Effect of Alumina on Crystallization Behavior of Calcium Ferrite in Fe2O3-CaO-SiO2-Al2O3 System

Al2O3 is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al2O3 effect on the crystallization of calcium ferrite is rarely reported. In this work, a crystallization device was designed to investigate the crystallization behavior of calcium ferrite in Fe2O3-CaO-SiO2-Al2O3 melt under non-isothermal conditions. XRD, SEM-EDS, and optical microscopy were used to identify the crystalline phase and the microstructure of samples. The result shows that the crystal morphology of SFCA changed in the order of strip, column, and needle as the Al2O3 content increased. The crystallization sequence of samples containing Al2O3 was observed as Ca4Fe14O25 (C4F14) → Fe2O3 → Ca3.18Fe15.48Al1.34O36 (SFCA-I) → CaFe2O4 (CF) → Ca5Si2(Fe, Al)18O36 (SFCA) → γ-Ca2SiO4 (C2S). The generation pathway of SFCA-I was found to be C4F14 + Si4+ + Al3+ → SFCA-I. Increasing the cooling rate can promote the formation of C4F14, SFCA-I, Fe2O3 and the amorphous phase. However, it prevented the crystallization of CF and SFCA while inhibiting the transformation of β-C2S to γ-C2S. When the Al2O3 content reached or exceeded 2.5 mass pct, the viscosity of Fe2O3-CaO-SiO2-Al2O3 melt increased sharply, resulting in the decrease in the crystal size of calcium ferrite.


Introduction
High-basicity sinter is mainly utilized as a critical iron-containing material for blast furnace ironmaking, where calcium ferrite is the predominant binding phase [1,2]. The mineral composition and microstructure of the binding phase has an important influence on the quality of the sinter [3][4][5]. Most of the binding phase is mainly complex calcium ferrite.
Recently, with the increasing consumption of high-alumina iron ores, the investigations focused on the role of Al 2 O 3 in the formation and crystallization of the binding phase have increased substantially [6][7][8]. Researchers [9][10][11] have found that adding a moderate quantity of Al 2 O 3 can promote the formation of complex calcium ferrite.
In sinter, some studies [12][13][14][15] revealed two primary crystal forms of complex calcium ferrite as SFCA (Ca 5 Si 2 (Fe, Al) 18 O 36 ) and SFCA-I (Ca 3.18 Fe 15.48 Al 1.34 O 36 ). Compared to SFCA (column and lath), needle-shaped SFCA-I is more favorable for releasing internal stress to improve the strength of the sinter [16]. Furthermore, the microstructure and morphology of complex calcium ferrite also have an important influence on the strength of sinter [17]. Webster et al. [18] investigated the effect of Al 2 O 3 on the formation process and thermodynamic stability of complex calcium ferrite. Liles et al. [19] investigated SFCA using the structural refinement approach, finding that Fe 3+ , Si 4+ , and Al 3+ tended to occupy the tetrahedral positions of SFCA, while Fe 3+ , Ca 2+ in the octahedral locations. The ion replacement is 2(Fe 3+ , Al 3+ ) = Ca 2+ + Si 4+ on electric neutrality. In addition, lowering the temperature is aided in replacing Al 3+ ↔ Fe 3+ .
Ding et al. [20] studied the crystallization kinetics of the CaO-Fe 2 O 3 binary system by the DSC method using Avrami and Mo models. In addition, the crystalline surface

Sinter Process
At room temperature, Fe 2 O 3 , CaO, SiO 2 , and Al 2 O 3 were mixed evenly, as stated in Table 1. For improving precision, CaCO 3 was used to replace CaO with an equal-molar quantity for precise weighing. 20.0 g of sample and an appropriate amount of anhydrous ethanol (≥99.7 pct, Sinopharm Chemical Reagent Co., Ltd.) were mixed evenly, then roasted at 200 • C for 3 h in a drying oven under an air atmosphere. The sample was compressed into a cylindrical shape (Ø 20 × 20 mm) and sintered in a platinum crucible.
From the previous research [25][26][27] it was found that if the sample was held above the TL for 2 h, a molten equilibrium liquid phase would be formed.
In this experiment, in order to obtain a complete equilibrium liquid phase, the sample was heated to 1350 • C at a heating rate of 5 • C/min and held for 4 h in air atmosphere. Subsequently, the samples were treated under the condition of various cooling rates (0.02 • C/s, 5 • C/s, 15 • C/s, and 65 • C/s.) [28] as presented in Figure 1. Table 1. For improving precision, CaCO3 was used to replace CaO with an equal-mola quantity for precise weighing. 20.0 g of sample and an appropriate amount of anhydrou ethanol (≥99.7 pct, Sinopharm Chemical Reagent Co., Ltd.) were mixed evenly, the roasted at 200 °C for 3 h in a drying oven under an air atmosphere. The sample was com pressed into a cylindrical shape (Ø 20 × 20 mm) and sintered in a platinum crucible. From the previous research [25][26][27] it was found that if the sample was held above the TL for h, a molten equilibrium liquid phase would be formed.
In this experiment, in order to obtain a complete equilibrium liquid phase, the sampl was heated to 1350 °C at a heating rate of 5 °C/min and held for 4 h in air atmosphere Subsequently, the samples were treated under the condition of various cooling rates (0.0 °C/s, 5 °C/s, 15 °C/s, and 65 °C/s.) [28] as presented in Figure 1. For obtaining the order of different crystallization phases, once the samples wer cooled to the target temperature at a cooling rate of 0.02 °C/s, water cooling was conducte to obtain an instantaneous mineral composition at the corresponding temperature.

Phase Determination
A part in each sample was ground to a particle size of less than 50 μm passin through the sieve completely for XRD determination. The mineral phase of the crystallin powder samples was identified using a Rigaku SmartLab X-ray diffractometer (Rigak Corporation, Tokyo, Japan). Cu Kα was used as the radiation source (40 kV, 150 mA) wit a graphite curved monochromator in the diffracted beam path. The wavelength is 0.1540 nm, with a scanning speed of 10°/min, a scanning step length of 0.02°, and a scannin range (2θ) from 10° to 100°. XRD data were matched using Crystallographica Search Match software (CSM3.0, Oxford Cryosystems Ltd., UK, Oxford).
The other part of the samples was embedded into the ethylenediamine-doping epox resin and polished for the microstructure observation. The mineral morphology and struc ture were observed by optical microscope (Optical Instrument Fifth Factory Co., Ltd Shanghai, China) and scanning electron microscope (Zeiss GeminiSEM500, Berlin, Ger many). The device is equipped with EDS (Ultim Max 170, Berlin, Germany) to detect ele mental composition. For obtaining the order of different crystallization phases, once the samples were cooled to the target temperature at a cooling rate of 0.02 • C/s, water cooling was conducted to obtain an instantaneous mineral composition at the corresponding temperature.

Phase Determination
A part in each sample was ground to a particle size of less than 50 µm passing through the sieve completely for XRD determination. The mineral phase of the crystalline powder samples was identified using a Rigaku SmartLab X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). Cu Kα was used as the radiation source (40 kV, 150 mA) with a graphite curved monochromator in the diffracted beam path. The wavelength is 0.15406 nm, with a scanning speed of 10 • /min, a scanning step length of 0.02 • , and a scanning range (2θ) from 10 • to 100 • . XRD data were matched using Crystallographica Search-Match software (CSM3.0, Oxford Cryosystems Ltd., UK, Oxford).
The other part of the samples was embedded into the ethylenediamine-doping epoxy resin and polished for the microstructure observation. The mineral morphology and structure were observed by optical microscope (Optical Instrument Fifth Factory Co., Ltd., Shanghai, China) and scanning electron microscope (Zeiss GeminiSEM500, Berlin, Germany). The device is equipped with EDS (Ultim Max 170, Berlin, Germany) to detect elemental composition.

Effect of Al 2 O 3
The composition change of the sample in equilibrium cooling process was obtained by heating to 1350 • C for 4 h and then cooling to room temperature at a rate of 0.02 • C/s.  Table 2 depicts the XRD patterns, optical micrograph, SEM, and EDS results of the crystalline samples containing varying amounts of Al 2 O 3 , respectively.
The composition change of the sample in equilibrium cooling process was obtained by heating to 1350 °C for 4 h and then cooling to room temperature at a rate of 0.02 °C/s.  Table 2 depicts the XRD patterns, optical micrograph, SEM, and EDS results of the crystalline samples containing varying amounts of Al2O3, respectively.        , CF, γ-Ca 2 SiO 4 (γ-C 2 S), SFCA-I, SFCA and Fe 2 O 3 were crystallized. The increase of Al 2 O 3 content led to the gradual decreases of C 4 F 14 , Fe 2 O 3 , and γ-C 2 S. SFCA-I first increased and then decreased. CF and SFCA increased gradually. The detailed result is as follows: (1) When Al 2 O 3 was not added, Ca 2+ reacted with Fe 3+ and O 2− to form C 4 F 14 and CF, while Ca 2+ reacted with Si 4+ and O 2− to form γ-C 2 S; (2) When Al 2 O 3 reached 0.5 mass pct, C 4 F 14 disappeared, and CF had gradually increased, indicating that the preferentially crystallized C 4 F 14 reacted with Al 3+ and Si 4+ to form SFCA-I; (3) When Al 2 O 3 reached 2.0 mass pct, CF and Fe 2 O 3 had gradually decreased, γ-C 2 S had not changed significantly, and SFCA-I increased gradually. It shows that CF also participated in the generation of SFCA-I. (4) When Al 2 O 3 reached 2.5 mass pct, the iron-rich SFCA-I was transformed into SFCA (high Si, high Al). Simultaneously, it promoted the precipitation of Fe 2 O 3 . Fe 2 O 3 and CF increased, and Si 4+ was mainly involved in generating SFCA, resulting in the decrease of γ-C 2 S. (5) When Al 2 O 3 reached 3.0 mass pct, CF and SFCA continued to increase, while γ-C 2 S decreased and Fe 2 O 3 disappeared.

The Sequence of Crystallization Phase
Due to the strong crystallization ability of calcium ferrite, the crystallization order in the liquid phase cooling process has yet to be understood. For obtaining the sequence of various phases crystallized in the Fe 2 O 3 -CaO-SiO 2 -Al 2 O 3 system, samples of No.1, No.6, and No.9 (0, 1.5, and 3.0 Al 2 O 3 mass pct) were selected to further research as cooled to the different target temperature at a cooling rate of 0.02 • C/s, and followed by water cooling. Figure 5 depicts the XRD patterns of the collected samples. It shows that adding Al 2 O 3 inhibited the formation of C 4 F 14 and SFC, while it promoted the formation of Fe 2 O 3 , CF, and γ-C 2 S simultaneous to the transformation of SFCA-I into SFCA. The detailed result is as follows: (1) When Al 2 O 3 was not added, the crystalline phase of quenched samples was Fe 2 Thermodynamically, the Gibbs free energy of formation of C 2 S is lower than that of CF [27], and the reactions are as Equations (1) and (2), respectively. It shows that C 2 S is more stable to form easier than CF. However, since the added SiO 2 content of 4mass pct is much smaller than Fe 2 O 3 , resulting in the probability of Si 4+ reacting with Ca 2+ is relatively small, simultaneously Si 4+ also participates in the formation of SFC, SFCA-I, and SFCA. Therefore, the crystallization sequence of C 2 S was late. Since the Gibbs free energy of C 4 F 14 , SFC, SFCA-I, and SFCA formations are not existing in the thermodynamic database, unfortunately it cannot be compared with other crystalline phases.
From the crystallization order of different samples containing Al 2 O 3 , a new generation path of SFCA-I was found in the crystallization process of the Fe 2 O 3 -CaO-SiO 2 -Al 2 O 3 quaternary system. The C 4 F 14 reacts with Si 4+ and Al 3+ in the melt to form SFCA-I (C 4 F 14 + Si 4+ + Al 3+ → SFCA-I), and SFCA-I reacts with Si 4+ and Al 3+ to form SFCA (SFCA-I + Si 4+ + Al 3+ → SFCA). Figure 6 shows the corresponding cross-sectional optical micrograph, where the experimental results are consistent with the XRD results. Seven phases of Fe 2 O 3 , C 4 F 14 , CF, SFC, γ-C 2 S, SFCA-I, and SFCA were co-precipitated in the melt. When the quenched temperature was lowered, equivalent to prolonging the crystallization time, the Fe 2 O 3 grew up in a lump, and calcium ferrite (CF, C 4 F 14 , SFC) developed from strip to short column. Simultaneously needle-shaped SFCA-I transformed into column-shaped SFCA, and γ-C 2 S developed from block to strip. γ-C 2 S was generated at 1200 • C while degraded when lowered to 1150 • C. various phases crystallized in the Fe2O3-CaO-SiO2-Al2O3 system, samples of No.1, No.6, and No.9 (0, 1.5, and 3.0 Al2O3 mass pct) were selected to further research as cooled to the different target temperature at a cooling rate of 0.02 °C/s, and followed by water cooling. Figure 5 depicts the XRD patterns of the collected samples. It shows that adding Al2O3 inhibited the formation of C4F14 and SFC, while it promoted the formation of Fe2O3, CF, and γ-C2S simultaneous to the transformation of SFCA-Ι into SFCA. The detailed result is as follows:        Figure 7 and Table 3 show the SEM photos and EDS results of the samples, confirming the XRD results. With the increase of the mass percentage of Al2O3 in the melt, the crystalline phase would change, and Al2O3 promoted the transition from SFCA-I to SFCA while inhibiting the formation of C4F14.   The corresponding optical micrographs of crystallized phases are presented in Figure 11. Table 4 shows six phases as CF, C 4 F 14 , SFCA-I, SFCA, Fe 2 O 3 , and β-C 2 S.

Effect of Cooling Rate on Crystallization
It can be seen that CF, C 4 F 14 , and Fe 2 O 3 were generated without Al 2 O 3 . With the increase of the Al 2 O 3 content, the crystallographic phase transformed significantly as follows. C 4 F 14 had gradually decreased as C 4 F 14 reacted with Al 3+ and Si 4+ to form columnar SFCA-I. With further increasing the content of Al 2 O 3 , SFCA-I transformed to SFCA. Compared with SFCA, SFCA-I had a higher ratio of Fe 2 O 3 to CaO in chemical composition. The crystallization of C 4 F 14 and Fe 2 O 3 was promoted during the transformation. Moreover, with the increase of Al 2 O 3 content, the complex calcium ferrite first increased and then decreased. The two-dimensional crystal morphology of the minerals shows that CF was skeletal or corroded. Meanwhile, C 4 F 14 , Fe 2 O 3 , SFCA-I, and SFCA were existed in the morphology of strip, irregular block, column and needle, and short column, respectively. The corresponding optical micrographs of crystallized phases are presented in Figure 11. Table 4 shows six phases as CF, C4F14, SFCA-I, SFCA, Fe2O3, and β-C2S.

Effect of Cooling Rate on Crystallization
It can be found that as increasing the cooling rate it shortens the crystal growth time, so the crystalline of some minerals would be inhibited, while the crystalline phase and morphology were also changed significantly. On the one hand, the crystal size would be narrowed. On the other hand, the formation of complex calcium ferrite and the conversion of SFCA-I to SFCA would be promoted, while the formation of SFC and γ-C 2 S would be inhibited. It also promoted the formation of C 4 F 14 , Fe 2 O 3 , and the amorphous phase that filled around the complex calcium ferrite in an imperfect crystallization state. But when the cooling rate reaches 65 • C/s, it was found that C 4 F 14 was easier to form than  It can be seen that CF, C4F14, and Fe2O3 were generated without Al2O3. With the increase of the Al2O3 content, the crystallographic phase transformed significantly as follows. C4F14 had gradually decreased as C4F14 reacted with Al 3+ and Si 4+ to form columnar SFCA-I. With further increasing the content of Al2O3, SFCA-I transformed to SFCA. Compared with SFCA, SFCA-I had a higher ratio of Fe2O3 to CaO in chemical composition. The crystallization of C4F14 and Fe2O3 was promoted during the transformation. Moreover, with the increase of Al2O3 content, the complex calcium ferrite first increased and then decreased. The two-dimensional crystal morphology of the minerals shows that CF was To further confirm the phase composition, the SEM-EDS analysis of the sample with a cooling rate of 5 • C/s is shown in Figure 12 and Table 5. When the Al 2 O 3 is not added, only C 4 F 14 and CF phases were formed. With Al 2 O 3 content increasing, C 4 F 14 , CF, SFCA-I, and SFCA appeared, which confirmed the experimental results in Figure 8. crystallization order of samples in the Fe2O3-CaO-SiO2-Al2O3 melt containing Al2O3 should be C4F14 → Fe2O3 → SFCA-I → CF → SFCA → γ-C2S.
To further confirm the phase composition, the SEM-EDS analysis of the sample with a cooling rate of 5 °C/s is shown in Figure 12 and Table 5. When the Al2O3 is not added, only C4F14 and CF phases were formed. With Al2O3 content increasing, C4F14, CF, SFCA-I, and SFCA appeared, which confirmed the experimental results in Figure 8.    To investigate the influence of cooling rate on the morphology of calcium ferrite (C 4 F 14 , SFCA-I, and SFCA) in the Fe 2 O 3 -CaO-SiO 2 -Al 2 O 3 melt under different Al 2 O 3 content, the grain size of calcium ferrite in each sample in Figure 11 was measured using the Nano Measurer 1.2 software [29]. Thirty positions in each sample were selected, measured, and an averaged value was calculated. Figure 13 illustrates that with the increased cooling rate, the crystal size of calcium ferrite decreased significantly. Furthermore, when the Al 2 O 3 content increased, the crystal size of calcium ferrite increased and subsequently decreased, which demonstrates that adding a small amount of Al 2 O 3 promoted the formation of complex calcium ferrite. At different cooling speeds (5 • C/s, 15 • C/s, and 65 • C/s), the grain size achieved the maximum value (corresponding to 22.15 µm, 13.85 µm, and 9.25 µm, respectively) when the Al 2 O 3 reached 2.0 mass pct. After Al 2 O 3 reached 2.5 mass pct, it would increase a viscosity of the melt, which could be a primary reason for the decrease in the crystal size of calcium ferrite. decreased, which demonstrates that adding a small amount of Al2O3 promoted the formation of complex calcium ferrite. At different cooling speeds (5 °C/s, 15 °C/s, and 65 °C/s), the grain size achieved the maximum value (corresponding to 22.15 μm, 13.85 μm, and 9.25 μm, respectively) when the Al2O3 reached 2.0 mass pct. After Al2O3 reached 2.5 mass pct, it would increase a viscosity of the melt, which could be a primary reason for the decrease in the crystal size of calcium ferrite.

Discussion on Crystallization Mechanism
To further explain the effect of Al2O3 on the crystallization of calcium ferrite, FactSage 8.2 software was used to perform thermodynamic equilibrium calculations on the Fe2O3-CaO-SiO2-Al2O3 system, even though C4F14, SFCA-I, and SFCA are lacking in the thermodynamic database. Future metallurgical workers are required to improve it. Figure 14 and Table 6 show the primary crystallization temperature and crystallization amount of the thermodynamic equilibrium phase with different Al2O3 content. The result shows that without adding Al2O3, the phases are M2O3 (≥99.50 mass pct Fe2O3 and ≤0.50 mass pct Al2O3), α'-Ca2SiO4(α'-C2S), and CaFe4O7(CF2). With the Al2O3 content increasing, CF2 disappeared while CF appeared. When Al2O3 content reached 2.0 mass pct, the primary crystallization phase transformed from M2O3 to α'-C2S, and the transition temperature was 1250 °C. When Al2O3 content reached 3.0 mass pct, CF and Ca(Al, Fe)6O10 appeared.

Discussion on Crystallization Mechanism
To further explain the effect of Al 2 O 3 on the crystallization of calcium ferrite, Fact-Sage 8.2 software was used to perform thermodynamic equilibrium calculations on the Fe 2 O 3 -CaO-SiO 2 -Al 2 O 3 system, even though C 4 F 14 , SFCA-I, and SFCA are lacking in the thermodynamic database. Future metallurgical workers are required to improve it. Figure 14 and Table 6  The corresponding crystallization amounts of M 2 O 3 , α'-C 2 S, and CF decreased from 39.01 to 24.19 mass pct, 11.41 to 11.12 mass pct, and 49.00 to 35.76 mass pct, respectively. Ca (Al, Fe) 6 O 10 increased from 0.579 to 28.924 mass pct.
As shown in Figure 11, many spherical holes appeared in samples when the Al 2 O 3 content reached 2.5 and 3.0 mass pct, which increased with the increase of the Al 2 O 3 content and the cooling rate. Simultaneously, the crystalline size of minerals decreased. It can be considered that the increase of the melt viscosity resulted in a slow crystalline rate due to the hard mass transferring.   The corresponding crystallization amounts of M2O3, α'-C2S, and CF decreased from 39.01 to 24.19 mass pct, 11.41 to 11.12 mass pct, and 49.00 to 35.76 mass pct, respectively. Ca (Al, Fe)6O10 increased from 0.579 to 28.924 mass pct.
As shown in Figure 11, many spherical holes appeared in samples when the Al2O3 content reached 2.5 and 3.0 mass pct, which increased with the increase of the Al2O3 content and the cooling rate. Simultaneously, the crystalline size of minerals decreased. It can be considered that the increase of the melt viscosity resulted in a slow crystalline rate due to the hard mass transferring. Figure 15 shows the viscosity diagrams of Fe2O3-CaO-SiO2-Al2O3 melts with Al2O3  Figure 15 shows the viscosity diagrams of Fe 2 O 3 -CaO-SiO 2 -Al 2 O 3 melts with Al 2 O 3 content at different temperatures, which is calculated using thermodynamical software, simultaneously combined by the Einstein-Roscoe formula (Equation (3)) [30], where the mass fraction of the solid phase was obtained as shown in Figure 14. The result shows that at all temperatures, the viscosity value increased with the increase of Al 2 O 3 content. The viscosity value increased obviously. The viscosity increase would hinder the crystallization and mass transfer of complex calcium ferrite in the melt, resulting in poor crystalline morphology. Specially, after Al 2 O 3 reached 2.5 mass pct, the viscosity of the melt increased sharply, which could be the main reason for the decrease in the crystal size of calcium ferrite.
η-solid-liquid mixing viscosity; η 0 -viscosity of pure liquid phase; c-a mass fraction of solid phase. ferrite.
-solid-liquid mixing viscosity; 0 -viscosity of pure liquid phase; c-a mass fraction of solid phase. The isothermal cross-sections of Fe2O3-CaO-SiO2-Al2O3 systems with varying Al2O3 content at different temperatures are shown in Figure 16. With the Al2O3 content increasing, the liquid phase is divided into three regions, named Lα, Lα + Lβ, and Lβ, where the content of Al2O3 increased from 1.0 to 3.0 mass pct. When the red component point is at 1250 °C, the primary crystal region is transformed from Lα + M2O3 to Lα + Lβ + α'-C2S. There may be two reasons for the deterioration of crystallization. On the one hand, the viscosity of Lα + Lβ + α'-C2S is higher than that of Lα+M2O3. In addition, the crystallization of α'-C2S leads to the reducing of initial Ca 2+ and Si 4+ in the melt, which is not conducive to the crystallization of complex calcium ferrite. It can be seen from Figure 16 that when Al2O3 is 2.0 mass pct, not only a certain amount of liquid phase is retained, but also Ca 2+ and Si 4+ are not reduced too much. This also explains that when Al2O3 was 2.0 mass pct, the crystal size of calcium ferrite was the largest, as shown in Figure 13.  Figure 16. With the Al 2 O 3 content increasing, the liquid phase is divided into three regions, named L α , L α + L β, and L β , where the content of Al 2 O 3 increased from 1.0 to 3.0 mass pct. When the red component point is at 1250 • C, the primary crystal region is transformed from L α + M 2 O 3 to L α + L β + α'-C 2 S. There may be two reasons for the deterioration of crystallization. On the one hand, the viscosity of L α + L β + α'-C 2 S is higher than that of L α +M 2 O 3 . In addition, the crystallization of α'-C 2 S leads to the reducing of initial Ca 2+ and Si 4+ in the melt, which is not conducive to the crystallization of complex calcium ferrite. It can be seen from Figure 16 that when Al 2 O 3 is 2.0 mass pct, not only a certain amount of liquid phase is retained, but also Ca 2+ and Si 4+ are not reduced too much. This also explains that when Al 2 O 3 was 2.0 mass pct, the crystal size of calcium ferrite was the largest, as shown in Figure 13.

Conclusions
In this work the influences of Al2O3 content, cooling rate and the crystallization sequence of the Fe2O3-CaO-SiO2-Al2O3 system during the cooling process were investigated. On this basis, the influence mechanism of Al2O3 content and cooling rate on the crystallization of complex calcium ferrite (C4F14, SFCA-I, SFCA) was also proposed. The main conclusions are as follows: (1) Al2O3 has an important effect on the composition of the crystal phase of the Fe2O3-CaO-SiO2-Al2O3 system. Adding alumina promoted the crystallization of Fe2O3, γ-C2S, SFCA-I, and SFCA, while it inhibited the crystallization of C4F14 and SFC. However, the content of CF first decreased and then increased. This is mainly because of the formation of complex calcium ferrite and the transformation of SFCA-I to SFCA. (2) The crystallization sequence in Fe2O3-CaO-SiO2-Al2O3 melt under different Al2O3 content was investigated, where the corresponding crystalline order is (Fe2O3, C4F14) → CF → (SFC, γ-C2S), (Fe2O3, C4F14) → SFCA-I → CF → SFCA → γ-C2S, and (Fe2O3, SFCA-I) → CF → SFCA → γ-C2S under the Al2O3 content of 0 mass pct, 1.5 mass pct, and 3.0 mass pct respectively. It can be concluded that the C4F14 reacts with Si 4+ and Al 3+ in the melt to form SFCA-I (C4F14 + Si 4+ + Al 3+ → SFCA-I), and then SFCA-I reacts with Si 4+ and Al 3+ to form SFCA (SFCA-I + Si 4+ + Al 3+ → SFCA). (3) As the cooling rate increase, C4F14, SFCA-I, Fe2O3, β-C2S, and the amorphous phases are increased while CF and SFCA are reduced, and the crystal transformation from β-C2S to γ-C2S can be effectively inhibited. However, when the cooling rate was increased from 15 °C/s to 65 °C/s, C4F14 was found to crystallize before Fe2O3.

Conclusions
In this work the influences of Al 2 O 3 content, cooling rate and the crystallization sequence of the Fe 2 O 3 -CaO-SiO 2 -Al 2 O 3 system during the cooling process were investigated. On this basis, the influence mechanism of Al 2 O 3 content and cooling rate on the crystallization of complex calcium ferrite (C 4 F 14 , SFCA-I, SFCA) was also proposed. The main conclusions are as follows: