Great attempts were made to reduce the amount of calcium aluminate cement
(CAC) content in refractory castables to improve their hot strength. Using more than 2-3
wt% CAC may cause low melting phases formation in the refractory matrix leading to
weak thermo-mechanical propereties of the castables. Colloidal Silica can affect the
structure of refractory castables to achieve superior thermo mechanical properties. Replacing
calcium aluminate cement (CAC) by colloidal silica as a water base binder,
speeds up drying, reduces the amount of liquid phase at high temperatures and may lead
to mullite formation, which will increase the hot strength of the refractory castables. In
this research, the influence of colloidal silica addition on bulk density, apparent porosity
and HMOR of a tabular alumina based refractory castable containing have been studied.
The results showed that samples containing colloidal silica have higher hot strength
compared to those containing only CAC as binder due to the better compaction, less
liquid phase formation at high temperature.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2057

Alireza, S.

Fatemeh, K.N.

Hossein, S.

SocioEconomic Challenges Journal (Sumy State University)

254                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
IMPROVING THERMO-MECHANICAL PROPERTIES OF 
TABULAR ALUMINA CASTABLES VIA USING  
NANO STRUCTURED COLLOIDAL SILICA 
Alireza Souri*, Fatemeh Kashani Nia, Hossein Sarpoolaky 
 
Iran University of Science and Technology, Tehran, Iran 
 
ABSTRACT 
Great attempts were made  to reduce the amount of calcium aluminate cement 
(CAC) content in refractory castables to improve their hot strength. Using more than 2-3 
wt% CAC may cause low melting phases formation in the refractory matrix leading to 
weak thermo-mechanical propereties of the castables. Colloidal Silica can affect the 
structure of refractory castables to achieve superior thermo mechanical properties. Re-
placing calcium aluminate cement (CAC) by colloidal silica as a water base binder, 
speeds up drying, reduces the amount of liquid phase at high temperatures and may lead 
to mullite formation, which will increase the hot strength of the refractory castables. In 
this research, the influence of colloidal silica addition on bulk density, apparent porosity 
and HMOR of a tabular alumina based refractory castable containing have been studied. 
The results showed that samples containing colloidal silica have higher hot strength 
compared to those containing only CAC as binder due to the better compaction, less 
liquid phase formation at high temperature. 
 
Keywords: Nano Silica, Alumina Castable, Cement, HMOR 
 
INTRODUCTION 
After more than 20 years of development and testing, low and ultra low-
cement castables have proven their suitability for many applications in different 
industries particularly in iron and steel production. The main difference be-
tween the traditional and the castables with reduced cement content can be 
stated as fraction of fine (a few microns) and ultrafine (sub-micron) material 
addition. 
These particles aid packing and increase thereby the compactness of the 
castables. Consequently, the cement and so the water needed for casting is 
lowered.[1] Using less water leads to less porosity when heated, so the castable 
loses less strength upon firing and will be less prone to gas and slag attack (Fig. 
1). [2, 3] 
Moreover, lowering cement conten reduces CaO in alumina castables, de-
creases the amount of low melting phases at around 1200°C[2] which in turn 
improves the corrosion resistance and creep strength in service.[4] 
                                               
* arsouri@gmail.com 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  255 
 
 
Fig. 1 – A comparison of porosity for cement bonded and colloidal silica bonded (a sub-
micron agent) refractories[3]. 
 
Colloidal Silica is a sole with ultra fine particles of silica (Fig. 2) which 
can affect the structure of refractory castables due to its superior thermo me-
chanical properties. Replacing calcium aluminate cement (CAC) by colloidal 
silica as a binder, provides all the advantages of the low and ultra low cement 
castables, while eliminating most of their disadvantages. [5].  Although the 
colloidal silica bonded products show a lower initial strength than cement 
bonded ones due to decreasrd hydraulic bond, they show a good progression in 
strength development similar to the cement bonded products at moderate tem-
peratures. At elevated temperatures, the strength differences become more 
dramatic. 
Low-melting-temperature 
CaO-Al2O3-SiO2 phases asso-
ciated with CAC castables are 
responsible for liquid phase 
formation which resulting 
lower hot strengths. Because 
the colloidal silica bonded 
materials are CaO-free, they 
do not generate these low 
melting phases and typically 
exhibit higher hot strengths 
which results in better in-
service erosion resistance for 
these materials. [3] Also Silica 
fumes with extremely small 
particle size (average size is 
0.15 mm) replace water by 
attaching themselves to cement particles because of their opposite charge; so, 
lower water requirement reduces the porosity while increases the density and 
 
Fig. 2 – Colloidal Silica under TEM [6]  
256                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
strength. Moreover, since the cement is partially hydrated, there is no loss of 
strength for the colloidal silica containing castable at intermediate tempera-
tures.[7] Another factor which increases the strength of colloidal silica bonded 
castables at high temperatures is the mullite bond formation. Mullite formation 
in refractory castables has been found dependant on several factors[8] such as 
using nano-sized silica which promotes it and causes mullite formation at lower 
temperatures. [9,10]  Mullite is a very much desirable phase in castables devel-
oping improved hot strength, creep resistance, good thermal stability as well as 
slag penetration resistance.[11] In the present study, apparent porosity, bulk 
density, hot modulus of rupture and phase analysis of a cement-free colloidal 
silica containing castable have been compared to those of the normal LCC one.  
EXPERIMENTAL 
1. Raw Materials 
The chemical analyses of the raw materials used in this study are listed in 
Table 1. 
 
Table 1- Chemical analyses of the raw materials 
 Tabular Alumina, 
(wt%) 
Reactive 
Alumina, (wt%) 
Calcium Aluminate 
Cement(wt%) 
Colloidal Silica, 
(wt%) 
Al2O3  99.4 99.5 69.8-72.2 0 
CaO  0.03 0.03 26.8-29.2 0 
SiO2  0.03 0.04 0.2-0.6 40.2 
Fe2O3  0.02 0.02 0.1-0.3 0 
Na2O  0.35 0.04  
 < 0.5 
0 
K2O  0 0 0 
The recipes of the studied castables (sample C, which is a low cement 
castable, LCC, containing 5 wt% cement and sample N, a no-cement castable 
containing  colloidal silica) are provided in Table 2. 
 
Table 2 – Composition of castabales  
Castable Composition (wt%) C N 
Tabular Alumina 85 87 
Reactive Alumina 10 10 
Cement 5 0 
Colloidal silica 0 7.5 
Dispersant 0.1 0.1 
water 5 4.5 
 
2. Castables Preparation  
For  each  sample,  a  3kg  batch  mixture  using  the  formulation  in  Table 2 
was prepared by dry mixing for 4 minutes at slow speed, using a Hobart mixer 
with 5 liter capacity mixing bowl. Then wet mixing was carried out by addition 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  257 
 
of water with further 4 minutes mixing at medium speed. Test samples were 
prepared by casting the mixture into the stainless steel molds for 1 minute, with 
the dimensions of 150*25*25 mm by vibrating at 50HZ, The castables were 
cured at 20°C and a relative humidity of 95% for 24 hours in the mold. After 
curing time, the samples were allowed to be dried at the temperature of 110°C, 
and then fired at 1400°C for 5 hours (by the rate of  300°C per hour). 
3. Testing  
Bulk density and apparent porosity were determined by ASTM standard 
methods. The given values of these properties in result section are the average 
of three samples for each formulation. Hot modulus of rupture, HMOR, was 
carried out according to DIN 51048 using the Netzsch 442D/3 model. These 
values are also the average of three measures. 
RESULT AND DISCUSSION 
1. Bulk density and apparent porosity 
Bulk density and apparent porosity of the samples are given in Fig 3 and 
Fig 4. 
  
Fig. 3 – Bulk density of the samples fired 
at 1400°C for 5 hrs 
Fig.4 – Apparent porosity of the samples 
fired at 1400°C for 5 hrs 
 
In LCC castables (Sample C), the cement content requires water for 
placement. This leads to high porosity when heated, the castable loses strength 
and becomes vulnerable to infiltration e.g. by slags. Besides, due to dehydration 
of the hydrate phases before the ceramic bond is formed, the castables also 
show a strength lowering at intermediate temperatures.[2]   Colloidal silica 
particles, due to their nano sizes (~15 nm), could behave as a liquid[12]  and so 
less water was needed in preparation of the sample N, which led to less amount 
of apparent porosity upon drying and firing.   
Colloidal silica, due to its high specific area, anticipates sintering and so 
the sample N shows higher bulk density than sample C. 
2. HMOR 
The HMOR amounts of the samples are shown in Fig. 5. Using fine and 
reactive powders such as reactive alumina and colloidal silica in the bond phase 
of sample N can precipitate mullite needles from 1300°C in which the mullite 
needle-like particles grow out of a the matrix promote a good reinforcement 
258                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II           
 
according to Schumacherrs report. [2]  In Fig. 5, It can be seen that the sample 
containing colloidal silica (Sample N) shows higher amount of HMOR, better 
compaction, lower apparent porosity and higher bulk density due to presence of 
colloidal silica and concequently, formation and growth of long, needle-shaped 
mullite crystals.  
  
Fig.5 – Hot Modulus of Rupture 
(HMOR) of the samples fired at 1400°C 
for 5 hrs 
Fig. 6 – XRD patterns of the sam-
ples fired at 1400°C for 5 hrs. A repre-
sents alumina and CA represents calcium-
hexaluminate(6Al2O3.CaO) 
 
3. Phase Analysis 
The XRD patterns of the samples can be seen in Fig. 6. 
In the XRD pattern of sample C, calcium aluminium oxide can be detected 
besides alumina, which is a cement phase and has a lower melting point than 
alumina and acts as a flux, so can lower the HMOR of the sample. 
Phase analysis of the sample N reveals mullite (3Al2O3.2SiO2) peaks, alt-
hough they are weak due to low amount in the total phase constituents of the 
castable, so  mullite was assumed as a trace phase. 
CONCLUSIONS 
Colloidal silica sol could behave as a liquid and so the castables 
containing colloidal silica need less water to be prepared, which leads to 
lower their apparent porosity than that of LCC ones. Colloidal silica, due 
to its high specific area, anticipates sintering and so leads to better com-
paction. Comparing the traditional low calcium aluminate cement casta-
bles, colloidal silica containing refractory castables attains better me-
chanical strength at high temperatures. 
REFERENCE 
[1] Bjorn Myhre, "Hot Strength and Bond-Phase Reactions in Low and Ultra-Low Ce-
ment Castables",  Elkem a/s Refractories, P.O. Box 126, N-4602 Kristiansand, Nor-
way. 
[2] Bjorn Myhre, "Tabular Alumina Based Refractory Castables" , Elkem a/s Refracto-
ries, P.O. Box 126, N-4602 Kristiansand, Norway. 
Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  259 
 
[3] Michael Anderson, "Better Refractories through Nanotechnology", Metrel, Inc., 
Addison, Ill. October 1, 2005 
[4] S. Ghosh, R. Majumdar, B.K. Sinhamahapatra et al., Ceramics International 29 
(2003),P. 671–677. 
[5] Ali Reza Souri, Behzad Mirhadi and Fatemeh Kashani Nia, The Effect of Nano-
Structured Collidal Silica on the Properties of Tabular Alumina Castables", XVI. In-
ternational Conference on Refractories, Prague, The Czech Republic, 2008. 
[6] Florence Sanchez, Konstantin Sobolev, "Nanotechnology in concrete- A review", 
Elsevier 2010. 
[7] Subrata Banerjee, Am. Ceram. Soc. Bull., 77 [10] (1998), ACerS Data Depository 
File No. 295. 
[8] C. Odegård, H. Fjeldborg and B. Myhre; "Mullite Formation In Refractory Casta-
bles", XIII. International Conference on Refractories, Prague, The Czech Republic, 
2000. 
[9] M. R. Ismael, R.D. dos Anjos, R. Salomão and V.C. Pandolfelli, Refractories Appli-
cations and News, Vol. 11, N. 4, July/August 2006.  
[10] S. Hosein Badiee, Sasan Otroj, Ceramics – Silikáty 53 (4) 2009, P. 297-302 
[11] S. Mukhopadhyay, S. Ghosh, M. K. Mahapatra et al., Ceramics International, 
Volume 28, Issue 7, 2002, P. 719-729  
[12] M. R. Ismael, R.D. dos Anjos, R. Salomão and V.C. Pandolfelli, "Colloidal 
Silica as a nano-structured binder for refractory castables", Refractories Applications 
and News, Volume 11, Number 4, July/August 2006. 
 
  


English

Improving thermo-mechanical properties of tabular alumina castables via using nano structured colloidal silica

Electronic Sumy State University Institutional Repository

254                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II            IMPROVING THERMO-MECHANICAL PROPERTIES OF TABULAR ALUMINA CASTABLES VIA USING  NANO STRUCTURED COLLOIDAL SILICA Alireza Souri*, Fatemeh Kashani Nia, Hossein Sarpoolaky  Iran University of Science and Technology, Tehran, Iran  ABSTRACT Great attempts were made  to reduce the amount of calcium aluminate cement (CAC) content in refractory castables to improve their hot strength. Using more than 2-3 wt% CAC may cause low melting phases formation in the refractory matrix leading to weak thermo-mechanical propereties of the castables. Colloidal Silica can affect the structure of refractory castables to achieve superior thermo mechanical properties. Re-placing calcium aluminate cement (CAC) by colloidal silica as a water base binder, speeds up drying, reduces the amount of liquid phase at high temperatures and may lead to mullite formation, which will increase the hot strength of the refractory castables. In this research, the influence of colloidal silica addition on bulk density, apparent porosity and HMOR of a tabular alumina based refractory castable containing have been studied. The results showed that samples containing colloidal silica have higher hot strength compared to those containing only CAC as binder due to the better compaction, less liquid phase formation at high temperature.  Keywords: Nano Silica, Alumina Castable, Cement, HMOR  INTRODUCTION After more than 20 years of development and testing, low and ultra low-cement castables have proven their suitability for many applications in different industries particularly in iron and steel production. The main difference be-tween the traditional and the castables with reduced cement content can be stated as fraction of fine (a few microns) and ultrafine (sub-micron) material addition. These particles aid packing and increase thereby the compactness of the castables. Consequently, the cement and so the water needed for casting is lowered.[1] Using less water leads to less porosity when heated, so the castable loses less strength upon firing and will be less prone to gas and slag attack (Fig. 1). [2, 3] Moreover, lowering cement conten reduces CaO in alumina castables, de-creases the amount of low melting phases at around 1200°C[2] which in turn improves the corrosion resistance and creep strength in service.[4]                                                * arsouri@gmail.com Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  255   Fig. 1 – A comparison of porosity for cement bonded and colloidal silica bonded (a sub-micron agent) refractories[3].  Colloidal Silica is a sole with ultra fine particles of silica (Fig. 2) which can affect the structure of refractory castables due to its superior thermo me-chanical properties. Replacing calcium aluminate cement (CAC) by colloidal silica as a binder, provides all the advantages of the low and ultra low cement castables, while eliminating most of their disadvantages. [5].  Although the colloidal silica bonded products show a lower initial strength than cement bonded ones due to decreasrd hydraulic bond, they show a good progression in strength development similar to the cement bonded products at moderate tem-peratures. At elevated temperatures, the strength differences become more dramatic. Low-melting-temperature CaO-Al2O3-SiO2 phases asso-ciated with CAC castables are responsible for liquid phase formation which resulting lower hot strengths. Because the colloidal silica bonded materials are CaO-free, they do not generate these low melting phases and typically exhibit higher hot strengths which results in better in-service erosion resistance for these materials. [3] Also Silica fumes with extremely small particle size (average size is 0.15 mm) replace water by attaching themselves to cement particles because of their opposite charge; so, lower water requirement reduces the porosity while increases the density and  Fig. 2 – Colloidal Silica under TEM [6]  256                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II            strength. Moreover, since the cement is partially hydrated, there is no loss of strength for the colloidal silica containing castable at intermediate tempera-tures.[7] Another factor which increases the strength of colloidal silica bonded castables at high temperatures is the mullite bond formation. Mullite formation in refractory castables has been found dependant on several factors[8] such as using nano-sized silica which promotes it and causes mullite formation at lower temperatures. [9,10]  Mullite is a very much desirable phase in castables devel-oping improved hot strength, creep resistance, good thermal stability as well as slag penetration resistance.[11] In the present study, apparent porosity, bulk density, hot modulus of rupture and phase analysis of a cement-free colloidal silica containing castable have been compared to those of the normal LCC one.  EXPERIMENTAL 1. Raw Materials The chemical analyses of the raw materials used in this study are listed in Table 1.  Table 1- Chemical analyses of the raw materials  Tabular Alumina, (wt%) Reactive Alumina, (wt%) Calcium Aluminate Cement(wt%) Colloidal Silica, (wt%) Al2O3  99.4 99.5 69.8-72.2 0 CaO  0.03 0.03 26.8-29.2 0 SiO2  0.03 0.04 0.2-0.6 40.2 Fe2O3  0.02 0.02 0.1-0.3 0 Na2O  0.35 0.04   < 0.5 0 K2O  0 0 0 The recipes of the studied castables (sample C, which is a low cement castable, LCC, containing 5 wt% cement and sample N, a no-cement castable containing  colloidal silica) are provided in Table 2.  Table 2 – Composition of castabales  Castable Composition (wt%) C N Tabular Alumina 85 87 Reactive Alumina 10 10 Cement 5 0 Colloidal silica 0 7.5 Dispersant 0.1 0.1 water 5 4.5  2. Castables Preparation  For  each  sample,  a  3kg  batch  mixture  using  the  formulation  in  Table 2 was prepared by dry mixing for 4 minutes at slow speed, using a Hobart mixer with 5 liter capacity mixing bowl. Then wet mixing was carried out by addition Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  257  of water with further 4 minutes mixing at medium speed. Test samples were prepared by casting the mixture into the stainless steel molds for 1 minute, with the dimensions of 150*25*25 mm by vibrating at 50HZ, The castables were cured at 20°C and a relative humidity of 95% for 24 hours in the mold. After curing time, the samples were allowed to be dried at the temperature of 110°C, and then fired at 1400°C for 5 hours (by the rate of  300°C per hour). 3. Testing  Bulk density and apparent porosity were determined by ASTM standard methods. The given values of these properties in result section are the average of three samples for each formulation. Hot modulus of rupture, HMOR, was carried out according to DIN 51048 using the Netzsch 442D/3 model. These values are also the average of three measures. RESULT AND DISCUSSION 1. Bulk density and apparent porosity Bulk density and apparent porosity of the samples are given in Fig 3 and Fig 4.   Fig. 3 – Bulk density of the samples fired at 1400°C for 5 hrs Fig.4 – Apparent porosity of the samples fired at 1400°C for 5 hrs  In LCC castables (Sample C), the cement content requires water for placement. This leads to high porosity when heated, the castable loses strength and becomes vulnerable to infiltration e.g. by slags. Besides, due to dehydration of the hydrate phases before the ceramic bond is formed, the castables also show a strength lowering at intermediate temperatures.[2]   Colloidal silica particles, due to their nano sizes (~15 nm), could behave as a liquid[12]  and so less water was needed in preparation of the sample N, which led to less amount of apparent porosity upon drying and firing.   Colloidal silica, due to its high specific area, anticipates sintering and so the sample N shows higher bulk density than sample C. 2. HMOR The HMOR amounts of the samples are shown in Fig. 5. Using fine and reactive powders such as reactive alumina and colloidal silica in the bond phase of sample N can precipitate mullite needles from 1300°C in which the mullite needle-like particles grow out of a the matrix promote a good reinforcement 258                  Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II            according to Schumacherrs report. [2]  In Fig. 5, It can be seen that the sample containing colloidal silica (Sample N) shows higher amount of HMOR, better compaction, lower apparent porosity and higher bulk density due to presence of colloidal silica and concequently, formation and growth of long, needle-shaped mullite crystals.    Fig.5 – Hot Modulus of Rupture (HMOR) of the samples fired at 1400°C for 5 hrs Fig. 6 – XRD patterns of the sam-ples fired at 1400°C for 5 hrs. A repre-sents alumina and CA represents calcium-hexaluminate(6Al2O3.CaO)  3. Phase Analysis The XRD patterns of the samples can be seen in Fig. 6. In the XRD pattern of sample C, calcium aluminium oxide can be detected besides alumina, which is a cement phase and has a lower melting point than alumina and acts as a flux, so can lower the HMOR of the sample. Phase analysis of the sample N reveals mullite (3Al2O3.2SiO2) peaks, alt-hough they are weak due to low amount in the total phase constituents of the castable, so  mullite was assumed as a trace phase. CONCLUSIONS Colloidal silica sol could behave as a liquid and so the castables containing colloidal silica need less water to be prepared, which leads to lower their apparent porosity than that of LCC ones. Colloidal silica, due to its high specific area, anticipates sintering and so leads to better com-paction. Comparing the traditional low calcium aluminate cement casta-bles, colloidal silica containing refractory castables attains better me-chanical strength at high temperatures. REFERENCE [1] Bjorn Myhre, "Hot Strength and Bond-Phase Reactions in Low and Ultra-Low Ce-ment Castables",  Elkem a/s Refractories, P.O. Box 126, N-4602 Kristiansand, Nor-way. [2] Bjorn Myhre, "Tabular Alumina Based Refractory Castables" , Elkem a/s Refracto-ries, P.O. Box 126, N-4602 Kristiansand, Norway. Nanomaterials: Applications and Properties (NAP-2011). Vol. 2, Part II                  259  [3] Michael Anderson, "Better Refractories through Nanotechnology", Metrel, Inc., Addison, Ill. October 1, 2005 [4] S. Ghosh, R. Majumdar, B.K. Sinhamahapatra et al., Ceramics International 29 (2003),P. 671–677. [5] Ali Reza Souri, Behzad Mirhadi and Fatemeh Kashani Nia, The Effect of Nano-Structured Collidal Silica on the Properties of Tabular Alumina Castables", XVI. In-ternational Conference on Refractories, Prague, The Czech Republic, 2008. [6] Florence Sanchez, Konstantin Sobolev, "Nanotechnology in concrete- A review", Elsevier 2010. [7] Subrata Banerjee, Am. Ceram. Soc. Bull., 77 [10] (1998), ACerS Data Depository File No. 295. [8] C. Odegård, H. Fjeldborg and B. Myhre; "Mullite Formation In Refractory Casta-bles", XIII. International Conference on Refractories, Prague, The Czech Republic, 2000. [9] M. R. Ismael, R.D. dos Anjos, R. Salomão and V.C. Pandolfelli, Refractories Appli-cations and News, Vol. 11, N. 4, July/August 2006.  [10] S. Hosein Badiee, Sasan Otroj, Ceramics – Silikáty 53 (4) 2009, P. 297-302 [11] S. Mukhopadhyay, S. Ghosh, M. K. Mahapatra et al., Ceramics International, Volume 28, Issue 7, 2002, P. 719-729  [12] M. R. Ismael, R.D. dos Anjos, R. Salomão and V.C. Pandolfelli, "Colloidal Silica as a nano-structured binder for refractory castables", Refractories Applications and News, Volume 11, Number 4, July/August 2006.    

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Improving thermo-mechanical properties of tabular alumina castables via using nano structured colloidal silica

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