Refractory Castable

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Panalytical X-Pert Pro model diffractometer with Nifiltered Cu K

a radiation and a scanning 2h between 10u

and 80u were used. The fracture surfaces of the castable

samples were investigated using the field emission

scanning electron microscopy JEOL JSM 7000F with

fully computer controlled with energy dispersive spec-

trometry (EDS), wavelength dispersive spectrometry,

electron backscattered diffraction attachments.

Results and discussion

Figure 2 shows the XRD results of the studied castablesat different firing temperatures between 110 and 1500uC.

As shown in Fig. 2a, the castable dried at 110uC for 24 h

consists of corundum (Al2O3, a rhombohedral structure

with lattice parameters a5b50?4758 nm, c51?2991 nm,

PDF no. 100173).14 This result indicates the excess of

alumina in the present castable composition. In additionto the phases of the corundum observed at 110uC

(Fig. 2a), one silicon oxide phase (SiO2, a cubic structure

with lattice parameters a5b5c50?713 nm, PDFno. 270605),15 and the phases of albite [Na(Si3Al)O8, a

triclinic structure with lattice parameters a50?8165 nm,

b51?2872 nm, c50?7111 nm, PDF no. 100393],16

anorthite (CaAl2Si2O8, a triclinic structure with

lattice parameters a50?81756 nm, b51?2872 nm, c51?41827 nm, PDF no. 411486)17 were detected from the

XRD pattern of the castable sample fired at 1000uC for

2 h, as seen in Fig. 2b. It should be mentioned here that

since both anorthite and albite phases have same crystal

structure with the lattice parameters very close to each

other, the respective X-ray peaks for both phases are

coincident with each other. Observing one diffraction

peak of the silicon oxide indicates a minor content of

cristobalite. This belongs to the devitrification process of

microsilica particles starting about 1075uC, as reported

by Chakravorty and Ghosh.18 When the firing tempera-

ture increases from 1000 to 1200uC, the peak intensity of

silicon oxide decreases, and there is no trace indicating

the formation of mullite phase, as shown in Fig. 2c.

However, the diffraction peak of silicon oxide disap-

peared at 1500uC (Fig. 2d ). In addition to the similar

Table 2 Composition and self-flowability of studiedcastable

Components Amount, wt-%

Brown fused alumina3–5 mm 161–3 mm 270–1 mm 26Calcined alumina HTM 30 10Rotary bauxite 0–0.063 mm 9Microsilica 971U 7Cement Secar71 5Dispersant D7S 0.05Water 4.6Self-flowability 160

Table 1 Composition of starting raw materials used in this study, wt-%

Components Brown fused alumina* Calcined alumina{ Rotary bauxite{ Microsilica1 Cement Secar71"

Al2O3 96.25 99.50 88.30 0.2–0.30 72.70Fe2O3 0.14 0.020 1.48 0.01–0.1 ,0.3TiO2 2.64 0.006 … … ,0.8SiO2 0.90 0.018 … 98–98.40 …Na2O … 0.030 … 0.15–0.20 …K2O … … … 0.2–0.30 …Na2OzK2O … … … … ,0.6MgO … … … 0.1–0.20 ,0.3CaO … … … 0.2–0.30 29.20H2O … … … … …

*Qing dao Asian Minerals Co. Ltd, China.{Eczacibasi Doga Madencilik San. Tic. A.S. Turkey.

{Qing dao Asian Minerals Co. Ltd, China.1Elkem Refractories, Norway."Lafarge, France.

a 110uC for 24 h; b 1000uC for 2 h; c 1200uC for 2 h; d 1500uC for 2 h

1 Castable samples after CCS tests

Go gtas et al. Preparation and characterisation of self-flowing refractory material

Adv an ces in Ap pli ed Ce ra mic s 2010 VOL 10 9 NO 1 7



diffraction peaks belonging to the corundum, albite andanorthite as those observed at 1200uC (Fig. 2c), the

measured main peak positions, namely 2h5

16?

4, 33?

1,40?8, 53?9, 70?3 and 74?1u for the XRD scan in Fig. 2d

matched the card values of the mullite (Al6Si2O13, an

orthorhombic structure with lattice parameters

a50?75456 nm, b50?76898 nm, c50?28842 nm, PDF

no. 150776).19 It is considered that the minor content

of silicon oxide has reactioned with corundum from the

matrix and formed mullite at high temperature

(>1400uC). The similar XRD profile has been reported

by Gerotto et al.9,12 who studied the refractory castable

containing 6 wt-% microsilica (MS6).

The microstructural investigation of the fracture

surfaces of the castable samples was performed using

the scanning electron microscope after CCS tests.

Figure 3a – d shows the SEM images of the castablesamples dried at 110 for 24 h, and fired at 1000, 1200

and 1500uC for 2 h respectively. The corundum grains

are homogeneously embedded in the castable matrix.

Furthermore, it is clear that increasing the temperature

from 110 through 1500uC reveals the formation of a

dense and compact microstructure of the castable

sample as seen from higher magnification micrographs

given in Fig. 3e and f . The EDS patterns of the points

indicated in Fig. 3a are given in Fig. 3 g and h, and

reveal the corundum phase and the average composition

of the matrix respectively.

Self-flowability is the typical characteristic of self-

flowing castables, which promotes these mixes auto-degasification and flowing without applying external

vibration.20,21 The amount of liquid in a SFC has a

significant effect on self-flowability, the more liquid in aSFC, the better its self-flowability.21 However, the excess

water can decrease strength and increase shrinkage,while less water can cause voids in the castable, con-

sequently poor consolidation and a weak, porous struc-

ture.21,22 As given in Table 2, the self-flow behaviour of

the studied castable is 160 where 4?6% water content is

the optimum addition to achieving an acceptable self-

flowability property. This value is higher than that of

similar type self-flowing low cement castable containing

920D type microsilica with 5?6 wt-% water addition.13 It

can be seen that the self-flowability is clearly influenced

by the amount of microsilica in the castable. It has been

known that the addition of higher amount of microsilica

in a castable mixture decreases flowability and increases

castable water demand since the microsilica occupies the

cement surface and thereby impedes the dissolution of the cement.9,23 In addition, comparison of the purity,

consequently, alkalis content in 920D and 971U type

microsilicas24 (SiO2: 92?4 wt-%, H2O: 0?4, LOI:

0?98 wt-% vs SiO2: 98?4 wt-%, H2O: 0?5, LOI:

0?80 wt-% respectively) may indicate that the microsilica

quality has a significant effect on the flow of the

castable. This observation conforms to Mhyre25 who

reported that low purity microsilica significantly

decreases flow and increases water addition in refractory

castable system. Furthermore, it should be noted that

improving the flow and workability related to the some

important well known tips, i.e. achieving properly

dispersed castable matrix, using raw materials with arounded particle shape and higher purity alumina,

adding high quality microsilica to the mix, increasing

a 110uC for 24 h; b 1000uC for 2 h; c 1200uC for 2 h; d 1500uC for 2 h

2 X-ray diffraction patterns of castables


8 Advances in Applied Ceramics 2010 VOL 1 09 NO 1



the number of coarser grades of alumina used, wideningthe particle size range of the formulation.7,11,20,25–29

The bulk density (BD) and apparent porosity (AP) of the studied castable containing 971U type microsilicawere in the range of 3?19–3?24 g cm23 and 7?10–11?25%respectively. Figure 4 shows the behaviour of both BDand AP of the castable as a function of the temperature.The BD of the castable dried at 110uC for 24 h, 3?24¡0?03 g cm23, decreases slightly to 3?21¡0?03 g cm23

after firing up to 1200uC, but increases 3?23¡0?03 g cm23 at 1500uC. Evaporation of water from thecastable creates porosity, which results in decreased BD

and increased AP (11?25¡0?52%) at 1200uC. The higherAP observed at 1200uC for the studied castable indicates

that pores obtained during the water evaporation couldnot be eliminated, even after firing at higher tempera-tures (.1000uC). The AP at 110uC, (10?07¡0?27%), is

similar to that of Chakraborty30 who investigated pro-perly designed low cement castables. However, above1200uC, due to converting most of the metastable liquidof the castable into the mullite, and the densification of

the composition, the AP significantly decreases to 7?1¡0?10% at 1500uC. This kind of decrease of the AP wasreported by Gerotto et al.9 Furthermore, the present AP

a 110uC; b 1000uC; c 1200uC; d 1500uC; e and f higher magnification micrographs at 110uC and 1500uC, respectively; g

EDS pattern at a specific point A in Fig.3 a; h the average composition of the whole matrix in Fig.3 a

3 Images (SEM) of the fractural surfaces of castable samples and EDS patterns





value is smaller than that of the authors’ previous

work13

on similar type brown fused alumina castablecontaining 920D type microsilica. Therefore, it is clear

that several factors such as total water content, initialpacking density (or porosity), consequently, the grainsize distribution, and the both amount and quality of the

microsilica have significant effects on densificationcharacteristics of the self-flowing low cement castables.

Figure 5 shows the relationship between the densifica-tion characteristic of the castable and firing temperature,also verifies the behaviour of both the AP and BD given

in Fig. 4. It appears that the WA increases graduallyfrom 3?10¡0?10% at 110uC to 3?51¡0?20% at 1200uC,but decreases to 2?23¡0?06% at 1500uC. Furthermore,the WA value (2?23¡0?06%) of the present castable

sample at 1500uC is similar to the WA value (y2?0%) of the self-flow ultralow cement high alumina castablecontaining 5 wt-% of microsilica.31

Figure 6 presents the cold crushing strength (CCS) of

the castable samples as a function of the firing tem-perature. The increase in CCS is observed from 924¡18 kg cm22 for 110uC dried samples to 1784¡

160 kg cm22 for the samples fired at 1500uC. Thecastable sample, dried at 110uC for 24 h, is about 0?5

times stronger than the similar type brown fused

alumina castable containing 920D type microsilica, itis also close to that of the self-leveling cement tabularalumina castable with 4?7% water addition.32 Further-

more, the present CCS values are significantly higherthan those of a lightweight low cement self-flowingcastable,33 i.e. 924¡18 kg cm22 for 110uC and 1072¡46 kg cm22 vs 296 kg cm22 at 110uC and 887 kg cm22

at 1000uC. The mechanical characteristics of ceramic

systems containing high alumina contents are related tothe phase formation (i.e. amount of corundum, a glassyphase composition, etc.) and microstructure (crystalsize, and shape, a glassy phase and secondary crystalline

phase distribution, porosity, etc.).34,35 As seen in Fig. 2band c, the anorthite, albite, and minor silicon oxidephases crystallise and gradual increase observed in CCS

indicates the ceramic bond formation. After firing at1500uC for 2 h, the castable exhibits the highest value of

the CCS due to not only the enhanced densification, butalso the formation of elongated needle shaped mullitecrystals which strengthens the structure at high heattreatment temperatures.36

Conclusions

In the present investigation, the authors have prepared aself-flowing low cement brown fused alumina basedrefractory castable containing 971U type microsilica.Water addition (4?6%) has been used to achieve full self-flowing characteristics. The castable sample self-flowedunder its own weight and easily filled intricate shaped

moulds without any vibration. At 110u

C, the matrixconsists of corundum phase. With increasing firingtemperature through 1200uC, the phases of corundum,anorthite, albite and silicon oxide were detected. At1500uC, the mullite formation was observed due to thereaction of silicon oxide with corundum. The investi-gated physicomechanical characteristics exhibited thatseveral factors such as total water content, initialpacking density (or porosity), therefore, the grain sizedistribution, and both amount and quality of themicrosilica have significant effects on densificationproperty of the self-flowing low cement castables.

Acknowledgements

This work was financially supported by the IstanbulTechnical University, Turkey, Research Fund (Project

4 Densification behaviour of castable samples as a func-

tion of firing temperature

5 Water absorption of castable samples as a function of

firing temperature

6 Cold crushing strength of castable samples as a func-

tion of firing temperature


10 Advances in Applied Ceramics 2010 VOL 1 09 NO 1



No. 30329). N. U nlu would like to thank ProfessorMohan J. Edirisinghe for his helpful discussion. Theauthors wish to thank Mr Huseyin Sezer for his help andcontributions during the SEM analysis of this study.

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