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Effect of zircon addition on the physical properties and coatability adherence of MgO–2CaO·SiO2–3CaO·SiO2 refractory materials
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Effect of zircon addition on the physical properties and coatability adherence of MgO–2CaO SiO2–3CaO SiO2 refractory materials Wei Meng, Chengliang Ma n, Tiezhu Ge, Xiangchong Zhong High Temperature Ceramics Institute, Zhengzhou University, Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou 450052, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2016 Received in revised form 17 February 2016 Accepted 23 February 2016
MgO–2CaO SiO2–3CaO SiO2 refractory compositions incorporated with zircon (ZrO2 SiO2) were obtained by solid state sintering at 1550 °C for 3 h. The effect of different ZrO2 contents (0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5%) on the physical properties and adherence ability to cement clinker were investigated. Specimens were characterized by bulk density, apparent porosity, cold crushing strength, cold and hot modulus of rupture and coatability adherence with the combination of crystalline phase formation (XRD) and microstructural analysis (SEM). The results showed that CaZrO3 was generated in the matrix grain boundaries and triple points, forming direct bonding between MgO and calcium silicate. Densification of the composites was promoted, and the apparent porosity decreased to 8.9% when the content of ZrO2 was 1.5%. The cold crushing strength increased steadily from 65 MPa to 124.2 MPa. The cold modulus of rupture slightly decreased in the ZrO2 content range of 0.5–1.5% and then reached 43.2 MPa, whereas the hot modulus of rupture reached its highest value of 4.4 MPa with 1.5% ZrO2 addition. The highest adherence strength (7.1 MPa) was obtained for 0.5% ZrO2 addition because the dissolution of CaZrO3 into the cement clinker increased the viscosity of the clinker and the bonding with the specimen. At high ZrO2 concentrations (1.0–2.5%), penetration of the clinker into the matrix was hindered by CaZrO3, thus resulting in lower adherence strength. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Zircon Calcium silicate Refractories Physical properties Coatability adherence
1. Introduction Currently, refractory linings for the burning zone of cement rotary kilns are exposed to aggressive service conditions [1,2], firing temperatures of approximately 1500 °C, and corrosive factors including alkali salts and atmospheres with high contents of alkali and sulphur from the substitution of fossil fuels such as rubber or other industrial wastes. These factors can strongly modify the microstructure and phase composition of the refractories in the work zone [3–6]. To achieve a suitable corrosion resistance, MgO-based materials have been implemented as the main refractory bricks in rotary cement kilns because they are hard-wearing under exposure to liquefied cement materials at high temperatures [7,8]. Magnesiachromite bricks have long been used as the typical lining materials for the burning zones because of their excellent thermal conductivity, resistance to thermal shock and low level of thermal expansion [9,10]. However, owing to the formation of water-soluble and carcinogenic hexavalent chromium (CrO42 ) during the production of clinker and the used bricks, which severely contaminates water and soil, the use of magnesia-chrome bricks is n
Corresponding author.
prohibited worldwide [11]. Therefore, chrome-free substitutes for magnesia brick, such as MgO–CaO, MgO–CaZrO3, MgO–MgAl2O4 and MgO–FeAl2O4 refractories, have been investigated and developed [12–15]. Magnesium–spinel (MgAl2O4, FeAl2O4) refractories have been developed and used in the cement industry [16–18]. Nevertheless, because liquids (such as 3CaO Al2O3, 12CaO 7Al2O3 and CaO Al2O3) are formed at approximately 1400 °C when combined with the components of cement clinkers and alkali salts, the composition of such materials is inadequate from the standpoint of thermodynamics [19–21]. In addition, magnesia-spinel refractories present relatively low corrosion resistance under the work conditions in cement kilns [2]. MgO–CaZrO3-based composite material is an alternative for replacing the MgO-spinel system owing to its enhanced refractoriness [22]. In particular, CaZrO3 is a phase without polymorphic transformations, which is compatible with MgO and Portland cement silicates, very resistant to the penetration of fluxes from the clinker [23]. MgO–CaZrO3 composite materials are characterized by high hot mechanical resistance and excellent corrosion resistance against alkaline salts and basic slag, demonstrating a higher corrosion resistance than MgO chrome or MgOspinel refractories, but they peel off easily in highly stressed areas [24–26].
http://dx.doi.org/10.1016/j.ceramint.2016.02.140 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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Doloma bricks have been used because of their chemical compatibility with basic environments because alkali compounds do not react with its two highly basic components. Because of the relatively high thermodynamic stability of MgO and CaO at high temperatures, they are highly resistant against reducing conditions [27–29]. However, the major disadvantage is the hydration susceptibility of doloma-based refractories; in contact with water or water vapor, they turn to powder and crumble, hindering their utilization [30,31]. The crucial factor influencing the service life of lining refractory bricks in the burning zone of cement rotary kilns is the stability of the coating on their surface [28,32–34]. Stabilized coating plays a key role in minimizing the effects of thermal load, corrosion, and thermal shock on the brick linings. The use of natural raw materials is attractive for the production of low-cost, MgO-based, high-temperature structural materials. In our previous work, MgO–2CaO SiO2–3CaO SiO2 composite ceramics have been prepared from dolomite, magnesite and forsterite mine ores, and they feature more favorable hydration resistance than normal doloma clinkers. Therefore, they are promising raw materials for use as refractory linings in the burning zone of rotary cement kilns. Hence, in the present work, MgO–2CaO SiO2–3CaO SiO2 refractory compositions incorporated with zircon (ZrO2 SiO2) were prepared. The influence of ZrO2 content on the physical properties and coatability adherence of MgO–2CaO SiO2–3CaO SiO2 refractory were investigated with an aim to explore the materials suitable for the burning zone of cement rotary kilns.
2. Experimental 2.1. Raw materials The starting materials employed were synthesized MgO– 2CaO SiO2–3CaO SiO2 raw materials (3–1 mm, 1–0 mm and 0.088 μm) and commercially available zircon powder (75 μm ¼93.6%, Saint Gobain, France) with 32.9% SiO2 and 66.2% ZrO2, 0.3% Al2O3 and 0.058% Fe2O3 as main impurities. The phase composition of the synthesized MgO–2CaO SiO2–3CaO SiO2 raw materials is shown in Table 1. 2.2. Synthesis of composites
2.3. Characterization The bulk density and apparent porosity were measured by the immersion method in water under vacuum using Archimedes’ principle. The cold crushing strength (CCS) was evaluated by a hydraulic compressive machine (NYL-2000A China) with a crosshead speed of 5 mm/min. At least three specimens were prepared for one test to ensure the reproducibility and the reliability of the test. The cold modulus of rupture (CMOR) at room temperature and hot modulus of rupture (HMOR) at 1400 °C were determined by a three-point bending test using a testing machine (MOR-03AG, China) with a span of 120 mm and a loading rate of 0.5 mm/s. A sandwich test was used to test the reactivity of the clinker towards the doloma bricks. The procedures were as follows: the as-received raw meal (Tianrui Group Cement Co., Ltd. China) was crushed and pulverized to pass a sieve of 150 mesh. The sandwich was prepared using the cut surfaces of brick. A paste mixed from equal amounts of water and the pulverized raw meal was placed between the two pieces cut out of the specimens with a thickness of 2–3 mm to complete the sandwich. At least three sandwiches were prepared for one test to ensure the reproducibility and the reliability of the test. The sandwiches were then introduced into an electrical muffle furnace and heated to 1500 °C for 3 h at a rate of 3 °C/min and then cooled to room temperature with the furnace. The mean value of the modulus of rupture at room temperature obtained from the three specimens was considered as the adherence strength of the cement clinker on the specimens. The chemical composition of the kiln hot meal used in the experiments is shown in Table 2. The main phases of the cement clinker were determined to be alite Ca3[SiO4]O (C3S), belite Ca2[SiO4] (C2S), calcium aluminate Ca3Al2O6 (C3A), and calcium alumina-ferrite phase Ca2AlFeO5 (C4AF) using X-ray diffraction (XRD). Table 3 The crystallized phases were identified by XRD (PHILIPS X'Pert Pro, the Netherlands) in Bragg-Brentano geometry with CuKα radiation (λ ¼1.5418 Å). Diffraction patterns were recorded in the 2θ range of 20–80°. Scanning electron microscopy (SEM, Carl ZEISS, EVO HD 15, England) equipped with an energy dispersive spectroscopy apparatus (EDS, Oxford Inca X-Max) were employed for microstructure observations and elemental analysis of sputtered gold-coated polished surfaces.
To evaluate the influence of zircon as an additive on the physical properties and coatability of MgO–2CaO SiO2–3CaO SiO2 refractory, six batch formulations were prepared: the first without zircon addition (denominated as Z0) and the rest with variations in the ZrO2 concentration of 0.5, 1.0, 1.5, 2.0, 2.5, which were labeled Z0.5, Z1, Z1.5, Z2 and Z2.5, respectively. After the homogenization process by mechanical mixing of masses with sulphite liquor as binders, green compacts with dimension of 25 mm 25 mm 150 mm were prepared by uniaxial cold pressing in a metallic mould under 180 MPa (YT63T, China). The samples were dried in an electric dry oven at 120 °C for 24 h and subsequently sintered at 1550 °C for 3 h in a MoSi2 electrical furnace (SX16-15-16, China) in air atmosphere with heating rate of 3–5 °C/min. The specimens were then furnace-cooled to room temperature.
The XRD patterns of the compositions are shown in Fig. 1. The phase analysis clearly indicated that they mainly consist of MgO, β-2CaO SiO2 (C2S) and 3CaO SiO2 (C3S). CaZrO3 was not observed in specimens Z0.5 and Z1.0, mainly because the amounts were too small to be detected. With increasing ZrO2 from 1.5 to 2.5, the peaks of CaZrO3 strengthened, accompanied by the slight weakening of C3S peaks. At high temperatures (above 1000 °C), zircon decomposes by a solid-state reaction into ZrO2 and SiO2, and then the two decomposition products react with tricalcium-silicate (C3S) to yield CaZrO3 and dicalcium silicate, respectively.
Table 1 Phase composition of starting material wt%.
Table 2 Chemical composition of kiln hot meal wt%.
3. Results and discussion 3.1. Phase composition
Composition
MgO
C3S
C2S
C4AF
C2F
Component
Al2O3
SiO2
CaO
MgO
Fe2O3
K2O
Na2O
SO2
P2O5
Content
44.5
11.6
35.6
7.4
1.0
Content
7.61
22.07
63.64
2.18
2.62
0.76
0.01
0.94
0.073
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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Table 3 EDS analysis of points in Fig. 8 at%. Point
O
Mg
Al
Si
Ca
Fe
Zr
1 2 3 4
40.04 56.36 50.94 61.94
59.96 1.56 14.19 8.03
0.72 0.85
7.81 6.71 9.35
34.26 26.45 18.68
0.99 0.32
0.83
Fig. 3. Variation of bulk density and apparent porosity versus ZrO2 content in the compositions.
Fig. 1. Comparative XRD phase analysis of the composites. MgO: periclase, ICDD: no. 01-077-2179; Ca3SiO5: tricalcium silicate, ICDD: no.00-031-0301; β-C2SiO4: dicalcium silicate, ICDD: no. 00-049-1673; CaZrO3: calcium zirconate, ICDD: no. 00035-0790.
3.2. Microstructure Backscattered electron (BSE) images of specimen Z2.5 are illustrated in Fig. 2. In combination with the EDX analysis, dark grey grains and the grey area were identified as MgO and calcium silicates, respectively, presenting good distribution in the microstructure as main phases. The white quasi-spherical shape particles were identified as CaZrO3 particles. Small particles of CaZrO3 were located between the matrix grain boundaries and triple points of the coarse aggregates of rounded MgO grains. It was observed that direct bonding (solid–solid bonding) between MgO and CaZrO3 particles was evident at high magnifications Fig. 2(b). 3.3. Physical properties The effect of zircon on the bulk density and apparent porosity
P
of specimens is shown in Fig. 3. As seen, the bulk density increased gradually with increasing zircon addition, and the apparent porosity decreased accordingly. This suggests that the addition of zircon is beneficial to the sintering and densification of the composites. When added by 2.5%, the bulk density reached 3.13 g cm 3 from 3.01 g cm 3. The apparent porosity decreased from 12.0% to 8.9% when the concentration of ZrO2 was 1.5% and then increased slightly to 9.8% when the content was 2.5%. Densification of the sintered specimens with the incorporation of zircon may be associated with the following theories: 1) a better compaction of the body upon filling the intergranular voids between magnesia and calcium silicates; 2) better mass transport during the decomposition of zircon and solid reaction with tricalcium silicate; 3) the formation of CaZrO3 acting as a ceramic bonding (a bridge) between the MgO and calcium silicate grains and therefore eliminating the porosity. However, owing to the different thermal expansion coefficients among MgO ( 13.5 10 6 °C 1), Ca2SiO4 (α E16.3 10 6 °C 1) and CaZrO3 ( 7.0 10 6 °C 1, from 20 to 1000 °C), it becomes a crucial factor in the development of physical properties. At high concentrations (Z2.0, Z2.5), excessive micro-cracks are generated in the matrix structure, thus provoking a porosity increase. Moreover, sintering was also impeded by the volume expansion (approximately 7–8%) during the formation of CaZrO3. Fig. 4 illustrates the variation of cold crushing strength and cold modulus of rupture. The results obtained for cold crushing strength exhibited a sharp increasing tendency in magnitude with increasing zircon addition. The maximum crushing strength value in the refractory specimens corresponded to the 2.5% addition, which was 124.2 MPa, nearly double that of the 0% addition
M
CZ
M CS M
CZ CS
200μm
10μm
Fig. 2. Microstructure of specimen Z2.5. M¼ magnesia; CS ¼ calcium silicate; CZ ¼calcium zirconate; P ¼ pores.
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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CS
CZ CS
M 10μm
Fig. 4. Variation of cold crushing strength and cold modulus of rupture versus ZrO2 content in the compositions.
Fig. 6. SEM micrograph of ruptured surface of specimen Z2.5 after HMOR test. M¼ magnesia; CS ¼ calcium silicate; CZ ¼calcium zirconate.
(65 MPa). The increase in the cold crushing strength can be ascribed to the enhanced ceramic bond linkage between the matrix of MgO and calcium silicates generated by CaZrO3. For ZrO2 content of 0–1.5 wt%, the CMOR values were almost maintained with a slight decrease in the range of 38.5–36.3 MPa. This may be due to the mismatch of thermal expansion coefficients, which gives rise to the micro-cracks in the matrix. For the concentration of 2.0 wt% and 2.5 wt%, CMOR increased gradually to 43.2 MPa. This can be explained by the reinforced bonding of the matrix by the formation of CaZrO3 and the increased compactness. Fig. 5 shows the hot modulus of rupture (HMOR) with different zirconium contents. The HMOR of the specimens with the addition of zircon increased compared with sample Z0. The value for specimen Z2.0 increased steadily to 4.4 MPa from 3.3 MPa (Z0) and was maintained for specimen Z2.5 (4.3 MPa). The added zircon may fill in the inter-granular voids, and the formed CaZrO3 can act as a ceramic bonding (a bridge) between the MgO and calcium silicate grains. CaZrO3 avoids abnormal grain growth, providing the matrix with a mechanical reinforcement. However, the microcracks generated by the formation of CaZrO3 and the different thermal expansion coefficients lead to a slight decrease in
strength. Therefore, the HMOR diminished slightly for specimen Z2.5. Fig. 6 reveals the microstructure of the ruptured surface (specimen Z2.5) after the HMOR test. The existence of a direct bond between CaZrO3 and the matrix is observed. CaZrO3 grains in the refractory mixtures produced a grain refinement in the matrix. The results showed that this was the typical trans-granular fracture mode for the specimen. The much stronger CaZrO3/MgO (calcium silicate) interface bonding may be the principal mechanism responsible for the trans-granular fracture in the composites, owing to the strong bonding between CaZrO3 and MgO (calcium silicate) also inhibiting the crack to propagate along the interface.
Fig. 5. Variation of hot modulus of rupture versus ZrO2 content in the compositions.
3.4. Coatability adherence Fig. 7 presents the adherence strength of the specimens to cement clinker. A sharp increase in the adherence strength was obtained with the addition of ZrO2 in the range of 0.5–2.0%. The maximum coating adherence was found for 0.5% ZrO2 addition (7.1 MPa), giving an improvement of ca. 136.7% to that of the formulation Z0 (3.0 MPa). With higher ZrO2 content, the adherence strength decreased gradually, and the lowest value was observed for specimen Z2.5, corresponding to 2.75 MPa, slightly lower than
Fig. 7. Effect of ZrO2 content on the adherence strength of specimens.
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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P
Speccimens
C4AF F
M
Inteerface
1 C2S
nker Clin
4 1m mm
C3S 3
2 50μm
Fig. 8. Microstructure observed by backscattered electron microscopy and EDS mapping results of interface clinker/specimen Z0.5. M¼ magnesia; C2S¼ dicalcium silicate; C3S¼tricalcium silicate; C4AF¼ calcium aluminate-ferrite; P ¼ pores.
that of specimen Z0. The microstructure at the interface between the specimen and clinker is shown in Fig. 8 and is mainly composed of dicalcium silicate, which adhered tightly with the specimen matrix. The partially melted clinkers can be observed near the reaction boundary of clinker and substrate. The compatibility of the clinker with the matrix may promote the adherence with specimens. At high temperatures, the components in the clinkers may penetrate into the specimen through the open pores and the grain boundaries. CaO reacts with dicalcium silicate to yield tricalcium silicates. The stabilizer calcium phosphate in the preparation of the starting materials stabilizes the dicalcium silicate in the β form, avoiding the crystal transformation into the γ form and pulverization. The calcium aluminate-ferrite (C4AF, Tm ¼ 1415 °C) and tricalcium aluminate (C3A, Tm ¼1539 °C) in the liquid phase promote the diffusion of the cement clinker through the grain boundaries and thus facilitate sintering. As can clearly be seen from the EDS results, CaZrO3 was dissolved into the cement clinker after treatment at 1500 °C. The dissolution can increase the viscosity of the clinker and promote the bonding of the clinker with the matrix, so the coating adherence strengthened. However, at high concentrations, the existence of CaZrO3 crystals at the grain boundaries may hinder the penetration of cement clinker into the specimen matrix. The reaction and sintering may be restrained to some extent, and the bonding between the clinker and specimen was weakened. As a result, lower adherence strength was observed for specimens with ZrO2 content of 1.0–2.5%.
4. Conclusions The addition of zircon into the MgO–2CaO SiO2–3CaO SiO2 refractory materials improved their physical properties and coatability adherence. The microstructural analysis revealed that
CaZrO3 was generated in the matrix grain boundaries and triple points, forming direct bonding between MgO and calcium silicate. The densification of the composites was promoted, and the lowest apparent porosity was 8.9% for 1.5% ZrO2 addition. At the ZrO2 content of 2.5%, CCS and CMOR obtained their highest values of 124.2 MPa and 43.2 MPa, respectively. HMOR reached 4.4 MPa at the ZrO2 content of 1.5%. The optimal adherence strength (7.1 MPa) was found for 0.5% ZrO2 addition because the dissolution of CaZrO3 increased the viscosity of the clinker and promoted the adherence with the matrix. The penetration of the clinker into the matrix was hindered by CaZrO3, resulting in decreased adherence strength at higher ZrO2 concentrations (1.0–2.5%). The MgO– 2CaO SiO2–3CaO SiO2 refractory materials with ZrO2 content of 0.5–2.5% are promising lining materials for the burning zone of cement kilns.
Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (General program, 51172210, 51472220), Technology Research and Development Program of Zhengzhou (112PCXYZ114).
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Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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Effect of zircon addition on the physical properties and coatability adherence of MgO–2CaO SiO2–3CaO SiO2 refractory materials Wei Meng, Chengliang Ma n, Tiezhu Ge, Xiangchong Zhong High Temperature Ceramics Institute, Zhengzhou University, Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou 450052, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2016 Received in revised form 17 February 2016 Accepted 23 February 2016
MgO–2CaO SiO2–3CaO SiO2 refractory compositions incorporated with zircon (ZrO2 SiO2) were obtained by solid state sintering at 1550 °C for 3 h. The effect of different ZrO2 contents (0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5%) on the physical properties and adherence ability to cement clinker were investigated. Specimens were characterized by bulk density, apparent porosity, cold crushing strength, cold and hot modulus of rupture and coatability adherence with the combination of crystalline phase formation (XRD) and microstructural analysis (SEM). The results showed that CaZrO3 was generated in the matrix grain boundaries and triple points, forming direct bonding between MgO and calcium silicate. Densification of the composites was promoted, and the apparent porosity decreased to 8.9% when the content of ZrO2 was 1.5%. The cold crushing strength increased steadily from 65 MPa to 124.2 MPa. The cold modulus of rupture slightly decreased in the ZrO2 content range of 0.5–1.5% and then reached 43.2 MPa, whereas the hot modulus of rupture reached its highest value of 4.4 MPa with 1.5% ZrO2 addition. The highest adherence strength (7.1 MPa) was obtained for 0.5% ZrO2 addition because the dissolution of CaZrO3 into the cement clinker increased the viscosity of the clinker and the bonding with the specimen. At high ZrO2 concentrations (1.0–2.5%), penetration of the clinker into the matrix was hindered by CaZrO3, thus resulting in lower adherence strength. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Zircon Calcium silicate Refractories Physical properties Coatability adherence
1. Introduction Currently, refractory linings for the burning zone of cement rotary kilns are exposed to aggressive service conditions [1,2], firing temperatures of approximately 1500 °C, and corrosive factors including alkali salts and atmospheres with high contents of alkali and sulphur from the substitution of fossil fuels such as rubber or other industrial wastes. These factors can strongly modify the microstructure and phase composition of the refractories in the work zone [3–6]. To achieve a suitable corrosion resistance, MgO-based materials have been implemented as the main refractory bricks in rotary cement kilns because they are hard-wearing under exposure to liquefied cement materials at high temperatures [7,8]. Magnesiachromite bricks have long been used as the typical lining materials for the burning zones because of their excellent thermal conductivity, resistance to thermal shock and low level of thermal expansion [9,10]. However, owing to the formation of water-soluble and carcinogenic hexavalent chromium (CrO42 ) during the production of clinker and the used bricks, which severely contaminates water and soil, the use of magnesia-chrome bricks is n
Corresponding author.
prohibited worldwide [11]. Therefore, chrome-free substitutes for magnesia brick, such as MgO–CaO, MgO–CaZrO3, MgO–MgAl2O4 and MgO–FeAl2O4 refractories, have been investigated and developed [12–15]. Magnesium–spinel (MgAl2O4, FeAl2O4) refractories have been developed and used in the cement industry [16–18]. Nevertheless, because liquids (such as 3CaO Al2O3, 12CaO 7Al2O3 and CaO Al2O3) are formed at approximately 1400 °C when combined with the components of cement clinkers and alkali salts, the composition of such materials is inadequate from the standpoint of thermodynamics [19–21]. In addition, magnesia-spinel refractories present relatively low corrosion resistance under the work conditions in cement kilns [2]. MgO–CaZrO3-based composite material is an alternative for replacing the MgO-spinel system owing to its enhanced refractoriness [22]. In particular, CaZrO3 is a phase without polymorphic transformations, which is compatible with MgO and Portland cement silicates, very resistant to the penetration of fluxes from the clinker [23]. MgO–CaZrO3 composite materials are characterized by high hot mechanical resistance and excellent corrosion resistance against alkaline salts and basic slag, demonstrating a higher corrosion resistance than MgO chrome or MgOspinel refractories, but they peel off easily in highly stressed areas [24–26].
http://dx.doi.org/10.1016/j.ceramint.2016.02.140 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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Doloma bricks have been used because of their chemical compatibility with basic environments because alkali compounds do not react with its two highly basic components. Because of the relatively high thermodynamic stability of MgO and CaO at high temperatures, they are highly resistant against reducing conditions [27–29]. However, the major disadvantage is the hydration susceptibility of doloma-based refractories; in contact with water or water vapor, they turn to powder and crumble, hindering their utilization [30,31]. The crucial factor influencing the service life of lining refractory bricks in the burning zone of cement rotary kilns is the stability of the coating on their surface [28,32–34]. Stabilized coating plays a key role in minimizing the effects of thermal load, corrosion, and thermal shock on the brick linings. The use of natural raw materials is attractive for the production of low-cost, MgO-based, high-temperature structural materials. In our previous work, MgO–2CaO SiO2–3CaO SiO2 composite ceramics have been prepared from dolomite, magnesite and forsterite mine ores, and they feature more favorable hydration resistance than normal doloma clinkers. Therefore, they are promising raw materials for use as refractory linings in the burning zone of rotary cement kilns. Hence, in the present work, MgO–2CaO SiO2–3CaO SiO2 refractory compositions incorporated with zircon (ZrO2 SiO2) were prepared. The influence of ZrO2 content on the physical properties and coatability adherence of MgO–2CaO SiO2–3CaO SiO2 refractory were investigated with an aim to explore the materials suitable for the burning zone of cement rotary kilns.
2. Experimental 2.1. Raw materials The starting materials employed were synthesized MgO– 2CaO SiO2–3CaO SiO2 raw materials (3–1 mm, 1–0 mm and 0.088 μm) and commercially available zircon powder (75 μm ¼93.6%, Saint Gobain, France) with 32.9% SiO2 and 66.2% ZrO2, 0.3% Al2O3 and 0.058% Fe2O3 as main impurities. The phase composition of the synthesized MgO–2CaO SiO2–3CaO SiO2 raw materials is shown in Table 1. 2.2. Synthesis of composites
2.3. Characterization The bulk density and apparent porosity were measured by the immersion method in water under vacuum using Archimedes’ principle. The cold crushing strength (CCS) was evaluated by a hydraulic compressive machine (NYL-2000A China) with a crosshead speed of 5 mm/min. At least three specimens were prepared for one test to ensure the reproducibility and the reliability of the test. The cold modulus of rupture (CMOR) at room temperature and hot modulus of rupture (HMOR) at 1400 °C were determined by a three-point bending test using a testing machine (MOR-03AG, China) with a span of 120 mm and a loading rate of 0.5 mm/s. A sandwich test was used to test the reactivity of the clinker towards the doloma bricks. The procedures were as follows: the as-received raw meal (Tianrui Group Cement Co., Ltd. China) was crushed and pulverized to pass a sieve of 150 mesh. The sandwich was prepared using the cut surfaces of brick. A paste mixed from equal amounts of water and the pulverized raw meal was placed between the two pieces cut out of the specimens with a thickness of 2–3 mm to complete the sandwich. At least three sandwiches were prepared for one test to ensure the reproducibility and the reliability of the test. The sandwiches were then introduced into an electrical muffle furnace and heated to 1500 °C for 3 h at a rate of 3 °C/min and then cooled to room temperature with the furnace. The mean value of the modulus of rupture at room temperature obtained from the three specimens was considered as the adherence strength of the cement clinker on the specimens. The chemical composition of the kiln hot meal used in the experiments is shown in Table 2. The main phases of the cement clinker were determined to be alite Ca3[SiO4]O (C3S), belite Ca2[SiO4] (C2S), calcium aluminate Ca3Al2O6 (C3A), and calcium alumina-ferrite phase Ca2AlFeO5 (C4AF) using X-ray diffraction (XRD). Table 3 The crystallized phases were identified by XRD (PHILIPS X'Pert Pro, the Netherlands) in Bragg-Brentano geometry with CuKα radiation (λ ¼1.5418 Å). Diffraction patterns were recorded in the 2θ range of 20–80°. Scanning electron microscopy (SEM, Carl ZEISS, EVO HD 15, England) equipped with an energy dispersive spectroscopy apparatus (EDS, Oxford Inca X-Max) were employed for microstructure observations and elemental analysis of sputtered gold-coated polished surfaces.
To evaluate the influence of zircon as an additive on the physical properties and coatability of MgO–2CaO SiO2–3CaO SiO2 refractory, six batch formulations were prepared: the first without zircon addition (denominated as Z0) and the rest with variations in the ZrO2 concentration of 0.5, 1.0, 1.5, 2.0, 2.5, which were labeled Z0.5, Z1, Z1.5, Z2 and Z2.5, respectively. After the homogenization process by mechanical mixing of masses with sulphite liquor as binders, green compacts with dimension of 25 mm 25 mm 150 mm were prepared by uniaxial cold pressing in a metallic mould under 180 MPa (YT63T, China). The samples were dried in an electric dry oven at 120 °C for 24 h and subsequently sintered at 1550 °C for 3 h in a MoSi2 electrical furnace (SX16-15-16, China) in air atmosphere with heating rate of 3–5 °C/min. The specimens were then furnace-cooled to room temperature.
The XRD patterns of the compositions are shown in Fig. 1. The phase analysis clearly indicated that they mainly consist of MgO, β-2CaO SiO2 (C2S) and 3CaO SiO2 (C3S). CaZrO3 was not observed in specimens Z0.5 and Z1.0, mainly because the amounts were too small to be detected. With increasing ZrO2 from 1.5 to 2.5, the peaks of CaZrO3 strengthened, accompanied by the slight weakening of C3S peaks. At high temperatures (above 1000 °C), zircon decomposes by a solid-state reaction into ZrO2 and SiO2, and then the two decomposition products react with tricalcium-silicate (C3S) to yield CaZrO3 and dicalcium silicate, respectively.
Table 1 Phase composition of starting material wt%.
Table 2 Chemical composition of kiln hot meal wt%.
3. Results and discussion 3.1. Phase composition
Composition
MgO
C3S
C2S
C4AF
C2F
Component
Al2O3
SiO2
CaO
MgO
Fe2O3
K2O
Na2O
SO2
P2O5
Content
44.5
11.6
35.6
7.4
1.0
Content
7.61
22.07
63.64
2.18
2.62
0.76
0.01
0.94
0.073
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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Table 3 EDS analysis of points in Fig. 8 at%. Point
O
Mg
Al
Si
Ca
Fe
Zr
1 2 3 4
40.04 56.36 50.94 61.94
59.96 1.56 14.19 8.03
0.72 0.85
7.81 6.71 9.35
34.26 26.45 18.68
0.99 0.32
0.83
Fig. 3. Variation of bulk density and apparent porosity versus ZrO2 content in the compositions.
Fig. 1. Comparative XRD phase analysis of the composites. MgO: periclase, ICDD: no. 01-077-2179; Ca3SiO5: tricalcium silicate, ICDD: no.00-031-0301; β-C2SiO4: dicalcium silicate, ICDD: no. 00-049-1673; CaZrO3: calcium zirconate, ICDD: no. 00035-0790.
3.2. Microstructure Backscattered electron (BSE) images of specimen Z2.5 are illustrated in Fig. 2. In combination with the EDX analysis, dark grey grains and the grey area were identified as MgO and calcium silicates, respectively, presenting good distribution in the microstructure as main phases. The white quasi-spherical shape particles were identified as CaZrO3 particles. Small particles of CaZrO3 were located between the matrix grain boundaries and triple points of the coarse aggregates of rounded MgO grains. It was observed that direct bonding (solid–solid bonding) between MgO and CaZrO3 particles was evident at high magnifications Fig. 2(b). 3.3. Physical properties The effect of zircon on the bulk density and apparent porosity
P
of specimens is shown in Fig. 3. As seen, the bulk density increased gradually with increasing zircon addition, and the apparent porosity decreased accordingly. This suggests that the addition of zircon is beneficial to the sintering and densification of the composites. When added by 2.5%, the bulk density reached 3.13 g cm 3 from 3.01 g cm 3. The apparent porosity decreased from 12.0% to 8.9% when the concentration of ZrO2 was 1.5% and then increased slightly to 9.8% when the content was 2.5%. Densification of the sintered specimens with the incorporation of zircon may be associated with the following theories: 1) a better compaction of the body upon filling the intergranular voids between magnesia and calcium silicates; 2) better mass transport during the decomposition of zircon and solid reaction with tricalcium silicate; 3) the formation of CaZrO3 acting as a ceramic bonding (a bridge) between the MgO and calcium silicate grains and therefore eliminating the porosity. However, owing to the different thermal expansion coefficients among MgO ( 13.5 10 6 °C 1), Ca2SiO4 (α E16.3 10 6 °C 1) and CaZrO3 ( 7.0 10 6 °C 1, from 20 to 1000 °C), it becomes a crucial factor in the development of physical properties. At high concentrations (Z2.0, Z2.5), excessive micro-cracks are generated in the matrix structure, thus provoking a porosity increase. Moreover, sintering was also impeded by the volume expansion (approximately 7–8%) during the formation of CaZrO3. Fig. 4 illustrates the variation of cold crushing strength and cold modulus of rupture. The results obtained for cold crushing strength exhibited a sharp increasing tendency in magnitude with increasing zircon addition. The maximum crushing strength value in the refractory specimens corresponded to the 2.5% addition, which was 124.2 MPa, nearly double that of the 0% addition
M
CZ
M CS M
CZ CS
200μm
10μm
Fig. 2. Microstructure of specimen Z2.5. M¼ magnesia; CS ¼ calcium silicate; CZ ¼calcium zirconate; P ¼ pores.
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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CS
CZ CS
M 10μm
Fig. 4. Variation of cold crushing strength and cold modulus of rupture versus ZrO2 content in the compositions.
Fig. 6. SEM micrograph of ruptured surface of specimen Z2.5 after HMOR test. M¼ magnesia; CS ¼ calcium silicate; CZ ¼calcium zirconate.
(65 MPa). The increase in the cold crushing strength can be ascribed to the enhanced ceramic bond linkage between the matrix of MgO and calcium silicates generated by CaZrO3. For ZrO2 content of 0–1.5 wt%, the CMOR values were almost maintained with a slight decrease in the range of 38.5–36.3 MPa. This may be due to the mismatch of thermal expansion coefficients, which gives rise to the micro-cracks in the matrix. For the concentration of 2.0 wt% and 2.5 wt%, CMOR increased gradually to 43.2 MPa. This can be explained by the reinforced bonding of the matrix by the formation of CaZrO3 and the increased compactness. Fig. 5 shows the hot modulus of rupture (HMOR) with different zirconium contents. The HMOR of the specimens with the addition of zircon increased compared with sample Z0. The value for specimen Z2.0 increased steadily to 4.4 MPa from 3.3 MPa (Z0) and was maintained for specimen Z2.5 (4.3 MPa). The added zircon may fill in the inter-granular voids, and the formed CaZrO3 can act as a ceramic bonding (a bridge) between the MgO and calcium silicate grains. CaZrO3 avoids abnormal grain growth, providing the matrix with a mechanical reinforcement. However, the microcracks generated by the formation of CaZrO3 and the different thermal expansion coefficients lead to a slight decrease in
strength. Therefore, the HMOR diminished slightly for specimen Z2.5. Fig. 6 reveals the microstructure of the ruptured surface (specimen Z2.5) after the HMOR test. The existence of a direct bond between CaZrO3 and the matrix is observed. CaZrO3 grains in the refractory mixtures produced a grain refinement in the matrix. The results showed that this was the typical trans-granular fracture mode for the specimen. The much stronger CaZrO3/MgO (calcium silicate) interface bonding may be the principal mechanism responsible for the trans-granular fracture in the composites, owing to the strong bonding between CaZrO3 and MgO (calcium silicate) also inhibiting the crack to propagate along the interface.
Fig. 5. Variation of hot modulus of rupture versus ZrO2 content in the compositions.
3.4. Coatability adherence Fig. 7 presents the adherence strength of the specimens to cement clinker. A sharp increase in the adherence strength was obtained with the addition of ZrO2 in the range of 0.5–2.0%. The maximum coating adherence was found for 0.5% ZrO2 addition (7.1 MPa), giving an improvement of ca. 136.7% to that of the formulation Z0 (3.0 MPa). With higher ZrO2 content, the adherence strength decreased gradually, and the lowest value was observed for specimen Z2.5, corresponding to 2.75 MPa, slightly lower than
Fig. 7. Effect of ZrO2 content on the adherence strength of specimens.
Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i
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P
Speccimens
C4AF F
M
Inteerface
1 C2S
nker Clin
4 1m mm
C3S 3
2 50μm
Fig. 8. Microstructure observed by backscattered electron microscopy and EDS mapping results of interface clinker/specimen Z0.5. M¼ magnesia; C2S¼ dicalcium silicate; C3S¼tricalcium silicate; C4AF¼ calcium aluminate-ferrite; P ¼ pores.
that of specimen Z0. The microstructure at the interface between the specimen and clinker is shown in Fig. 8 and is mainly composed of dicalcium silicate, which adhered tightly with the specimen matrix. The partially melted clinkers can be observed near the reaction boundary of clinker and substrate. The compatibility of the clinker with the matrix may promote the adherence with specimens. At high temperatures, the components in the clinkers may penetrate into the specimen through the open pores and the grain boundaries. CaO reacts with dicalcium silicate to yield tricalcium silicates. The stabilizer calcium phosphate in the preparation of the starting materials stabilizes the dicalcium silicate in the β form, avoiding the crystal transformation into the γ form and pulverization. The calcium aluminate-ferrite (C4AF, Tm ¼ 1415 °C) and tricalcium aluminate (C3A, Tm ¼1539 °C) in the liquid phase promote the diffusion of the cement clinker through the grain boundaries and thus facilitate sintering. As can clearly be seen from the EDS results, CaZrO3 was dissolved into the cement clinker after treatment at 1500 °C. The dissolution can increase the viscosity of the clinker and promote the bonding of the clinker with the matrix, so the coating adherence strengthened. However, at high concentrations, the existence of CaZrO3 crystals at the grain boundaries may hinder the penetration of cement clinker into the specimen matrix. The reaction and sintering may be restrained to some extent, and the bonding between the clinker and specimen was weakened. As a result, lower adherence strength was observed for specimens with ZrO2 content of 1.0–2.5%.
4. Conclusions The addition of zircon into the MgO–2CaO SiO2–3CaO SiO2 refractory materials improved their physical properties and coatability adherence. The microstructural analysis revealed that
CaZrO3 was generated in the matrix grain boundaries and triple points, forming direct bonding between MgO and calcium silicate. The densification of the composites was promoted, and the lowest apparent porosity was 8.9% for 1.5% ZrO2 addition. At the ZrO2 content of 2.5%, CCS and CMOR obtained their highest values of 124.2 MPa and 43.2 MPa, respectively. HMOR reached 4.4 MPa at the ZrO2 content of 1.5%. The optimal adherence strength (7.1 MPa) was found for 0.5% ZrO2 addition because the dissolution of CaZrO3 increased the viscosity of the clinker and promoted the adherence with the matrix. The penetration of the clinker into the matrix was hindered by CaZrO3, resulting in decreased adherence strength at higher ZrO2 concentrations (1.0–2.5%). The MgO– 2CaO SiO2–3CaO SiO2 refractory materials with ZrO2 content of 0.5–2.5% are promising lining materials for the burning zone of cement kilns.
Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (General program, 51172210, 51472220), Technology Research and Development Program of Zhengzhou (112PCXYZ114).
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Please cite this article as: W. Meng, et al., Effect of zircon addition on the physical properties and coatability adherence of MgO– 2CaO SiO2–3CaO SiO2 refractory materials, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.140i