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Effect of micro-sized hydromagnesite addition on the properties of calcium aluminate cement-bonded castables
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of micro-sized hydromagnesite addition on the properties of calcium aluminate cement-bonded castables ⁎
Shuhe Hua,b, Liugang Chena,c, , Lingling Zhua, Dafei Dinga, Fei Zhaoa, Guotian Yea a
Henan Key Laboratory of High Temperature Functional Ceramics, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, China High Temperature Materials Research Institute, Henan University of Science and Technology, Luoyang, Henan 471003, China c Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, Leuven 3001, Belgium b
A R T I C LE I N FO
A B S T R A C T
Keywords: Refractory Castables Thermo-mechanical properties Hydromagnesite
To assess the viability of micro-sized hydromagnesite as a precursor in refractory castables bonded with calcium aluminate cement, the volumetric stability and thermo-mechanical properties of high-alumina castables containing different micro-sized hydromagnesite amounts (0–1.6 wt%) after firing at 1550 °C were investigated. Phase composition and microstructure evolution in fired castable matrices with different micro-sized hydromagnesite contents were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Dimensional stability, hot modulus of rupture and thermal shock resistance of castables were improved by adding micro-sized hydromagnesite. The microstructure evolution in castable matrix added with micro-sized hydromagnesite was discussed to understand the mechanism of enhanced volume stability and thermo-mechanical properties.
1. Introduction Alumina-containing refractory castables bonded with calcium aluminate cement (CAC) are prone to considerable expansion because of the formation of CaO·2Al2O3 (CA2) and CaO·6Al2O3 (CA6) [1–3]. Among these refractory castables, MgO is added to generate in situ magnesium aluminate (MgAl2O4, MA) spinel and thus to attain a higher slag corrosion resistance [4,5]. The in situ spinel formation is accompanied with about 8 vol% expansion and can consequently brought about an additional expansion of castables [4,5]. As a result, dimensional expansion occurs in CAC-bonded alumina-containing refractory castables, leading to the formation and propagation of micro-cracks and even the spalling of castables. Subsequently, both the thermo-mechanical properties and slag infiltration resistance of castables decrease and thus the life time of castables is shortened. Therefore, many approaches have been attempted to prepare alumina-based castables with a moderate volume expansion through the designed compositions and additives [6–15]. These attempts include many factors, such as the selection of binder system [6,7], and the introduction of nanoscaled and microsized particles [8–11], rare earth oxides [11], and carbonate precursors [13–15]. There are several merits by using carbonate precursors, i.e. CaCO3 and MgCO3 [13–15], to improve the dimensional stability of refractory
castables: (1) high melting point phases, i.e. CA6 and MA spinel, are in situ formed in castables; (2) volume expansions associated with in situ CA2, CA6 and spinel formation are counterbalanced by the porosity generated from the carbonate precursors decomposition; (3) carbonate precursors can be added in micro-sized level in refractory matrix, which are low cost and easier to be distributed compared with nano-scaled particles. However, drawbacks were also brought about in castables by adding micro-sized carbonate precursors. Higher expansions can take place in castables when the micro-sized CaCO3 and MgCO3 contents are higher than 1.0 wt% and 1.5 wt%, respectively [14,15]. This is most likely due to the generation of larger amounts of CA6 and in situ spinel. Additionally, the incorporation of micro-sized MgCO3 in castables could lead to a decreased hot modulus of rupture of castables, because the formation and growth of CA6 grains are retarded [15]. These demerits lead to the consideration of an alternative precursor addition used in alumina-containing castables to improve the volume stability. Hydromagnesite (basic magnesium carbonate, 4MgCO3·Mg (OH)2·4H2O), which has been employed to synthesize various MgObased refractories [16–19], would be a promising carbonate precursor. On one hand, compared with CaCO3 and MgCO3, more porosity can be generated from hydromagnesite decomposition to counterbalance dimensional expansions accompanied with CA6 and in situ spinel formation. On the other hand, MgO can be generated/produced with the
⁎ Corresponding author at: Henan Key Laboratory of High Temperature Functional Ceramics, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, China. E-mail address: [email protected] (L. Chen).
https://doi.org/10.1016/j.ceramint.2018.04.114 Received 31 March 2018; Received in revised form 12 April 2018; Accepted 13 April 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hu, S., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.04.114
Ceramics International xxx (xxxx) xxx–xxx
S. Hu et al.
Table 1 Formulation of high alumina castables with varied micro-sized hydromagnesite contents (wt%). Recipe number Tabular alumina
Reactive alumina Hydromagnesite Calcium aluminate cement Water (extra)
6–0 mm − 200 mesh − 325 mesh BL-2 Secar 71
B0
B1
B2
B3
B4
70 10 8 7.0 0 5
70 10 8 6.6 0.4 5
70 10 8 6.2 0.8 5
70 10 8 5.8 1.2 5
70 10 8 5.4 1.6 5
+ 4.2
+ 4.3
+ 4.4
+ 4.5
+ 4.6
identical morphologies of the nano-sized sheet-like structure of the hydromagnesite precursor [16,17] and generates nano-sized MA spinel at temperatures lower than 1200 °C [18]. The nano-sized MgO and MA spinel, may not physically obstacle the growth of CA6 grains much, compared with the added micro-sized MgCO3, thereby leading to a higher strength of castables. However, no experimental results are available to assess the viability of micro-sized hydromagnesite addition in CAC-bonded alumina-containing castables. Thus, the volumetric stability and thermo-mechanical properties of high alumina castables with different amounts of micro-sized hydromagnesite additions was investigated. The changes in properties of high alumina castables containing micro-sized hydromagnesite were discussed with respect to the phase development and microstructure evolution in castable matrices.
Fig. 1. Permanent linear changes (PLC) of castables with different micro-sized hydromagnesite addition after firing at 1550 °C for 3 h.
20 min, and dropped into ambient temperature water for the thermal shock resistance testing. Thermal shock resistance of castables was assessed by measuring the CMOR values of samples before and after water quenching. To evaluate the phase developments and microstructure evolution with the addition of micro-sized hydromagnesite, matrix samples after firing at 1550 °C were characterized by X-ray diffraction (XRD; D4 Endeavor, Bruker, Germany) and scanning electron microscopy (SEM, Nova NanoSEM 230; FEI, Hillsboro, OR, USA) coupled with X-ray energy-dispersive spectroscopy (EDS: Genesis XM2, EDAX, Mahwah, NJ, USA), respectively.
2. Experimental 3. Results and discussion
2.1. Sample preparation
Fig. 1 shows the permanent linear change (PLC) of castables added with different amount of micro-sized hydromagnesite fired at 1550 °C. Phase compositions of castable matrix after firing at 1550 °C are presented in Fig. 2. The intensity of peaks corresponding to CaO·6Al2O3 (CA6) is identical in all castable matrix. The observation indicates that the same amount of CA6 was generated in castables with different hydromagnesite contents since the same amount of calcium aluminate cement (CAC) was added (5 wt%, as shown in Table 1). The generation of CA6 consequently results in volume expansions of castables. For instance, a positive PLC value of 0.22% is found in the castable without hydromagnesite (Fig. 1). As shown in Fig. 2, peaks related to magnesium-aluminate (MgAl2O4, MA) spinel are observed in castables containing hydromagnesite, suggesting the in situ formation of MA spinel from the interaction of Al2O3 and MgO from the hydromagnesite decomposition. These MA spinel peaks continuously increase by
Table 1 gives the four formulations of high alumina refractory castables containing varied micro-sized hydromagnesite (MgO: 44.9 wt%, d50 = 7.1 µm, Sinopharm Chemical Reagent Co. Ltd., China) contents. Sintered alumina aggregates (JGS-99; Jiangsu Jingxin new material CO. Ltd., China), 325 mesh sintered alumina powder (JGS-99; Jiangsu Jingxin new material CO. Ltd, China), reactive alumina powder (BL-2; Shangdong Zhongnai CO. Ltd., China) were used as starting materials. Calcium aluminate cement (Secar 71; Kerneos, China) and 0.2 wt% FS10 were used as the binder and dispersant, respectively. Reactive alumina was partially substituted by micro-sized hydromagnesite to investigate the effect of hydromagnesite addition on the volumetric stability and properties of castables. In addition, matrix samples were prepared using fine sintered alumina (325 mesh), reactive alumina, cement, micro-sized hydromagnesite, and dispersant, according to the ratios shown in Table 1. The starting powder mixtures were dry-mixed for 30 s, and then wet-mixed for 180 s in a laboratory mixer. After mixing, prismatic castables were cast into molds of 40 mm × 40 mm × 160 mm with vibration, and cured at room temperature for 24 h. The castable samples were then de-molded, dried at 110 °C for another 24 h, and fired for 3 h at 1550 °C. 2.2. Sample characterization The permanent linear change (PLC) was measured by the percentage difference between the final and initial bar length (GB/T 59882007). Bulk density (BD) and apparent porosity (AP) of the castables were attained through the Archimedes method. Cold modulus of rupture (CMOR) and crushing strength (CCS) of castables were attained according to GB/T 3001-2007 and GB/T 5702-2008, respectively. The hot modulus of rupture (HMOR) of castables was tested using the threepoint bending method with the castable specimens heated at 1400 °C for 1 h in a high temperature strength testing furnace (model HMOR03A, China). The 1550 °C fired samples were heated to 1000 °C for
Fig. 2. X-ray diffraction (XRD) patterns of the castable matrix after firing at 1550 °C for 3 h. 2
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Fig. 3. Secondary electron images of the castable matrix (a) without and (b) with 1.6 wt% hydromagnesite after firing for 3 h at (b) 1550 °C.
increasing the hydromagnesite content, revealing that in situ spinel formation was favored. The PLC value, however, continuously decreases from 0.22% to − 0.06% with the increase of hydromagnesite content from 0 to 1.6 wt% (Fig. 1). The decreased PLC values suggest that volumetric expansions derived from the in situ CA6 and MA spinel formation are suppressed by increasing the hydromagnesite addition in castables. As a result, the volume stability of CAC-bonded castables is improved. The castables containing hydromagnesite after firing at 1550 °C would have an appropriate PLC value of less than 0.2%. Moreover, even a volume shrinkage (a negative PLC value of −0.06%) can occur in the castable with the addition of 1.6 wt% hydromagnesite. To understand the mechanism of reduced PLC values of castables added with micro-sized hydromagnesite, microstructures of castable matrix without and with 1.6 wt% hydromagnesite after firing at 1550 °C were characterized and are shown in Figs. 3 and 4. CA6 plates are observed in castable matrix without and with 1.6 wt% hydromagnesite. It can be seen from Fig. 4 that CA6 plates are thicker in the matrix without hydromagnesite than those in the matrix with hydromagnesite. Compared with the thicker CA6 grains, thinner CA6 plates could lead to less local volume expansion in castable matrix. As shown in Fig. 4 that, the sintering neck can be observed between two CA6 plates in the matrix without hydromagnesite, thereby forming strong bonding among particles, e.g. CA6, in castables. In contrast, in the matrix with 1.6 wt% hydromagnesite (Fig. 4b) thinner CA6 plates are physically separated by even dispersed micro-sized MA spinel gains. As a result, the CA6 growth and sintering between CA6 plates were retarded. Note in Figs. 3 and 4 that a compact microstructure can be seen in the hydromagnesite-free matrix. In contrast, the matrix containing 1.6 wt% hydromagnesite has more porosity, due to the release of CO2 and H2O from the hydromagnesite decomposition. The higher porosity
Fig. 4. Back-scattered electron images of the castable matrix without (a) and with 1.6 wt% hydromagnesite (b) after firing for 3 h at 1550 °C. Insert images are the energy disperse spectroscopy (EDS) measurements of the indicted points by arrows: CA6 = CaO·6Al2O3; MA = MgAl2O4 spinel; Am = alumina.
Fig. 5. Changes of bulk density and apparent porosity of castables added with different amount of micro-sized hydromagnesite after firing at 1550 °C for 3 h.
can be also proven by the increased porosity level and decreased bulk density of castables with the addition of hydromagnesite (Fig. 5). The generation of higher porosity by adding hydromagnesite in the castable matrix can therefore provide space to counterbalance the volume 3
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Fig. 8. Changes of residual CMOR ratio of castables with different micro-sized hydromagnesite contents after water quenching, showing the effect of microsized hydromagnesite addition on the thermal shock resistance (TSR) of castables.
Fig. 6. Changes of cold crushing strength (CCS) and modulus rupture (CMOR) with the addition of hydromagnesite.
expansion associated with in situ CA6 and MA spinel formation. As a result of the hindered CA6 growth and the increased porosity level by adding hydromagnesite, volume expansions accompanied with in situ CA6 and MA spinel formation were inhibited, thereby suppressing the overall expansion in castables (Fig. 1). The influence of micro-sized hydromagnesite addition on the cold modulus of rupture (CMOR) and crushing strength (CCS) of 1550 °C fired castables is shown in Fig. 6. Both the CMOR and CCS continuously decrease with the increase of hydromagnesite content (Fig. 3). For instance, the CMOR decreases from 38.9 MPa to 27.2 MPa when the added hydromagnesite is increased from 0 to 1.6 wt%. The increased porosity of castables (as shown in Figs. 3–5) caused by the added hydromagnesite is most likely the reason leading to the decrease of CMOR and CCS values. In contrast to the decrease of CMOR and CCS, the hot modulus of rupture (HMOR) is higher for the hydromagnesite-containing castables than for the hydromagnesite-free one (Fig. 7). Specifically, the HMOR of castables firstly augments from 12.39 MPa to 14.49 MPa with the increase of hydromagnesite level from 0 to 0.8 wt%, then slightly reduces to 14.0 MPa by further increasing the hydromagnesite content to 1.6 wt% (Fig. 7). This is probably because more micro-sized in situ MA spinel was generated in castable matrix with the addition of hydromagnesite, thereby providing more bonding among particles in castables at elevated temperatures. As a result, castables containing hydromagnesite exhibit a higher HMOR than the hydromagnesite-free
castable. Compared with castables added with micro-sized MgCO3 [15], HMOR values increased for the castables containing micro-sized hydromagnesite. This is probably because CA6 and in situ MA spinel particles grown in micro size in hydromagnesite-containing castables (Fig. 4b), whereas they are nanometric in the MgCO3-containing castables [15]. Fig. 8 presents the influence of micro-sized hydromagnesite addition on the thermal shock resistance (TSR) of castables, which is assessed by the residual CMOR ratio of castables after water quenching. The residual CMOR ratio of castable increases from 17.5% to 30% by increasing the hydromagnesite level from 0 to 1.6 wt%. The increased residual CMOR ratio implies that likewise the addition of micro-sized MgCO3 [15], the added micro-sized hydromagnesite also improves the TSR of castables. This is most likely because of the generation of even distributed micro-sized MA spinel particles and CA6 plates (Figs. 3 and 4) and of more porosity in castables, thereby suppressing the formation and propagation of micro-cracks in castables. 4. Conclusions The permanent linear expansion of calcium aluminate cement (CAC) bonded castables was reduced by increasing micro-sized hydromagnesite content from 0 to 1.6 wt%, due to the retarded CaO·6Al2O3 (CA6) growth and the formation of more porosity caused by the release of CO2 and H2O from hydromagnesite decomposition in the castable fired at 1550 °C. This is favorable to the volumetric stability of the castables. The optimal amount of hydromagnesite addition would be between 0.4% to 1.2 wt% to have a permanent leaner change of less than 0.2%. The hot modulus of rupture of castable was also improved with the addition of micro-sized hydromagnesite up to 1.6 wt%. This is probably because in situ magnesium aluminate (MA) spinel formation increased bonding among particles in castables at elevated temperatures. The thermal shock resistance of castables was also improved due to the enhanced formation of uniformly distributed micro-sized MA spinel particles and CA6 plates and of more porosity in the castables. These results suggest that micro-sized hydromagnesite could be used as an alternative carbonate precursor in alumina-containing castables bonded with CAC to improve volume stability and thermo-mechanical properties. Acknowledgement
Fig. 7. Changes of hot modulus of rupture (HMOR) of castables added with different micro-sized hydromagnesite addition.
The authors acknowledge the financial support from the National Natural Science Foundation of China (51572244, U1604252 and 4
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51372230).
Process. Technol. 209 (2009) 5552–5557. [10] W. Yuan, H. Tang, Y. Zhou, D. Zhang, Effects of fine reactive alumina powders on properties of alumina-magnesia castables with TiO2 additions, Ceram. Int. 44 (2018) 5032–5036. [11] W. Yuan, H. Tang, H. Shang, J. Li, C. Deng, H. Zhu, Effects of TiO2 addition on kinetics of in situ spinel formation and properties of alumina-magnesia refractory castables, J. Ceram. Sci. Technol. 8 (2017) 121–128. [12] W. Yuan, Q. Zhu, C. Deng, H. Zhu, Effects of rare earth oxides additions on microstructure and properties of alumina-magnesia refractory castables, Ceram. Int. 43 (2017) 6746–6750. [13] A.P. Luz, A.H.G. Gabriel, L.B. Consoni, C.G. Aneziris, V.C. Pandolfelli, Self-reinforced high-alumina refractory castables, Ceram. Int. 44 (2018) 2364–2375. [14] Q. Wang, G. Ye, Y. Wang, A. Fu, C. Zhang, X. Shang, Effect of micro-sized calcium carbonate addition on volumetric stability and strength of corundum-based castables, Int. J. Appl. Ceram. Technol. 12 (2015) E166–E171, http://dx.doi.org/10. 1111/ijac.12273. [15] S. Hu, G. Ye, L. Chen, W. Gu, P. Zhai, Q. Wang, Effect of micro-sized MgCO3 addition on properties of MgAl2O4 spinel containing castables, Ceram. Int. 43 (2017). [16] L. Chen, G. Ye, W. Zhou, J. Dijkmans, B. Sels, A. Malfliet, M. Guo, Influence of MgO precursors on mechanically activated forsterite synthesis, Ceram. Int. 41 (2015) 12651–12657. [17] L. Chen, G. Ye, Q. Wang, B. Blanpain, A. Malfliet, M. Guo, Low temperature synthesis of forsterite from hydromagnesite and fumed silica mixture, Ceram. Int. 41 (2015) 2234–2239. [18] K.J.D. Mackenzie, J. Temuujin, T. Jadambaa, M.E. Smith, P. Angerer, Mechanochemical synthesis and sintering behaviour of magnesium aluminate spinel, J. Mater. Sci. 35 (2000) 5529–5535. [19] L. Chen, G. Ye, P. Zhai, L. Zhu, Y. Zhang, A. Fu, Synthesis of cordierite from different MgO source-containing mixtures via mechanical activation, Naihuo Cailiao/ Refract. 47 (2013) 14–18.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effect of micro-sized hydromagnesite addition on the properties of calcium aluminate cement-bonded castables ⁎
Shuhe Hua,b, Liugang Chena,c, , Lingling Zhua, Dafei Dinga, Fei Zhaoa, Guotian Yea a
Henan Key Laboratory of High Temperature Functional Ceramics, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, China High Temperature Materials Research Institute, Henan University of Science and Technology, Luoyang, Henan 471003, China c Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44, Leuven 3001, Belgium b
A R T I C LE I N FO
A B S T R A C T
Keywords: Refractory Castables Thermo-mechanical properties Hydromagnesite
To assess the viability of micro-sized hydromagnesite as a precursor in refractory castables bonded with calcium aluminate cement, the volumetric stability and thermo-mechanical properties of high-alumina castables containing different micro-sized hydromagnesite amounts (0–1.6 wt%) after firing at 1550 °C were investigated. Phase composition and microstructure evolution in fired castable matrices with different micro-sized hydromagnesite contents were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Dimensional stability, hot modulus of rupture and thermal shock resistance of castables were improved by adding micro-sized hydromagnesite. The microstructure evolution in castable matrix added with micro-sized hydromagnesite was discussed to understand the mechanism of enhanced volume stability and thermo-mechanical properties.
1. Introduction Alumina-containing refractory castables bonded with calcium aluminate cement (CAC) are prone to considerable expansion because of the formation of CaO·2Al2O3 (CA2) and CaO·6Al2O3 (CA6) [1–3]. Among these refractory castables, MgO is added to generate in situ magnesium aluminate (MgAl2O4, MA) spinel and thus to attain a higher slag corrosion resistance [4,5]. The in situ spinel formation is accompanied with about 8 vol% expansion and can consequently brought about an additional expansion of castables [4,5]. As a result, dimensional expansion occurs in CAC-bonded alumina-containing refractory castables, leading to the formation and propagation of micro-cracks and even the spalling of castables. Subsequently, both the thermo-mechanical properties and slag infiltration resistance of castables decrease and thus the life time of castables is shortened. Therefore, many approaches have been attempted to prepare alumina-based castables with a moderate volume expansion through the designed compositions and additives [6–15]. These attempts include many factors, such as the selection of binder system [6,7], and the introduction of nanoscaled and microsized particles [8–11], rare earth oxides [11], and carbonate precursors [13–15]. There are several merits by using carbonate precursors, i.e. CaCO3 and MgCO3 [13–15], to improve the dimensional stability of refractory
castables: (1) high melting point phases, i.e. CA6 and MA spinel, are in situ formed in castables; (2) volume expansions associated with in situ CA2, CA6 and spinel formation are counterbalanced by the porosity generated from the carbonate precursors decomposition; (3) carbonate precursors can be added in micro-sized level in refractory matrix, which are low cost and easier to be distributed compared with nano-scaled particles. However, drawbacks were also brought about in castables by adding micro-sized carbonate precursors. Higher expansions can take place in castables when the micro-sized CaCO3 and MgCO3 contents are higher than 1.0 wt% and 1.5 wt%, respectively [14,15]. This is most likely due to the generation of larger amounts of CA6 and in situ spinel. Additionally, the incorporation of micro-sized MgCO3 in castables could lead to a decreased hot modulus of rupture of castables, because the formation and growth of CA6 grains are retarded [15]. These demerits lead to the consideration of an alternative precursor addition used in alumina-containing castables to improve the volume stability. Hydromagnesite (basic magnesium carbonate, 4MgCO3·Mg (OH)2·4H2O), which has been employed to synthesize various MgObased refractories [16–19], would be a promising carbonate precursor. On one hand, compared with CaCO3 and MgCO3, more porosity can be generated from hydromagnesite decomposition to counterbalance dimensional expansions accompanied with CA6 and in situ spinel formation. On the other hand, MgO can be generated/produced with the
⁎ Corresponding author at: Henan Key Laboratory of High Temperature Functional Ceramics, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, Henan 450001, China. E-mail address: [email protected] (L. Chen).
https://doi.org/10.1016/j.ceramint.2018.04.114 Received 31 March 2018; Received in revised form 12 April 2018; Accepted 13 April 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hu, S., Ceramics International (2018), https://doi.org/10.1016/j.ceramint.2018.04.114
Ceramics International xxx (xxxx) xxx–xxx
S. Hu et al.
Table 1 Formulation of high alumina castables with varied micro-sized hydromagnesite contents (wt%). Recipe number Tabular alumina
Reactive alumina Hydromagnesite Calcium aluminate cement Water (extra)
6–0 mm − 200 mesh − 325 mesh BL-2 Secar 71
B0
B1
B2
B3
B4
70 10 8 7.0 0 5
70 10 8 6.6 0.4 5
70 10 8 6.2 0.8 5
70 10 8 5.8 1.2 5
70 10 8 5.4 1.6 5
+ 4.2
+ 4.3
+ 4.4
+ 4.5
+ 4.6
identical morphologies of the nano-sized sheet-like structure of the hydromagnesite precursor [16,17] and generates nano-sized MA spinel at temperatures lower than 1200 °C [18]. The nano-sized MgO and MA spinel, may not physically obstacle the growth of CA6 grains much, compared with the added micro-sized MgCO3, thereby leading to a higher strength of castables. However, no experimental results are available to assess the viability of micro-sized hydromagnesite addition in CAC-bonded alumina-containing castables. Thus, the volumetric stability and thermo-mechanical properties of high alumina castables with different amounts of micro-sized hydromagnesite additions was investigated. The changes in properties of high alumina castables containing micro-sized hydromagnesite were discussed with respect to the phase development and microstructure evolution in castable matrices.
Fig. 1. Permanent linear changes (PLC) of castables with different micro-sized hydromagnesite addition after firing at 1550 °C for 3 h.
20 min, and dropped into ambient temperature water for the thermal shock resistance testing. Thermal shock resistance of castables was assessed by measuring the CMOR values of samples before and after water quenching. To evaluate the phase developments and microstructure evolution with the addition of micro-sized hydromagnesite, matrix samples after firing at 1550 °C were characterized by X-ray diffraction (XRD; D4 Endeavor, Bruker, Germany) and scanning electron microscopy (SEM, Nova NanoSEM 230; FEI, Hillsboro, OR, USA) coupled with X-ray energy-dispersive spectroscopy (EDS: Genesis XM2, EDAX, Mahwah, NJ, USA), respectively.
2. Experimental 3. Results and discussion
2.1. Sample preparation
Fig. 1 shows the permanent linear change (PLC) of castables added with different amount of micro-sized hydromagnesite fired at 1550 °C. Phase compositions of castable matrix after firing at 1550 °C are presented in Fig. 2. The intensity of peaks corresponding to CaO·6Al2O3 (CA6) is identical in all castable matrix. The observation indicates that the same amount of CA6 was generated in castables with different hydromagnesite contents since the same amount of calcium aluminate cement (CAC) was added (5 wt%, as shown in Table 1). The generation of CA6 consequently results in volume expansions of castables. For instance, a positive PLC value of 0.22% is found in the castable without hydromagnesite (Fig. 1). As shown in Fig. 2, peaks related to magnesium-aluminate (MgAl2O4, MA) spinel are observed in castables containing hydromagnesite, suggesting the in situ formation of MA spinel from the interaction of Al2O3 and MgO from the hydromagnesite decomposition. These MA spinel peaks continuously increase by
Table 1 gives the four formulations of high alumina refractory castables containing varied micro-sized hydromagnesite (MgO: 44.9 wt%, d50 = 7.1 µm, Sinopharm Chemical Reagent Co. Ltd., China) contents. Sintered alumina aggregates (JGS-99; Jiangsu Jingxin new material CO. Ltd., China), 325 mesh sintered alumina powder (JGS-99; Jiangsu Jingxin new material CO. Ltd, China), reactive alumina powder (BL-2; Shangdong Zhongnai CO. Ltd., China) were used as starting materials. Calcium aluminate cement (Secar 71; Kerneos, China) and 0.2 wt% FS10 were used as the binder and dispersant, respectively. Reactive alumina was partially substituted by micro-sized hydromagnesite to investigate the effect of hydromagnesite addition on the volumetric stability and properties of castables. In addition, matrix samples were prepared using fine sintered alumina (325 mesh), reactive alumina, cement, micro-sized hydromagnesite, and dispersant, according to the ratios shown in Table 1. The starting powder mixtures were dry-mixed for 30 s, and then wet-mixed for 180 s in a laboratory mixer. After mixing, prismatic castables were cast into molds of 40 mm × 40 mm × 160 mm with vibration, and cured at room temperature for 24 h. The castable samples were then de-molded, dried at 110 °C for another 24 h, and fired for 3 h at 1550 °C. 2.2. Sample characterization The permanent linear change (PLC) was measured by the percentage difference between the final and initial bar length (GB/T 59882007). Bulk density (BD) and apparent porosity (AP) of the castables were attained through the Archimedes method. Cold modulus of rupture (CMOR) and crushing strength (CCS) of castables were attained according to GB/T 3001-2007 and GB/T 5702-2008, respectively. The hot modulus of rupture (HMOR) of castables was tested using the threepoint bending method with the castable specimens heated at 1400 °C for 1 h in a high temperature strength testing furnace (model HMOR03A, China). The 1550 °C fired samples were heated to 1000 °C for
Fig. 2. X-ray diffraction (XRD) patterns of the castable matrix after firing at 1550 °C for 3 h. 2
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Fig. 3. Secondary electron images of the castable matrix (a) without and (b) with 1.6 wt% hydromagnesite after firing for 3 h at (b) 1550 °C.
increasing the hydromagnesite content, revealing that in situ spinel formation was favored. The PLC value, however, continuously decreases from 0.22% to − 0.06% with the increase of hydromagnesite content from 0 to 1.6 wt% (Fig. 1). The decreased PLC values suggest that volumetric expansions derived from the in situ CA6 and MA spinel formation are suppressed by increasing the hydromagnesite addition in castables. As a result, the volume stability of CAC-bonded castables is improved. The castables containing hydromagnesite after firing at 1550 °C would have an appropriate PLC value of less than 0.2%. Moreover, even a volume shrinkage (a negative PLC value of −0.06%) can occur in the castable with the addition of 1.6 wt% hydromagnesite. To understand the mechanism of reduced PLC values of castables added with micro-sized hydromagnesite, microstructures of castable matrix without and with 1.6 wt% hydromagnesite after firing at 1550 °C were characterized and are shown in Figs. 3 and 4. CA6 plates are observed in castable matrix without and with 1.6 wt% hydromagnesite. It can be seen from Fig. 4 that CA6 plates are thicker in the matrix without hydromagnesite than those in the matrix with hydromagnesite. Compared with the thicker CA6 grains, thinner CA6 plates could lead to less local volume expansion in castable matrix. As shown in Fig. 4 that, the sintering neck can be observed between two CA6 plates in the matrix without hydromagnesite, thereby forming strong bonding among particles, e.g. CA6, in castables. In contrast, in the matrix with 1.6 wt% hydromagnesite (Fig. 4b) thinner CA6 plates are physically separated by even dispersed micro-sized MA spinel gains. As a result, the CA6 growth and sintering between CA6 plates were retarded. Note in Figs. 3 and 4 that a compact microstructure can be seen in the hydromagnesite-free matrix. In contrast, the matrix containing 1.6 wt% hydromagnesite has more porosity, due to the release of CO2 and H2O from the hydromagnesite decomposition. The higher porosity
Fig. 4. Back-scattered electron images of the castable matrix without (a) and with 1.6 wt% hydromagnesite (b) after firing for 3 h at 1550 °C. Insert images are the energy disperse spectroscopy (EDS) measurements of the indicted points by arrows: CA6 = CaO·6Al2O3; MA = MgAl2O4 spinel; Am = alumina.
Fig. 5. Changes of bulk density and apparent porosity of castables added with different amount of micro-sized hydromagnesite after firing at 1550 °C for 3 h.
can be also proven by the increased porosity level and decreased bulk density of castables with the addition of hydromagnesite (Fig. 5). The generation of higher porosity by adding hydromagnesite in the castable matrix can therefore provide space to counterbalance the volume 3
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Fig. 8. Changes of residual CMOR ratio of castables with different micro-sized hydromagnesite contents after water quenching, showing the effect of microsized hydromagnesite addition on the thermal shock resistance (TSR) of castables.
Fig. 6. Changes of cold crushing strength (CCS) and modulus rupture (CMOR) with the addition of hydromagnesite.
expansion associated with in situ CA6 and MA spinel formation. As a result of the hindered CA6 growth and the increased porosity level by adding hydromagnesite, volume expansions accompanied with in situ CA6 and MA spinel formation were inhibited, thereby suppressing the overall expansion in castables (Fig. 1). The influence of micro-sized hydromagnesite addition on the cold modulus of rupture (CMOR) and crushing strength (CCS) of 1550 °C fired castables is shown in Fig. 6. Both the CMOR and CCS continuously decrease with the increase of hydromagnesite content (Fig. 3). For instance, the CMOR decreases from 38.9 MPa to 27.2 MPa when the added hydromagnesite is increased from 0 to 1.6 wt%. The increased porosity of castables (as shown in Figs. 3–5) caused by the added hydromagnesite is most likely the reason leading to the decrease of CMOR and CCS values. In contrast to the decrease of CMOR and CCS, the hot modulus of rupture (HMOR) is higher for the hydromagnesite-containing castables than for the hydromagnesite-free one (Fig. 7). Specifically, the HMOR of castables firstly augments from 12.39 MPa to 14.49 MPa with the increase of hydromagnesite level from 0 to 0.8 wt%, then slightly reduces to 14.0 MPa by further increasing the hydromagnesite content to 1.6 wt% (Fig. 7). This is probably because more micro-sized in situ MA spinel was generated in castable matrix with the addition of hydromagnesite, thereby providing more bonding among particles in castables at elevated temperatures. As a result, castables containing hydromagnesite exhibit a higher HMOR than the hydromagnesite-free
castable. Compared with castables added with micro-sized MgCO3 [15], HMOR values increased for the castables containing micro-sized hydromagnesite. This is probably because CA6 and in situ MA spinel particles grown in micro size in hydromagnesite-containing castables (Fig. 4b), whereas they are nanometric in the MgCO3-containing castables [15]. Fig. 8 presents the influence of micro-sized hydromagnesite addition on the thermal shock resistance (TSR) of castables, which is assessed by the residual CMOR ratio of castables after water quenching. The residual CMOR ratio of castable increases from 17.5% to 30% by increasing the hydromagnesite level from 0 to 1.6 wt%. The increased residual CMOR ratio implies that likewise the addition of micro-sized MgCO3 [15], the added micro-sized hydromagnesite also improves the TSR of castables. This is most likely because of the generation of even distributed micro-sized MA spinel particles and CA6 plates (Figs. 3 and 4) and of more porosity in castables, thereby suppressing the formation and propagation of micro-cracks in castables. 4. Conclusions The permanent linear expansion of calcium aluminate cement (CAC) bonded castables was reduced by increasing micro-sized hydromagnesite content from 0 to 1.6 wt%, due to the retarded CaO·6Al2O3 (CA6) growth and the formation of more porosity caused by the release of CO2 and H2O from hydromagnesite decomposition in the castable fired at 1550 °C. This is favorable to the volumetric stability of the castables. The optimal amount of hydromagnesite addition would be between 0.4% to 1.2 wt% to have a permanent leaner change of less than 0.2%. The hot modulus of rupture of castable was also improved with the addition of micro-sized hydromagnesite up to 1.6 wt%. This is probably because in situ magnesium aluminate (MA) spinel formation increased bonding among particles in castables at elevated temperatures. The thermal shock resistance of castables was also improved due to the enhanced formation of uniformly distributed micro-sized MA spinel particles and CA6 plates and of more porosity in the castables. These results suggest that micro-sized hydromagnesite could be used as an alternative carbonate precursor in alumina-containing castables bonded with CAC to improve volume stability and thermo-mechanical properties. Acknowledgement
Fig. 7. Changes of hot modulus of rupture (HMOR) of castables added with different micro-sized hydromagnesite addition.
The authors acknowledge the financial support from the National Natural Science Foundation of China (51572244, U1604252 and 4
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