The effect of additives on Al O –SiO –SiC–C ramming ...

The effect of additives on Al2O3–SiO2–SiC–C

ramming monolithic refractories

E. Karamian, A. Monshi and S.M. Siadati*

Department of Materials Engineering, Isfahan University of Technology (IUT), Isfahan,84156-83111, Iran

*E-mail: [email protected]

ABSTRACT

The aim of this study is to describe the effect of containing additives on increasing cold crushing

strength (CCS) and bulk density (BD) of Al2O3–SiO2–SiC–C monolithic refractories. Two series of

carbon containing monolithics were prepared from Iranian chamotte (Samples A) and Chinese bauxite

(Samples B), as 65 wt.% in each case together with, 15 wt.% SiC-containing material regenerates

(crushed sagger) – 10 wt.% fine coke (a total of 90% aggregate) and 10 wt.% resole (phenol

formaldehyde resin) as a binder. Different types of additives (such as silicon and ferrosilicon

metal) are added to a batch of 100 g of mixture and the physical and mechanical properties (such

as BD, apparent porosity and CCS) are measured after tempering at 200�C for 2 h and firing at

1100�C and 1400�C for 2 h. After low temperature tempering at 200�C, silicon and ferrosilicon

contribute to the formation of stronger cross linking in the resulting structure and provides CCS

values as high as 65 MPa. After high temperature sintering, at 1400�C, SiC whiskers of nano sized

diameter are formed due to the presence of Si and FeSi2 and increases the CCS values of the

refractories as high as 3 – 4 times in sample containing 6 wt.% ferrosilicon metal as additive,

compared to the material without additive. The temperature of 1100�C is a transient temperature,

used in high temperature sintering.

Keywords: silicon additives, ferrosilicon additives, ramming, monolithic refractories, carbon

1. INTRODUCTION

The development of Al2O3–SiO2–SiC–C resin bonded

ramming monolithic refractory for iron making applications

(such as iron and slag runners in blast furnaces, furnace

bottom and electric-furnace spouts) [1] is a very important

recent refractory development which can be considered to

have evolved from the high performance chamotte–carbon

and bauxite–carbon refractories. This class of refractory not

only shows superior slag corrosion and erosion (wear)

resistance, but also excellent thermal shock resistance and

mechanical properties. Monolithic refractories containing

bauxite aggregate has shown higher densification and

mechanical properties due to the presence of some impu-

rities such as iron which help liquid phase sintering, and

consequently, improved mechanical properties [2].

Probably, the high glassy content of the matrix of chamotte

results in high surface tension and higher amounts of

needles of mullite in chamotte, and has caused more

nucleation of SiC whiskers at high temperature, conse-

quently, resulting in improved strength and higher density

[3]. These refractories consist mainly of aluminosilicate and

carbon, which are bonded by carbon bonds formed by

carbonisation of phenol formaldehyde-resin (resole) during

firing of the refractories [1,3].

Carbon does not wet molten metal and will not melt, that

is an advantage excellent for refractory use. Its great

weakness is that it will oxidise at low temperatures,

above 600�C, which creates high porosity, leading to

lower density and reduced strength [1,4]. In order to

improve the oxidation resistance of carbon-containing

refractories, antioxidants (such as aluminum and silicon

metal) have been often added [5]. However, by examining

the behaviour of Al in such refractories, the following

problems have been observed: (1) Low melting point Al

particles, gives decreasing refractoriness; (2) the reaction of

Al with water, therefore Al cannot be used in carbon-

containing monolithic refractories in which water-

containing binders are used. Based on the above considera-

tion, it is necessary to develop new antioxidants with

excellent hydration resistance and high melting point,

such as Si-containing antioxidants. Mostly, metallic

silicon and aluminum are added to the material as a

sintering agent to improve the mechanical strength and

abrasion resistance under oxidation. The effect of metallic

silicon grain size on the properties of alumina–carbon brick

has been studied, and it is reported that the added metallic

silicon reacts under the reducing atmosphere with carbon in

the brick and produces b-SiC bond which contributes to

improvements of the mechanical strength and the abrasion

MATERIALS AT HIGH TEMPERATURES 26(4) 000–000 1# 2009 Science Reviews 2000 Ltddoi: 10.3184/096034009X12570016241884

MAHT 090263

resistance under oxidation. The influence of b-SiC bond on

the mechanical strength has yet to be clarified [6].

Generally, the modulus of rupture vs. firing temperature is

increased at a given temperature (1100, 1200, 1300, 1500�C)

by increasing the grain size of silicon up to more than 100

microns, but the elastic modulus is decreased. The amount of

SiC formed decreases and the remaining Si increases for

coarse particles. It is concluded that the strength and

durability of the slide gate plates can be improved, not by

increasing the amount of silicon metal, but by decreasing the

grain size to enhance silicon reaction. The results of other

workers show that SiC additions in the range from 4 to 16%

are beneficial to the improvement of mechanical properties

as well as the thermal shock resistance [7]. From the

investigation of the thermodynamics and from observation

of the results after use, it is evident that the SiC materials

have a large influence on oxidation prevention. SiC beha-

viour in Al2O3–SiC–C bricks is illustrated in reactions below

[8].

SiCðsÞ þ COðgÞ ¼ SiOðgÞ þ 2CðsÞ ð1Þ

SiOðgÞ þ COðgÞ ¼ SiO2ðsÞ þ CðsÞ ð2Þ

The most successful and commonly used ramming mixes

were based on phenol-formaldehyde resins. The cured

strength of these products was high because of polymerisa-

tion of the resin, which is called a ‘resit’ structure, at about

200�C. At elevated temperatures, however, the products

become weak because of the oxidation in the carbonaceous

bond phase, which creates high porosity, lower density and

reduced strength [1].

Advantages of a resole binder is that it is environmental

friendly, easy to mix, with high residual carbon. A disad-

vantage of a resole binder is the pyrolization of the ‘resist’

structure, that is called polymer carbon, at temperatures

about 300 – 500�C, and the products became weak

(Figure 1). The usage of additives such as silicon and

ferrosilicon is supposed to reinforce the ‘resit’ structure by

increasing the cross linking. The cross linking phenomenon

was discovered by Charles Goodyear in 1839 and involved

heating the raw rubber with sulfur (the Vulcanisation

process). The mechanism appears to involve addition of

sulfur units to the double bonds in the rubber with the

production of cross links between the polymer chains [9].

Cross-linking reactions in general are those that lead to the

formation of insoluble and infusible polymers in which

chains are joined together to form a three dimensional

network structure. A sample cross linking reaction is exem-

plified by polymer chains with several functional groups,

designated A, that are capable of reacting among themselves

to form chemical bonds, A22A, (Figure 2).

If these polymer chains are exposed to conditions such

that the functional groups do react, then all the chains in the

reaction vessel will be tied to each other through A22A

bonds [10]. In principle, the polymer molecules in the

reaction vessel will have formed giant molecules. Cross-

linking changes and generally improves physical properties.

A chemical reaction that leads to formation of a chemical

bond can be used for cross linking of polymers if the

requirements of effective average functionally per molecule

are met. Phenol-formaldehyde resins including resoles and

novolacs are technologically useful cross-linking systems.

Cross linking agents must carry two or more functional

groups per molecule [10].

2. MATERIALS AND METHODS

Two series of compositions were prepared. The first compo-

sition (samples A) containing as aggregates a combination of

The effect of additives on Al2O3-SiO2-SiC-C Ramming Monolithic Refractories: S.M. Siadati et al.

2 MATERIALS AT HIGH TEMPERATURES www.scilet.com

Figure 1 Linear polymer and cross-linked polymer.

linear polymer

cross-linked polymer

��?

Figure 2 Effect of additives on chamotte based cured at 200�C for

2 h. (Samples A).

chamotte (0 – 3 mm, 65 wt.%), crushed sagger which is a

SiC-containing material regenerates (15 wt.%) and fine coke

(10 wt.%) with different amounts of additives such as Silicon

and ferrosilicon metal. The second one (samples B)

containing as aggregates a combination of bauxite

(20 wt.% coarse bauxite (1– 3 mm), 45 wt.% fine bauxite

(0 – 1 mm)), crushed sagger (15 wt.%) and fine coke

(10 wt.%) with different amounts of additives such as

Silicon and ferrosilicon metal. At low temperature of

curing (200�C), three other additives such as sulfur, carbon

black and silica fume are also used. Since silica fume and

carbon black were not so effective in increasing of CCS

values, they were eliminated from higher temperature tests

(1100�C and 1400�C). Chamotte, bauxite and sagger are

used to increase the strength of the refractory. Coke is used

to lower the wettability with the iron or steel melt. Both

series consisted of 10 wt.% resole (phenol formaldehyde

resin) as the binding agent. The chemical compositions and

phase contents of the raw materials are given in Tables 1 and

2 respectively.

Specimens were prepared by mixing 90 wt.% aggregate

and 10 wt.% resole as a binder with different amounts of

additives on top of 100 wt.%. A twin blade mixer was used

for dry mixing and kneading. The raw materials were dry

mixed for 5 min and then the resole liquid was added

gradually. Mixing continued for another 5 min. The

mixture was packed into metal moulds with a dimension of

50650650 mm and then formed by using a hydraulic press

under a low pressure of 4 MPa, to act similar to ramming.

Pressed samples were cured at 200�C for 2 h. Finally, some

of the samples were fired for 2 h at either 1100�C or 1400�Cin a coke environment to avoid an oxidising atmosphere. The

specimens cured or fired were characterised according to

ASTM standards. The following tests were carried out: bulk

density (BD), apparent porosity (AP), cold crushing strength

(CCS), also X-ray diffraction (XRD). Scanning electron

microscopy (SEM) were performed in some cases. CCS

was measured by using a Hydraulic Testing Machine, type

Amsler model D3010y2E.

BD and AP were determined by using the following

equations:

BD ¼ WyWf ð3Þ

AP ¼ ðWs �WÞyWf ð4Þ

where W, Ws and Wf respectively are the weight of dried

sample, saturated weight with water and weight of the

sample in water suspended by a thin thread without

contacting the vessel walls, but fully inside the water and

reading by a digital balance under the vessel. In fact Wf is the

weight reduction of the saturated sample in water, which is

equivalent to its volume.

An XRD instrument (Philips Xpert-MPD System) with Cu

Ka1(l1 ¼ 1:5405 A) and Cu Ka2(l2 ¼ 1:5443 A) and Ni

filter was used for phase analysis. A voltage of 30 kV and

current of 25 mA were employed. In order to study precisely

the phases, the XRD experiments were carried out with a rate

of 0.04 degreesysec between 5 – 80 degrees (2Y).

SEM Philips XL30 was used for microstructure investiga-

tions. X-ray fluorescence (XRF) was also used for elemental

analysis and oxides content calculations (Table 1).

3. RESULTS

3.1 Cold crushing strength and bulk density results

Figures 2 – 5 show the influence of additives on CCS of

resole bond, Al2O3–SiO2–SiC–C refractory. Figures 6 – 8

illustrate BD variations.


www.scilet.com MATERIALS AT HIGH TEMPERATURES 3

Table 1 The chemical composition of raw materials (wt.%)

Material Al2O3 SiO2 CaO MgO TiO2 SiC C Si Fe S Grain size(mm)

Chinese bauxite 85 6.9 0.15 0.1 3.9 – – – – – 0 – 1,1– 3Iranian chamotte 42 53 0.5 0.5 – – – – – – 0 – 3Crushed sagger 45 15 – – – 38 – – – – 0 –3Fine coke 10 5 – – – – 80 – – – 0 – 0.3Ferrosilicon metal – – – – – – 2 72 23 – 0 – 0.15Silicon metal – – – – – – 1.0 97 1.0 – 0 – 0.15Silica fume – 92 – – – – – – – – 0 – 0.1Carbon black – – – – – – – 98 – – 0 – 0.1Sulfur powder – – – – – – – – – 97 0 – 0.3

Table 2 Phase analysis of raw materials (XRD results)

Material Major phase Medium phase Minor phase(s)

Chinese bauxite aAl2O3 Mullite(3Al2O3.2SiO2) TiO2, SiO2

Iranian chamotte Mullite(3Al2O3.2SiO2) – Cristobalite (SiO2)Crushed sagger aSiC Mullite(3Al2O3.2SiO2) a-Al2O3

Fine coke Graphite – Al2O3, SiO2

Ferrosilicon metal FeSi2 Si –Silicon metal Si – –



Figure 3 Effect of additives on bauxite based cured at 200�C for

2 h. (Samples B).

Figure 4 Effect of additives on bauxite and chamotte based fired at

1100�C for 2 h.

Figure 5 Effect of additives on bauxite and chamotte based fired at

1400�C for 2 h.

Figure 6 Effect of additives on chamotte and bauxite based cured at

200�C for 2 h.

Figure 7 Effect of additives on chamotte and bauxite based fired at

1100�C for 2 h.

Figure 8 Effect of additives on chamotte and bauxite based fired at

1400�C for 2 h.

Figure 9 Micrograph of chamotte – carbon refractory resole bond

containing 6 wt.% ferrosilicon metal as additive, fired at 1100�C,

where small quantities of SiC whisker are observed.

3.2 Microstructure

Figures 9 and 10 show fired microstructures of 1100�C firing

for 2 h. Figures 11 and 12 show fired microstructures of

1400�C for 2 h.

3.3 Phase compositions

Phase compositions by XRD results are given in Figures 13

to 16.

4. DISCUSSION

The effect of additives on sample A containing Iranian

chamotte as the main aggregate (65 wt.%) is such that

silicon metal and ferrosilicon metal generally increase cold

crushing strength at all heat treatments of 200�C, 1100�Cand 1400�C and also increase bulk density. Sample B based

on Chinese bauxite also shows the CCS increases at 200�Cand 1400�C, but the CCS is reduced at 1100�C. SEM

photomicrographs have shown that SiC whiskers, of nano

sized diameter, have developed in sample A, but not in

sample B at 1100�C. The reason for the increase of strength

in sample A, which has not been clarified in some previous

studies [5], is now evident and is related to the development

of nano-sized whiskers of SiC within the microstructure. The

reason these develop only in sample A, may well depend on

the higher amount of mullite and glassy phase in chamotte,

when compared bauxite. Needles of mullite in a glassy

matrix phase provides good nucleation sites from which to

growth SiC whiskers at the low temperature of 1100�C. At

1400�C, the thermodynamic driving force is high enough for

the production of SiC whiskers in both samples A and B. But

the concentration of whiskers at a comparable level of

6 wt.% ferrosilicon metal (FeSi2) is more for the chamotte

matrix (Figure 11) than for the bauxite matrix (Figure 12). It

seems that these whiskers are formed on chamotte aggregates

as can be seen in Figure 11, but for the bauxite matrix the

whiskers are nucleated on the FeSi2 particles, spherical

particles seen in Figure 12, formed on the outside of the

bauxite particles. These figures support the hypothesis that

SiC whisker formation nucleates on mullite needles, formed

in the chamotte aggregates, due to the surface tension of the

glassy matrix. The reason for the strength increase at high

temperatures is evidently related to the formation of SiC

whiskers. However at 200�C the mechanism for CCS

increase cannot be related to SiC whisker formation but is

related to cross linking formation in the phenol-formalde-

hyde resole structure or resit. For this temperature, 5 different

additions of silicon metal, ferrosilicon metal, sulfur, silica

fume and carbon black were used. The first two were capable

of increasing the cross linking, but the other three were not

so effective (see Figures 2 and 3). The Si has a strong effect,

when in a small amount of 1%, due to the large number of

covalent bonds related to pure silicon. The effect is reduced

for 2% and above, probably due to an interfering effect of the

Si atoms. By considering Figure 1, it is probable that

formaldehyde (CH2O), which acts as cross linking agent of

A for the main monomer of phenol (C6H5OH), is interfered

with when a lot of Si atoms are present. Each is associated



Figure 10 Micrograph of bauxite –carbon refractory resole bond

containing 5 wt.% silicon metal as additive, fired at 1100�C, which

shows that no SiC whiskers are formed.

Figure 11 Micrograph of chamotte –carbon refractory resole bond


showing SiC whiskers of about 75 nanometres in diameter in the

microstructure.

Figure 12 Micrograph of bauxite –carbon refractory resole bond


which shows SiC whiskers are developed from ferrosilicon metal

particles in the microstructure.

with covalent bonds, at the expense of, and hence

decreasing, the cross links of the phenol-formaldehyde

monomers. Formaldehyde is very reactive and readily poly-

merises with ionic initiation [11]. Either anionic or cationic

materials can initiate the polymerisation reaction.

�R þ CH2O?RCH2O�; ðR � O� ¼ CH2Þ ð5Þ

þR þ CH2O?RCH2OþðR � Oþ ¼ CH2 or R � O� CHþ2 Þ

ð6Þ

Ferrosilicon is less effective than silicon and shows a

maximum CCS at 2% and reduces the strength for 3% and

above. The mechanism of the effect is the same as for silicon.

Micro silica or silica fume (small amorphous spherical

particles of SiO2) and carbon black, amorphous carbon, do

not contribute to a stronger cross linking, probably due to

them having an amorphous and non crystalline nature and

the submicron size of these amorphous particles. Instead,

they have an interfering effect and reduce CCS. Almost the

same effects are observed when these additives are used in

bauxite based refractories of sample B (Figure 3). Results for

samples A and B provide confirmation to each other. The

role of sulfur, in increasing of CCS, was good to a degree

(Figures 2 and 3), but was not as effective as Si or

ferrosilicon. At high temperature, above 800�C, the resit

structure is destroyed and graphite phase, as a crystalline

carbon state, is formed. XRD results show higher peaks of

the graphite phase at 1400�C firing (Figures 13 and 14) when

compared to 1100�C firing (Figures 15 and 16).

5. CONCLUSIONS

1. The addition of silicon or ferrosilicon metal to alumina-

silicate-carbon ramming monolithics, with resole bond,

for Iron making applications, improves mechanical

properties and develop a high strength and therefore

more wear resistance for applications such as blast

furnace runners and troughs.

2. For low temperature heat-treatments, of 200�C, these

additions contribute to the formation of the resit structure

in resole, by increasing the cross linking, and provides

CCS values as high as about 65 MPa , which is remark-

able for a carbon containing monolithic.

3. There is a limit of 1 wt.% for silicon, and 2 wt.% for

ferrosilicon, to increase the cross linking phenomena and

thus raise the strength of the refractory. Beyond this the

additions interfere with the formaldehyde hence the

effectiveness of cross linking is reduced and the strength

is lowered.

4. Three additives, silica fume, carbon black and sulfur,

were not so effective as compared with silicon and

ferrosilicon metal.

5. SiC whiskers of nano-sized diameter are observed to

form at 1400�C in both samples containing chamotte

and bauxite. At 1100�C these whiskers are only

observed in the chamotte based refractory. It is proposed

that probably the higher glassy content of the matrix of

chamotte has caused a higher surface tension. This,



Figure 13 XRD results of fired chamotte – carbon refractory, resole

bond. (1400�C, 2 h).

NOTA

S COP

Y SUPPLI

ED

Figure 14 XRD results of fired bauxite – carbon refractory, resole

bond. (1400�C, 2 h).

NOTA

S COPY

SUPPLI

ED

Figure 15 XRD results of fired chamotte – carbon refractory, resole

bond. (1100�C, 2 h).

NOTAS COPY SUPP

LIED

Figure 16 XRD results of fired bauxite – carbon refractory, resole

bond. (1100�C, 2 h).

NOTAS COPY SUPP

LIED

together with the higher amounts of needles of mullite

formed in chamotte when compared to bauxite, has

caused more nucleation of SiC whiskers. The tempera-

ture of 1100�C is a transient temperature, where any

cross linking is destroyed yet SiC whisker formation is

not fully formed.

REFERENCES

[1] Bosall, S.B. and Henry, D.K. (1985) New developments in

monolithic refractories. The American Ceramic Society,

Westerville, OH, Vol. 13, pp. 331 –340.

[2] Zawrah, M.F. (2007) Effect of zircon additions on low and

ultra-low cement alumina and bauxite castables. Ceramics

International, Vol. 33, no. 5, pp. 751 – 759.

[3] Chan, C.F., Aregent, B.B. and Lee, W.E. (1988) Influence of

additives on slag resistance of Al2O–SiO2–SiC–C refractory

bond phase under reducing atmosphere. J. Am. Ceram. Soc.,

No. 81, pp. 3177– 3188.

[4] Parsons, J.R., Forest, P. and Rechter, H.L. (1974) Refractory

composition comprising coarse particles of clay or bauxite and

carbon. U.S. Patent 3 810 768.

[5] Yamaguchi, K., Zhang, S. and Hashmato, J.YU. (1991)

Behavior of antioxidant added to carbon-containing refrac-

tories, J. Ceram. Soc. Japan, pp. 341 – 348.

[6] Akamine, K., Suruga, T., Yoshitomi, J. and Asona, K. (2006)

Effect of metallic silicon size and heat temperature on proper-

ties of Al2O3–C brick. Internationals Feuerfest-kolloquium,

Aachen.

[7] Fangbao, Ye, Rigaud, M., Liu, X. and Zhong, X. (2004)

High temperature mechanical properties of bauxite-based

SiC-containing castable. Ceramics International, Vol. 30,

pp. 801 – 805.

[8] Nishio, H. and Matsuo, A. (1995) Al2O3–SiC–C Bricks for

Torpedo Car, Taikabutsu Overseas, Vol. 15, No. 4, pp. 39 – 46.

[9] Streitwieser, Jr. A. and Heathcock, C.H. (1981) Introduction to

organic chemistry, second edition. Macmillan Publishing Co.

Inc., USA, pp. 595, Chapter 20 (conjugation).

[10] Kroschwitz, J.I. (1990) Concise encyclopedia of polymer

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[11] Ibid, page 4.



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