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Technical Paper 

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NEW STRONGER BONDS FOR MONOLITHIC CASTABLES 

THROUGH SURFACE REACTIVE CALCIUM ALUMINATE 

AGGREGATES 

C. Wöhrmeyer, C. Parr, H. Fryda, E. Frier 

Kerneos GmbH, Oberhausen, Germany, www.kerneos.com 

Presented at The International Colloqium on Refractories, AACHEN, Germany, September 8 th and 9 th , 2010. 

8 rue des Graviers – 92521 Neuilly sur Seine Cedex 

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ABSTRACT 

This investigation studies the influence of calcium aluminate aggregates on refractory castable 

properties in comparison to bauxite and fireclay aggregates. Calcium aluminate aggregates with a 

mineralogical composition similar to CAC are able to develop stronger bonding forces with the 

cement than aggregates from the alumina-silica system. The chemical affinity between the calcium 

aluminate aggregates and the matrix enables direct chemical linkages via calcium aluminate 

hydrate bridges. During the de-hydration period and the re-crystallisation of anhydrous calcium 

aluminate phases, the linkages between the calcium aluminate matrix and the surface of the 

calcium aluminate aggregates remain stronger since the aggregates are of a similar mineralogical 

nature as the anhydrous cement. This effect results in higher strength compared to bauxite even 

though the density is lower with the fused calcium aluminate aggregate R50. For a given furnace 

design, 10% less material is needed to fill the forms, and better insulation properties and energy 

savings are expected. This makes R50 interesting for many applications where good heat 

containment, abrasion resistance and low liquid penetration in combination with low accretion 

build-up are required. In aluminium contact areas, it can even be used with less or without antiwetting 

additives which makes it more temperature resistant. 

The sintered calcium aluminate aggregate R60 has a higher intrinsic open porosity than R50. It 

makes castables even lighter than with fireclay but without negative effects with respect to bending 

strength. 

Both R50 and R60 are calcium aluminate aggregates which suit applications up to 1350°C- 

1400°C, for example, in secondary aluminium furnaces, back-up linings in many industrial 

furnaces, DRI-production, boilers and re-heating furnaces. 


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1 Introduction 

In recent years, the refractory raw material 

situation has become a critical and even political 

subject mainly driven by the high demand for 

refractory aggregates as consequence of the 

booming steel industry in China. In 2009, China 

counted for 47% of the global crude steel 

production (568 Mio t out of global steel output of 

1220 Mio t) vs. only 15% just 10 years back when 

China produced only 127 Mio t of the total of 848 

Mio t [1]. On the other hand, China is one of the 

very few countries with very important geological 

reserves for refractory grade bauxite [2]. Only 10 

years ago, a large amount of this bauxite was 

available for the global refractory industry at low 

prices. But now, China needs a large portion of this 

bauxite for its own aluminium and steel production. 

China’s steel production has more than quadrupled 

during the last 10 years. This situation has even 

become a political issue. The Commission of the 

European Communities launched a position paper 

in 2008 with the title “The raw materials initiative – 

Meeting our critical needs for growth and jobs in 

Europe” [3]. However, political processes need 

their time and can only try to improve the trade 

rules and stimulate research in new raw materials. 

The European refractory and raw materials 

industries need to develop strategies to adjust to 

the dramatically changed global situation and need 

to turn it into new opportunities. An example for the 

use of new calcium aluminate aggregate based 

products was given by [4] which proposed calcium 

aluminate aggregates for tin bath bottom bricks to 

overcome problems with alkalis when aggregates 

from alumina-silica systems are used. Other 

publications discussed the application of calcium 

aluminate aggregates in bricks [5] and castables 

[6, 7] for non-ferrous and other industries. This 

present paper will specifically study the calcium 

aluminate aggregates and their surface reaction 

with the binder phase. When calcium aluminate 

aggregates consist of hydraulic phases like CA 

and CA2, they can interact chemically during 

hydration and de-hydration with the calcium 

aluminate binder, unlike those calcium aluminate 

aggregates which consist of CA6, corundum or 

mullite. Specifically, the effect of this stronger bond 

on the physical properties of castables will be 

investigated in this paper, in comparison to bauxite 

and fireclay in similar formulations. 

Calcium Aluminate Aggregates 

Calcium aluminate cement is well-known for many 

years as the binder for refractory castables. Also, 

calcium aluminate aggregates are known from 

peri-refractory applications (ALAG ® ) and from 

fluxing applications for metallurgical slags (LDSF ® ). 

Both are fused materials and start to melt below 

1400°C, ALAG ® due to its relatively high iron oxide 

content combined in different aluminate phases 

and LDSF ® due to its high C12A7 mineral phase 

content (Tab. 1, 2). ALAG ® is, for example, used in 

the back-up zones of blast furnace cast houses 

and the blast furnace slag granulation station. 

Other fields of application are the ramps in coke 

oven plants and industrial floors in the aluminium 

industry where high abrasion resistance is 

required. A low iron oxide, fused calcium aluminate 

aggregate (R50) for castables has been presented 

by [6]. Compared to ALAG® the R50 product 

contains more Al 2 O 3 . This results, unlike what is 

stated in [8], in a very low C12A7 phase content 

(



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Tab.1: Chemistry and mineralogy of aggregates 

Calcium aluminate Fireclay Bauxite 

R60 R50 ALAG ® LDSF ® FC B 

Sintered Fused Fused Fused Sintered Sintered 

Chemistry 

Al 2O 3 62 52,5 39,5 41 45,6 89,5 

SiO 2 6 5,5 4,5 3,5 51,5 4 

CaO 26 37 37,5 50,5 0 0,1 

Fe 2O 3 2,5 2 15,5 2 0,9 1,5 

TiO 2 2,5 2 2 2 1,7 3,7 

Mineralogy (A= Al 2O 3, S=SiO 2, C=CaO, F=Fe 2O 3) 

A - - - - - XXX 

A3S2 

S 

CA2 

CA 

C12A7 

C3A 

C2S 

C2AS 

C4AF 

- - - - XXX X 

- - - - XX X 

XXX - - - - - 

X XXX XXX - - - 

- - X XXX - - 

- - - X - - 

- - - X - - 

XX XX X - - - 

- - X - - - 

XXX= main phase, XX=secondary phase, 

X= minor phase, - = below 1% 

2 Test Methods 

Classical castable test methods such as vibration 

flow, porosity by water immersion and strength 

measurements have been used to study the basic 

castable properties. Quantitative chemical and 

mineralogical analyses have been conducted using 

XRF and XRD methods. Model regular castables 

(Tab. 2, 3) and deflocculated medium cement 

castables (MCC) have been investigated (Tab. 4). 

Tab. 2: Model regular castable recipes CC-FC and 

CC-R60 

mm CC-FC CC-R60 

R60 3-6 % - 28 

R60 1-3 % - 25 

R60 0-1 % - 22 

Fireclay 3-6 % 28 - 



Secar 51 % 25 25 

H 2O % 8,5 10,5 

Working time min 240 220 

3 Test Results 

Calcium aluminate aggregate (R60) in regular 

castable 

The R60 aggregate is a sintered calcium aluminate 

with about 30% open porosity, higher than for 

bauxite or dense fireclay aggregates. Fig. 1 

compares the rheology of the fireclay-based 

castable CC-FC with the R60-based CC-R60. Both 

need the same amount of water to reach identical 

initial flow properties. While fluidity under vibration 

slightly increases during the first 30 minutes and 

then stays almost stable when fireclay is used, the 

flow decays in the case of CC-R60 due to the 

higher pore volume into which the water can 

migrate over time. Then, less water is available to 

fluidise the cement matrix and the castable starts 

to stiffen. It is assumed that the drop in flow is not 

caused by the mineralogy of R60 since it contains 

mineral phases which have low hydraulic 

properties at 20°C (Tab. 1). The observed 

stiffening effect with R60 could be of interest for 

gunning mixes but has not been further studied 

here. 

After firing at 800, 1100 and 1350°C, the bulk 

density of CC-R60 is about 5% lower than CC-FC 

(Fig. 2) and the open porosity is almost 40% higher 

(Fig. 3). With classical aggregates from the 


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alumina-silica system, one would expect a drop in 

strength [9]. But in this case, despite the lower bulk 

density and higher porosity, flexural strength is the 

same as CC-FC (Fig. 4). 

Vibration flow (mm) 

240 

220 

200 

180 

160 

140 

120 

100 

CC-FC 

0 30 60 90 

Time (min) 

CC-R60 

Fig. 1: Vibration flow of regular castables CC-FC and 

CC-R60 at 20°C 

Bulk density (g/cm3) 

2,5 

2,3 

2,1 

1,9 

1,7 

1,5 

CC-FC 

CC-R60 

800 1100 1350 

Temperature (°C) 

Fig. 2: Bulk density of regular castables CC-FC and 

CC-R60 

Open porosity (Vol.%) 

30 

28 

26 

24 

22 

20 

18 

16 

14 

12 

10 

CC-FC 

CC-R60 

800 1100 1350 


Fig. 3: App. porosity of regular castables CC-FC and 

CC-R60 

CMOR (MPa) 

10 

9 

8 

7 

6 

5 

4 

3 

2 

1 

0 

CC-FC 

CC-R60 

800 1100 1350 


Fig. 4: CMOR of regular castables CC-FC and CC- 

R60 

R50 in regular castables 

The fused and dense R50-aggregate has been 

compared with Chinese bauxite aggregate in a 

model recipe as shown in Tab. 3. As can be seen 

in Fig. 5, both casables exhibit a very similar flow 

profile with the same amount of mixing water. Due 

to the dense R50-grains, no flow decay has been 

observed. The dense grains, together with the 

mineralogy of R50, with its high calcium 

monoaluminate and low C12A7-content (



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Tab. 3: Model regular castable recipes CC-R50 and 

CC-BX 


mm CC-R50 CC-BX 

R50 3-5 % 31 - 

R50 1-3 % 20 - 

R50 0-1 % 29 - 

Bauxite 3-6 % - 31 

Bauxite 1-3 % - 20 

Bauxite 0-1 % - 29 

Secar 51 % 20 20 

H 2O % 9.7 9.7 

Working time min 240 240 

230 

220 

210 

200 

190 

180 

170 

160 

CC-R50 

T0 T30 T60 T90 T120 

Time (min) 

CC-BX 

Fig. 5: Vibration flow of regular castables CC-R50 

and CC-BX at 20°C 

Bulk density (g/cm3) 

2,8 

2,7 

2,6 

2,5 

2,4 

2,3 

2,2 

CC-R50 

CC-BX 

CMOR (MPa) 

12 

10 

8 

6 

4 

2 

0 

CC-R50 

CC-BX 

20 (6 h) 20 (24h) 110 800 1200 


Fig. 7: CMOR of regular castables CC-R50 and CC- 

BX 

delta L (%) 

1 

-1 

-2 

-3 

-4 

-5 

CC-R50 

CC-BX 

0 

0 200 400 600 800 1000 1200 1400 


Fig. 8: Refractoriness under load (0.2 MPa) of regular 

castables CC-R50 and CC-BX 

R50 in deflocculated medium cement castables 

(MCC) 

The effect of R50 in a medium cement containing 

model recipe (MCC) with different amounts of 

barium sulphate (as an anti-wetting material for 

aluminium contact areas) is shown in Tab. 4. 

Whilst the barium sulphate is absolutely necessary 

to get the required anti-wetting effect for the 

bauxite mix, the amount can be reduced or even 

completely eliminated when R50 is used [6]. 

2,1 

110 800 1200 


Fig. 6: Bulk density of regular castables CC-R50 and 

CC-BX 


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Tab. 4: Model MCC recipes with bauxite (B1-B3) and 

R50 (R1-R3) with different barium sulphate contents. 

Bauxite 

R50 

B1 B2 B3 R1 R2 R3 

R50 3-5 mm % - 33 

R50 1-3 mm % - 12 

R50 0-1 mm % - 20 

SECAR ® 51 % - - 5 5 

Bauxite 3-6 mm % 21 - 



Bauxite 0-0.09 % - 5 5 - 

Calc. Alumina % - - 5 - - 5 

BaSO 4 % 10 5 - 10 5 - 

React. Alumina % 10 10 

SECAR ® 71 % 15 15 

PP fibres % 0.05 0.05 

Peramin® AL 200 % 0.15 0.12 

H 2O % 5 4.5 

containing mix B1 with 10% BaSO 4 (which can be 

seen as a reference for this type of application). In 

the case of R50, less barium sulphate can be used 

(R2, R3) which has a positive effect in that the 

castable can be heated to higher temperatures 

without drastic volume changes (caused by the 

disintegration of the barium sulphate as in case of 

B1). Even R50 with 10% barium sulphate (mix R1) 

shows only moderate volume expansion at 

1400°C, whilst the mix with bauxite (mix B1) 

expands strongly. Heated to 1400°C, the MCC with 

R50 keeps a significant lower porosity level than 

the bauxite based mixes (Fig. 13). It offers an 

additional safety measure should unexpectedly 

high temperatures occur in the course of 

aluminium production. 


230 

220 

210 

200 

190 

180 

170 

160 

150 

0 30 60 

Time (min) 

Fig. 9: Flow of MCC with R50 (R1-R3) and bauxite 

(B1-B3) 

R1 

R3 

R2 

B1 

B3 

B2 

A microsilica-free MCC has been chosen to 

minimise the potential reactions between the 

aluminium melt and the castable matrix. The 

polycarboxylate ether-based deflocculant 

(Peramin® AL 200) fluidifies the mixes very 

efficiently with 5% water in case of bauxite and 

only 4.5% in case of R50. With R50, the dosage of 

the superplasticizer could even be reduced. 

Despite the lower water addition and lower 

deflocculant content, better flow properties have 

been achieved for all R50 containing MCC’s (R1- 

R3) as can be seen in Fig. 9. Here, open porosity 

after heating at 800 and 1200°C, the critical 

temperature range for the aluminium application, 

is, in all cases, significantly lower with R50 despite 

the lower bulk density (Fig. 10, 11). The cold 

crushing strength is, independently of the barium 

sulphate content, higher than with the bauxite 

App. Porosity (Vol.%) 

21 

19 

17 

15 

13 

11 

9 

7 

5 

800°C 1200°C 

B1 B2 B3 R1 R2 R3 

Castable 

Fig. 10: App. Porosity of MCC with R50 (R1-R3) and 

bauxite (B1-B3) 


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Bulk density (Vol.%) 

3 

2,95 

2,9 

2,85 

2,8 

2,75 

2,7 

2,65 

2,6 

2,55 

2,5 

800°C 1200°C 

B1 B2 B3 R1 R2 R3 

Castable 

Fig. 11: Bulk density of MCC with R50 (R1-R3) and 


CCS (MPa) 

250 

200 

150 

100 

50 

0 

800°C 1200°C 

B1 B2 B3 R1 R2 R3 

Castable 

Fig. 12: Cold crushing strength of MCC with R50 (R1- 

R3) and bauxite (B1-B3) 

App. Porosity (Vol. %) 

35 

30 

25 

20 

15 

10 

5 

B3 

R3 

R2 

B2 

R1 

0 

-2,00 -1,00 0,00 1,00 2,00 3,00 4,00 5,00 

Permanent linear change 110°C --> 1400°C (%) 

Fig. 13: Permanent linear change and open porosity 

after firing at 1400°C of MCC with R50 (R1-R3) and 


B1 

4 Summary 

Calcium aluminate aggregates show very particular 

bonding properties due to their surface reaction 

with the cement. Despite the lower density and 

higher porosity of the sintered R60 aggregate 

compared to fireclay, equivalent strengths have 

been found with a regular castable. The fused R50 

aggregate has very little open porosity but reduced 

density when compared to bauxite due to its 

calcium aluminate nature. In addition, strength is 

even higher with R50 despite the lower density, 

due to the better bonding between the matrix and 

calcium aluminate aggregate. Thus a given 

furnace lining design could be cast with less 

material without sacrificing the strength and/or 

abrasion resistance. It is expected that the thermal 

heat containment will also be improved, which 

would, in turn, save energy and costs. R50 used in 

microsilica-free medium cement castables for 

aluminium applications offers new ways to 

formulate the castable with better temperature 

resistance. R50 reduces the pore structure of the 

castable, with very low open porosity recorded 

even after firing at 1400°C. 

Barium sulphate starts to decompose at 1200°C 

which leads to a massive volume expansion in 

bauxite containing castables at 1400°C. Volume 

stability is much better when R50 is used, even in 

combination with barium sulphate. However, due 

to the improved matrix/aggregate bonding 

properties and the low wettability of calcium 

aluminates with aluminium, the barium sulphate 

content can be reduced when R50 is used. This 

further improves the temperature stability. 

Both R50 and R60 calcium aluminate aggregates 

can be used up to 1350°C to 1400°C depending on 

the castable type and the calcium aluminate 

cement and filler used. 


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5 References 

[1] World Steel Association (www.worldsteel.org) 

[2] G. Bardossy, P.J. Combes: Karst bauxites: 

Interfingering of deposition and 

paleoweathering. Spec. Publication No. 27 of 

Int. Ass. Sedimentologists, p. 189-206, 1999. 

[3] Commission of the European Communities. 

Communication from the commission to the 

European parliament and council: The raw 

materials initiative – meeting our critical 

needs for growth and jobs in Europe. 

Brussels, COM (2008)699, SEC(2008)2741, 

2008. 

[4] Patent US005420087A (Didier-Werke AG), 

1995. 

[5] K. Santowski, K. Gamweger, B. 

Schmalenbach, T. Weichert: Ceramic bonded 

refractories based on calcium aluminate 

minerals for the use in glass, aluminium and 

lime furnaces. Unitecr congress, p. 386-388, 

Dresden, 2007. 

[6] C. Wöhrmeyer, N.Kreuels, C.Parr, M.Vialle: 

Calcium aluminate: Zemente und neue 

Zuschlagstoffe für den Einsatz in der 

Aluminiumindustrie. 41st Int. Colloquium on 

Refractories, p. 80-86, Aachen, 1998. 

[7] C. Wöhrmeyer, N.Kreuels, C.Parr, T.Bier: The 

use of calcium aluminate solutions in the 

aluminium industry. Unitecr congress, Berlin, 

1999. 

[8] Patent WO2005 021434 A2 (MCGOWAN), 

2005. 

[9] G. Routschka: Feuerfeste Werkstoffe, Vulkan 

Verlag, 1996. 


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