The fabrication of porous corundum spheres with core-shell structure for corundum-spinel castables

Materials and Design 85 (2015) 574–581

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The fabrication of porous corundum spheres with core-shell structure for corundum-spinel castables Huan Chen a, Lei Zhao a,⁎, Xuan He a, Wei Fang a, Zhong-xing Lei b, Hui Chen a a b

The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, PR China Wisco Refractory Col., Ltd., Wuhan 430081, PR China

a r t i c l e

i n f o

Article history: Received 28 January 2015 Received in revised form 1 July 2015 Accepted 6 July 2015 Available online 11 July 2015 Keywords: Porous corundum spheres Core-shell Corundum-spinel castables Corrosion resistance Thermal conductivity

a b s t r a c t Porous corundum spheres (PCSs) that possess core-shell structure were fabricated by foaming and wet granulation method and used as aggregates to prepare the porous corundum-spinel castables (PCSCs). The microstructure and properties of PCS and PCSC have been studied. The results showed that the microporous shell and macroporous core were formed in PCS with the bulk density of 3.08 g · cm−3 and the apparent porosity of 10.9%. In addition, the median pore diameter of PCS is 6.47 μm. The thermal conductivity of PCSC is 0.753 W/m · K at 1000 °C, and PCSC showed the higher corrosion index and better resistance to slag penetration than corundum-spinel castables (CSC) using the tabular corundum as aggregates. Compared with CSC, the bulk density of PCSC is decreased by 6.3% at 110 °C, 4.6% at 1550 °C, and the thermal conductivity decreased by 13.3–21.4% from 200 °C to 1000 °C. The results demonstrated that PCS with designed core-shell structure can achieve the trade-off between thermal conductivity and corrosion resistance. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Corundum-spinel castables (CSCs) are widely used as working linings of steel ladles due to their excellent properties, such as good thermal stability, high corrosion, and wear resistance [1–4]. However, the high thermal conductivity of CSC usually results in a striking decrease of temperature of the molten steel in the steel ladles and leads to high temperature of the shell at the external surface of steel ladles [5,6]. Therefore, the heat insulation refractories of wear lining urgently need to be concerned for the refractory industry. It is well known that heat insulation materials with low thermal conductivity usually possess high porosity. However, the high porosity might bring high apparent porosity to refractory material and decrease its corrosion resistance. In previous studies, researchers devoted significant effort [7–10] to decrease the thermal conductivity of wear lining. Yan et al. [8] prepared porous MgO–Al2O3 refractory aggregates containing 30–92 wt.% Al2O3 via an in-situ decomposition pore-forming route with magnesite and Al(OH)3 being used as starting raw materials. The use of these lightweight aggregate has positive effect on the heat-shielding performance of wear lining of steel ladles. However, the high apparent porosity and large pore size of the prepared refractories resulted in serious erosion. The key challenge for the development of wear lining of steel ladles lies in achieving guaranteed resistance against slag corrosion. Fu et al. [10] focused on impact factors on the properties of alumina–magnesia ⁎ Corresponding author at: 947 Heping Road, Wuhan, Hubei 430081, PR China. E-mail address: [email protected] (L. Zhao).

http://dx.doi.org/10.1016/j.matdes.2015.07.033 0264-1275/© 2015 Elsevier Ltd. All rights reserved.

castable prepared with microporous corundum aggregate. Although the performance of slag resistance of this castable is better than that of a common one, the thermal conductivity hardly reduces significantly. Therefore, in order to achieve the trade-off between thermal conductivity and corrosion resistance, porous corundum spheres (PCSs) constructed by microporous shell and macroporous core were designed by our research group as shown in Fig. 1. The microporous shell is expected to exhibit good slag resistance, and at the same time. The macroporous core is a great help to reducing thermal conductivity. In this work, the designed PCSs with the core-shell structures were fabricated through foaming and wet granulation method. Both the microstructure and pore size distribution of PCS were characterized. Moreover, used as aggregates, a novel type of porous corundum-spinel castable (PCSC) was prepared. The physical properties at room temperature were studied and thermal conductivity and performance of slag resistance of samples were characterized as well. 2. Experimental 2.1. Preparation of porous corundum spheres (PCSs) The foam was prepared according to the literature [11]. The morphology and microstructure of prepared stable foam with multilayered liquid film structure are shown in Fig. 2. Industrial alumina (γ-Al2O3, d50 ~44 μm) was used as raw materials. The route for the preparation of the PCS is shown in Fig. 3. Firstly, the alumina slurry and foam were uniformly dispersed in cement mixer to obtain the alumina foam slurry. Then, the as-prepared slurry was

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Table 1 Main chemical constituents of the RH refining furnace slag (wt.%). SiO2

Al2O3

Fe2O3

CaO

MgO

MnO

15.92

1.71

24.35

42.92

5.14

3.36

(AutoPore IV 9500, Micromeritrics Instrument Corporation, and USA). The apparent porosity (πa) and bulk density (ρb) of PCS were analyzed according to ISO 5017:1998 and 5014:1997. The bulk density of broken PCS was analyzed according to 5014:1997. The true density (ρt) of PCS was measured by automatic true density analyzer (ACCUPYC 1330, Micromeritrics Instrument Corporation, and USA). The closed porosity (πf) of PCS was calculated by the equation: ρt −ρb  100% ρt

ð1Þ

Closed porosity ðπ f Þ ¼ πt −πa :

ð2Þ

Porosity ðπt Þ ¼ Fig. 1. The schematic diagram of the structure of porous corundum spheres.

poured into sugarcoating machine in which dry alumina powder had been arranged in advance. After a 5-minute run of the turntable of sugarcoating machine, the alumina foam slurry was coated completely by dry alumina powder, and the alumina spheres were manufactured. After 6 h curing at room temperature, alumina spheres were dried at 110 °C for 24 h, and sintered at 1800 °C for 3 h. The corundum spheres were obtained after final furnace-cooling. The microstructures of PCS were observed using scanning electron microscopy (FESEM, Nova NanoSEM400, FEI Company, USA) equipped with an energy dispersive X-ray spectroscope (EDS, Noran 623 M-3SUT, Thermo Electron Corporation, Japan). The phase compositions of PCS were investigated by X-ray diffraction (XRD, X'Pert, Philips, Netherlands). The pore-size distribution and average porediameter of PCS were studied using mercury intrusion porosimetry

2.2. Preparation of porous corundum-spinel castables (PCSCs) The PCS, sintered alumina–magnesia spinel (Al2O3, 78 wt.%), tabular alumina and sintered alumina–magnesia spinel (Al2O3, 78 wt.%) powder (b0.088 mm), α-Al2O3 micro-powder, and calcium aluminate cement were used as raw materials. PCSCs were prepared into 125 mm × 25 mm × 25 mm-sized for the physical property measurements at room temperature, 70 mm × 70 mm × 70 mm-sized cubes with an aperture of Ф (20–30 mm) × 40 mm for slag resistance study, and Ф180 mm × H20 mm for the thermal conductivity investigation. Then, the samples were dried at 110 °C for 24 h and heated at 1550 °C for 3 h after curing at room temperature for 24 h., The furnace was then cooled down to the room temperature. The tabular corundum

Fig. 2. The morphology and microstructure of the foam.

Fig. 3. The preparing route of porous corundum spheres.

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The slag corrosion tests of samples were carried out by static crucible method. RH refining furnace slag (30 g) was put in the cube (70 × 70 × 70 mm) with an aperture of Ф (20–30 mm) × 40 mm. The slag composition is provided in Table 1, basicity m(CaO)/m(SiO2) = 2.7. After heating at 1550 °C for 3 h, corroded samples were then cut horizontally into two pieces and corrosion/ penetration indices were calculated using equations [12]: Corrosion Index ¼

S1−S0  100% S0

Penetration Index ¼

S2−S0  100% S0

ð3Þ

ð4Þ

where S0 is the cross-sectional area of aperture, S1 is the corrosion area, and S2 is the penetration area. 3. Results Fig. 4. The XRD patterns of PCS and TC.

3.1. Characterizations of porous corundum spheres (PCS) 3.1.1. Phase identification As shown in Fig. 4, the PCS and TC are identified from the XRD patterns. The results show that only corundum phase is detected both in PCS and in TC. Upon heating, Wefers [13] showed that γ-Al2 O 3 undergoes a series of polymorphic transformations: γAl2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O3 (corundum). When temperature is higher than 1500 °C, only corundum phase is left in the alumina sphere. This outcome can be testified for the experimental findings. 3.1.2. Pore size distribution Fig. 5 shows the pore size distribution patterns of PCS and TC. It exhibits that the pore size of PCS distributes mainly in a concentrated range of 6–7 μm. In the case of TC, the pore size is mainly in the range of 1–5 μm which indicates the existence of pores in different sizes in TC. In addition, the median pore diameters of TC and PCS are 0.95 μm and 6.47 μm, respectively.

Fig. 5. Pore size distributions of the PCS and TC.

(TC) was used as aggregate to fabricate CSC which was regarded as a reference system. The apparent porosity and bulk density of samples were analyzed according to ISO 5017:1998 and 5014:1997. The thermal conductivity of samples at 200, 400, 600, 800, and 1000 °C were tested according to ISO 8301:1991.

3.1.3. Microstructure The SEM images of PCS cross-section are shown in Fig. 6. Fig. 6(b) is the amplified image of the areas marked by white solid line in Fig. 6(a). From Fig. 6(a), it can be seen clearly that PCS possess the microporous shell and macroporous core structures. Insets a1 and a2 showed that in the core domain of PCS, pores coming from foam are about 45 μm in size, while the pores are 6–7 μm in size in the shell after sintering at 1800 °C. The boundary between the core and shell is marked with a round white dotted line. The macroporous core of PCS may originate from the alumina foam slurry and the microporous shell may form

Fig. 6. The SEM images for cross-sectional of the porous corundum spheres (a): the images of shell and core; inset (a1): amplified SEM images of white dotted line box in (a); inset (a2): amplified SEM images of white solid line box in (a); (b): enlarged the areas that been marked by white solid line box in (a).

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Table 3 The physical properties of PCSC and CSC at room temperature.

PCSC CSC

Fig. 7. The SEM images of the cross-section of TC.

through the dry alumina powders coating on the spheres of alumina foam slurry. The SEM images in Fig. 7 show the cross-section of TC. It is clearly observed that a lot of pores with diameter around 1 μm are generated between grains.

3.1.4. The physical properties at room temperature Table 2 shows the physical properties of PCS and TC at room temperature. The bulk density of PCS is 3.08 g · cm− 3, which decreases by 12.3% comparing with that of TC. When the PCS is broken, the coreshell structures of PCS are damaged and macroporous core is exposed to the air. As a result, the apparent porosity of PCS increases from 10.9% to 15.5% and closed porosity declines from 13.6% to 6.6%. Therefore, the bulk density of the broken PCS increases by 3.2% in contrast with the unbroken one, which further illustrated that the microporous shell and macroporous core structures have been well constructed in PCS. Meanwhile, the closed porosity of unbroken PCS is four times as large as that of TC, indicating that the core-shell structure of PCS can refrain from the substantial increase of apparent porosity and lead to the high closed porosity. The porosity of PCS is greater than that of TC which is dependent on the sintering temperature as well (approximately at 1900 °C) [14].

Permanent linear change/%

Apparent porosity/%

1550 °C +0.34 +0.64

110 °C 18.7 12.6

Bulk density/g · cm−3 1550 °C 22.6 17.7

110 °C 2.96 3.16

1550 °C 2.90 3.06

3.2.2. Thermal conductivity Fig. 8 showed the thermal conductivities of PCSC and CSC at different temperatures. It can be seen that the thermal conductivity of PCSC is generally lower than that of CSC by 13.3–21.4% from 200 °C to 1000 °C and is 0.753 W/m · K at 1000 °C. It was reported that high porosity, small sized pores, complicated pore structure and small sized grains could be the main reasons for low thermal conductivity [15]. In our case, the present bubbles can not only enlarge the pore size of PCSC, but also change other pore parameters such as pore structure and porosity. Consequently, the generated spherical pores and high porosity in PCSC give rise to a relatively low thermal conductivity in comparison with that of CSC [16–18]. Moreover, most of spherical pores preserved from foam in the size of 45 μm mainly induce the closed pores which dispersed in the core of PCS. These pores normally block the heat conduction pathways, and resulting in a low thermal conductivity in further. These aforementioned distinct results of the porosity dependence on thermal conductivity can be partially explained by the rule-of-mixtures: kT ðTÞ ¼ πt kg ðTÞ þ ð1−πt Þks ðTÞ

ð5Þ

where πt is the porosity, kg (T) is the gaseous thermal conductivity (e.g., 2.53 × 10− 5 W/m · K of air at room temperature), and ks (T) is the intrinsic thermal conductivity of the solid material. The value of kg (T) is relatively small comparing with the thermal conductivity of CSC and could be ignored, especially at room temperature. 3.2.3. Performance of slag resistance The cross sections of PCSC and CSC after the slag attacked for 3 h at 1550 °C are shown in Fig. 9. S0, S1, and S2 were marked in corroded samples. According to Eqs. (3) and (4), the corrosion and penetration index were calculated and the results of corrosion measured by static slag resistance method were given in Fig. 10. PCSC showed the higher

3.2. Characterizations of porous corundum-spinel castables (PCSCs) 3.2.1. The physical properties at room temperature Table 3 gave the detailed information of the physical properties of PCSC at room temperature. It can be seen that both PCSC and CSC have good volume stability since their permanent linear change is lower than 1% at 1550 °C. In comparison to the CSC, the bulk density of PCSC decreases by 6.3% at 110 °C and 4.6% at 1550 °C, which might be due to the higher apparent porosity of PCS than that of TC.

Table 2 The physical properties of PCS and TC at room temperature. Corundum aggregate PCS TC

Unbroken Broken

True Apparent Bulk density/g · cm−3 density/g · cm−3 porosity/%

Closed porosity/%

3.08 3.18 3.51

13.6 6.6 3.4

4.08 4.08 3.94

10.9 15.5 7.5

Fig. 8. The thermal conductivities of PCSC and CSC at different measure temperatures.

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Fig. 9. Cross-sections of PCSC and CSC after slag resistance test.

Fig. 10. Corrosion and penetration index of PCSC and CSC after slag resistance test.

corrosion index than CSC and showed better resistance to slag penetration. Its resistance to slag corrosion was not good as CSC because of its high specific surface area with reactivity with slag.

Fig. 11(a) and (b) show the elemental line scan of specimens of PCSC and CSC from corrosion layer to original layer after slag corrosion test at 1550 °C. The layer of complex spinel (labeled a) is marked in Fig. 11. The original layer is on the right side of dotted line b. The middle section is the penetration layer. It is observed that the variation of Fe/Ca/Si elements' concentration is much greater than that of Mn element which indicated that Fe/Ca/Si elements easily penetrate in the specimens as shown in Table 4. It may originate from the reaction of CaO in the slag with Al2O3 (especially free Al2O3) of the spinel clinker. The reaction may result in the saturation of slag in Al2O3 and the increase of viscosity and melting temperature, which impeded the slag penetration (Point 1 and 5). Meanwhile, the spinel grains in the castable matrixes are fully dissolved and recrystallized. When they contact with liquid slag, they entrap slag ions such as Fe and Mn to increase the local concentration of silica in the slag (Point 2 and 6) and then its viscosity inhibit the further penetration. In the PCSC and CSC containing Al2O3-rich spinel, free MgO hardly exists (as it is probably in Al2O3-rich spinel), no continuous layer of spinel that could act as a physical barrier is detected due to the low content of magnesia [19]. Furthermore, since the CaO–Al2O3 phases (Points 3 and 7) form, the CaO–Al2O3 compounds, such as calcium dialuminate (CA2) and calcium hexa-aluminate (CA6), are precipitated at corundum aggregate surface to block the infiltration path of Fe and to further hinder the penetration of slag. As identified by the inset EDS spectrum in Fig. 11, there is almost no Fe at the Points 4 and 8 in original layer.

Fig. 11. Line scan back scattered electron images of PCSC and CSC from corrosion layer to original layer after slag corrosion test at 1550 °C for 3 h, (a) PCSC, (b) CSC.

H. Chen et al. / Materials and Design 85 (2015) 574–581 Table 4 The EDS analysis (wt.%) of PCSC and CSC of insets in Fig. 11.

MgO Al2O3 SiO2 CaO MnO Fe2O3

1

2

3

4

5

6

7

8

16.74 58.06 0.03 1.17 3.67 20.33

0.39 56.45 10.57 23.34 – 9.25

1.42 89.44 – 8.00 – 1.14

1.08 89.70 1.38 7.52 – 0.32

17.93 61.84 0.07 0.14 2.98 17.04

1.32 58.25 12.13 21.71 – 6.59

2.37 89.11 0.95 6.08 – 1.49

1.71 89.44 1.11 7.07 – 0.67

Fig. 12(a) and (c) reveal the microstructure of the corroded samples, while Fig. 12(b) and (d) show the SEM images of the compacted layers with the higher resolution. It can be observed from Fig. 12(b) and (d) that the compact layers in PCS and TC are both made of flake-like CaO–Al2O3 compounds which indicates the interaction between the unreacted aggregates and the molten slag. It also can be seen that a suitable number of aggregates coated by CA6 appear before the slag attack (Points 7 and 8 in Fig. 11). 4. Discussions 4.1. Formation mechanism of PCS core-shell structure Fig. 13 shows the schematic illustration of the formation of pores when increasing the temperature. The α-Al2O3 phase transformation occurs during the process of nucleation and growth [20]. Anja Terzić et al. [21] reported that the transition of θ-Al2O3 appeared at approximately 900 °C and transformed completely into α-Al2O3 at 1200 °C.

579

During the reconstructive transformation from θ-Al2O3 to α-Al2O3, there is a specific volume reduction (from 28.6 to 25.6 cm3/mol Al2O3) due to the increase of density (from 3.6 to 3.986 g/cm3 Al2O3) [13,22]. The low intrinsic nucleation density results in large space among single crystal α-Al2O3 grains surrounded by continuous pore channels. When temperature is higher than 1500 °C, the size of α-Al2O3 grains is almost unchanged all the time [23]. Then, the pores in scale of micrometers form in grain clusters which are composed of many grains when increasing the sintering temperature. Consequently, the size of micropore PCS is bigger than that of TC due to lower sintering temperature. Generally, a layer is present between the core and the shell because the linear shrinkage of the alumina foam slurry is larger than that of dry alumina powders after drying and sintering. However, from the domain circled by white dotted line in Fig. 6(b), a continuous phase can be observed. It may be attributed to (1) the stable bubble and the good chemical stability of alumina ceramic to form the macroporous core [24] and (2) the occurrence of mass-transfer from matter to the pores which play the role of vacancy sources. The stress between particles when they contact each other drives directional migrations of phase. Therefore, the adjacent grains moved in the opposite direction at the boundary between the microporous shell and macroporous core, and the layer didn't appear. Finally, a complete core-shell structure was shown in Fig. 6. 4.2. Analysis of the thermo-physical properties of PCSC According to the study of E.Y. Sako et al. [25], CA6 normally forms at roughly 1400 °C and achieves maximum value of expansion rate at nearly 1500 °C in cement-bonded high alumina refractory castables.

Fig. 12. SEM images of the reaction interface between slag and PCS (a) or TC (c), (b) and (d) enlarged the areas of “P” and “L” in (a) and (c).

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Fig. 13. Schematic illustration for the formation of pores.

Two different mechanisms took place during the corundum-spinel castables' linear change behavior when increasing the temperature: one is the sintering shrinkage and the other is CA6 generation. According to the previous work [26], the CA6 expansion depends mainly on the crystal growth through a liquid-phase diffusion procedure which results in high linear change. However, since there is little free microsilica in the corundum-spinel castables, the lack of liquid phase leads to good volume stability for PCSC and CSC. 4.3. Slag resistance mechanism of PCS and TC Fig. 14 shows the schematic diagram of the reaction between molten slag and different corundum aggregates. During the reaction, slag penetrates into aggregate pores resulting in the formation of CaO–Al2O3 system phases. When the corundum aggregates are coated by CA6 crystals, they react with the high-CaO slag and with CaO in calcium aluminate cement. As a consequence, a dense CA2 layer will form as an indirect dissolution product at the interface. This compacted layer acts as a protective barrier to prevent further infiltration, providing excellent slag corrosion indexes. According to Ostwald's supersaturation theory [27], nucleation free energy depends on the supersaturation of the solution (S), net interfacial energy of nucleation (σ), temperature (T), and superficial area of grains (A): ΔG ¼ −T lnS þ σA:

ð6Þ

According to Eq. (6), increasing the supersaturation of the solution leads to nucleation of the second phase.

Flaky crystals are mainly composed of CaO and Al2O3. We consider CaO to be the matrix while Al2O3 to be the second phase. In the following parts, the dissolution and supersaturated precipitation of Al2O3 in CaO are discussed. The slag resistance experiment was carried out at 1550 °C in this study. As shown in Fig. 15, theoretically, solid phase calcium aluminates (CA) appear at this temperature when the mass ratio of Al2O3 and CaO is 1.57. This means that when the mass ratio is greater than 1.57, the slag will be supersaturated and calcium aluminates will precipitate out. Considering the differences of pore size and porosity of aggregates between the two samples, the observed difference in slag resistance of the two samples is mainly caused by the relation between pores and slag penetration. When the aggregates are exposed to the slag, the pores, which are the weakest parts, are the first to be penetrated by the slag. Assuming that the voids in the aggregates are of spherical shape and that the slag penetration along each direction in the two aggregates occurs at the same rate, the mass ratio of Al2O3 and CaO in each pore can be calculated from Eq. (7):
 4 4 4 πðx þ rÞ3 − πðrÞ3 ρa þ πðx þ rÞ3 ρs ωAl2 O3 3 3 3  100% ω¼ 4 3 πðx þ rÞ ρs ωCaO 3

ð7Þ

where ω is the mass ratio of Al2O3 and CaO, x is the dissolved depth of the aggregate in slag, r is the radius of the pore, ρa is the true density of the aggregate, ρs is the density of the slag, and ωAl2 O3 and ωCaO are the mass fraction of Al2O3 and CaO in slag, respectively. The true densities of the aggregates are listed in Table 2. The density of the slag can be estimated using Eq. (8) [28]: ρs ¼

Xn i¼1

ðρi ωi Þ

ð8Þ

where ρi and ωi are the density and mass fraction of compounds in the slag, respectively. The values for the densities of the compounds in the slag are listed in Table 5, while the mass fractions are listed in Table 1. The calculated value for ρs is equal to 3.504 g/cm3. The median pore diameters of porous and tabular corundum are 6.47 and 0.95 μm (Fig. 5), respectively. Thus, the radii of the pores are 3.235 and 0.475 μm, respectively. Introducing these values into Eq. (7), the calculation indicates that when the slag reaches its saturation point, the critical dissolved

Fig. 14. Schematic diagram of the reaction between molten slag and PCS (a), TC (b).

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Acknowledgments The authors thank the final support from the China Postdoctoral Science Foundation (No. 2015M572210).

References

Fig. 15. CaO–Al2O3 phase diagram.

Table 5 The densities of compounds in the slag. Compounds

SiO2

Al2O3

Fe2O3

CaO

MgO

MnO

Density (g/cm3)

2.32

3.97

5.24

3.32

3.59

5.40

depths of 4.08 and 0.60 μm are obtained for porous and tabular corundum, respectively. In fact, the pore size of TC is centered on the value of 1–5 μm. That distribution indicates the critical dissolved depth of tabular corundum in slag should be larger than 0.60 μm. Hence, due to the concentrated pore size distribution (6–7 μm) of PCS, the second phases are more likely to achieve supersaturation when the aggregate reacts with the slag, and massive crystal nuclei are immediately generated. Therefore, PCSC shows the higher corrosion index and better resistance to slag penetration than CSC. 5. Conclusions Porous corundum spheres (PCS) with microporous shell and macroporous core structures were successfully fabricated by foaming and wet granulation method. The bulk density of HPCS is 3.08 g · cm− 3 and the apparent porosity of PCS is 10.9%. In addition, the median pore diameter of PCS is 6.47 μm. Using PCS as aggregates, a novel type of corundum-spinel castables (PCSCs) was prepared. PCSC has a thermal conductivity of 0.753 W/m · K at 1000 °C and shows the higher resistance to slag penetration than corundumspinel castables (CSC). Compared to the CSC using the tabular corundum as aggregates, the bulk density of PCSC was decreased by 6.3% at 110 °C and 4.6% at 1550 °C, respectively. The thermal conductivity also drops by 13.3–21.4% compared with CSC in temperature range of 200 °C to 1000 °C. Due to uniform pore size (6–7 μm) and the larger porosity of PCS, PCSC showed the higher corrosion index and better resistance to slag penetration than CSC. It was demonstrated that the designed PCS with core-shell structure can achieve the trade-off between thermal conductivity and corrosion resistance.

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