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N. Traon*, J. Schnieder*, A. Villalba*, T. Tonnesen*, R. Telle*, M. Huger**, T. Chotard**

Influence of Andalusite, Al2O3-ZrO2-SiO2 and Al2O3-ZrO2 Addition on Elastic and Mechanical Properties of High Alumina Castables

1 IntroductionThe choice of available raw materials for high-performance castables with excellent strength and thermo-mechanical properties is central to the refractory field. The fabrica-tion of such materials requires the develop-ment of stable and thermal shock-resistant interfaces between matrices and aggregates. Several surveys have been conducted with the purpose of improving both the chemical and mechanical resistance of the matrix by enhancing ebonite or calcium hexaalumi-

nate formation in standard castables [1]. With this theoretical point of view in mind, the works focussed on the reactivity of the fine materials and more precisely of the reactive alumina and the cement [2–3]. The other alternative would be the addition of aggregates of different nature in calcium aluminate cement (CAC)-bonded refracto-ry castables. Tabular alumina, white fused alumina, sintered pre-formed spinels, mag-nesia or partially stabilized zirconia are commonly added to these standard refrac-tory formulations in order to attain a suit-able performance during the application [4–5]. The nature of such coarse grains as well as their distribution within the matrix has a significant effect on thermal shock re-sistance. As demands on refractory materi-

als have increased, new and purer raw mate-rials have been used for their fabrication, although the importance of recycling mate-rials has grown too. A crucial development in recent years has been the progressive use of unshaped products, as they have several advantages over shaped products. For exam-ple, a previous sintering is not required and furnace lining can be performed quickly and without joints. However, there are dis-advantages, for example that a successful lining is more difficult and also the initial strength of these refractories is lower. Casta-bles with hydrate bonding play an impor-tant role in the field of unformed products; they receive their green strength due to the formation of hydrates, as they contain an amount of refractory cement. The trend is

For many years, the optimization of refractory castable formulations has been a key factor in improving the service life of unshaped materials during their application in the steel and iron industry as well as in the metallurgical industry.The incorporation of functional aggregates in common low cement casta-bles based on tabular alumina raw materials was investigated within the framework of the study presented herein. Such an addition of eutectic aggregates aims to modify the elastic and mechanical properties of the castable leading to eventual thermal shock resistance amelioration.Different high alumina-based formulations were investigated. Starting with a reference tabular alumina-based castable, the largest grain frac-tion of this first formulation was replaced by three different aggregates. Andalusite was incorporated to favour microcrack formation due to in situ mullite and ensuing glass formation during the sintering process. Both mechanisms may explain the depletion of elastic and mechanical properties and furthermore the decrease of the stored elastic energy of the castable. Al2O3-ZrO2

addition may also result in crack formation be-cause of expansion mismatch between the calcium aluminate matrix and the zirconia-containing aggregate. The martensitic transformation of zir-conia at high temperature characterized by a considerable volume expan-sion is the second factor explaining the increase in mechanical stresses at the level of the grain boundaries between aggregates and matrix. An alternative to these materials was the examination of Al2O3-ZrO2-SiO2 eutectic aggregates in order to study the influence of zirconia in the presence of a silica glassy phase. This study will focus on the modifica-tion of the elastic and thermo-mechanical properties of the andalusite-based castable in comparison with the reference castable (without func-tional aggregate incorporation) to discuss the impact of such aggregates on thermal shock resistance.

functional aggregates, thermal shock resist-ance, stored elastic energy, microcracks INTERCERAM 63 (2014) [6]

The corresponding author, Nicolas Traon (1986), studied Ceramic Engineering at the Ecole Nationale Supérieure de Céramique Industrielle of Limoges (ENSCI), France. In 2009, he graduated after con-ducting his final-year thesis

project at the Institut für Gesteinshüttenkunde of Aachen (GHI), Germany, where he is currently working as a Ph.D. student. His work focuses on the determination of the damage induced in refractory materials by thermal shocks through resonant frequency damping analysis. Since 2011, he has been working in close co-operation with his colleague Jonas Schnieder on model high alumina castables with addition of functional ag-gregates. Thereby, the authors tackle the impact of such aggregates addition on the outgoing properties of refractory materials. The research activities of the corresponding author on ceramic microstructure changes have led to an enhanced understanding of the elastic property results presented in this work.E-Mail: traon@ghi.rwth-aachen.de

The auThor absTracT Keywords

to move towards smaller amounts of ce-ment, on the one hand for reasons of cost, on the other, because cement binds water and during dehydration water loss causes pore formation.The emphasis in this study is laid on the effect of functional aggregates integrated into a tabular-based low cement castable formulation (LCC) on mechanical and elas-tic properties, key parameters in view of thermal shock resistance. The cornerstone of good thermal shock resistance is a high strain-to-rupture ratio, which can be ob-served in materials exhibiting non-elastic behaviour [6]. It is already known from pre-vious studies that andalusite leads to low values of Young’s modulus and non-elastic behaviour. However, the mechanisms, par-ticularly in terms of the non-linear stress-strain curve, are not fully understood. An improvement in thermal shock resistance of high alumina castables can also be provoked by adding zirconia aggregates [7]. Both an-

* Institute of Mineral Engineering, RWTH University Aachen, Mauerstraße 5, 52064 Aachen (Germany)

** SPCTS UMR 7315 CNRS, Centre Européen de la Céramique, Limoges cedex (France)

Table 1 • Composition of the tested castables

Component Details Refractory castablesRef / mass-% And / mass-%

CA cement CA Secar 71 5 5

Reactive alumina PFR 12.5 12.5

Tabular alumina 0–0.045 mm 10 10

0–0.3 mm 10 10

0.2–0.6 mm 10 10

0.5–1 mm 17.5 17.5

1–2.24 mm 22.5 22.5

2.24–3.0 mm 12.5 –

Andalusite 2.24–3.0 mm – 12.7

Total 100 100

Mixing water H2O 5 5

Deflocculant FS 40 0.15 0.15

Retarder Citric acid 0.03 0.03

Fig. 1 • Raw (upper row) and thermal treated

(lower row) aggregates with grain sizes 2.24–3 mm:

tabular alumina (TA), andalusite (And)

1

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N. Traon*, J. Schnieder*, A. Villalba*, T. Tonnesen*, R. Telle*, M. Huger**, T. Chotard**

Influence of Andalusite, Al2O3-ZrO2-SiO2 and Al2O3-ZrO2 Addition on Elastic and Mechanical Properties of High Alumina Castables

als have increased, new and purer raw mate-rials have been used for their fabrication, although the importance of recycling mate-rials has grown too. A crucial development in recent years has been the progressive use of unshaped products, as they have several advantages over shaped products. For exam-ple, a previous sintering is not required and furnace lining can be performed quickly and without joints. However, there are dis-advantages, for example that a successful lining is more difficult and also the initial strength of these refractories is lower. Casta-bles with hydrate bonding play an impor-tant role in the field of unformed products; they receive their green strength due to the formation of hydrates, as they contain an amount of refractory cement. The trend is

functional aggregates, thermal shock resist-ance, stored elastic energy, microcracks INTERCERAM 63 (2014) [6]

Keywords

to move towards smaller amounts of ce-ment, on the one hand for reasons of cost, on the other, because cement binds water and during dehydration water loss causes pore formation.The emphasis in this study is laid on the effect of functional aggregates integrated into a tabular-based low cement castable formulation (LCC) on mechanical and elas-tic properties, key parameters in view of thermal shock resistance. The cornerstone of good thermal shock resistance is a high strain-to-rupture ratio, which can be ob-served in materials exhibiting non-elastic behaviour [6]. It is already known from pre-vious studies that andalusite leads to low values of Young’s modulus and non-elastic behaviour. However, the mechanisms, par-ticularly in terms of the non-linear stress-strain curve, are not fully understood. An improvement in thermal shock resistance of high alumina castables can also be provoked by adding zirconia aggregates [7]. Both an-

dalusite and zirconia aggregates lead to mi-crocrack formation [8]. This observed phe-nomenon of microcrack formation can be explained by mismatches in expansion coef-ficient between the matrix and aggregates and, additionally, by the martensitic phase transformation in the case of zirconia. The microcracks are responsible for the decrease in Young’s modulus. AZS (Alumina-Zirco-nia-Silica) and AZ (Alumina-Zirconia) could be an alternative to these materials, because of the zirconia they contain. AZS and AZ are synthetically produced materials and have been barely studied as aggregates in castables. Recycled materials could also play a greater role in the future because of their cost advantages over pure zirconia. In this paper herein, only the comparison of the structural and mechanical properties of the reference tabular alumina based castable and those of the andalusite based castable are highlighted.

2 Experimental2.1 Materials and elaborationTwo different low cement castables were investigated in this study. To be able to com-pare the impact of the andalusite aggregates,

both castables were prepared using the same basic formulation but in one the largest grain fraction was replaced by andalusite. The tested castables therefore included the reference formulation “Ref ”, containing only tabular alumina aggregates, and a for-mulation containing andalusite aggregates called “And”; the exact formulations are il-lustrated in Table 1, while grain shape before and after heat treatment is shown in Fig. 1. After 48 h in the humidity chamber to final-ize the hydration, the castables were sintered at 1500 °C/6 h. The sample dimensions were 160 mm × 40 mm × 40 mm.Alumina raw materials were procured from the company Alteo. The deflocculant FS 40 is produced by BASF, and the cement Secar 71 by Kerneos. The andalusite aggregates were delivered from South Africa by Dam-rec, an affiliated company of Imerys. The chemical composition of andalusite is given in Table 2.Since in practice refractory castables are not fired before application, the elastic proper-ties were also recorded for the unsintered materials after drying at 110 °C. Young’s modulus was measured using ultrasonic methods at room temperature.

Table 1 • Composition of the tested castables

Component Details Refractory castablesRef / mass-% And / mass-%

CA cement CA Secar 71 5 5

Reactive alumina PFR 12.5 12.5

Tabular alumina 0–0.045 mm 10 10

0–0.3 mm 10 10

0.2–0.6 mm 10 10

0.5–1 mm 17.5 17.5

1–2.24 mm 22.5 22.5

2.24–3.0 mm 12.5 –

Andalusite 2.24–3.0 mm – 12.7

Total 100 100

Mixing water H2O 5 5

Deflocculant FS 40 0.15 0.15

Retarder Citric acid 0.03 0.03

Table 2 • Chemical composition (mass-%) of andalusite aggregates

Al2O3 61.5

SiO2 37.7

Fe2O3 0.4

K2O 0.1

Other oxides 0.3

Table 3 • Young’s modulus of dense ceramic materials [15]

Material E / GPa

Al2O3 410

Amorphous SiO2 70

Mullite 100

Fig. 1 • Raw (upper row) and thermal treated

(lower row) aggregates with grain sizes 2.24–3 mm:

tabular alumina (TA), andalusite (And)

1

Fig. 2 • a) Scheme of the indenter penetrating the grain, b) typical load-depth curve [11]

2 Load P

Depth d

Depth d (nm)

App

lied

load

P (m

N)

S = dP dh

a) b)

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Young’s moduli of the individual aggre-gates were determined by nanoindentation, as each compound (aggregate) contributes elasticity to the overall elasticity of the castable [9].Besides Young’s modulus also the mechani-cal strength and fracture toughness is of great importance in predicting the thermal shock behaviour of materials [10]. For the determination of mechanical properties, compression tests and four-point bending tests were carried out. Crack formation dur-ing thermal shocks is principally caused by tensile stresses. Therefore tensile tests were carried out. By applying load–unload tensile cycles, not only the tensile strength could be approximated but also the energy consump-tion by deformation, which is related to fracture toughness.

2.2 NanoindentationThe nanoindentation method allows the determination of the local Young’s modulus and hardness of the investigated aggregates. The technique consists of applying succes-sive loading–unloading cycles on an indent-er that is in contact with the grain’s surface (Fig. 2). The depth varies with the applied load (from 0.01 to 10 N) but remains rather

small (from a nanometer to several tens of micrometers) [11].The utilized experimental device Nano-indentation TM II was fitted with a dia-mond indenter in pyramid form with a triangular base (Berkovich type). After embedding the aggregates in synthetic resin, the samples were first polished (~1 µm) and then glued onto a support.The conditions concerning the loading– unloading cycles were defined according to the nature of the investigated materials. For all materials a velocity of 10 nm/s and a maximal depth of 1200 nm were chosen. The estimation of Young’s modulus was car-ried out by determining the system rigidity S formed by the indenter–material couple, corresponding to the slope of the curve (dP/dh) during unloading (Fig. 3).From this system rigidity S (Eq. 1), the pro-jected contact surface A, and a form factor ß depending on the tip used, a reduced Young’s modulus Er, corresponding to the indenter–sample contact and the local Young’s modulus of the grain Eexp could be calculated in equation (2).

(1)

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

(2)

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

whereνi – Poisson’s ratioE – Young’s modulusν – Poisson’s ratio (assumed value for all materials ν = 0.25)5 Aggregates on each material were investi-gated and 5 indentations were repeated on each aggregate for an accurate estimation of the aggregate Young’s modulus.Tabular alumina and andalusite raw materi-als were measured plus andalusite aggre-gates after heat treatment. Tabular alumina aggregates were not analyzed after heat treatment, as no phase transformation oc-curs in these aggregates at these tempera-tures and the Young’s modulus should not change significantly.

2.3 Ultrasonic methods2.3.1 Transmission method at low temperaturesThis method utilizes two transducers: an emitter and a receptor. The specimen was placed in casting direction between the two transducers. An ultrasonic wave with a fre-

quency of 500 kHz was emitted through the material and the time τ needed by the wave to pass through the material was measured by an oscilloscope. This procedure was con-ducted with longitudinal and transversal waves, using two different transducers.Young’s modulus was then calculated by equation (3).

(3)

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

whereρ – material densityvi – wave velocity for longitudinal and trans-versal wavesThe velocities were calculated (equation 4 and 5) by means of the width d of the sam-ple and the time τ needed to pass the mate-rial.

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

(4)

(5)

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

Four samples of each material were meas-ured, with six measurements for each sam-ple (three with longitudinal waves, three with transversal waves).

3 Results and discussion3.1 Young’s modulus values for the aggregatesAccording to the indentation depth, the measured volume of the sample will vary. As a manufacturing characteristic of tabular alumina aggregates is their closed porosity, such pores may explain the considerable deviation of the different indentation meas-urements. The obtained Young’s moduli show good correlation with the theoretical data of dense ceramic materials summed up in Table 3.

Fig. 3 • Young’s modulus values with standard deviation at an indentation

depth of 200–300 nm

3

Fig. 6 • Comparison of peak tensile strength

6

Fig. 7 • Comparison of maximal strain obtained by tensile loading/unloading

cycles

7

Fig. 4 • Young’s modulus after drying at 110 °C

4

Youn

g’s

mo

dul

us /

GP

a

Fig. 5 • Young’s modulus after sintering at 1500 °C

5

Youn

g’s

mo

dul

us /

GP

a

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(2)

whereνi – Poisson’s ratioE – Young’s modulusν – Poisson’s ratio (assumed value for all materials ν = 0.25)5 Aggregates on each material were investi-gated and 5 indentations were repeated on each aggregate for an accurate estimation of the aggregate Young’s modulus.Tabular alumina and andalusite raw materi-als were measured plus andalusite aggre-gates after heat treatment. Tabular alumina aggregates were not analyzed after heat treatment, as no phase transformation oc-curs in these aggregates at these tempera-tures and the Young’s modulus should not change significantly.

2.3 Ultrasonic methods2.3.1 Transmission method at low temperaturesThis method utilizes two transducers: an emitter and a receptor. The specimen was placed in casting direction between the two transducers. An ultrasonic wave with a fre-

quency of 500 kHz was emitted through the material and the time τ needed by the wave to pass through the material was measured by an oscilloscope. This procedure was con-ducted with longitudinal and transversal waves, using two different transducers.Young’s modulus was then calculated by equation (3).

(3)

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

whereρ – material densityvi – wave velocity for longitudinal and trans-versal wavesThe velocities were calculated (equation 4 and 5) by means of the width d of the sam-ple and the time τ needed to pass the mate-rial.

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

(4)

(5)

rEAdhdPS ⋅⋅==

πβ 2

(1)

i

i

r EE

E 2

2

exp 111

νν−

−

−=

(2)

1

432

22

−

−=

T

L

TL

vv

vvvE ρ (3)

TT

dvτ

= (4)

LL

dvτ

= (5)

Four samples of each material were meas-ured, with six measurements for each sam-ple (three with longitudinal waves, three with transversal waves).

3 Results and discussion3.1 Young’s modulus values for the aggregatesAccording to the indentation depth, the measured volume of the sample will vary. As a manufacturing characteristic of tabular alumina aggregates is their closed porosity, such pores may explain the considerable deviation of the different indentation meas-urements. The obtained Young’s moduli show good correlation with the theoretical data of dense ceramic materials summed up in Table 3.

The addition of andalusite aggregates in a tabular alumina matrix aims to decrease the modulus of elasticity of the castables while not drastically reducing their mechanical strength. Thereby, the stored elastic energy of the sample will decrease, which improves the behaviour of the refractory piece in regard to resistance to crack propagation. Also, such an addition will provoke the formation of a crack network in the matrix because of the mismatch in thermal coeffi-cient of expansion which is responsible for stress fields between the grog grains and the matrix.

3.2 Young’s modulus values for the castablesYoung’s modulus values of the refractory castables should increase over the heat treat-ment through a sintering process as shown in Figs. 4–5 with “Ref”. However, it is worth mentioning that “And” presents a lower val-ue of elasticity after the sintering. The in situ transformation of andalusite into mullite was characterised by a considerable volume expansion and the release of glassy silica phases in the matrix explains the depletion of elastic properties in “And”.Furthermore, the thermal expansion mis-match between the aggregates and the

matrix is an additional factor at the origin of crack nucleation atthe level of the inter-faces aggregates/matrix.

3.3 Mechanical properties3.3.1 Tensile strengthA decrease in tensile strength is expected with the addition of andalusite aggregates as such grains favour the formation of cracks throughout the matrix and more particular-ly at the level of the grain boundary corre-sponding to the part of the microstructure exhibiting the lowest tensile strength. This depletion of strength is all the more consid-erable where the presence of cracks is signif-icant (Fig. 6). Moreover, “And” is character-ised by higher maximal strain values than those of “Ref”, which reveals a certain flexi-bility of this material (Fig. 7).With regards to thermal shock resistance, it is noticeable that the addition of andalusite aggregates reduces the stored elastic energy of the material leading to a theoretical im-provement in crack growth resistance. In addition, the castables can present anelastic behaviour and respond to thermal stresses with deformation.In terms of cold crushing strength, the qual-ity of the aggregate is the most important criterion. It has been proved previously that

Fig. 6 • Comparison of peak tensile strength

Fig. 7 • Comparison of maximal strain obtained by tensile loading/unloading

cycles

7

Fig. 8 • Compression strength of all castables

8

Fig. 4 • Young’s modulus after drying at 110 °C

9

Fig. 9 • Flexural

strength of all

castables

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during sintering, microcracking occurs within the andalusite aggregates because of expansion mismatch and mullite formation. The microstructural damage explains the drastic depletion in cold crushing strength of the andalusite-based castable in compari-son with that of “Ref” (Fig. 8).

3.3.2 Bending strengthDuring the bending test the material is un-der both tensile and compression stress, therefore the differences between the differ-ent castables are not as obvious as in the tensile test, but higher than that in the crushing test (Fig. 9).

4 ConclusionsIn conclusion, this study shows the impact of andalusite aggregates addition to high alumina castables on the ensuing structural, mechanical and elastic properties. The in situ transformation of the andalusite aggregate into mullite, characterized by a considerable volume expansion, was the cause of radial stresses within the matrix that led to the formation of a network of cracks both around and inside the grains.The transformation of andalusite aggregates into mullite during the sintering treatment as well as the thermal expansion mismatch between the aggregates and the matrix are mainly responsible for the damaging of those aggregates and their surrounding areas. Indeed, transgranular cracks through-out the mullite transformed aggregates after the heat treatment. This results in a deple-tion of the cold crushing strength.

Nevertheless, crack formation at the grain boundary between those mullite trans-formed aggregates and the matrix reduces bending strength as well as toughness of the samples. Furthermore, the And formulation showed non-linear behaviour characterized by a considerable maximal strain during the tensile strength test; it can thus respond to thermal stress with deformation. The rela-tive free motion between the mullite trans-formed grains and the matrix induces a low flexibility of the refractory material with an improved resistance to crack propagation.

AcknowledgmentsThe authors would like to express their sin-cere thanks to the Federation for Interna-tional Refractory Research and Education (FIRE) for having supported and funded this academic research study and for having promoted this student exchange.

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Received: 05.08.2014

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