content) may be due to better UV absorption ability (related to with the improvement of TiO2 particle dispersion) and the close TiO2 support interaction, which can eventually cause a photocatalysis-enhancing shift towards more negative oxidation potentials. The SSS also acted as an efficient adsorption trap for organic compounds. The pollutant was thus transferred to the TiO2 surface and photodegraded more rapidly and efficiently. The outstanding synergetic adsorption-photocatalysis capacities of TiO2 waste stainless steel slag composites for dye water treatment made the proposed conversion approach have great potential in practical applications.

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Different stainless steel slags have been successfully employed in previous experiments, for the treatment of industrial acidic wastewaters. Although, before this technology can be implemented on an industrial scale, upscaled pilot experiments need to be performed. In this study, the parameters of the upscale trials, such as the volume and mixing speeds, are firstly tested by dispersing a NaCl tracer in a water bath. Mixing time trials are used to maintain constant mixing conditions when the volumes are increased to 70, 80 and 90 L, compared to the 1 L laboratory trials. Subsequently, the parameters obtained are used in pH buffering trials, where stainless steel slags are used as reactants, replicating the methodology of previous studies. Compared to laboratory trials, the study found only a minor loss of efficiency. Specifically, in previous studies, 39 g/L of slag was needed to buffer the pH of the acidic wastewaters. To reach similar pH values within the same time span, upscaled trials found a ratio of 43 g/L and 44 g/L when 70 and 90 L are used, respectively. Therefore, when the kinetic conditions are controlled, the technology appears to be scalable to higher volumes. This is an important finding that hopefully promotes further investments in this technology.

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This work develops the manufacture of self-compacting concrete (SCC) with 50% cement reduction. As an alternative binder to cement, the viability of using an alkali-activated combination of stainless steel slag (SSS) and fly ash (FA) has been demonstrated. SSS was processed applying three different treatments. Binders were manufactured mixing 35% SSS with 65% FA, as precursors, and a hydroxide activating solution. This binder was replaced by the 50% cement for the manufacture of SCC. The results obtained show good mechanical properties and durability. The study shows a reduction in the use of cement in the manufacture of SCC reusing two wastes.

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Adsorbent materials for the control of dye pollutants in water were synthetized from stainless steel slag (SSS) using different acid-base treatments. Using HCl (SS-Cl) and HNO3 (SS-NO3) produced high-capacity adsorbents, with BET areas of 232 m2/g and 110 m2/g respectively. Specifically, the SS-Cl had a structure of amorphous silica sponge. Treatment with H2SO4 (SS-SO4) did not enhance the adsorption capabilities with respect to the raw sample (SSS). Activated carbon (AC) was also tested as reference. The materials were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 adsorption-desorption isotherms, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) zeta potential, and infrared spectroscopy (FTIR). Batch adsorption experiments with methylene blue (MB) showed that the maximum sorption capacities were 9.35 mg/g and 8.97 mg/g for SS-Cl and SS-NO3 at 240 h, respectively. These values, even at slower rate, were close to the adsorption capacity of the AC (9.72 mg/g). This behavior has been attributed to the high porosity in the range of nanopores (0.6-300 nm) and the high-surface area for both samples. Preferential involvement of certain functional groups in the adsorption of dye ions on their surface indicative of chemisorption has been found. Although optimization, repeatability, and reproducibility of the process and environmental assessment have to be done before practical applications, these preliminary results indicate that application of these cost-effective adsorbents from raw SSS may be used in water pollution treatment and contribute to the sustainable development of the steel manufacturing industry.

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The volume of slags generated from the steel industry is a source of possible resources which is constantly increasing. Specifically, in the production of stainless steel, specific and singular slags with unique characteristics are obtained, which allows considering an approach aimed at their use in new recycling ways. This work shows the feasibility of using stainless steel slag as a substitute for limestone filler in the manufacture of self-compacting concrete. The influence of different treatments applied to slags on physical and chemical properties was studied. On the other hand, the mechanical behaviour, as well as the durability acquired in self-compacting concrete, has been analysed. Very encouraging results were obtained, since this research demonstrates the possible application of this stainless steel slag as a construction material, improving sustainability and promoting circular economy processes, which are achieved through the minimisation of the waste disposal and accumulation.

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Copper slag and stainless steel slag of Electric Arc Furnace (EAF) are two typical metallurgical solid wastes, which contain a large number of valuables, such as Fe, Cr, and Cu. The transition metal elements in the waste slags, such as Cr and Fe, can be recycled as the coloring ions in the black ceramic tile. In this study, the Fe/Cr molar ratio in the raw materials of copper slag and stainless steel slag was adjusted, and the black ceramic tile was subsequently prepared by sintering. The results show that the optimum process parameters for the preparation of black ceramic tiles are the Fe/Cr molar ratio of 2.0, the sintering temperature of 1150 °C, and the sintering time of 30 min. The compressive strength of the black ceramic tile at optimum sintering conditions exceeds the minimum compressive strength of the Chinese national standard for standard polished tiles, and the concentrations of harmful elements, for example, Cr, Cu, Ni, As, Zn, Pb, and Cr(VI) are within the regulation thresholds specified by the Chinese national standard.

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In order to compare the properties of alkali-activated carbon steel slag (CSS) and stainless steel slag (SSS), the effects of sodium hydroxide/sodium silicate solution mass ratio (NH/NS), liquid/solid ratio and blast furnace slag (BFS) dosage on the compressive strength, hydration products and hydration degree of CSS and SSS were studied. Furthermore, a combination of X-ray diffraction (XRD), thermo-gravimetric analysis coupled with differential thermal analysis (TGA-DTA), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope-energy dispersive spectrometer (SEM-EDS) were used to characterize the morphology and structure of alkali-activated CSS-BFS and SSS-BFS cementitious materials. As the results revealed, the primary hydrate of alkali-activated CSS and SSS is C-(A)-S-H with Q2 [SiO4] units, which has a low Ca/Si ratio and includes inert phases like a CaO-FeO-MnO-MgO solid solution (RO) in CSS while cuspidine, magnesiochromite etc. in SSS. More active C3S and ß-C2S promote the alkali activation of CSS, whereas the less active γ-C2S hinders the depolymerization of SSS. The incorporation of BFS does not change the hydrate, whose seed effect is helpful for accelerating the depolymerization and polycondensation of CSS and SSS, especially for SSS, and makes the hydrate increase significantly. Owing to the high SiO2 and Al2O3 contents of SSS, the C-(A)-S-H chain length is increased, thus facilitating the polycondensation effect. In this study, the optimal NH/NS of CSS and SSS is NH/NS= 1:2, and the optimal liquid/solid ratio is 0.29. Compared to CSS-BFS, the C-(A)-S-H gel produced by SSS-BFS has lower Ca/Si and Al/Si ratios. Unlike CSS, pure SSS is inappropriate as an alkali-activated precursor and needs to be co-activated with BFS.

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This paper presents a new soil amendment used for immobilization of soil Pb, produced from vinegar residue, stainless steel slag, and weathered coal. The pH value measuring, granulation and adsorption experiments were carried out to determine the optimal composition of soil amendment. Optimizing soil amendment B2 was composed of vinegar residue, weathered coal (humic acid 61.53 wt%), and stainless steel slag with the ratio of 80∶16∶4, and particle size was in the range of 2-4 mm. In the leaching column experiment, B2 addition reduced the Pb release from the soil as well as increasing leachate pH and decreasing the bioavailable Pb concentration. The leachate Pb concentration decreased with lengthened leaching time under lower pH, but such a phenomenon disappeared in the rebounding period. Compared to control, the DTPA extractable Pb content in soil decreased by 12.41, 13.20, and 8.78% with the B2 addition amount of 1.00, 2.00, and 2.00 wt%, respectively. In addition, the total Pb content of each soil layer generally rose as B2 addition increased. It was concluded that application of B2 led to lower transport and transformation of Pb in soil. Based on the single chemical extraction, the environmental risk of Pb was decreased after application of B2. Meanwhile, soil amendment was also a new way to recycle vinegar residue, stainless steel slag, and weathered coal.

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With an aim of producing high value cementitious binder, stainless steel refining slag containing a high amount of CaO in γ-dicalcium silicate form was activated with NaOH and Na-silicate as well as KOH and K-silicate solutions, followed by steam curing at 80 °C. Higher levels of alkali-silicate in the activating solution resulted in higher cumulative heat suggesting accelerated reaction kinetics. With respect to compressive strength, higher levels of alkali silicate resulted in higher strength and the mortars with Na activator were found to have higher early strength than the ones with K activator. The long term strength was found to be similar, regardless of the alkali metal. Thermogravimetric, QXRD and FTIR analyses showed an increase in the amount of reaction products (C-S-H type) over time, further confirming the reactivity of the crystalline slag. Batch leaching results showed lower leaching of heavy metals and metalloids with K activator compared to the Na activator. These results demonstrate that the alkali type and the ratio of hydroxide to silicates have a significant impact on the hydration and mechanical strength development of the stainless steel slag. The above findings can aid in the recycling and valorization of these type of slags which otherwise end up landfilled.

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