High-Temperature Chemical Stability of Cr(III) Oxide Refractories in the Presence of Calcium Aluminate Cement

Al2O3-CaO-Cr2O3 castables are used in various furnaces due to excellent corrosion resistance and sufficient early strength, but toxic Cr(VI) generation during service remains a concern. Here, we investigated the relative reactivity of analogous Cr(III) phases such as Cr2O3, (Al1−xCrx)2O3 and in situ Cr(III) solid solution with the calcium aluminate cement under an oxidizing atmosphere at various temperatures. The aim is to comprehend the relative Cr(VI) generation in the low-cement castables (Al2O3-CaO-Cr2O3-O2 system) and achieve an environment-friendly application. The solid-state reactions and Cr(VI) formation were investigated using powder XRD, SEM, and leaching tests. Compared to Cr2O3, the stability of (Al1−xCrx)2O3 against CAC was much higher, which improved gradually with the concentration of Al2O3 in (Al1−xCrx)2O3. The substitution of Cr2O3 with (Al1−xCrx)2O3 in the Al2O3-CaO-Cr2O3 castables could completely inhibit the formation of Cr(VI) compound CaCrO4 at 500–1100 °C and could drastically suppress Ca4Al6CrO16 generation at 900 to 1300 °C. The Cr(VI) reduction amounting up to 98.1% could be achieved by replacing Cr2O3 with (Al1−xCrx)2O3 solid solution. However, in situ stabilized Cr(III) phases as a mixture of (Al1−xCrx)2O3 and Ca(Al12−xCrx)O19 solid solution hardly reveal any reoxidation. Moreover, the CA6 was much more stable than CA and CA2, and it did not participate in any chemical reaction with (Al1−xCrx)2O3 solid solution.


Introduction
Cr 2 O 3 -containing refractories possess remarkable corrosion resistance due to their extremely low solubility and high chemical stability against molten slag. Therefore, they are widely used as lining materials in incinerators, gasifiers, glass furnaces, non-ferrous smelting, etc. [1][2][3][4][5][6]. In addition, refractories as castables have become a popular choice in recent decades because of the energy-saving manufacturing process, convenient for installation and repair works, where binders' chemistry plays a crucial role [7][8][9]. Calcium alumina cement (CAC) binders are the most widely used since they exhibit fast setting and strength development, stable thermo-mechanical behaviour, and resistance to slag attack [10]. However, Cr 2 O 3 can oxidize into toxic Cr(VI) products at high temperatures upon reaction with alkali or alkaline earth metals/oxides/compounds under an oxidizing atmosphere [11][12][13]. The Cr(VI) compounds pose a severe threat to humans and the environment since they are toxic, carcinogenic, highly soluble in water, and quickly enter the food cycle [14]. Therefore, it is of significant environmental and practical significance to inhibit the generation of Cr(VI) when applying Al 2 O 3 -CaO-Cr 2 O 3 castables as lining materials.
The Al 2 O 3 -CaO-Cr 2 O 3 system was not investigated in detail earlier though numerous Cr(VI) reduction techniques were described for other applications [15][16][17]. Generally, Cr(VI) formation was closely related to the atmosphere and basicity of other components in the

Materials and Methods
Tabular alumina (Al 2 O 3 ) of various size fractions, 5-3 mm, 3-1 mm, 1-0 mm, and ≤0.045 mm, were procured from Zhejiang Zili Alumina Materials Technology Co., Ltd., Shangyu, China. Reactive α-alumina fines of size fraction ≤ 0.005 mm were purchased from Kaifeng Tenai Co., Ltd., Kaifeng, China. Industrial-grade fused chromium oxide (Cr 2 O 3 ) (size ≤ 0.074 mm) was obtained from Luoyang Yuda Refractories Co., Ltd., Luoyang, China. The hydraulic calcium aluminate cement binder, Secar 71 (CA and CA 2 phases), was procured from Imerys Aluminates, Tianjin, China. An organic defloculant, FS 65 (Wuhan Sanndar Chemical Co., Ltd., Wuhan, China), was used as the dispersant. The detailed chemical composition of raw materials is shown in Table 1.  3 and Al 2 O 3 fine powders with a mass ratio of 8:17 were dry-mixed, pressed into pellets, and then treated at 1300, 1600, and 1650 • C for 3 h in the air to obtain the mixture of Al 2 O 3 , Cr 2 O 3 and (Al 1−x Cr x ) 2 O 3 solid solution and pre-synthesized (Al 1−x Cr x ) 2 O 3 solid solution. Thus, obtained pellets were then pulverized to 200-mesh powders before adding them into the castables. The specimen with Cr 2 O 3 and Al 2 O 3 powders as initial raw materials was labelled as R, while specimens with (Al 1−x Cr x ) 2 O 3 solid solution presynthesized at 1300 • C, 1600 • C, and 1650 • C were designated as S13, S16, and S165, respectively. Specimen R was pre-heated at 1500 • C for 3h (labelled as F15) to produce the in situ formed (Al 1−x Cr x ) 2 O 3 , whose effect on the Cr(VI) formation in the castables at various temperatures was also evaluated then. The castables were formulated based on the Andreasen distribution coefficient (q) value of 0.31, and the specific formulation is shown in Table 2. Each batch was dry-mixed for 3 min in a Hobart mixer followed by wet-mixing (4.0 wt% water, 25 • C) for further 3 min, and then castables were moulded in a vibrating table (1 min) into bars of size 160 mm × 40 mm × 40 mm at room temperature. All specimens were cured at 25 • C and 75% ± 5% relative humidity for 24 h in a standard cement maintainer and dried at 110 • C for 24 h in an electric air oven after demoulding. Dried specimens R, S13, S16, S165, and specimen F15 were finally heated in the temperature range of 300-1500 • C for 3h at peak temperature in air. To figure out relative oxidation, the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions, fine powders of CAC and CA 6 were mixed with Cr 2 O 3 and pre-synthesized (Al 1−x Cr x ) 2 O 3 (Table 3). Then, the mixed powders were pressed into Φ20 mm × 20 mm cylindrical specimens under a pressure of 50 MPa. After being treated at 900 • C and 1300 • C for 3 h in the air atmosphere, the phase compositions and microstructures of the specimens were analyzed by XRD and SEM, respectively. the software of X'pert Pro High Score (Philips, Almelo, Netherlands). Lattice parameters were calculated using X'pert Pro High Score (Philips, Almelo, Netherlands) and Celref 2.0 software. Microstructure morphology was analyzed by scanning electron microscopy (SEM, Nova 400 Nano-SEM, FEI Company, Hillsboro, OR, USA) equipped with energy dispersive spectroscopy (EDS, Oxford, High Wycombe, UK). Cr(VI) leachability was evaluated using the leaching test according to TRGS 613 standard procedure, which is suitable for determining water-soluble Cr(VI) compounds in cement and products containing cement. Leaching specimens were prepared by crushing and grinding thoroughly before passing through a 200-mesh sieve (≤74 µm). Fine samples were stirred with deionized water as a leaching solution using a magnetic stirrer at a speed of 300 rpm for 15 min (at room temperature) with a solid-liquid ratio of 1:20. Then, leachates were obtained through a 0.45-µm membrane filter with a glass fibre by vacuum. The Cr(VI) concentration in the leachates was determined using a colorimetric method. The Cr(VI) can react in acid condition with the 1,5-diphenylcarbazide (DPC) to form 1,5-diphenylcarbazone, a red complex (0.02-0.2 mg/L chrome). Then, the absorbance of the leachates after the DPC method was recorded at 540 nm, using a 722 Vis spectrophotometer (Jinghua Instruments, Shanghai, China).

Results and Discussion
3.1. Pre-Synthesis of (Al 1−x Cr x ) 2

O 3 Powders
The pre-synthesized powders of the (Al 1−x Cr x ) 2 O 3 solid solution at different temperatures are observed by XRD ( Figure 1). It could be found that both corundum and eskolaite existed as separate phases after dry mixing at 25 • C. After treated at 1300 • C, the eskolaite disappeared with a noticeable reduction of the peak intensity of corundum, while a new phase identified as (Al 1−x Cr x ) 2 O 3 solid solution was generated. With the increase in the heat treatment temperature, the peak intensity of corundum reduced gradually until disappearance at 1650 • C, while the peak intensity of (Al 1−x Cr x ) 2 O 3 solid solution increased steadily. After treated at temperatures up to 1650 • C, only the (Al 1−x Cr x ) 2 O 3 solid solution could be detected in the specimens. So, it could be inferred that we have added the (Al 1−x Cr x ) 2 O 3 solid solution with the remnant of corundum and eskolaite (sample S13 and S16), while that of S165 is a complete (Al 1−x Cr x ) 2 O 3 solid solution. In addition, the lattice parameters of the (Al 1−x Cr x ) 2 O 3 solid solution were calculated in comparison with Al 2 O 3 (reference code: JCPDS 01-081-2266, a = b = 4.7569 Å and c = 12.9830 Å) and Cr 2 O 3 (reference code: JCPDS 00-038-1479, a = b = 4.9540 Å and c = 13.5842 Å). Since Al 2 O 3 has smaller lattice parameters than Cr 2 O 3 , the (Al 1−x Cr x ) 2 O 3 solid solution reveals smaller lattice parameters than Cr 2 O 3 . With the increasing temperature, the (Al 1−x Cr x ) 2 O 3 solid solution showed decreasing lattice parameters as more Al 2 O 3 dissolution is expected at higher temperatures. For example, the lattice parameter a = b = 4.8607 Å at 1300 • C (for sample S13) decreased to a = b = 4.8291 Å at 1600 • C (for sample S16).

Cr(VI) Leachability
The Cr(VI) concentration in Al 2 O 3 -CaO-Cr 2 O 3 castables treated at various temperatures was evaluated by leaching test according to the TRGS 613 standard procedure ( Figure 2). The details of Cr(VI) reduction compared to the reference specimen R is presented in Table 4. With the addition of the pre-synthesized (Al 1−x Cr x ) 2 O 3 solid solution, a noticeable decrease in the Cr(VI) concentration was observed. The specimens with (Al 1−x ,Cr x ) 2 O 3 pre-synthesized at higher temperature exhibited relatively lower Cr(VI) concentrations at the same heat treatment temperature (exception for specimen S165 at 1300 and 1500 • C). For example, at 700 • C, the total amount of Cr(VI) reduced drastically from 1233.2 mg/kg in specimen R (without (Al 1−x Cr x ) 2 O 3 ) to 223.7 mg/kg in specimen S13 (a reduction of 81.9%), and reduced further to 24.0 mg/kg in specimen S165 (a decrease of 98.1%). However, at 1300 • C, specimen S165 exhibited an even higher Cr(VI) concentration than the reference specimen. Moreover, the temperature corresponding to the maximum Cr(VI) concentration shifted from 900 • C for R to 1100 • C for the pre-synthesized (Al 1−x Cr x ) 2 O 3 . The specimen F15, pre-heated at 1500 • C, exhibited extremely low Cr(VI) concentration at all heat treatment temperatures studied. It was concluded that the chromium would present as Cr(III) together within the solid solution of (Al 1−x Cr x ) 2 O 3 and Ca(Al,Cr) 12 O 19 after the pre-heating treatment at 1500 • C [20]. Therefore, it is plausible that the reoxidation of these stable solid solution phases did not occur. Although the midtemperature (700-1100 • C) was favourable for Cr(VI) formation, the total amount of Cr(VI) in F15 was still only 13.0-17.3 mg/kg (a decrease of~98.9-99.1% compared to specimen R). These values are below the allowable Cr(VI) limit of the Environmental Protection Agency (EPA), United States (5 mg/L is equivalent to 100 mg/kg) [34].

Cr(VI) Leachability
The Cr(VI) concentration in Al2O3-CaO-Cr2O3 castables treated at various temperatures was evaluated by leaching test according to the TRGS 613 standard procedure ( Figure 2). The details of Cr(VI) reduction compared to the reference specimen R is presented in Table 4. With the addition of the pre-synthesized (Al1-xCrx)2O3 solid solution, a noticeable decrease in the Cr(VI) concentration was observed. The specimens with (Al1-x,Crx)2O3 pre-synthesized at higher temperature exhibited relatively lower Cr(VI) concentrations at the same heat treatment temperature (exception for specimen S165 at 1300 and 1500 °C). For example, at 700 °C, the total amount of Cr(VI) reduced drastically from 1233.2 mg/kg in specimen R (without (Al1-xCrx)2O3) to 223.7 mg/kg in specimen S13 (a reduction of 81.9%), and reduced further to 24.0 mg/kg in specimen S165 (a decrease of 98.1%). However, at 1300 °C, specimen S165 exhibited an even higher Cr(VI) concentration than the reference specimen. Moreover, the temperature corresponding to the maximum Cr(VI) concentration shifted from 900 °C for R to 1100 °C for the pre-synthesized (Al1-xCrx)2O3. The specimen F15, pre-heated at 1500 °C, exhibited extremely low Cr(VI) concentration at all heat treatment temperatures studied. It was concluded that the chromium would present as Cr(III) together within the solid solution of (Al1-xCrx)2O3 and Ca(Al,Cr)12O19 after the pre-heating treatment at 1500 °C [20]. Therefore, it is plausible that the reoxidation of these stable solid solution phases did not occur. Although the mid-temperature (700-1100 °C) was favourable for Cr(VI) formation, the total amount of Cr(VI) in F15 was still only 13.0-17.3 mg/kg (a decrease of ~98.9-99.1% compared to specimen R). These values are below the allowable

Phase Evolution of the Castables
To study the effect of the pre-synthesized (Al1-xCrx)2O3 solid solution on the phase evolution of the castables, phase compositions of the specimens treated at 110-1500 °C were analyzed ( Figure 3). In all samples, the main phase corundum and the NaAl11O17 impurity could be detected at all temperatures, and hydrate phase C3AH6 was generated at 110 °C but then disappeared at 300 °C due to dehydration. For specimen R, the CaCrO4 phase could be detected at 300 °C, whose peak intensity increased with the increase in temperature from 300 °C to 900 °C but then decreased with further increasing temperature until disappearance at 1300 °C. The Ca4Al6CrO16 was generated at 900 °C, whose peak intensity reached a maximum at 1100 °C but dropped down with further increasing temperature until disappearance at 1500 °C. Moreover, eskolaite existing in the range of 110 °C to 1100 °C reduced in peak intensity with temperature and disappeared at 1300 °C, while the (Al1-xCrx)2O3 solid solution and CaAl12O19 increased in peak intensity after generating at 1100 °C and 1300 °C, respectively. However, for specimens S13, S16, and S165, no CaCrO4 phase was detected at 300-1100 °C, indicating chromium that in the (Al1-xCrx)2O3, the CAC in this temperature range would not oxidize the solid solution. At 900-1300 °C, although the Ca4Al6CrO16 phase was still formed in these specimens with presynthesized (Al1-xCrx)2O3, the peak intensity of Cr(VI) compound was much lower compared with sample R. The peak intensity of the Ca4Al6CrO16 phase reached the maximum at 1100 °C in Al2O3-CaO-Cr2O3 castables, and therefore, the highest Cr(VI) concentration for the specimens with pre-synthesized (Al1-xCrx)2O3 were detected at 1100 °C (Figure 3b). In general, the substitution of Cr2O3 with (Al1-xCrx)2O3 in the Al2O3-CaO-Cr2O3 castables can almost completely restrict the formation of CaCrO4 compounds at 300-1100 °C and effectively lower the Cr(VI) compound Ca4Al6CrO16 formation at 900-1300 °C, which was

Phase Evolution of the Castables
To study the effect of the pre-synthesized (Al 1−x Cr x ) 2 O 3 solid solution on the phase evolution of the castables, phase compositions of the specimens treated at 110-1500 • C were analyzed (Figure 3). In all samples, the main phase corundum and the NaAl 11 O 17 impurity could be detected at all temperatures, and hydrate phase C 3 AH 6 was generated at 110 • C but then disappeared at 300 • C due to dehydration. For specimen R, the CaCrO 4 phase could be detected at 300 • C, whose peak intensity increased with the increase in temperature from 300 • C to 900 • C but then decreased with further increasing temperature until disappearance at 1300 • C. The Ca 4 Al 6 CrO 16 was generated at 900 • C, whose peak intensity reached a maximum at 1100 • C but dropped down with further increasing temperature until disappearance at 1500 • C. Moreover, eskolaite existing in the range of 110 • C to 1100 • C reduced in peak intensity with temperature and disappeared at 1300 • C, while the (Al 1−x Cr x ) 2 O 3 solid solution and CaAl 12 O 19 increased in peak intensity after generating at 1100 • C and 1300 • C, respectively. However, for specimens S13, S16, and S165, no CaCrO 4 phase was detected at 300-1100 • C, indicating chromium that in the (Al 1−x Cr x ) 2 O 3 , the CAC in this temperature range would not oxidize the solid solution. At 900-1300 • C, although the Ca 4 Al 6 CrO 16 phase was still formed in these specimens with pre-synthesized (Al 1−x Cr x ) 2 O 3 , the peak intensity of Cr(VI) compound was much lower compared with sample R. The peak intensity of the Ca 4 Al 6 CrO 16 phase reached the maximum at 1100 • C in Al 2 O 3 -CaO-Cr 2 O 3 castables, and therefore, the highest Cr(VI) concentration for the specimens with pre-synthesized (Al 1−x Cr x ) 2 O 3 were detected at 1100 • C (Figure 3b). In general, the substitution of Cr 2 O 3 with (Al 1−x Cr x ) 2 O 3 in the Al 2 O 3 -CaO-Cr 2 O 3 castables can almost completely restrict the formation of CaCrO 4 compounds at 300-1100 • C and effectively lower the Cr(VI) compound Ca 4 Al 6 CrO 16 formation at 900-1300 • C, which was following the results of Cr(VI) leachability shown in Figure 2. After being treated at 1500 • C, only the corundum (with NaAl 11 O 17 impurity), the (Al 1−x Cr x ) 2 O 3 solid solution, and the CA 6 phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al 1−x Cr x ) 2 O 3 pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA 6 phase after being treated at 1300 • C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 • C, showed hardly any phase changes with the subsequent heat treatment temperature. °C, only the corundum (with NaAl11O17 impurity), the (Al1-xCrx)2O3 solid solution, and the CA6 phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al1-xCrx)2O3 pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA6 phase after being treated at 1300 °C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O3-CaO-Cr2O3 castables, chromium and calcium would exist in the state of Cr2O3/(Al1-xCrx)2O3, and CAC/CA6, respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO4 and Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine powders of CAC/CA6 were mixed with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 °C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figures 4 and 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 °C and 1300 °C are listed in Table 5. In addition, the qualitative EDS spot analysis (atomic%) was shown in Table 6, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology. CA6 phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al1-xCrx)2O3 pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA6 phase after being treated at 1300 °C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O3-CaO-Cr2O3 castables, chromium and calcium would exist in the state of Cr2O3/(Al1-xCrx)2O3, and CAC/CA6, respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO4 and Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine powders of CAC/CA6 were mixed with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 °C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figures 4 and 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 °C and 1300 °C are listed in Table 5. In addition, the qualitative EDS spot analysis (atomic%) was shown in Table 6, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology.
-CaCrO 4 , CA6 phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al1-xCrx)2O3 pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA6 phase after being treated at 1300 °C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O3-CaO-Cr2O3 castables, chromium and calcium would exist in the state of Cr2O3/(Al1-xCrx)2O3, and CAC/CA6, respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO4 and Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine powders of CAC/CA6 were mixed with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 °C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figures 4 and 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 °C and 1300 °C are listed in Table 5. In addition, the qualitative EDS spot analysis (atomic%) was shown in Table 6, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology.
-Hauyne (Ca 4 Al 6 CrO 16 ), CA6 phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al1-xCrx)2O3 pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA6 phase after being treated at 1300 °C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O3-CaO-Cr2O3 castables, chromium and calcium would exist in the state of Cr2O3/(Al1-xCrx)2O3, and CAC/CA6, respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO4 and Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine powders of CAC/CA6 were mixed with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 °C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figures 4 and 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 °C and 1300 °C are listed in Table 5. In addition, the qualitative EDS spot analysis (atomic%) was shown in Table 6, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology.
-NaAl 11 O 17 , CA6 phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al1-xCrx)2O3 pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA6 phase after being treated at 1300 °C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O3-CaO-Cr2O3 castables, chromium and calcium would exist in the state of Cr2O3/(Al1-xCrx)2O3, and CAC/CA6, respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO4 and Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine powders of CAC/CA6 were mixed with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 °C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figures 4 and 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 °C and 1300 °C are listed in Table 5. In addition, the qualitative EDS spot analysis (atomic%) was shown in Table 6, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology.

Reaction Mechanism
The above results demonstrated that in the Al 2 O 3 -CaO-Cr 2 O 3 castables, chromium and calcium would exist in the state of Cr 2 O 3 /(Al 1−x Cr x ) 2 O 3, and CAC/CA 6 , respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO 4 and Ca 4 Al 6 CrO 16 at mid-temperature (700-1100 • C). Fine powders of CAC/CA 6 were mixed with Cr 2 O 3 /(Al 1−x Cr x ) 2 O 3 (pre-synthetized at 1650 • C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 • C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figures 4 and 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 • C and 1300 • C are listed in Table 5. In addition, the qualitative EDS spot analysis (atomic%) was shown in Table 6, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology. Table 5. Chemical reaction equations in cylindrical specimens.
CH-C -(5) (6) CH-S --    CA6 phases were found in specimens R, S13, S16, and S1 the castables (Figure 3c) indicated that samples with higher temperature exhibited relative lower peak inten treated at 1300 °C. In addition, specimen F15, which ha the other four specimens treated at 1500 °C, showed hard sequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O and calcium would exist in the state of Cr2O3/(Al1-xCrx which affects the formation and concentration of Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine p with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to Cr(VI) generation in the castables and the correspond mixed powders were treated at 900 and 1300 °C for 3 h SEM microstructure are summarized in Figures 4 and 5, ical reaction equations discussed below in various sam are listed in Table 5. In addition, the qualitative EDS sp in Table 6, corresponding to the fractured surface in F the uneven surface of the specimen would only reveal th to identify the different phases associated with differen -CaCrO 4 , CA6 phases were found in specimens R, S13, S16, and S the castables (Figure 3c) indicated that samples wit higher temperature exhibited relative lower peak inte treated at 1300 °C. In addition, specimen F15, which h the other four specimens treated at 1500 °C, showed har sequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O and calcium would exist in the state of Cr2O3/(Al1-xC which affects the formation and concentration of Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to Cr(VI) generation in the castables and the correspond mixed powders were treated at 900 and 1300 °C for 3 SEM microstructure are summarized in Figures 4 and 5 ical reaction equations discussed below in various sam are listed in Table 5. In addition, the qualitative EDS s in Table 6, corresponding to the fractured surface in F the uneven surface of the specimen would only reveal t to identify the different phases associated with differe     °C, only the corundum (with NaAl11O17 impurity), the (Al1-xCrx)2O3 solid solution, and the CA6 phases were found in specimens R, S13, S16, and S165. The enlarged XRD patterns of the castables (Figure 3c) indicated that samples with (Al1-xCrx)2O3 pre-synthesized at higher temperature exhibited relative lower peak intensity of the CA6 phase after being treated at 1300 °C. In addition, specimen F15, which had the same phase compositions as the other four specimens treated at 1500 °C, showed hardly any phase changes with the subsequent heat treatment temperature.

Reaction Mechanism
The above results demonstrated that in the Al2O3-CaO-Cr2O3 castables, chromium and calcium would exist in the state of Cr2O3/(Al1-xCrx)2O3, and CAC/CA6, respectively, which affects the formation and concentration of Cr(VI) compounds CaCrO4 and Ca4Al6CrO16 at mid-temperature (700-1100 °C). Fine powders of CAC/CA6 were mixed with Cr2O3/(Al1-xCrx)2O3 (pre-synthetized at 1650 °C) to figure out the mechanisms of the Cr(VI) generation in the castables and the corresponding chemical reactions. Then, the mixed powders were treated at 900 and 1300 °C for 3 h in the air; the XRD patterns and SEM microstructure are summarized in Figures 4 and 5, respectively. The plausible chemical reaction equations discussed below in various samples heated at 900 °C and 1300 °C are listed in Table 5. In addition, the qualitative EDS spot analysis (atomic%) was shown in Table 6, corresponding to the fractured surface in Figure 5. Needless to mention that the uneven surface of the specimen would only reveal the non-stoichiometric composition to identify the different phases associated with different morphology.     After being treated at 900 • C, the CA phase disappeared in specimen C-C with the formation of many granular CaCrO 4 grains (Figure 5a) via reaction 1. However, the sample C-S was still composed of the initial main phases (CA, CA 2 , and (Al 1−x Cr x ) 2 O 3 ) (Figure 5b) in addition to forming minute amounts of Ca 4 Al 6 CrO 16 (reaction 2). As the heat treatment temperature increased to 1300 • C, plenty of chrome-hauyne and (Al 1−x Cr x ) 2 O 3 solid solution (Figure 5e) were generated in specimen C-C (via reactions [3][4][5], accompanied by the disappearance of CA and significant reduction in CA 2 phase, while specimen C-S possessed relative lower peak intensity of Ca 4 Al 6 CrO 16 although it had similar phases as C-C. Combining the observations of Cr(VI) in Figure 2, with the phase evolution results (Figures 3 and 4), it can be deduced that compared with Cr 2 O 3 , the (Al 1−x Cr x ) 2 O 3 solid solution was more stable that would not form CaCrO 4 and could effectively hinder the Ca 4 Al 6 CrO 16 formation when contacted with CAC. Therefore, the substitution of Cr 2 O 3 with (Al 1−x Cr x ) 2 O 3 can effectively lower the Cr(VI) concentration of the castables after being treated at various temperatures ( Figure 2). Furthermore, the castables with (Al 1−x Cr x ) 2 O 3 pre-synthesized at higher temperature exhibited lower Cr(VI) concentration, implying that the stability of the (Al 1−x Cr x ) 2 O 3 improved gradually with the Al 2 O 3 proportion in the solid solution. In addition, in comparison with the CA 2 phase, CA was more likely to react with Cr 2 O 3 /(Al 1−x Cr x ) 2 O 3 resulting in the formation of Cr(VI) compounds.
For specimens CH-C, no new phases occurred after heat treatment at 900 • C, and only a minuscule amount of chrome-hauyne was generated at 1300 • C (Figure 5g) via Eqs. 6, which also produced Al 2 O 3 that subsequently interacted with Cr 2 O 3 to develop the (Al 1−x Cr x ) 2 O 3 solid solution via Eqs. 5. It is worth mentioning that no changes in the phase compositions were detected in specimen CH-S after heat treatment at both 900 • C and 1300 • C ( Figure 4). These observations demonstrated that calcium in CA 6 was much more stable than in CA and CA 2 , which only caused slight oxidation of Cr 2 O 3 and would not take chemical reaction with (Al 1−x Cr x ) 2 O 3 solid solution. Therefore, specimen F15, in which chromium and calcium existed in (Al 1−x Cr x ) 2 O 3 and CA 6 , respectively, showed no changes in phase composition and extremely low Cr(VI) concentration at various heat treatment temperatures. In the Al 2 O 3 -CaO-Cr 2 O 3 castables, CA 6 could be generated from the reaction between CAC and Al 2 O 3 powders in the matrix at 1300 • C ( Figure 4). However, for specimen S165, since no Al 2 O 3 existed in the (Al 1−x Cr x ) 2 O 3 powder pre-synthesized at 1650 • C, the calcium would still exist as CA and CA 2 rather than CA 6 at 1300 • C. As a result, specimen S165 possessed an even higher Cr(VI) concentration than the reference specimen R at 1300 • C, suggesting that CA and CA 2 can more easily react with (Al 1−x Cr x ) 2 O 3 to produce Ca 4 Al 6 CrO 16 compared with CA 6 .

Conclusions
In the present work, (Al 1−x Cr x ) 2 O 3 solid solution was pre-synthesized at a different temperatures for the inhibition of the formation of Cr(VI) in Al 2 O 3 -CaO-Cr 2 O 3 castables was systematically investigated. The summarized conclusions are as follows: (1) Compared with Cr 2 O 3 , the stability of the (Al 1−x Cr x ) 2   Institutional Review Board Statement: Not applicable.

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Data Availability Statement: Data sharing not applicable.