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Article

Enhancement of Intermediate-Temperature Strength of Corundum-Spinel Castables via Incorporation of Zn(OH)2 Powders

by
Yifan Dong
,
Mantang He
,
Mengyang Sang
,
Xin Qiu
,
Pengyu Xu
*,
Xinhong Liu
and
Quanli Jia
*
Henan Key Laboratory of High Temperature Functional Ceramics, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China
*
Authors to whom correspondence should be addressed.
Materials 2025, 18(12), 2777; https://doi.org/10.3390/ma18122777
Submission received: 10 May 2025 / Revised: 5 June 2025 / Accepted: 10 June 2025 / Published: 12 June 2025
(This article belongs to the Section Advanced and Functional Ceramics and Glasses)
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Versions Notes

Abstract

Corundum-spinel based purging plugs are extensively employed in steel ladle refining processes. Traditionally, these plugs are manufactured through a high-temperature firing process that not only demanded substantial energy consumption but also resulted in a dense microstructure with higher strength; however, they often led to undesirable consequences such as fracture and thermal spalling, significantly impeding the enhancement of their service life. In recent years, the steel industry has witnessed the emergence of unfired purging plugs as an alternative solution. Unfortunately, there are some shortcomings including low strength at intermediate-temperature and poor volume stability, which easily lead to a short life and lower blowing rate of the unfired purging plug, thereby restricting their utilization. Aiming to improve the intermediate-temperature properties of the unfired purging plug, the effect of Zn(OH)2 on the properties of the castables was investigated. The results show that the cold strength of the specimens sintered at different temperatures remarkably increased with rising Zn(OH)2 content, for instance, CMOR values of the specimens sintered at 800 °C escalated from 3.19 MPa to 14.98 MPa. Furthermore, the incorporation of Zn(OH)2 led to a reduction in permanent linear change and a marked increase in hot strength. The remarkable improvement in intermediate-temperature strength can be attributed to the formation of ZnCr2O4 and ZnAl2O4 spinel phases originating from the reaction between ZnO derived from the decomposition of Zn(OH)2, and the existing Cr2O3 or Al2O3. These spinel phases create a reinforcing effect, thereby substantially enhancing the mechanical properties of the specimens after firing at intermediate temperatures.

1. Introduction

To date, high-performance steel products have been utilized in many areas and are increasingly on the rise; therefore, they need to be further improved by long-time refining steel metallurgical processes. As important functional parts, purging plugs played a curious role on the refining process in steel ladle [1,2,3,4]. Pre-fired corundum-spinel purging plugs have been commonly utilized in refining steel ladles because of their higher cold and high-temperature strength, good slag resistance and volume stability [5,6,7,8], furthermore, their hot strength and slag resistance can be further enhanced by adding Cr2O3 powders. However, these pre-fired purging plugs also possessed some shortcomings including higher energy consumption derived from firing at a high temperature (>1500 °C), longer production period, poorer thermal shock resistance, etc. In addition, cracks, fracture or thermal spalling easily occurred because of their dense microstructure and higher strength, which noticeably restricted their service life improvement, thereby limiting the enhancement of steel ladle heats [7,8]. To overcome these shortcomings, many studies have been conducted, such as using modified aggregates (resin coated alumina [9], alumina bubble [10], bonite [11,12], zirconia-alumina aggregates [13]), different types of spinel [14,15], etc.; however, their properties still needed to be enhanced.
Recently, calcium aluminate cement (CAC)-bonded corundum-spinel purging plugs (CSPPs) have been fabricated at a lower temperature (about 600 °C) [5,16]. However, they also have some shortcomings including low strength at intermediate-temperature, poor volume stability, which easily led to a shorter service life and lower blowing rate, thereby restricting their application in steel ladle refining processes [17,18,19]. Previous works [20,21,22] found that intermediate-temperature strength of corundum castables can be notably enhanced via incorporating Zn(OH)2. Furthermore, their hot strength was also increased, which was attributed to the formation of ZnAl2O4 in the matrix, creating a strengthening effect. In addition, thermal shock resistance of the specimen was also slightly enhanced because small pores were generated by the decomposition of Zn(OH)2. Our previous work demonstrated that CAC bonded CSPPs with ZnO addition also possessed higher physical properties and high-temperature properties [16].
Aiming to enhance the intermediate-temperature strength and volume stability of unfired CSPPs, Zn(OH)2 powders were applied as an additive to CSPPs. The effects of firing temperatures and Zn(OH)2 content on the physical properties, hot strength and thermal shock resistance, phase compositions, and microstructures of CSPPs were investigated. Results show that the cold strength of the CSPPS after heat treatment at 600–1600 °C was remarkably enhanced, and strength improvement mechanisms were attributed to forming ZnCr2O4 and ZnAl2O4 spinel phases at 600–1000 °C, and (Mg1−xZnx)Al2O4 or Mg(Al,Cr)2O4 spinel solid solution were performed at 1400–1600 °C, creating an enforcing effect on the strength improvement.

2. Experimental

Tabular alumina aggregates (6–0.08 mm, from Almatis, Qingdao, China), fused corundum powders (<0.088 mm, from Kaifeng Tenai Co., Kaifeng, China), fused MgAl2O4 spinel powders (<0.044 mm, from Kaifeng Hecheng Co., Kaifeng, China), ultrafine alumina (D50 = 1.2 μm, from Kaifeng Tenai Co., China), chromia powders (Cr2O3: 99%, <20 μm, from Luoyang Zhengjie Co., Luoyang, China), calcium aluminate cement (CAC, Secar 71, from Imerys Co., Shanghai, China), and Zn(OH)2 fines (AR, <0.044 mm, from Aladdin Co., Shanghai, China) were utilized as starting materials, while Castment FS 10 was selected as the dispersant. The ratio of aggregates and matrices in CSPPS was 70:30, and the samples with various Zn(OH)2 additions (0, 0.3%, 0.6%, 0.9% and 1.2%) were labeled sample ZH0, ZH1, ZH2, ZH3, ZH4, respectively. The castables matrix ZH4 without spinel fines, labeled as sample ZH5, was also investigated as a reference sample, which were presented in Table 1.
The castable mixtures were blended with 4.20% water and cast into 25 mm × 25 mm × 150 mm bars, thereafter they were cured at room temperature and dried at 110 °C. Finally, the samples were heated at 600 °C, 800 °C, 1000 °C, 1400 °C, and 1600 °C for 3 h, respectively. Physical properties including apparent porosity (A.P.), bulk density (B.D.), cold modulus of rupture (CMOR), cold crushing strength (CCS), and permanent linear change (PLC) of the samples after firing at different temperatures were conducted according to Chinese standards. Thermal shock resistance (TSR) of the specimens pre-fired at 1000 °C and 1600 °C was measured by air-cooling (ΔT = 1100 °C, 3 cycles), and retained strengths were tested and residual CMOR ratios of the samples were calculated. The hot modulus of rupture (HMOR) of the bar specimens after drying at 110 °C was tested with a 3-point bending technique at 1400 °C for 1 h. Crystalline phases and microstructures of pre-firing samples were characterized using X-ray diffraction (XRD, Philips Xpert, Cu Kα), scanning electron microscope (SEM, Zeiss EVO H15, Jena, Germany), and energy dispersive spectrum (EDS, INCA X-act) analysis.

3. Results and Discussion

Influences of Zn(OH)2 content and firing temperature on the physical properties of the samples are illustrated in Figure 1. As the Zn(OH)2 content increased from 0 to 1.20%, A.P. values of the samples heat treatment at 110–1600 °C gradually decreased, while their B.D. values increased. In particular, A.P. values of the samples after firing at 110 °C, 1000 °C, and 1400 °C noticeably decreased from 11.44% to 10.23%, from 13.28% to 12.05%, and from 15.43% to 13.57%, respectively. Their PLC exhibited minimal variation at firing temperatures ≤ 1000 °C, but decreased from 0.40% to 0.26% at 1400 °C. As shown in Figure 1d, the CMOR values of the samples with higher Zn(OH)2 content presented a substantial improvement. The CMOR value of the samples rose remarkably from: 5.56 MPa to 10.96 MPa at 110 °C, 4.05 MPa to 12.13 MPa at 600 °C, 3.19 MPa to 14.98 MPa at 800 °C, 10.56 MPa to 23.72 MPa at 1000 °C, 30.50 MPa to 41.97 MPa at 1400 °C, 35.96 MPa to 54.97 MPa at 1600 °C, respectively. Their CCS values also increased greatly (Figure 1e), which followed a comparable trend to that of CMOR values.
As shown in Figure 1, it was found that the CMOR value of sample ZH0 exhibited a sharp decline from 5.56 MPa at 110 °C to 4.05 MPa at 600 °C, and further down to 3.19 MPa at 800 °C. This reduction in cold strength may be attributed to the decomposition of CAC hydrates, coupled with the absence of the new bonding phase formation in the matrix [15,16]. However, the cold strength (CMOR and CCS values) of the samples containing Zn(OH)2 after firing at 600–800 °C was remarkably enhanced, which were much higher than that of at 110 °C. These findings presented that cold strength of the specimens at 600–800 °C was dramatically enhanced via Zn(OH)2 addition. This finding was noticeably different from that of Zn(OH)2-containing spinel-free corundum castables [20], which may be attributed to the newly formed ZnCr2O4 bonding phases in the castables containing Cr2O3. The CMOR value of sample ZH4 fired at 1400 °C and 1600 °C was 37.61%, 52.86% higher than that of sample ZH0. The reasons for this will be discussed in following section.
The TSR of the samples after firing at 1000 °C and 1600 °C were conducted by air-cooling at 1100 °C. Residual CMOR values and residual CMOR ratios of the samples ZH0–ZH4 are revealed in Figure 2. Residual CMOR values depicted a slight decline with rising Zn(OH)2 content from 0 to 1.20%. Specifically, the CMOR retention ratios presented a decrease from 49.26% to 33.75% in samples pre-fired at 1000 °C and from 13.37% to 7.07% for those pre-fired at 1600 °C, indicating a marginal degradation in TSR with Zn(OH)2 addition. This negative effect on the TSR of the samples may be attributed to their lower A.P., denser microstructure and higher strength, thereby degrading the TSR of Zn(OH)2-bearing castables.
Figure 3 illuminates the HMOR values at 1400 °C of the samples ZH0–ZH4 as a function of Zn(OH)2 amount. On the increasing Zn(OH)2 contents (0→1.20%), the HMOR value of the samples noticeably enhanced, which was 30.19 MPa, 35.95 MPa, 49.85 MPa, 50.39 MPa, and 44.92 MPa, respectively, for sample ZH0, ZH1, ZH2, ZH3, and ZH4. They confirmed that the high-temperature strength of the samples was enhanced via adding Zn(OH)2 powders. It is noted that HMOR values at 1400 °C were higher than the cold strength of the samples pre-fired at 1400 °C, which was similar to that of nitride-bonded SiC materials [23]. The reasons can be ascribed to the fact that the hot strength is mostly controlled by crystal effect rather than glass effect [24]. After cooling, thermal expansion mismatch between corundum aggregates and spinel generates stresses and microcracks. These may weaken the structure, thereby slightly decreasing the cold strength. However, the microcracks may heal via thermal expansion of the aggregates and matrix via a thermally induced crack-healing mode. Therefore, hot strength is enhanced, which is the reason why the HMOR at 1400 °C is higher than that of the cold strength of the samples after firing at 1400 °C [24].
Physical properties, HMOR, and TSR of the specimens were significantly influenced by Zn(OH)2 addition, which can be attributed to changing in their crystalline phases and microstructures. Therefore, we further investigated them using XRD and SEM analysis. Figure 4 depicts the crystalline phases in castable matrices with varying Zn(OH)2 content after heat treatment from 600 to 1400 °C. At 600 °C (Figure 4a), the main phases were corundum, MgAl2O4, and Cr2O3, with minor phases of CA (CaO·Al2O3), CA2 (CaO·2Al2O3), and ZnO (from Zn(OH)2 decomposition), and trace ZnAl2O4 was also detected in Zn(OH)2-containing samples. The phase compositions were similar to that of at 800 °C, ZnAl2O4 peaks were detected in sample ZH4 (Figure 4b). By 1000 °C (Figure 4c), ZnO peaks disappeared completely, while ZnAl2O4 peaks intensified significantly. At 1400 °C (Figure 4d), ZnCr2O4 disappeared, and CA and CA2 phases vanished; they were transformed to CA6. Compared to sample ZH4, ZnAl2O4 peaks can be detected in sample ZH5 after firing at 600–1000 °C, and no MgAl2O4 peaks were found. Furthermore, the displacement of MgAl2O4 peaks in sample ZH4 after firing at 1400 °C shifted from 37.09° to 36.66°, indicating that (Mg1−xZnx)Al2O4 or Mg(Al,Cr)2O4 spinel solid solution were generated at 1400 °C. This can be attributed to ZnAl2O4 or ZnCr2O4 dissolving into the MgAl2O4 spinel structure to form spinel solid solution at elevated temperatures [16,20,21,22]. These solid solutions are beneficial for enhancing the cold and hot strength of the Zn(OH)2-bearing castables.
Figure 5 demonstrates the microstructural evolution and corresponding EDS analysis of sample ZH2, sintered at 600–1400 °C. At 600 °C (Figure 5a,b), granular aggregates were observed in the ZH0 matrix, and EDS analysis of point 1 (Figure 5(P1)) revealed that the presence of Al, O, Cr, and Zn was detected. This may be owing to ZnO incorporated into the Cr2O3 and Al2O3 grains. Combined with XRD results, these aggregates likely comprised Al2O3, ZnCr2O4, and residual ZnO, explaining the reason why the cold strength of the Zn(OH)2-containing samples was higher than sample ZH0. Similar aggregated particles were observed at 800 °C (Figure 5c,d). EDS analysis of point 2 (Figure 5(P2)) confirmed that these particles contained ZnAl2O4 and ZnCr2O4, consistent with the XRD analysis. This demonstrates that the in situ formation of ZnAl2O4 and ZnCr2O4 contributed to improving their physical properties. Increased density and visible sintering necks can be found in the samples after tfiring at 1000 °C (Figure 5e,f), indicating particles bonding content was enhanced. These particles comprised Al, O, Mg, Ca, Cr, and Zn (Figure 5(P3)). At 1400 °C (Figure 5g), the plate-like CA6 grains formed an interconnected network. The particles were well sintered, and this microstructure provided a substantial strengthening, resulting in marked improvements in both cold and hot strength of sample ZH2 after treating at 1400 °C. EDS analysis depicted that they were composed of Al, O, Mg, Ca, Cr, and Zn (Figure 5(P4)), an indication that (Mg,Zn)Al2O4 or Mg(Al,Cr)2O4 spinel solid solutions was found, which matched well with the XRD analysis. The bonding content between corundum and MgAl2O4 spinel in the matrix was intensified via the formation of spinel solid solutions, which contributed to augmenting their cold and hot strength [16,20,25].
As the firing temperature increased, ZnCr2O4 and ZnAl2O4 formed from reactions between ZnO (decomposition of Zn(OH)2) and Al2O3 and Cr2O3 dispersed in the matrix fired at 600 °C to 1000 °C. These newly formed ZnCr2O4 and ZnAl2O4 can link among the corundum and spinel particles in the matrix, which can compensate for the strength loss as a result of decomposition of CAC hydration products. Therefore, the bonding degree of the matrix was strengthened, thereby improving cold strength of the sample at 600–1000 °C. At 1400–1600 °C, flake CA6 grains were formed by the reaction of Al2O3 and CA2, while ZnAl2O4 and ZnCr2O4 particles interspersed within the plate-like CA6 and spinel structure to generate an interwoven structure. The interface bonding strength between particles was greatly improved, thereby notably enhancing the strength of the castables containing Zn(OH)2) [16,20,25,26].

4. Conclusions

The corundum-spinel castables were prepared using tabular alumina aggregates, corundum fines, fused spinel fines, ultrafine alumina, and calcium aluminate cement as the matrix. To enhance the intermediate-temperature strength of the castables, Zn(OH)2 fines were incorporated as an additive, and the effects of Zn(OH)2 content on the phase composition, microstructure, and properties of the castables were systematically investigated. The results are as follows:
(1)
The bulk density and cold strength of the specimens increased significantly with rising of Zn(OH)2 content and increasing of heat-treatment temperature. Apparent porosity decreased, and PLC gradually decreased after firing at 1400–1600 °C. HMOR values also presented a notable improvement. These may be ascribed to enhanced sintering densification behavior induced by incorporating Zn(OH)2, which decreased porosity and increased bulk density; therefore, their physical properties were enhanced.
(2)
The enhanced intermediate-temperature strength of Zn(OH)2-containing castables after firing at 600–1000 °C was attributed to in situ formation of ZnAl2O4 and ZnCr2O4 bonding phases in the matrixes. These newly formed ZnAl2O4 and ZnCr2O4 mixed phases created a reinforcing effect that outweighs the strength reduction from CAC hydrate decomposition. In addition, the strengthening effect intensified with rising heat-treatment temperature, which could be the reason for the strength improvement.

Author Contributions

Methodology, Y.D., M.H., M.S., X.Q., X.L. and Q.J.; Software, Y.D.; Formal analysis, Y.D. and P.X.; Resources, Q.J.; Data curation, Y.D., X.Q. and Q.J.; Writing—original draft, Y.D. and P.X.; Writing—review & editing, P.X. and Q.J. All authors have read and agreed to the published version of the manuscript.

Funding

The authors are grateful to the National Natural Science Foundation of China (Grant No. 52172029), the Collaborative Innovation Major Special Project of Zhengzhou (Grant No. 20XTZX12025), and Postgraduate Education Reform and Quality Improvement Project of Henan Province (Grant No. YJS2024JD03) for supporting this work.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Materials 18 02777 g001
Figure 1. Variation in physical properties as a function of Zn(OH)2 content. (a) B.D., (b) A.P., (c) PLC, (d) CMOR, and (e) CCS.
Figure 1. Variation in physical properties as a function of Zn(OH)2 content. (a) B.D., (b) A.P., (c) PLC, (d) CMOR, and (e) CCS.
Materials 18 02777 g001
Materials 18 02777 g002
Figure 2. TSR of samples after treating at (a) 1000 °C and (b) 1600 °C.
Figure 2. TSR of samples after treating at (a) 1000 °C and (b) 1600 °C.
Materials 18 02777 g002
Materials 18 02777 g003
Figure 3. HMOR of the samples tested at 1400 °C for 1 h.
Figure 3. HMOR of the samples tested at 1400 °C for 1 h.
Materials 18 02777 g003
Materials 18 02777 g004
Figure 4. XRD patterns of the samples after treating at (a) 600 °C, (b) 800 °C, (c) 1000 °C, and (d) 1400 °C, and (e) comparison with the sample ZH4 and sample ZH5.
Figure 4. XRD patterns of the samples after treating at (a) 600 °C, (b) 800 °C, (c) 1000 °C, and (d) 1400 °C, and (e) comparison with the sample ZH4 and sample ZH5.
Materials 18 02777 g004
Materials 18 02777 g005
Figure 5. SEM images of matrix samples ZH0(a,c,e,g) and ZH2(b,d,f,h) after treating at different temperatures: (a,b) 600 °C, (c,d) 800 °C, (e,f) 1000 °C, (g,h) 1400 °C. And EDS spectrum of points 1–4.
Figure 5. SEM images of matrix samples ZH0(a,c,e,g) and ZH2(b,d,f,h) after treating at different temperatures: (a,b) 600 °C, (c,d) 800 °C, (e,f) 1000 °C, (g,h) 1400 °C. And EDS spectrum of points 1–4.
Materials 18 02777 g005
Table 1. Formulation of the castables matrix with Zn(OH)2 powders addition (wt.%).
Table 1. Formulation of the castables matrix with Zn(OH)2 powders addition (wt.%).
Raw MaterialsMass Ratio (%)
ZH0ZH1ZH2ZH3ZH4ZH5
Corundum powders54.704.404.103.8013.80
MgAl2O4 powders10101010100
α-Al2O3 ultrafine888888
Cr2O3 powders333333
CAC444444
Zn(OH)2 powders00.300.600.901.201.20
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MDPI and ACS Style

Dong, Y.; He, M.; Sang, M.; Qiu, X.; Xu, P.; Liu, X.; Jia, Q. Enhancement of Intermediate-Temperature Strength of Corundum-Spinel Castables via Incorporation of Zn(OH)2 Powders. Materials 2025, 18, 2777. https://doi.org/10.3390/ma18122777

AMA Style

Dong Y, He M, Sang M, Qiu X, Xu P, Liu X, Jia Q. Enhancement of Intermediate-Temperature Strength of Corundum-Spinel Castables via Incorporation of Zn(OH)2 Powders. Materials. 2025; 18(12):2777. https://doi.org/10.3390/ma18122777

Chicago/Turabian Style

Dong, Yifan, Mantang He, Mengyang Sang, Xin Qiu, Pengyu Xu, Xinhong Liu, and Quanli Jia. 2025. "Enhancement of Intermediate-Temperature Strength of Corundum-Spinel Castables via Incorporation of Zn(OH)2 Powders" Materials 18, no. 12: 2777. https://doi.org/10.3390/ma18122777

APA Style

Dong, Y., He, M., Sang, M., Qiu, X., Xu, P., Liu, X., & Jia, Q. (2025). Enhancement of Intermediate-Temperature Strength of Corundum-Spinel Castables via Incorporation of Zn(OH)2 Powders. Materials, 18(12), 2777. https://doi.org/10.3390/ma18122777

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