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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., (Copyright © 2014 Elsevier B.V. All rights reserved.)

The influence of the Al2O3 level in CaO-SiO2-MgO-Al2O3 (CSMA) stainless steel refining slags on the degradation of magnesia-doloma refractories was investigated through static refractory finger corrosion tests. The tests were performed at 1620 °C under Ar atmosphere, using slags with Al2O3 contents of 5, 10, and 20 wt pct, respectively. The results indicate that the formation of 2CaO·SiO2 at the slag/refractory interface was suppressed by increasing the Al2O3 content to 20 wt pct, thereby changing the corrosion mechanism from an indirect dissolution to a direct dissolution of CaO and MgO from the refractories. The increased solubility limit of MgO by the Al2O3-rich CSMA slags results in an overall higher corrosion rate of the MgO-doloma refractory. [ABSTRACT FROM AUTHOR]

Crystalline argon oxygen decarburization slag, in powdery form, was investigated for its hydration potential by alkali activation and curing at 80°C. Na-silicate and K-silicate of the same modulus were used as activators. Isothermal calorimetry at 80°C indicated exothermic reactions in the slag pastes. When the slag mortars were cured under steam at 80°C appreciable gain in compressive strength was measured. This was attributed to C-S-H which was detected in TG, FTIR, and 29Si NMR analyses. Upon hydration at 90 d, the amount of crystalline phases decreased, whereas the XRD amorphous content in the slag increased. Electron microscopy showed the formation of different morphologies of reaction products depending on the alkaline activator employed. Presence of reaction rims around the crystalline phases with a major presence of Ca, Si, and O in the reacted matrix was observed in elemental maps. [ABSTRACT FROM AUTHOR]

The aim of this work was to investigate the binding potential of non-hydraulic, crystalline continuous casting stainless steel slag, which is a by-product from stainless steel production, by alkali activation with sodium hydroxide and potassium hydroxide. Calorimetric analysis of the activated slag showed low cumulative heat evolution at 20°C. Slag mortars did not harden at this temperature. Considerable enhancements in heat evolution and hardening were observed at 80°C. The resulting hydrated slag showed the formation of calcium silicate hydrate type reaction products and brucite in thermogravimetric analysis. X-ray diffraction analysis showed a reduction in the amount of -dicalcium silicate and periclase. Elemental mapping showed the presence of calcium, silicon and minor amounts of magnesium in the hydrated phase. An optimum activator dosage of 7·5M of sodium hydroxide or potassium hydroxide was found to generate the highest amount of reaction products and compressive strength. [ABSTRACT FROM AUTHOR]

This article investigates the effect of chemical composition and cooling rate during solidification on the mineralogy and hydraulic properties of synthetic stainless steel slags. Three synthetic slags, covering the range of typical chemical composition in industrial practice, were subjected to high cooling rates, by melt spinning granulation or quenching in water, and to low cooling rates, by cooling inside the furnace. Both methods of rapid cooling led to volumetrically stable slags unlike the slow cooling which resulted in a powder-like material. Stabilized slags consisted predominantly of lamellar β-dicalcium silicate ( β-CS) and Mg, Ca-silicates (merwinite and bredigite); the latter form the matrix at low basicity and are segregated along the CS grain boundaries at high basicities. Slowly cooled slags consist of the γ-CS polymorph instead of the β-CS and of less Mg, Ca-silicates. Isothermal conduction calorimetry and thermogravimetric analysis indicate the occurrence of hydration reactions in the stabilized slags after mixing with water, while calcium silicate hydrates (C-S-H) of typical acicular morphology are identified by SEM. The present results demonstrate that the application of high cooling rates can result in a stable, environmental-friendly, hydraulic binder from stainless steel slags, rich in β-CS, without the necessity of introducing any additions to arrest the β polymorph. [ABSTRACT FROM AUTHOR]

The ladle treatment of a 18 pct Cr-9 pct Ni stainless steel, with desulfurization as its main purpose, was simulated on a laboratory scale. The influence of the top slag chemistry on the steel cleanliness was evaluated. A higher steel cleanliness was obtained with an optimized lime-alumina-based slag than with a lime-fluorspar-based slag. The inclusions were found to be mainly in the form of oxysulfide; the alumina content in the inclusions first increased and subsequently showed a slow drop, while the sulfide content decreased during the treatment. The equilibrium between steel and inclusions was found to be more easily reached than that between slag and steel. A slag-steel kinetic model was used to predict the steel chemistry evolution during the treatment. Furthermore, a slag-steel-inclusions interaction kinetic model was developed to calculate the change of alumina content in the inclusions during the ladle treatment. The sulfide content of inclusions was also calculated and compared with the measured values. [ABSTRACT FROM AUTHOR]

The aim of this work was to investigate the hydraulic behaviour of a stainless steel converter slag after changing its chemical composition and cooling path. The target slag was designed to resemble ground granulated blast-furnace slag (GGBFS). A synthetic slag with a chemical composition close to stainless steel converter slags was mixed with 22, 30 and 38 wt% fly ash (FA) from lignite combustion, heated up to 1550°C and then granulated by quenching in water; the solidified new slags were named FA22, FA30 and FA38 respectively. Quantitative X-ray diffraction on FA22 revealed that the amorphous phase was approximately 40 wt%, the rest being bredigite and merwinite. For FA addition of 30 wt% or more, the amorphous phase reached almost 100 wt%. The resulting slags showed significant hydraulic activity when mixed with sodium-based activators, with C-S-H, hydrotalcite and hydrogarnet being the main hydration products formed. The calorimetric behaviour and the mechanical properties of blended cements with 30 wt% FA30 and FA38 were comparable to a blended cement with GGBFS. Assuming that FA addition will take place during the liquid state of the slag, the proposed process can result in a new hydraulic binder. [ABSTRACT FROM AUTHOR]

Disintegration of stainless-steel slag during cooling is caused by the β→γ transformation of dicalcium silicate (2CaO·SiO2 or C2S). It is well established that disintegration can be avoided by borate additions, which are commonly believed to stabilize the high-temperature polymorphs by forming a solid solution with C2S. In this study, the borate distributions in slowly cooled synthetic and industrial slag samples are determined. The slag microstructures are characterized using electron probe microanalysis (EPMA), quantitative X-ray diffraction (QXRD), and wavelength dispersive spectroscopy (WDS). The results show that only a limited fraction of the added borates forms a solid solution with the C2S phase, while the main fraction is found in a calcium borosilicate phase. In combination with literature data, this suggests that slag stabilization involves more than the chemical stabilization of the high-temperature C2S polymorphs. [ABSTRACT FROM AUTHOR]

The article discusses the results of experiments on the microstructural evolution of the slag during the electric arc furnace (EAF) austenitic stainless steel steelmaking and refining with focus on chromium recovery and slag foaming. The microstructure of the slag samples was characterized with electron probe microanalysis using energy dispersive spectroscopy and X-ray diffraction.

Abstract: The degradation of three stainless steel grades (23.8–25.0wt.% Cr and 3.8–19.0wt.% Ni) has been investigated after immersion in PbO–CaO–SiO2–As2O3 slag for 3h at 1200°C. The degradation mechanisms are related to the steel and slag composition. The effect of As2O3 is most pronounced in the slag with the highest PbO/SiO2 ratio, where dissolution of the steel in liquid Pb, coming from the reduction of PbO, is most severely present. The reduction of As2O3 to As causes alloying of the steel with As and subsequent liquid phase formation. The degradation degree increases with increasing Ni content of the steel. [Copyright &y& Elsevier]

Abstract: Degradation of stainless steel in contact with PbO–CaO–SiO2 slag at 1200°C is experimentally investigated. The contact between steel and slag leads to the reduction of PbO from the slag to liquid Pb, and concurrent oxidation of the steel. The degradation mechanisms can be divided into liquid slag and liquid metal corrosion and oxidation. High Cr content is beneficial as it promotes the formation of a Cr2O3 scale which shields the steel from the slag and liquid Pb. The high solubility of Ni in liquid Pb is responsible for the increasing dissolution rate with increasing Ni content of the steel. [Copyright &y& Elsevier]

Ladle metallurgy (LM) slag is generated during the refining process step in stainless steel making. In view of its very fine particle size, due to the transformation of β- to γ-C 2 S, and the Cr content, the slag is challenging in terms of handling and disposal and is used in only few applications. The current work explores the utilisation of the LM's fine fraction in Ordinary Portland Cement (OPC) clinker production. Three sets of samples containing LM slag, in 0 wt% (OPC – reference), 6 wt% (OPC6) and 14 wt% (OPC14), were investigated. The sintering temperature was 1450 °C with 40 min soaking time. The clinkers containing LM slag, showed a slight increase in C 3 S and a decrease in C 3 A and C 4 AF. The formation of the C 3 S initiated at a lower temperature probably due to the presence of F and C 2 S. Longer setting times were found for OPC6 and OPC14. The compressive strength results were found to be comparable at 2 d (22–25 MPa), 7 d (36–38 MPa) and 28 d (44 MPa) of curing, falling into 42.5R CEM I category. The emission of kgCO 2 /t of clinker during production, is estimated to decrease at least 12% for OPC14. A processing step for the pre-treatment of the slag is also presented, aiming to reduce the initial Cr content. [ABSTRACT FROM AUTHOR]

This study evaluates commercially available magnesia–chromite, magnesia–carbon and magnesia–doloma bricks for their use in a Vacuum Oxygen Decarburisation ladle. The corrosion behaviour of these bricks by stainless steelmaking slags is, therefore, investigated through crucible tests in a vacuum induction furnace at elevated temperatures (1650 and 1750 °C) and low oxygen partial pressures (5.3 and 3.0×10 −11 atm). The results reveal that magnesia–carbon bricks are severely corroded due to the high dissolution of MgO, while magnesia–chromite and magnesia–doloma refractories exhibit an excellent corrosion resistance. The MgO enrichment in the slag is believed to be the reason of the low wear rate of the MgO–doloma refractories. Rebonded and direct-bonded MgO–chromite refractories show similar corrosion resistance against the slags because of the ‘secondary chromite inactivation’. Decreasing the slag basicity enhances the dissolution of MgO into the slag, thereby increasing the corrosion of the magnesia-based refractories. [ABSTRACT FROM AUTHOR]

The binding properties of continuous casting stainless steel slag were studied when activated by a combination of Na silicate and 5 M NaOH as well as K silicate and 5 M KOH at steam curing temperatures of 60, 70, 80, 90, 100 and 110 °C. The slag mortars developed moderate compressive strength (∼5 MPa) at 60 °C which initially increased moderately and then steeply (>30 MPa) at higher curing temperatures. The porosity of K activated mortars was lower than for Na activated mortars. Thermal and ATR-FTIR analysis showed the formation of C–S–H type of reaction products for both activators at all temperatures, while the formation of brucite was promoted at higher curing temperatures. A much denser hydrated matrix with a better bonding with aggregates was formed at higher curing temperatures. Heavy metal leaching from the slag was dependent on its type and curing temperature of samples. [ABSTRACT FROM AUTHOR]

Highlights: [•] AOD slag can be valorized as building material by mineral carbonation. [•] Depending on the process strengths of more than 30MPa can be achieved. [•] XRD analysis show formation of amorphous carbonation products. [•] SEM analysis shows different morphology of calcite at different pressures. [•] Leaching of several heavy metals is reduced on carbonation. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this work is to clarify the effect of various activation methods on the hydraulic properties of γ-C2S, one of the main phases in stainless steel slags. Synthetic C2S, sintered at 1450°C for 20h, consisted of 88wt.% γ-C2S and 12wt.% β-C2S. Two activation paths were followed: mechanical activation, by means of high energy milling, and chemical activation, by means of NaOH and Na2CO3 addition. Results indicated that both mechanically and chemically activated γ-C2S hydrated at ambient temperature. XRD analysis showed that the amount of amorphous phase increased during hydration, whereas TGA/DTG data revealed weight loss in the decomposition region of C–S–H. Analyses by FTIR and SEM also support that C–S–H is one of the main hydration products. Mortars prepared with mechanically and chemically activated γ-C2S developed mechanical strength, reaching the values of 9.1MPa and 10.3MPa at 90days, respectively. [Copyright &y& Elsevier]

Abstract: This work aims to assess the possibility of using ladle metallurgy and argon oxygen decarburization stainless steel slag as a hydraulic binder after mechanical activation. Prolonged milling in ethanol suspension resulted in 10-fold increase of the surface area and increase of the amorphous phase. Calorimetric analysis of slags mixed with water indicated the occurrence of exothermic reactions. XRD results revealed that periclase, merwinite, γ-C2S and bredigite, decreased with hydration time. Thermogravimetric analyses indicated that the main hydration products are most probably C–S–H, CH and MH. The hydrated products in both slags were similar to C–S–H gel. WDS analysis demonstrated Ca and Si to be widespread in the structure. Formation of M–S–H gel or incorporation of Mg in the C–S–H gel remains uncertain. The 90days compressive strength of mortars prepared from slags reached approximately 20% for LM and 10% for AOD of the compressive strength of mortars prepared from OPC. [Copyright &y& Elsevier]

Abstract: In order to identify the influence of MnO on the wear rate of MgO–C bricks for the production of high Mn stainless steel, MgO–C refractory samples were exposed to slags containing different MnO levels (up to 26wt.%). Although the investigations of the worn brick microstructures revealed the presence of numerous Mn-rich metal particles and the formation of a (Mg,Mn)O solid solution at the slag/refractory interface, no clear evidence of wear rate enhancement was observed due to high MnO concentration in the slags. With respect to refractory wear, the MgO content in the slag is the dominant factor. The degradation processes are discussed by combining experimental results and thermodynamic calculations. [Copyright &y& Elsevier]