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Permeability of High-Alumina Refractory Castables Based onVarious Hydraulic Binders

Murilo D. M. Innocentini, Arturo R. F. Pardo, and Victor C. Pandolfelli*

Department of Materials Engineering, Federal University of São Carlos, São Carlos, São Paulo 13565–905, Brazil

Bruno A. Menegazzo

Alcoa Alumı́nio S.A., São Paulo, São Paulo 05804–900, Brazil

Luis R. M. Bittencourt and Roberto P. Rettore

Magnesita Research Center, Contagem, Minas Gerais, 32210–050, Brazil

This work investigated the changes in the permeability of high-alumina self-flowing refractory castables based on cal-cium aluminate cement (CAC) and hydratable alumina binder(HAB) pretreated between 110° and 1650°C. Permeability

constants k1 and k2 were fitted from Forchheimer’s equationbased on airflow tests conducted at room temperature. Theresults indicated that dehydration was the main contributingfactor for increased permeability in bodies pretreated up to600°C and that sintering effects prevailed between 900° and1650°C. Castables based on HAB were less permeable thanthose based on CAC, a behavior ascribed to the type of hydrates formed and to the particle-packing features of thematrix and the matrix–aggregate interfaces.

I. Introduction

THE development of high-alumina castables based on ultra-low-cement, on calcia-free, and, more recently, on binder-freecompositions has resulted in remarkable gains in refractoriness andmechanical properties at high temperatures, through the decreaseof the amount of low-melting-point phases in the alumina–silica–calcia ternary system.1–3 However, formulations of improvedparticle-packing design, particularly those based on high-strengthbinders, are almost invariably lower in porosity and permeabilitythan the products they replace. Numerous studies have suggestedthat this is the reason these castables are relatively more difficultto dewater, increasing the likelihood of explosive spalling duringthe initial heat-up.4,5 In addition to this problem, the literature hasnot yet established conclusively whether, in this case, the lowpermeability level of green castables remains after their sinteringat high temperatures, ensuring they are less vulnerable to infiltra-tion by corrosive fluids and, thus, a longer service life formonolithic products in foundries and steelmaking plants.

In the present work, the permeability to airflow of high-aluminaself-flowing castable compositions based on a commercial calciumaluminate cement (CAC) and a hydratable alumina binder (HAB)was measured at room temperature in samples pretreated between110° and 1650°C. The objective was to assess the permeabilitychanges caused by thermal phenomena, such as dehydration and

sintering, providing information on the causes of the different fluiddynamic performances mentioned in the literature for CAC- andHAB-based castables.4

II. Experimental Procedure

(1) Sample Preparation

For this investigation, two castable compositions were preparedwith a mixture of calcined aluminas as matrix powders ( d p 100m), white fused alumina as coarse aggregate grains (d p 5.6mm), and two types of hydraulic binders. The batch named CACcontained 2 wt% of calcium aluminate cement (CA-270, AlcoaChemicals, Little Rock, AK) and the batch named HAB contained2 wt% of hydratable alumina binder (Alphabond 300, AlcoaChemicals). The particle-size distribution of both compositionswas adjusted to a theoretical curve based on Andreasen’s packingmodel. A coefficient of distribution q of 0.21 was chosen to obtainpotentially self-flowing castables.

Castables were prepared by gradually adding water to the dryraw materials in a planetary mixer for 10 min. An optimumcontent of 0.26 mg/m2 of citric acid (Labsynth, Brazil) was addedto promote the dispersion of fine particles during mixing. Table Isummarizes the tested compositions.

Samples were cast as cylindrical disks (7.5 cm diameter 2.5cm thickness) for permeability measurements and as bars (2.5cm 2.5 cm 15.0 cm) for mechanical strength and porositytests. The samples were cured at 25°C in a moistened environmentfor 24 h, heat-treated in an electric furnace for 12 h at temperaturesof 110°–1650°C (1°C/min to 300°C, 2°C/min to 900°C, and3°C/min to 1650°C), and cooled to room temperature inside thefurnace.

(2) Physical Characterization

Permeability to air flow was measured at room temperatureusing an apparatus described elsewhere.6 Permeability constants

D. P. Butt—contributing editor

Manuscript No. 188030. Received January 8, 2001; approved March 11, 2002.Supported by the Brazilian research funding institutions FAPESP and CNPq and

by Magnesita S.A. and Alcoa S.A., Brazil.*Member, American Ceramic Society.

Table I. Compositions of Tested Castables

Material

Castable composition (wt%)

CAC HAB

Calcined aluminas† 22.79 22.57White fused aluminas‡ 75.11 75.43Calcium aluminate cement§ 2.00Hydratable alumina binder ¶ 2.00Water (dry weight basis) 4.93 4.12

†A1000 SG, A3000 FL, APC3017 SG, Alcoa Alumı́ nio S.A., Brazil. ‡AEB 4/10,8/20, 10/36, 20/40, 8/F, 40/F, 60/F, 200/F, Alcoa Alumı́nio S.A. §CA 270, AlcoaChemicals, Little Rock, AK. ¶Alphabond 300, Alcoa Chemicals.

J. Am. Ceram. Soc., 85 [6] 1517–21 (2002)

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were calculated from Forchheimer’s equation, expressed for flowof compressible fluids as6–9

Pi2 Po

2

2Po L

k 1vs

k 2vs

2

where Pi and Po are, respectively, the absolute air pressures at theentrance and exit of the sample, vs the fluid velocity, determinedby dividing the exiting volumetric flow rate by the total free-flowarea (25.4 cm2), L the sample thickness, the fluid viscosity, and the fluid density, evaluated for Po 690 mmHg (0.092 MPa)and T 25°C. The parameters k 1 and k 2 are, respectively, theDarcian and non-Darcian permeability constants.7,9

The first term of Forchheimer’s equation (vs / k 1) representsviscous energy losses and prevails at low fluid velocities, while thesecond (quadratic) term (vs

2 / k 2) represents kinetic energy lossesand is more significant at higher velocities. The relative contribu-tion of each term on the total pressure drop also depends on thevalues of k 1 and k 2, which can change significantly during thethermal treatment because of phenomena that modify the porousstructure, including dehydration, thermal expansion mismatch,sintering, and microcracking.

k 1 and k 2 were obtained by fitting the experimental data of (Pi

2 – Po2)/2Po L vs vs in Eq. (1), using the least-squares method. At

least 15 sets of Pi, Po, and vs values were acquired in a stationary

airflow regime for each test, and three samples of each castablewere tested to ensure data reliability.

Apparent porosity was determined using the water-displacement method, based on the Archimedes principle (“Stan-dard Test Methods for Apparent Porosity, Water Absorption,Apparent Specific Gravity, and Bulk Density of Burned RefractoryBrick and Shapes by Boiling Water,” ASTM Specification C20 –00. American Society for Testing and Materials, West Consho-hocken, PA). Mercury porosimetry (Model No. 9320 PoreSizer,Micromeritics Instrument Corp., Norcross, GA) provided data of pore-size distribution of bodies treated between 1200° and1650°C. The microstructural observation was conducted usingscanning electron microscopy (SEM; Model No. 440 Stereoscan,Leica Microsystems, Cambridge, U.K.) on gold-coated samples.The mechanical strength (f ; 2 5 mm 25 mm 150 mm

samples) was determined through modulus of rupture measure-ments (“Standard Test Method for Cold Crushing Strength andModulus of Rupture of Refractories,” ASTM Specification C133–94.), using a three-point bending device (span 140 mm,displacement rate 10 mm/min, Model 810, MTS Systems,Minneapolis, MN). The results were based on an average of fivesamples for each temperature.

III. Results and Discussion

(1) Dehydration Effects

Figure 1 illustrates the influence of the thermal treatment on k 1and k 2. The permeability increase observed for CAC-bondedcastables up to 600°C may be partially associated with the changes

in crystal structure and density of the hydrates present in thecalcia–alumina–water system cured at 25°C, especially of CAH10(1720 kg/m3) and C2AH8 (1950 kg/m

3),† which rapidly convert toC3AH6 (2520 kg/m

3) above 110°C.10-12 The latter hydrate, in turn,reacts with AH3 (2420 kg/m

3) to yield mostly C12A7 (2690 kg/m3)

at 600°C.10–14 This sequence of reactions leads to an increase inthe porosity of the matrix paste, contributing to the increasedpermeability shown in Fig. 1.15

Comparatively, the curing of HAB-based castables yieldslarge amounts of gel and crystalline AH3 (2420 kg/m

3) ashydrated phases. The agglutinative alumina gel contributes to

fill interstitial pores in the matrix paste.10,13,15,16 Thus, al-though thermal treatment leads to the formation of denseralumina phases (3980 kg/m3), the greater consolidation of thematrix ensures a lower permeability level throughout thedehydration process.

In addition to the changes in the matrix properties, othermicrostructural features relating to the castable’s particle pack-ing exert a significant effect on permeability. Ideally, a castableshould be considered as a two-phase composite, comprised of amatrix paste surrounding a skeleton of coarse aggregate grains.

However, because of the packing constraints imposed by theaggregate surface, a region of higher porosity is formed at thematrix–aggregate interface.17–19 The thickness and porosity of this interfacial zone depends on the relative size of matrixparticles and aggregates and on the strength of the chemicalbonds provided by the cement hydrates. This type of arrange-ment of matrix around aggregates produces a gradient of properties, particularly those involved in fluid flow.18,19 Thelarge pores at the interface offer much less resistance to thediffusive and convective transport of fluids than do the matrixpores.

The HAB and CAC compositions tested in this work are basedon the same size distribution of matrix and aggregates, resulting incastables with similar amounts of interfacial pores. However, thefeatures of the hydraulic binder used may have influenced the

thickness of the porous zone at the interface. In the HAB-basedcastables, the small particle size of the Alphabond 300 (d 50 2.2m and d 90 5.0 m) and the formation of AH3 gel may haveensured better packing in the interfacial region, a further reason forthe lower permeability level compared with the samples based onCA-270 (d 50 4.2 m and d 90 35 m).

One important implication of this analysis regards the use of permeability results obtained for prefired samples to distinguishbetween the dewatering behaviors of green HAB and CACcastables. In reality, there is a significant difference betweenthe air flowing through the castable structure during a perme-ability test and the water vapor being driven off it during anactual dewatering process. In the former case, air percolatesthrough the shortest and fastest pathways, which are mainlylocated in the interfacial zone,18 as previously discussed.

Therefore, permeability is expected to be very sensitive tovariations in the number of interfaces, which depend on the

Fig. 1. Influence of thermal treatment on the permeability of high-alumina refractory castables: (a) Darcian permeability constant k 1 and (b)non-Darcian permeability constant k 2 (deviation bars refer to an average of three samples for each temperature).

†Cement notation: C is CaO; A is Al2O3; and H is H2O.

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volumetric fraction and size distribution of aggregates in thecastable. On the other hand, during the dewatering process, thesteam must percolate a more difficult path, first exiting thepores within the matrix bulk, where it is generated by theboiling of free admixing water, and then percolating through theinterfacial zone to exit from the castable. In this case, the denser

the bulk of the matrix, the slower the release of steam.An electrical analogy can be used here to distinguish betweenthe castable’s resistance to airflow and its resistance to steamrelease. The forces of resistance to airflow are aligned in parallel(either through the matrix or the matrix–aggregate interfaces),whereas the forces of resistance to steam release are aligned inseries (first the matrix, then the interfaces). Hence, the overallresistance to dewatering is always higher than to airflow, whichexplains why the permeability constants of prefired samples areonly partially successful in assessing the drying behavior of greenrefractory castables. Hence, castables containing the same aggre-gate distribution can present a comparable airflow behavior, albeitwith different performances for dewatering, which are expected,depending on the cement type, water content, and other matrixmodifiers.

(2) Sintering Effects

Although dehydration is the major contributing factor forincreased permeability in castables preheated to 900°C, thesintering effects above this temperature begin to control thechanges in the castable’s properties.

The effect of sintering on permeability is strongly dependent onthe changes in porosity and pore shape that occur in the castablethroughout the thermal treatment. Sintering causes densificationand, usually, some volumetric change in the matrix bulk. On theother hand, the skeleton of fused aggregate grains is inherentlyslightly reactive only in the compositions studied here and nor-mally undergoes reversible thermal expansion effects during thesintering process. However, during heating, the relative movementcaused by the simultaneous densification of the matrix and the

reversible expansion of aggregates may, after cooling, result in afurther detachment of the boundaries between matrix and aggre-gates, where the more permeable pores of the castable are located.

From a fluid dynamics standpoint, the overall result of suchphenomena is an increase in the flow resistance through the matrix,which is already less permeable at lower temperatures, and adecrease in the resistance through interfacial pores, which are themain flow path in the castable. Therefore, unless strong andthermally stable adhesion is provided between aggregates andmatrix, a fraction of interfacial permeable pores always remainsafter cooling, and the expected net effect of sintering is a relativelyhigh permeability level of burned bodies.

This hypothesis is reinforced by the results illustrated in Fig.1. Thermal treatment promoted a more pronounced increase inpermeability constants above 900°C. HAB and CAC samples

displayed similar trends, although the use of hydratable aluminaas hydraulic binder resulted in significantly lower permeability

levels. Comparatively, for these two castables, the Darcianconstant k 1 increased by 50% with treatments between 110° an d600°C and by 50 times between 900° and 1650°C. In these two

intervals, the non-Darcian constant k 2 increased, respectively,by 248% and by almost 4 orders of magnitude. Two importantconclusions could be inferred from these results. First, theresults confirmed that sintering did, in fact, exert a muchstronger effect than dehydration on the permeability of pre-treated castables. Second, the results corroborated that thegreater increase in constant k 2, especially above 900°C, wasindirect evidence that the net permeable porosity became lessresistant to airflow because of a greater contribution of inter-facial pores.7

No conclusive difference in apparent porosity was verified incastables bonded with Alphabond 300 and CA-270, as shown inFig. 2. On the other hand, a mercury porosimetry analysis (Fig.3) confirmed that pore size increased progressively in bothcastables as the thermal treatment temperature increased from

1200° to 1650°C. This suggested that sintering caused adecrease in the fraction of small-sized voids within the matrixbulk, which was, however, accompanied by the appearance of larger and more-permeable pores, by the coalescence of matrixpores, and/or by a further detachment at the matrix –aggregateinterface.

The detachment possibility was confirmed by microstructuralSEM observations of samples treated at 1650°C. The SEM resultsin Fig. 4 show the existence of detachments in matrix–aggregateinterfaces in both types of castables.

Figure 5 illustrates the influence of thermal treatment onmechanical strength. In general, f decreased throughout thedehydration process through the loss of chemical bonds be-tween matrix particles in the castable. The relatively low f level in CAC bodies at 300°C occurred within the typical

temperature range for most of the dehydration reactions of CACs.10 Accordingly, an evident decrease in f was observedfor HAB-based samples pretreated at 600°C, which coincidedwith the temperature suggested for the decomposition of AHand AH3 hydrates. On the other hand, sintering conductedabove 900°C promoted structural densification and consequentgains in strength.

The above-described trends lead to the following remarks.(1) The decrease of f in HAB and CAC castables took place

in a temperature range in which the bodies were only slightlypermeable, which represents two major favorable conditions forthe occurrence of spalling in green castables subject to thedewatering process.

(2) Although the sintering process promoted densification,which resultied in the improved strength of both castables, it also

produced bodies with high levels of permeability. Figure 6 showsthat, regardless of the cement type used, there was a clear

Fig. 2. Influence of thermal treatment on the apparent porosity of castables (deviation bars refer to an average of three samples for eachtemperature).

Fig. 3. Pore-size distribution obtained using mercury porosimetry forcastables treated above 1200°C.

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correlation between the gains in mechanical strength and theincrease in k 1 and k 2 caused by thermal treatment.

IV. Conclusions

The purpose of this work was to investigate the causes of

permeability changes resulting from the thermal treatment of

high-alumina castables based on various types of hydraulic bind-ers. The results showed that dehydration was an important con-

tributing factor for permeability increases in bodies pretreated up

to 600°C. Castables based on HAB were less permeable than those

based on CAC, and this behavior was attributed to the particle-packing features of the matrix and the matrix–aggregate interface.

In bodies treated from 900° to 1650°C, sintering caused densifi-

cation of the matrix bulk, which resulted in changes in the size of

permeable voids in the castable, particularly because of detach-ment in the interfacial zone between the matrix bulk and aggre-

gates, which caused a remarkable increase in permeability. A

correlation was verified between the gains in mechanical strength

and the increase in k 1 and k 2.

References

1W. E. Lee and R. E. Moore, “Evolution of in Situ Refractories in the 20th

Century,” J. Am. Ceram. Soc., 81 [6] 1385–410 (1998).2

A. R. Studart, W. Zhong, and V. C. Pandolfelli, “Rheological Design of Zero-Cement Self-Flow Castables,” Am. Ceram. Soc. Bull., 78 [5] 65–72 (1999).3W. Ma and P. W. Brown, “Mechanisms of Reaction of Hydratable Aluminas,”

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Fig. 4. SEM micrographs of refractory castables treated at 1650°C: (a) CAC and (b) HAB.

Fig. 5. Flexural strength of prefired high-alumina refractory castables(deviation bars refer to an average of five samples for each temperature).

Fig. 6. Relationship between permeability constants and flexural strength

of refractory castables prefired at various temperatures (lines refer toexponential fittings).

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10K. M. Parker and J. H. Sharp, “Refractory Calcium Aluminate Cements,” Trans. J. Br. Ceram. Soc., 81, 35– 42 (1982).

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[1] 35–38 (1988).17D. P. Bentz, E. J. Garboczi, and P. E. Stutzman, “Experimental and Simulation

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