Characterization of the FeCr Slag
Figure 1 presents X-ray diffraction (XRD) pattern for FeCr slag. Spinel phases (chromian, ferroan) (Mg)(Al,Cr)2O4 appeared as main crystalline peaks. Other identified phases included forsterite Mg2SiO4 and enstatite MgSiO3. Additionally, the XRD spectra showed a slight increase in the overall intensity of the curve at low 2θ values which indicates the presence of an amorphous phase.
Figure 2 shows FESEM (AsB) micrographs of the FeCr slag cross section with two different magnifications. Microstructural examinations revealed clearly that microstructure of the slag includes several crystalline phases surrounded by an amorphous glassy phase. Elemental maps disclosed that aluminium is concentrated, together with chromium and magnesium, on larger separate quadrilateral-shaped crystals, most likely spinel (Mg)(Al,Cr)2O4 phase. Magnesium was concentrated on elongated needle-like crystals together with silicon, most likely being forsterite, Mg2SiO4, or enstatite, MgSiO3, phases detected by XRD analyses. Iron was detected in separate spots, seen in the brightest contrast in SEM images. Crystalline phases were surrounded by silicon-rich amorphous glass phase, seen in darker contrast in SEM images. The shape of the XRD curve supports the presence of amorphous phase besides the crystalline phases. The mineralogy of FeCr slag was characterized in detail by Makkonen and Tanskanen  who have suggested similarly that common phases in the slag are Fe–Mg–Cr–Al-spinels, forsterite (Mg2SiO4), Mg-silicates and metal droplets. With fast cooling rates, the slag is not totally crystalline, with amorphous glass phase being solidified between the grains. The amount of amorphous glass phase depends on the cooling rate being typically between 60 and 70% in FeCr slag .
Figure 3 shows the TG/DSC measurement curves of the FeCr slag measured in argon and air atmospheres. In both atmospheres, an exothermic peak was detected in DSC curves above 900 °C similarly than in the study Zelic . Additionally, in air, the weight loss curve started to increase slightly above 400 °C showing also a small exothermic peak in the DSC curve. In air, the weight loss curve continued to increase with increase in temperature, relating most probably to metal iron droplets oxidation detected in microstructure observations. In both atmospheres, peaks observed in DSC curves above 1200 °C were probably related to liquid slag phase formation, with temperature increases increasing the content of liquid phase. In the study by Zelic  FeCr slag melting point was reported to fall in the temperature range of 1200–1400 °C. The liquid phase formation at high temperatures restrains the use of FeCr slag-based materials for structural applications. Refractory systems should be designed in such a way that the maximum temperature attainable in the system is lower than the softening or melting temperature of the refractory ingredients: the binder and the aggregates . Thus these results suggest that maximum service temperature for these type of novel FeCr slag based refractory material is 1200 °C.
Properties of the Sintered Refractory Specimens
Figure 4 shows macroscopic cross-sectional images of sintered refractory castable specimens for (a) Reference (b) Castable1 (c) Castable1 after reheating to 1260 °C and (d) Castable1 after reheating to 1320 °C. Images revealed that both the Reference and Castable1 specimens showed typical refractory material cross-sections, including aggregates and surrounding binder phases. Reheating tests for sintered Castable1 specimens up to temperatures 1260 °C and 1320 °C were made in order to investigate the liquid phase formation which was observed in DSC curves for FeCr aggregates. Reheating to 1320 °C revealed clearly the melting of both slag aggregates and surrounding binder phases, which strengthens the observation of maximum service temperature in DSC curves.
Figure 5 shows FESEM images of sintered cross-sectional specimens of (a) Reference and (b) Castable2. FESEM micrographs revealed that for the Reference, the compatibility between the binder and the aggregates is good, as charged based on the sharp phase interfaces in Fig. 5a. This is likely a result from long development and surface optimization work for commercial product. For slag-based Castable2 specimen (Fig. 5b), clear cracks at the binder-aggregate interfaces were detected. However, the binder was evenly distributed between the aggregates of different size fractions.
Strength Development Curves
Figure 6 shows strength development curve for Castable1 composition mixture as a function of temperature. Ceramic bonded or fired refractories are formed at high temperatures using binders and a sintering process . As observed at 1000 °C no ceramic bonding has not yet developed. At 1200 °C temperature, higher strength values suggested that ceramic bonds have been formed and full strength of refractory castables has been achieved.
Cold Crushing Strengths
The cold compressive strength or cold crushing strength (CCS) of a refractory material is an indication of its suitability for the use as refractory. It is a combined measure for the strength of the aggregate grains and the bonding system . CCS values recorded for sintered specimens are presented in Fig. 7. The results are average of four specimens. It can be observed that the best performing slag-based Castable2 showed average CCS value of 91 MPa. Strength value was clearly below the strength recorded for the commercial refractory reference material, 149 MPa, but notably higher than the highest strength value, 60 MPa, reported in the studies by Kumar et al. [13, 14] for FeCr slag-utilizing castables. According to CCS values, the use of dispersant (in Castable2) increased the average compressive strength value from 79 (Castable1) to 91 MPa (Castable2). Commercial dispersant (BASF Castament) has been developed to disperse calcium aluminate cement particles and aggregates in order to improve the rheological properties of castables in refractory applications . The differences between the performance of Castable2 and the Reference specimen may be explained by the still poorer compatibility of the binder and aggregate phases of Castable2, as evaluated based on the phase interface appearance (Fig. 5) although the recorded strength values as high as 91 MPa indicate that the strength of the aggregate grains and also of the bonding system are at the acceptable level. By further improving the binder-aggregate compatibility with optimal additives, it can be assumed that even higher strength values may be reached. According to CCS results, it seems that 10% cement replacement by fine EAF slag (Castable3) is possible, still keeping almost the same average strength level for the castables, 75 MPa vs. 79 for Castable1. The higher cement replacement (Castable4) resulted in lower average strength value of 71 MPa.
In recent years, more importance has been given to high-temperature strengths of refractories rather than cold strength values, since refractories are used at elevated temperatures . Figure 8 shows the compressive strengths of sintered slag based castables and the Reference measured at the temperature of 1200 °C. Recorded strengths at 1200 °C were one tenth of corresponding value measured at room temperature. Also at 1200 °C the highest average strength value for slag-based castable was obtained for Castable2, 9.2 MPa, followed by Castable 1 (6.0 MPa). Strength results measured at 1200 °C revealed more clearly that cement replacement by fine EAF slag decreased the strength values. 10% cement replacement by fine EAF slag (Castable3) decreased the average compressive strength to 4.9 MPa and 20% cement replacement (Castable4) even lower, to 3.1 MPa.
Apparent Solid Density, Bulk Density and Open Porosity
The values of apparent solid density, bulk density, and open porosity for slag-based castables and the Reference are shown in Table 3. The results are average of three samples. Typically, the values of density and porosity are used to recommend the refractories for specific applications . In general, the higher the density, the lower the porosity and also, other physical properties, such as strength, are typically related to the density and porosity values. Table 3 shows that bulk density values were of the same order for slag-based castables and the commercial Reference, around 2.35 g/cm3. These values are typical for the alumino-silicate-based, dense refractory castables used as structural components in heat-treatment furnaces and kilns . The Castable2 showed a slightly higher density value of 2.39 g/cm3 but featured also the highest variation in the values, which could partly explain the result. Nevertheless, for the best performing (highest strength) slag-based castable, Castable2, the open porosity value of 17.3% was of the same order with the commercial Reference, 16.1%. For other slag-based castables, the porosity values were approximately 20% (Castable1 and Castable3; or even higher (22% for Castable4). Thus by increasing the cement replacement with fine EAF slag fraction, the porosity is increased and the density values of the castable are lowered.
Figure 9 shows the linear thermal expansion of sintered specimens measured by dilatometry. Thermal expansion coefficients, α, of the specimens were determined in the temperature interval from room temperature up to 1100 °C. Most refractory materials expand when heated and thus, when refractories are installed at room temperature, the whole structure expands and tightens up when heated . Figure 9 shows that all the FeCr slag-based specimens exhibited linear expansion and the recorded thermal expansion coefficient values were at the level comparable to those for the Reference.
Specific Heat Capacity and Thermal Conductivity
Figures 10, 11 and 12 show thermal diffusivity, thermal conductivity and specific heat capacity as a function of temperature for the sintered specimens. In all cases, thermal diffusivity values in Fig. 10 revealed a slight decrease with increase in temperature. Slag-based castables featured somewhat lower thermal diffusivity values as compared to commercial Reference, for which the values ranging between 1.3 and 0.7 mm2/s were obtained. Among the slag-based castables, the thermal diffusivity values closest to the commercial reference were recorded for Castable1, ranging from 1.1 to 0.5 mm2/s. The thermal diffusivity values for the slag-based Castable2 (with the best strength properties) showed the lowest thermal diffusivity values, from 0.8 to 0.4 mm2/s.
Thermal conductivity is a measure of the refractory materials’ ability to conduct heat from the hot to the cold face when it is exposed to high temperatures . Low thermal conductivity values implies good thermal insulation capability for refractory materials. Typically when thermal insulation is needed for refractory material, insulating refractory materials are formulated for low relative thermal conductivity and not for strength resistance. The majority of insulating castables are alumino-silicate- based or high alumina castables, having densities between 0.4 and 1.45 g/cm3 and corresponding porosities of 45–85%. Thus insulating castables show much lower densities and higher porosities than dense castables. Generally when density increases and porosity decreases, thermal conductivity will increase . Thermal conductivity values shown in Fig. 11 disclosed a slight overall decrease in with a shift towards higher temperatures. Slag-based Castable2 exhibited the lowest thermal conductivity values, from 1.3 to 0.9 W/mK, which were much lower than for the commercial Reference (2.4–2.1 W/mK). However, all slag-based castables showed thermal conductivity values lower than the Reference. Thermal conductivity values of slag-based Castable2 are comparable to values shown in literature for alumina based insulating castables which show thermal conductivity value of 1.2 W/mK at 200 °C but have much lower density value of 1.2 g/cm3 . These results indicate a good thermal insulation capability for FeCr slag based castables and their suitability in the use as aggregate in refractory castables for insulation purposes and simultaneously having sufficient strength for structural component also. Lower thermal conductivity values for FeCr slag compared with natural material granite has been reported also by Niemelä and Kauppi  for civil engineering and road construction uses.
The values of specific heat shown in Fig. 12 revealed a slightly increasing values with rise in temperature. Similar results were obtained for EAF slag reported in the literature .
Exposure Tests for Corrosive Atmosphere
Corrosion resistance is one of the most important characteristics of refractories, thus the test conditions have to be designed so that they closely simulate the conditions that the refractories experience during the use . Thermogravimetric analyses of the FeCr slag-based refractories indicate that their maximum service temperature is 1200 °C. Thus in common metallurgical processes that operate at temperatures above 1200 °C, direct exposure to molten metal and slag should be avoided. However, potential applications may involve exposure to gaseous atmosphere, the temperature of which is typically lower than that of liquid phases. For that reason, corrosion tests were performed to predict the behaviour of FeCr based castables in acidic gaseous atmosphere. During the test procedure, samples were exposed to the gas mixture of 15% humidity and HCl/SOx at 700 °C temperature for 168 h. With visual inspection, no corrosion effects were seen on the surface of the test samples after the exposure. Mechanical tests were performed for the exposed corrosion samples in order to evaluate the effect of corrosive atmosphere on mechanical properties. According to results, corrosion exposure caused no effect on strength values—neither for the slag-based castables nor for the commercial reference. These investigations indicate that the resistance to acidic gaseous atmosphere for FeCr slag based materials is at least at comparable level to the commercial materials.