Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables

Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables

Author’s Accepted Manuscript Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables Feng Wang, P...

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Author’s Accepted Manuscript Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables Feng Wang, Pingan Chen, Xiangcheng Li, Boquan Zhu www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)32842-6 https://doi.org/10.1016/j.ceramint.2018.10.049 CERI19761

To appear in: Ceramics International Received date: 8 September 2018 Revised date: 6 October 2018 Accepted date: 7 October 2018 Cite this article as: Feng Wang, Pingan Chen, Xiangcheng Li and Boquan Zhu, Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of micro-spinel powders addition on the microstructure and properties of alumina refractory castables Feng Wang, Pingan Chen*, Xiangcheng Li*, Boquan Zhu The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China * Corresponding author: [email protected] and [email protected]; Tel: +86-027-68862616; Fax: +86-027-68862616 Abstract The microstructural evolution and comprehensive properties of alumina refractory bonded with calcium aluminate cement and silica sol have been studied. Results have been correlated with the microstructural and phase evolutions using X-ray diffraction and scanning electron microscopy, as function of the pre-formed spinel powders. Matrix samples were obtained for phase and microstructural characterization in details, and the results were compared with those corresponding to the refractory castables. The room temperature and high temperature properties, including permanent of linear change, mechanical properties, hot modulus of rupture (HMOR) and slag resistance were measured. The castables exhibited a microstructural optimization and properties enhancement due to the addition of pre-formed spinel. A lot of secondary spinel and CA6 (CaO6Al2O3) with small size were produced in the castables, and the contents of micro pores were greatly increased. As a result, the permanent of linear change of castables was decreased by 61%, while cold modulus of rupture (CMOR) and HMOR were increased more than 45% and 100%, respectively. The penetration indexes in the static slag resistance were decreased from 28.8% to 12.2%. Keyword: Pre-formed spinel; Secondary spinel; Calcium aluminate cement and silica sol; Mechanical properties

1. Introduction Refractory castables are widely used in the high temperature field due to their many advantages, such as environmental protection, simple construction, high 1

production efficiency and so on [1-3]. One of the most important components in the castables is the binder, which provides the workability and early strength for the castables. Calcium aluminate cement (CAC), one of the extensive used binders, could provide high early strength, excellent working ability and resistance to many chemically aggressive conditions. The high early strength of calcium aluminate cement is obtained by the formation of hydrates such as CAH10 (CaOAl2O310H2O ), C2AH8 (2CaOAl2O38H2O) and C3AH6 (3CaOAl2O36H2O) [4, 5], though the strength of castables at mid-temperature could be drastically reduced due to the hydrates conversion and dehydration process during curing process in hot and humid conditions. These processes would lead to the collapse and spalling of castables. In order to solve these problems, many methods have been used to improve the mid-temperature of castables bonded with calcium aluminate cement. One of them is to utiliz the nano/micro SiO2 powders in the matrix. By this method, the hydrates conversion reaction can be inhibited and some kinds of compounds with better mechanical properties than C3AH6 are obtained [6-10]. For example, Mostafa [11] revealed that C2ASH8 (2CaOAl2O3SiO28H2O, stratllingite) compound had better mechanical properties than C3AH6 (hydrogarnet ) by adding nano SiO2 powders in the cement, while Rupasinghe [12] also found that nanosilica enhanced the mechanical properties of castables due to the filler effect of nano particles through inducing more nucleation sites and the formation of stratlingite. Li [13] further utilized the synergistic effects of micro- and nano-silica to improve the strength of concrete. The addition of nano/micro SiO2 powders indeed improves the mid-temperature strength of castables, though the high temperature properties are drastically deteriorated due to the formation of low melting phase in the CaO-Al2O3-SiO2 system. And our previous experimental results showed that there was obvious transformation and shrinkage for castables after high temperature heating treatment. The transformation and shrinkage, as well as reduction of high temperature properties (such as high temperature mechanical properties, slag resistance and thermal shock resistance), would have a bad impact on the application of castables. Therefore, the use of SiO2 powders in the castables is strictly restrained to avoid these disadvantage factors. 2

Spinel has excellent properties of high-temperature strength, slag resistance and thermal shock resistance [14-16]. Thus, they are often used in the refractory industry, such as ladles and purging plugs, among others to enhance the high temperature properties. The castables containing spinel powders had less cracks and high bonding strength between CA6 (CaO6Al2O3) and spinel grains, resulting in high mechanical properties and high hot strength [17-19]. Braulio [20] thought that spinel grains were helpful in promoting the formation of a protective spinel layer at the liquid-solid interface during the corrosion process and preventing further attack to the refractory. Korgul [21] and Mori [22] further showed that alumina-spinel castables could effectively remove MnO/FeO/Fe2O3 from slag to prevent slag corrosion, the life of which was twice as long as that of high-alumina bricks. Besides the traditional advantages of spinel, it is found that ternary compounds can be formed between spinel and liquid phase in Al2O3-CaO-SiO2 system [23-27]. So the low melting phase in Al2O3-CaO-SiO2 system is consumed, which is beneficial for improving high temperature properties of the castables. And these ternary compounds with high melting point could absorb the Ca2+ in the slag and release Mg2+ and Al3+ forming CA2( CaO2Al2O3) and spinel layers, so the slag viscosity was increased and the slag penetration was retained to obtain high refractoriness. The above research findings reveal that the addition of pre-formed spinel not only has a positive influence on the slag resistance of castables, but also may improve the mechanical properties and high temperature

strength

of

castables,

especial

for

the

castables

containing

Al2O3-CaO-SiO2 system. However, little research has been reported on the castables bonded with calcium aluminate cement and silica sol to improve their properties by the addition of pre-formed spinel. Thus, to make full use of the advantages of spinel and double binder system (calcium aluminate cement and silica sol), pre-formed spinel powders were incorporated into castables to improve properties in this paper. Both room temperature and high temperature properties were measured, and the microstructures of castables were characterized in details. Based on the properties change and microstructure evolution of castables bonded with calcium aluminate cement and 3

silica sol, the influence mechanism of pre-formed spinel on the castables was discussed. 2. Experimental The raw materials of castables are included as follows: Fine tabular alumina (98.85%, ≤0.088mm; Zhejiang Zili Co., Ltd., China) as matrix powders, coarse tabular alumina (98.85%; 5~3mm, 3~1mm, and 1~0mm; Zhejiang Zili Co., Ltd., China) as aggregate powders, α-Al2O3 micro powders (99.23%, d50=4.1μm; Zhejiang Zili Co., Ltd., China), calcium aluminate cement (Secar71; Kerneos, France), silica sol (10-20nm, 30% solid content, Jinan Yinfeng Silicon Products Co., Ltd., China) and dispersants (FS10; BASF SE, Germany). The pre-formed spinel powders (99%, d50=37μm; Zhejiang Zili Co., Ltd., China) were used. The compositions of pre-formed spinel powders and castables are shown in Table 1 and Table2, respectively. Table 1 Compositions of the pre-formed spinel powders Compositions

Al2O3

MgO

R2O (NaO, K2O)

SiO2

Fe2O3

wt.%

88.96

9.65

0.23

0.20

0.17

Table 2 Compositions of the refractory castables Specimen Tabular corundum

5~0.088 mm

≤0.088 mm Pre-formed spinel powder α-Al2O3 powder Secar71 cement FS10 (additive) Silica sol

wt.% A

B

C

D

E

65

65

65

65

65

20 0 10 5 0.15 5

15 5 10 5 0.15 5

10 10 10 5 0.15 5

5 15 10 5 0.15 5

0 20 10 5 0.15 5

All the raw materials were poured into a cement mortar mixer (JJ-5; Wuxi Jianyi Instrument and Machinery Co., Ltd, China) and mixed for about 5-8 min. Then, the mixture of water and silica sol was added gradually into the mixture. The water content of silica sol was considered as a part of mixed water. After that, the samples were cast into a bar-shaped mold (25mm×25mm×140mm), cured in air for 24h, and 4

then dried at 110°C for 24h. Finally, the castable samples were heated in an electric furnace (Luoyang Precondar Heat Resistant Test Equipment Co., Ltd, China) at 1100°C and 1500°C for 3h, respectively. The apparent porosity, cold modulus of rupture (CMOR), and cold compressive strength (CCS) of the castables after heating treatment were determined. The CMOR and CCS reported were the average values of three measurements. The apparent porosity was measured based on the Archimedes method by using water as fluid. The castables fired at 1500°C were used in the thermal shock resistance test. During the test, the castables were heated to 1100°C and then held for 20min according to the Chinese National Standard (YB/T 376.1-1995). Then the castables were placed in cold water. After recycling three times, the residual CMOR were measured to evaluate the thermal shock resistance. The slag resistance was carried out by the static crucible method. The 30g slag was added into hole of the crucible and then fired at 1500°C for 3h in a furnace. After cooling, the crucible was cut along the axis of the hole. The section across of crucible was measured and the penetration index was calculated by the Adobe Acrobat Pro software. Also the sectioned crucible was fixed by resin and polished to characterize the slag resistance by SEM. The slag composition is shown in Table 3. Table 3 Chemical composition of ladle slag Composition

CaO

SiO2

Al2O3

MgO

Fe2O3

MnO2

Content (wt%)

48.5

15.2

3.1

4.9

25.7

2.7

XRD analysis was performed by X-ray diffraction (X’Pert Pro, Philips, Netherland) at a scanning rate of 2/min in the scanning range of 10-90 to characterize the phase composition and peaks shift of the specimen. The microstructure and element distribution of the specimen were analyzed using a scanning electron microscope (SEM, Nova nano 40, FEI company, USA) equipped with an energy dispersive spectrometer (INCA PentaFET-x3, Oxford Instruments, UK). 3. Results and discussion 5

3.1 Microstructure and phase characterization of castables Fig. 1 shows the microstructure of castables fired at 1500°C. Many large size pores can be seen in the castables without spinel addition in the Fig. 1 (a), and the bonding between aggregates and matrix is loose. Also, there exists liquid phase in the pore, as shown in the Fig. 1 (b). The liquid phase is composed of Si, O, Ca and Al, which would be shown in the latter results. Compared with that, there are no obvious cracks in the castables added with spinel in Fig. 1 (c), the pores are less, the aggregates and matrix are bonded tightly without obvious gaps, though there is still liquid phase in the castables in Fig. 1 (d). The SEM results suggest that the addition of pre-formed spinel powders not only obtain closely packing of particles in the castables, but also prompt the sintering behavior between particles.

Fig. 1 The microstructure of castables with pre-formed spinel powders. (a-b) 0%; (c-d) 10% 6

Fig. 2 is the fracture morphologies of castables fired at 1500°C added with 10% pre-formed spinel powders. It can be seen from Fig. 2 (a) that the matrix and aggregates are bonded tightly. There are liquid phase surrounded with powders in Fig. 2 (b), which is composed of Ca-Na-Al-Si from EDS spectrum in Fig. 2 (d). In details, the octahedral spinel particles are distributed among the matrix and liquid phase, and their grains size is about 1 μm. What’s more, octahedral shape and small size spinel can also be seen at the surface of pre-formed spinel powders in Fig. 2 (c).

Fig. 2 The morphologies of castables added with 10% pre-formed spinel powders. (a-c) the morphologies, (d) EDS spectrum at point 1 The d50 of pre-formed spinel powders is about 37μm and they are irregular shape, which is much different with the above octahedral shape spinel particles, indicating that these regular shape and small size grains are in-situ formed. It is reported [26] that the MA (MgOAl2O3, spinel) would react with liquid phase CAS2 (CaOAl2O3SiO2,) (CaO2MgO8Al2O3)

to

produce and

the

intermediate

CAM-II

compound

(2CaO2MgO14Al2O3)

CaO-Al2O3-SiO2-Na2O system at 1200°C according to the equation (1).

CAS2 (Na-ss)Liq +MA  CAM-I 7

(1)

of

CAM-I in

the

CAM-I  MA  CAM-II+Liq

(2)

CAM-II  MA  CA6 +Liq

(3)

When the heating temperature rises up to 1500°C, most of CAM-I and CAM-II compounds with high concentration of cation defects are decomposed to spinel and CA6 according to the equations (2) and (3). The fast diffusion of Mg2+ leads to the secondary spinel formation not only at the surface of spinel powders, but in the corundum matrix, as the spinel powders are the only source of Mg2+ for the secondary spinel. Therefore, the secondary spinel grains are octahedron shape and their grains size is small. The fracture morphologies of castables are shown in Fig. 3 to analyze the microstructural differences due to the addition of pre-formed spinel. It can be seen that there are a lot of hexagonal plate CA6 with more than 20μm in the castables, and these CA6 particles are interlocked with powders in Fig. 3 (a-b). After added with 10% pre-formed spinel powders in Fig. 3 (c-e), the overall fracture morphologies of castables are similar with those castables without spinel. But there are some secondary spinel and CA6 particles with small size around pre-formed spinel powders, which the size of CA6 particles is about 5 μm, and there also exists liquid phase around these CA6 particles. The two kinds of CA6 grains in the castables added with pre-formed spinel can be due to the different formation mechanisms. When the heating temperature rises to 1200°C, CA (CaO·Al2O3 ) and CA2 (CaO·2Al2O3) in the cement are reacted with Al2O3 to produce CA6 according to the equations (4) and (5). This part of CA6 grains could fully grow into big size with more than 20μm with temperature increasing. However, another part of CA6 with small grain size in Fig. 3 (d-e) can be due to the decomposition of CAM-I/II at 1500°C according to equation (3), which there is no enough condition for CA6 to fully grow. And the coexistence of secondary spinel and liquid phase with small size CA6 grains also confirms the formation mechanism of the latter CA6 grains.

8

CA+A  CA2 (V / V =+13.6%)

(4)

CA2 +A  CA6 (V / V =+3.01%)

(5)

Fig. 3 The morphologies of castables fired at 1500°C with pre-formed spinel powders. (a-b) 0%, (c-e) 10%

In order to analyze the phase and microstructure evolution of spinel in the castables, pre-formed spinel powders, Al2O3 powders, silica sol and calcium aluminate cement were used to prepare the matrix castables without aggregates. The phase characterizations of matrix castables fired at different temperatures are shown in Fig. 4. It can be seen that the spinel and corundum are the main phases in the 9

matrix castables, while there are weak diffraction peaks of CAS2(CaOAl2O32SiO2) at 1100C in Fig. 4 (a). With heating treatment temperature rising, the strongest peak intensity of spinel is increased firstly and then decreased at 1500°C, and it continuously shifts to high angle in Fig. 4 (b). The angle shifting of spinel is corresponding to the decrement of the interplanar spacing in Fig. 4 (c). In Fig. 4 (d), the diffraction peak of low melting phase CAS2 only occur at 1100C, and it is almost disappeared after temperature rises to 1300C.

Fig. 4 The phase characterizations of matrix castables fired at different temperatures. (a) The overall phase; (b) The intensity change of spinel’s peak; (c) The interplanar spacing of spinel; (d) the diffraction peak change of anorthite There are wide solid solution range between spinel and Al2O3, making that it is easy for Al3+ to substitute Mg2+ in the spinel. However, the diameter of Al3+ (0.0535nm) is much smaller than Mg2+ (0.072nm), and two Al3+ ions have to substitute three Mg2+ ions to balance the electric neutrality during the substitution process, so many vacancy sites are left resulting in the interplanar spacing decreasing. 10

Furthermore, the amount of substitution of Al3+ to Mg2+ is increased with heating temperature increasing, giving rise to the interplanar spacing continuous decreasing. At the same time, with solid solution continuously proceeding and temperature rising to 1400°C, the grains sizes of spinel are increased, the peak intensity is enhanced. However, when the temperature rises to 1500°C, many secondary spinel grains with small size are produced according to equation (2-3), so the peak width at half height of spinel is increased according to Scherer equation. Regarding to the CAS2 diffraction peaks disappearing above 1100C, it is ascribed to the reaction between CAS2 and spinel according to equation (1). The CAS2 is reacted with spinel powders forming ternary compounds when the temperature is above 1100C but low 1500C.

Fig. 5 is the morphologies of matrix castables fired at 1500C. It can be seen that hexagonal plate CA6 grains with size of 20m are distributed around the pre-formed spinel powders, the spinel and corundum powders have been sintered together in Fig. 5 (a-b). Octahedral shape secondary spinel particles and CAM-I grain (confirmed by EDS 1 and 2) are displayed around pre-formed spinel powders in Fig. 5 (c). Fig. 5 (d) further reveals that CA6 and secondary spinel are coexistence in the same area, and the grain sizes of secondary spinel and CA6 are about 3-5m and 5m, respectively. It is worthy noted that the CA6 particles sizes in Fig. 5 (c-d) are much smaller than those in Fig. 5 (b). The differences of size for CA6 grains result from the different formation processes. As mentioned before, the former CA6 grains with big size are synthesized by CA/CA2 in the cement and corundum powders. The latter CA6 grains in Fig. 5 (c-d) are the products of CAM-I/II decomposition according to equation (3). The fracture morphologies of matrix castables in Fig. 5 indicate that the addition of pre-formed spinel powders are beneficial for the formation of secondary spinel and CA6 with small sizes and these microstructure change could improve the room temperature and high temperature properties as well as slag resistance for castables.

11

Fig. 5 The morphologies of matrix castables fired at 1500C. (a-b) BSE image, (c-d) SE image

The pore size distributions of castables are displayed in Fig. 6. As can be seen in Fig. 6 (a), all the d50 of castables is about 10μm. However, there are mainly 10μm pores in the castables without spinel in Fig. 6 (b), while the amounts of 10μm and 100μm pores are greatly increased with the pre-formed spinel addition. The pore diameter curves in Fig. 6 reveal that the addition of pre-formed spinel powders is beneficial for increasing the content of micro pores in castables. This can be attributed to several factors. Firstly, the d50 of pre-formed spinel powders is only 37μm, which is prone to fill into the large pores and attain small pores in the castables. Secondly, 3.01% volumetric expansion can be generated during the formation process CA6 according to the equation (5). What’s more, the nano SiO2 in the silica sol filling in the pores of castables can be the nucleus site of hydrates, and these hydrates are decomposed and produced small size pores during the heating treatment. 12

Fig. 6 The pore size distribution of castables added with pre-formed spinel powders. (a) Cumulative distribution; (b) Frequency distribution.

3.2 Room temperature physical properties of castables Fig. 7 presents the photos of castables fired at 1500C and the permanent of linear change of castables added with pre-formed spinel powders after heating treatment. It can be seen from Fig. 7 (a) that the castable is almost bent after heating treatment at 1500°C, while the deformation is drastically reduced when the castable is added with 15% pre-formed spinel powders. The permanent of linear change bar graph in Fig. 7 (b) reveals all the castables fired at 1100 and 1500°C occurs to shrink, and the shrinkage rate of castables fired at 1500°C is almost -1% when spinel powders are not added. However, the shrinkage rate of castables is significantly decreased and reached the minimum value of -0.39% when 15% pre-formed spinel powders are added. The shrinkage of castables bonded with calcium aluminate cement and silica sol can be ascribed to the SiO2 nano particles in the silica sol. SiO2 nano particles with 10-20nm have high sintering activity to greatly prompt the powders sintering during the heating treatment. So the castables after fired at 1100 and 1500°C could shrink. Furthermore, there is low melting phases production in the SiO2-CaO-Al2O3 system, such as CAS2 and containing Na compounds. These low melting phases could enhance the mass transferring and diffusion to prompt the sintering of castables at high temperature, which intensifies the shrinkage rate of castables. However, the shrinkage rate is reduced with the addition of pre-formed spinel contents increasing, 13

because volumetric expansion is produced during the process of CA2 and CA6 formation, according to the equations (4-5). And the amount of CA6 is increased due to the decomposition of CAM-I/II, which further reduces the shrinkage rate. Additionally, the low melting phase content can also be reduced due to the solid solution of Na in CA6 [28]. Subsequently, the shrinkage rate of castables is reduced.

Fig. 7 (a) Photos of castables fired at 1500C; (b) The permanent of linear change of castables

The low melting phase contents in the castables are displayed in Fig. 8. As can be seen, the low melting phase contents are decreased with pre-formed spinel contents increasing. When there is no addition of spinel, the low melting phase content in the castables is 3.55%, nevertheless, the content is decreased to 2.05% as 10% pre-formed spinel powders are added. The low melting phase in the castables is composed of the stabilizer of Na in the silica sol and the compounds in SiO2-Al2O3-CaO system such as CAS2 (CaOAl2O32SiO2, anorthite ). It is reported [28] that Na would be solid solution in the CA6 to reduce the liquid phase content in the castables. Thus, the containing Na compounds content could be decreased with pre-formed spinel contents increasing as the CA6 production amount is increased according to the equation (3). Besides that, the CAS2 compound can be reacted with spinel forming CAM-I/II compounds, which could also reduce low melting phase content at high temperature. The reduction of low melting phase content is beneficial for the volumetric stable and high temperature strength of castables. 14

Fig. 8 The low melting phase contents in the castables fired at 1500°C

Fig. 9 displays the apparent porosity and bulk density curves of castables. It can be seen that the apparent porosities of castables are firstly decreased, and then increased a little as pre-formed spinel powders are added, and the apparent porosities at 1100°C are obvious higher than those at 110 and 1500°C. The density curves show the opposite trends with apparent porosity. When 5% pre-formed spinel powders are added, all the apparent porosities of castables reach the minimum values of 14.02%, 15.95% and 13.84%, respectively, after heating treatment at 110, 1100 and 1500°C. The d50 of pre-formed spinel powders is 37μm, which is much smaller than the corundum powders (74μm). So the spinel powders can be filled into the large holes between aggregates in the castables obtaining high packing density. As a result, the apparent porosities of castables are firstly decreased with pre-formed spinel added. However, there may be bridging effect for powders as pre-formed spinel powders are continuously added, leading to the apparent porosity increasing a little. Further, more amounts of CA2 and CA6 are formed in the castables with addition of pre-formed spinel, leading volumetric expansion increasing. Therefore, the apparent porosities are increased slightly with pre-formed spinel powders continuously increasing.

15

Fig. 9 The apparent porosity and density of castables. (a) Apparent porosity; (b) Bulk density

The CMOR and CCS curves of castables are presented in Fig.10. The CMOR of castables dried at 110°C is held at 8.0MPa as pre-formed spinel contents are added in Fig. 10 (a). After fired at 1100 and 1500°C, CMOR values are gradually increased with spinel contents increasing and they are reached the maximum values of 20MPa and 37MPa, respectively, which are much higher than those of castables without spinel addition. However, the CCS values show no obvious trends with pre-formed spinel contents increasing. The CMOR are mainly affected by the porosity and bonding strength between particles in the castables. So the high CMOR values of castables can be attributed to the factors as follows. As shown in Fig. 9, the apparent porosities of castables added with pre-formed spinel are all lower than those of castables without spinel addition, so more stress could be undertook by particles as the loading are put on the castables to obtain high fracture strength. In addition, much more small size pores in the castables (as shown in Fig. 6) are also in favor of absorbing the energy of crack propagation and obtaining high CMOR values. On the other hand, besides large amounts of CA6 with big size are interlocked between particles, a lot of CA6 gains with small size are inserted in matrix and aggregates (shown in Fig. 3), which could effectively improve the bonding strength between powders.

16

Fig. 10 The mechanical strength of castables. (a) CMOR; (b) CCS

3.3 High temperature properties of castables Fig. 11 shows the HMOR of castables with different contents of pre-formed spinel. Though the HMOR curve presents up and down trend, the HMOR of castables added with pre-formed spinel powders are all higher than that of castables without spinel powders. When 5% pre-formed spinel powders are added, the HMOR of castables are obtained the maximum value of 2.3MPa, which is twice of castables without spinel powders. The HMOR are mainly influenced by the microstructure and liquid phase contents in the castables. As can be seen in Fig. 8, the low melting phase contents are greatly reduced from 3.55% to 2.05% due to the addition of pre-formed spinel powders. So when the castables are heated at high temperature, less liquid phase is existed to deteriorate the fracture strength. Additionally, it is reported [29] that the interlocking bonding between CA6 and corundum could enhance the HMOR of castables. In this work, besides the formation of CA6 grains with big size, a lot of CA6 grains with small size are produced and distributed in the matrix, as shown in Fig.3 and Fig.5. The latter CA6 grains are coexistence with liquid phase and spinel, which could inhibit the liquid phase and powders sliding and improve of HOMR [24, 30].

17

Fig. 11 The HMOR of castables

The elastic modulus and residual strength ratio curves of castables after three cycles thermal shock are shown in Fig. 12. It can be seen from Fig. 12 (a) that the elastic modulus values are decreased with the thermal shock times increasing. Before the thermal shock test, all the elastic modulus of castables is about 160GPa, and it is decreased severely to 40GPa after the first thermal shock test. After that, the decrement rate of elastic modulus is reduced with thermal shock times increasing. The residual strength ratio curve in Fig. 12 (b) presents that the residual strength ratio is decreased firstly, and then increased with pre-formed spinel contents increasing. When 20% pre-formed spinel powders are added, the residual strength ratio reaches the maximum value of 13.97%.

Fig. 12 The elastic modulus and residual strength of castables after thermal shock. (a) Elastic modulus, (b) Residual strength ratio 18

The R′′′′ proposed by Hasselman (equation (6)) is used to investigate the effect of pre-formed spinel powders on the thermal shock properties of castables. R=

 wof E  f (1  ) 2

(6)

R′′′′ is the index of fracture resistance, γwof is the fracture surface energy, ν is poisson's ratio, σf is the CMOR. According to the reference [31], the fracture surface energy is the positive relationship with CMOR, while poison’s ratio is increased with the apparent porosity decreasing. Therefore, combined with results of apparent porosity and CMOR curves of castables, the apparent porosities are decreased and CMOR values are increased as the pre-formed spinel contents increasing. So both the poison’s ratio ν and fracture surface energy γwof are increased with pre-formed spinel contents, resulting in the 2

increasing of E/ f, as shown in Table 4. Thus, the index values of fracture resistance R′′′′ are increased with pre-formed spinel content increasing, indicating that the addition of pre-formed spinel powders is indeed beneficial for the improvement of thermal shock resistance for the castables. The cracks in the castables due to the thermal shock lead to the decrease of mechanical properties. However, certain amount of pores in the castables can absorb the cracks to resist their propagation [32]. The CA6 are interlocked to strengthen the bonding between particles, which helps to improve the thermal shock resistance of castables. Furthermore, the micro diameter size pores are greatly increased after the pre-formed spinel powders are added, these micro pores could relieving the tip stress and blunt crack. Subsequently, the residual strength ratio is increased with the addition of pre-formed spinel powders. Table 4 The parameters changes in the equation (6) MA(wt.%) wof 2 E/ f (10-5Pa-1)  R′′′′

0 + 7.3 + +

5 ++ 7.1 ++ ++

10 +++ 8.9 +++ +++

Note: “+” indicates the size of value. 19

15 ++++ 9.6 ++++ ++++

20 ++++ 8.5 ++++ ++++

3.4 Slag resistance of castables The cross section photos of castables after slag resistance are presented in Fig. 13, and the penetration index has been obtained by Adobe Acrobat Pro software to quantify the penetration behavior of castables, as shown in Fig.14. Though the erosion area is almost the same in Fig.13, the penetration areas are dramatically different in Fig.14. With the pre-formed spinel contents increasing, the penetration areas of castables are gradually decreased. It is clearly shown that the penetration index for castables with 20% pre-formed spinel is 12.2%, which is much less than 28.8% of castables without spinel powders.

Fig. 13 The photos of castables after static slag resistance

Fig. 14 The penetration indexes of castables

20

Fig. 15 and Fig. 16 display the different microstructures of castables after slag resistance by static crucible. The dark areas are the resin; the bright and gray ones are the low melting phase, and corundum/spinel, respectively. Both samples can be classified three layers including the erosion layer (I), penetration layer (II) and original layer (III). When there is no addition of pre-formed spinel in the castables in Fig. 15 (a), the thickness of erosion layer and penetration layer are 4mm and 5mm, respectively. And a lot of pores are filled with resin due to the erosion of slag in the erosion layer. The enlargement image in Fig. 15 (b) reveals that the aggregates have almost been surrounded by low melting phase, which the low melting phase can be determined to be CAS2 and its solid solution with Fe/Mn in the slag by EDS. In the penetration layer in Fig. 15 (c), there are many micro pores in the castables, and the aggregates are surrounded by low melting phase, the EDS confirms that the Fe content has been decreased dramatically and element Mn is disappeared. The comparison of EDS at point 1 and 2 proves that the elements of Fe/Mn are largely absorbed during the penetration process. When 20% pre-formed spinel is added in the castables in Fig. 16 (a), the thickness of erosion layer and penetration layer are drastically reduced to 1mm and 2mm, respectively. The results are consistent with the penetration index in Fig. 14. The excellent slag resistance of castables added with pre-formed spinel can be attributed to several factors. Firstly, it is well known [33] that spinel can effectively absorb the ions of Fe2+/Fe3+ and Mn2+ in the slag forming complex spinels, i.e. (Mg, Mn, Fe)O(Fe, Al)2O3. At the same time, the CaO and Al2O3 are used to form CA2 and CA6 compounds, the relative amount of silica are increased to generate a high viscosity and high melting temperature slag, thus the erosion and penetration of slag are inhibited. What’s more, due to the in-situ formation of secondary spinel in the castables, the dissolution of spinel into the molten liquid slag could happen at an earlier stage, increasing the local concentration of Mg2+ and Al3+ ions, favoring the slag saturation in these components and giving rise to the re-precipitation of a continuous spinel layer[26]. Secondly, it is clearly that the pore size distribution and apparent porosity of castables affect the liquid penetration behavior at high 21

temperatures. Considering that the castables added with pre-formed spinel obtain less porosity and more small size pores (as shown in Fig. 6 and 9), the second phase is more likely to achieve saturation during the reaction between aggregate and slag, speeding up the formation of second phase, so the molten slag is difficult to penetrate into the castables, increasing the corrosion resistance of castables. The last but not the least, there is some content of liquid phase (mainly as CAS and containing Na compounds) in the castables, as shown in Fig. 8. When these liquid phases are contacted with slag at high temperature, they would blend in the molten slag and corrode the castables. Thus, the less content of liquid phase in the castables is also favor of obtaining good slag resistance.

Fig. 15 The SEM images of castables without spinel after slag resistance by static crucible

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Fig.16 The SEM images of castables with 20% pre-formed spinel after slag resistance by static crucible

4. Conclusion The use of double binder system (calcium aluminate and silica sol) could improve the mid-temperature strength for castables, though the high temperature properties were drastically deteriorated. In order to solve it, the pre-formed spinel was added to optimize the microstructure, subsequently to improve properties of castables in this work. The pre-formed spinel powders were not only filled into the pores in the castables, but also reacted with low melting phase (CAS2 and containing Na compounds) forming secondary spinel and CA6 particles with small size. As a result, the comprehensive properties of castables were greatly enhanced. (1) The castables added with pre-formed spinel powders were possessed with densified structure without obvious gaps between particles. The macro-pores were transformed to micro-pores, resulting in the amounts of micro-pores were greatly increased. (2) The shrinkage rate of castables was decreased from 0.998% to 0.39%, and the amount of low melting phase was decreased from 3.55% to 2.05%. The formation of secondary spinel and CA6 particles with small size contributed to the shrinkage rate 23

and low melting phase decreasing in the castables. (3) The CMOR and HMOR were increased by 45.8% and 110%, respectively, after castables were fired at 1500C. The residual strength ratio was increased from 10% to 13.97%, while the penetration indexes were decreased from 28.8% to 12.2%. The improvement of room temperature properties can be attributed to the promoting sintering of low melting phase, while a large amount of CA6 particles and spinel (including the pre-formed spinel and secondary spinel powders) in the castables were responsible for the high temperature properties enhancement.

Acknowledgments This work was supported by the National Natural Science Foundation of China (grant numbers 51374162, 51602231, 51674182, 51774218).

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