Topological considerations on some properties of refractory products

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TOPOLOGICAL CONSIDERATIONS ON

SOME PROPERTIES OF REFRACTORY PRODUCTS

PROEFSCHRIFT TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECH-NISCHE WETENSCHAP AAN DE TECHTECH-NISCHE HOGESCHOOL TE DELFT OP GEZAG VAN DE RECTOR MAGNIFICUS DR. R. KRONlG. HOOG-LERAAR IN DE AFDELING DER TECHNISCHE NATUURKUNDE. VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAG 13 JANUARI 1960 DES NAMIDDAGS TE 2 UUR

DOOR

AERNOUDT WILLEM VAN HAEFTEN

MIJNINGENIEUR

GEBOREN TE TJIMAHI

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DIT PROEFSCHRIFT WERD GOEDGEKEURD DOOR DE PROMOTOREN PROF. DR. M. J. DRUYVESTEYN EN PROF. DR. IR. C. SCHOUTEN

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Aan mijn moeder Aan mijn vrouw

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ACKNOWLEDGEMENTS

lam

indebted to Ir.

K.

KOOIJ, Director of Chamotte Unie N.V., for the opportunity afforded to make this study and the permission kindly granted to publish the results of the investigations carried out by the author at the laboratory of Chamotte Unie N.V. at Geldermalsen.

I wish to express my appreciation to all the members of the Research Department of Chamotte Unie N.V. for their co-operation during the work. Valuable assistance was given by Mr. A. H. ZWEMSTRA in the preparation and execution of the investigations.

Finally, I particularly wish to thank Ir. G. VAN GIJN, advisory engineer to Chamotte Unie N.V., for the invaluable help so willingly givenduring many discussions, which not only revealed his intense interest in the topo-ceramic conception, but also inspired and encouraged me greatly.

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4.5.3 Execution of the investigation 4.5.4 Results . . . . 5 Refractoriness-under-Ioad 5.1 Introduction . . . . . 5.2 Method of investigation. . . 5.3 Apparatus... . . 5.4 Evaluation of the experimental results. 5.4.1 Curves of identical suhsidence. . . 5.4.2 Curves of identical test temperature 5.5 Discussion . . . .

5.5.1 Temperature at which a given suhsidence occurs 5.5.2 Suhsidence occurring at a given test temperature 5.5.3 Practical value of the data discussed above. . .

6 Relation between the topology and the refractoriness-under-Ioad

6.1 6.2

lntroduction

Topological interpretation of the temperature/suhsidence curves 6.2.1 Introduction

6.2.2 6.2.3 6.3

Test temperature at which the subsidence is 0,6%

Test temperature at which the suhsidence is 10% Opinions expressed in literature

Summary . . Samenvatting Appendix: 3 plates 65 67 67 68 69 72 74 78 78 78 79 79 82 82 83 84 86 87 87 87 88 89 90 91 91 92 95 97 100 103

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CHAPTER 1

PRELIMINARIES

1.1 T opo-ceramic conception

Refractory ceramics are known gene rally to be composed of various crystalline and amorphous phases and of a gaseous phase (pores), their composition, ratio and distribution depending on the considered product. Yet this knowledge has not saved these materials from being described or investigated as though they we re homogeneous. Only during the last 15 years the idea has developed that the properties and behaviour. of these inhomogeneous materials do not depend on the mean chemical and mineralogical composition nor on the porosity, but on the way in which the composing phases occur, distributed according to nature, form and size. In principle it would be necessary to give the composition and distribution of every phase and then try to explain or predict the behav-iour of refractory ceramics from the properties of every phase and their distribution. If the distribution according to nature, shape and size of the phases and the consequent distribution of the properties is called the topology of the product, then the behaviour of the refractory ceramics considered here, may be said to be determined by their topology under test conditions (topo-ceramic conception). Owing to the large number of variables (every phase has for instance a distributive function regarding the si ze of the grains) a method of this kind offers few concrete pos si-bilities for explaining or modifying the behaviour of refractory materials.

For this reason it is necessary to classify the distribution of the proper-ties in order to come to a more usable description. Except in some special cast or sintered ceramics the coarse, partly or completely crystalline con-stituents in refractories are embedded in a residual substance called matrix which consists of a high percentage of glassy phase, very fine crystalline particles and pores as shown in the figure overleaf.

Everything within the dotted lines, except the parts of the grains A, B, C, D, E and F which all have a diameter of more than 150 fLm, is considered to form the matrix.

This classification of the topology has been chosen because it is suitable for the explanation of the refractoriness-under-load, which at elevated temperatures is one of the most important properties of refractory ceramics. The reason being that the refractoriness-under-load of

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refrac-- refrac-- refrac-- refrac-- \ refrac-- refrac-- refrac-- , l m clrphous phase

_ _ ... - -·fine grain diam. < 150(.lm } matrix a.--~--pore

~~~~~--...,çoar:se grain diam. > 150(.lm

tory ceramics is thought to be strongly affected by the topology - the topo-ceramic texture - of the matrix.

Although knowingly or unknowingly this concept ion is the basis of the preparation and execution of practically every ceramic process and also of the interpretation of the results achieved therein, its fundamental value has so far not been investigated systematically.

An investigation of this kind is being carried out by the Physical and Chemical Laboratory of Chamotte Unie N.V. at Geldermalsen, The Netherlands, part of the investigation being the subject of this thesis. 1.2 Choice of the material to be examined

Verification of the above conception requires the investigation of products of which the matrices have a highly divergent topology. A suitable investigation would have been that of products on an alumino-silicate basis having highly different Al20a 1) contents, in which the start-ing point could have been the current quality gradstart-ing accordstart-ing to Al20a content. Starting from a product consisting of kaolinite and

cal-cined kaolinite, the calcal-cined kaolinite (chamotte) could be replaced by quartz. Thus from a so-called basic chamotte brick with 40% Al20a one would arrive at a so-called semi-acid chamotte brick with 15% Al20a. 1) In continental refractory practice the "alumina content" of a material is given as the total sum of the AlZ03 content and the small amount of TiOz generally also present. This practice is followed throughout this thesis.

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By replacing the kaolinite by hydrargillite, boehmite or diaspore and the chamotte by y or a Al20 a or minerals of the sillimanite type, products would be obtained having an average Al20 s content of approx. 60%. The

main objection to this procedure is that in this way a direct approach of the problem is not possible, since another variable, the chemical compo-sition, is introduced.

In order to exclude beforehand the influence of the Al20 a content as

a variable and also in order to arrive at a more systematic procedure, products were made having the same average Al20 a content of approx.

40%. To this end we started from an industrial product consisting of chamotte and clay. By systematically replacing certain grain si ze fractions of chamotte by an equal weight of quartzite and corundum of the same grain size, it proved to be possible to vary the Al20 a content of the matrix

from 15 to 60%, while the average chemical composition remained the same. Owing to the also highly divergent mineralogical composition of the matrix, products were in fact obtained whose matrices showed a greatly different topology. A possible objection to this method of in-vestigation is that the picture thus obtained is exaggerated and that prod-ucts of this kind have no practical value. In defence it may be said that it will be to the advantage of the investigation if the systematic variation in the topology of the matrix is at its maximum as long as the average Al20 a content is equal. The investigation wi11 also have a practical value

because as said before, besides products consisting of chamotte and clay only, there are also ceramic products which also contain either quartzite or corundum or both. Another motive is the fact that instead of using for instance little bonding clay and possibly a high firing temperature, it is sometimes possible and of ten cheaper to attain certain properties by using quartzite and/or corundum and more bonding clay and thus avoiding high firing temperatures. It is therefore of particular importance to know how these quartzite or corundum fractions have to be added and whether after the firing process these fractions must be considered to be the coarse grains or the matrix.

1.3 Choice of the methods of investigation

In the investigation of these test materials a distinction must be made between:

a. investigations of the topology, which are mainly of scientific im-portance.

b. investigation of the behaviour at high temperature, which is of practical importance.

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Re a. Investigations for the purpose of establishing the topology of the test materiais.

Data on the average phase composition were obtained by: 1. mineralogical analysis

2. porosity determination

Information on the distribution of the phases was obtained by: 3. microscopie investigatian

4. permeability determination

5. determination of pare size distribution

Re b. Investigation of the behaviour of the material at elevated tem-peratures.

Chosen in this case was the resistance of the materials to permanent deformation as a function of the temperature and time under a constant load. The determination of the deformation was chosen as the method of investigation. The motive for this choice was that in practice construc-tions consisting of refractory materials have to withstand forces, without a permanent deformation detrimental to the life of the construction.

As to the choice of the method of investigation, the determination of the refractoriness-under-Ioad is in Holland the sole standardized method producing some decisive information on this behaviour. How-ever, the equipment and method used in this test deviate from standard N 412, because it was found that in this way more exact and more re-producible data could be obtained than would have been possible if the standardized method had been followed.

1.4 Purpase af the investigatian

In

view of the above, the purpose may be formulated as follows: T 0 establish the relation between the topology of the matrix before the

test and the refractoriness-under-Ioad of produets of the alumino-silicate type, all having a mean Al20 a content of 40

%

and the same porosity in unfired condition.

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CHAPTER 2

TOPOLOGY OF THE UNFIRED TEST CYLINDERS

2.1 Choice of the basic mixture - motivation

The starting point for all the test mixtures was a mixture of chamotte

and c1ay with an average Al20 3 content of approx. 40%. The

considera-tions which prompted this choice have al ready been mentioned in

CHAPTER 1. In order to obtain products in which at relatively low firing

temperatures sufficient ceramic bond will be formed, a refractory clay

with approx. 32% Al20 3 was chosen which meets this requirement.

A hard and dense kaolinite chamotte with approx. 41

%

Al20 3 was chosen

as grog. The chemical composition of these two raw materials is shown

in TABLE 1. As usual the chemical analysis of the refractory clay was

determined af ter calcining at 1100

o

e.

The weight losses due to drying and calcining were approx. 9%.

TABLE 1

Chemical composition of the raw materials

Chamotte Quartzite Corundum Clay

(% wt) (% wt) (% wt) (% wt) Si02 55.9 96.4 0.3 63.3 Al20s+Ti02 40.9 2.1 99.5 32.5 Fe,Os 0.9 0.5 0.1 1.6 CaO+MgO 0.4 0.3 0.0 0.9 K2O+Na2O 2.3 0.3 0.0 1.3

The desired average chemical composition of 40% Al20 3 and the given

mean chemical composition of the chamotte and the refractory clay led to a mixture with a high chamotte content of 84% against 16% clay. This basic mixture was denoted as no. 4 in TABLE 2. The partic1e size distribu-tion of the chamotte in the basic mixture is laid down in a cumulative grain size distribution-by-weight curve shown in FIG. 1. This curve is practically identical to the so-called LITZOW curve 1).

On the one hand the choice of this grain distribution was motivated by the consideration that a similar distribution is usually chosen in the

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f

100 ]---,;.--- ---

!

~ -~---.

-

---

-

---

----

I

- I I

I

I I - I I

j---

I : I I 50

J ______

I i

!

. ~ I

~--

I

- I I

I

I : - I I

i :

- : :

:

I

-!

!

I o

29 '

f"042

-=-

0 - - -

-.7:

₁JOO=-=---=2~OOO::-::----~28oo

sleve opening II-'m] Fig. 1. Cumulative grain size distribution by weight.

manufacture of alumino-silicate products by the dry pressing process. On the other hand the above distribution was chosen because such a grain mixture can be composed of six fractions of equal weight with the aid of standard screens. The smalle st particle diameter chosen here was approx. 20 microns, a size which can still be identified by microscope. Particles with a diameter smaller than 20 microns were separated with the aid of a classifier. Taking into consideration the diameter (= 35 mm) and the height ( ~ 50 mm) of the test cylinders, the largest particle dia-meter chosen was 2.8 mmo FIG. 1 shows that one third of the chamotte grains has a particle diameter of minus 150 microns (= fine grains), one third a diameter minus 1.2 mm and plus 150 microns and one third a diameter minus 2.8 mm and plus 1.2 mm (= coarse grains). The refractory clay particles have a grain size of minus 150 microns.

2.2 Campasitian of the ather mixtures

Starting from the composition of basic mixture no. 4, other mixtures were obtained by replacing an ever increasing portion of the chamotte by an equal weight of coarse crystalline quartzite and fused corundum in such a way that the chemical composition and grain size distribution of the non-plastic components remained the same.

The chemical compositiop. of the chamotte, the quartzite and the corundum is shown in TABLE 1. In order to keep the Al₂0_acontent of the

overall mixture at approx. 40%, 14% chamotte was substituted each time by 8.12% quartzite and 5.88% corundum. In conformity with the

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requirements, a series of mixtures was obtained by replacing to an in-creasing degree fine chamotte grains by fine quartzite grains and at the same time coarse chamotte grains by coarse corundum grains, or by replacing the fine chamotte by fine corundum and at the same time coarse chamotte by coarse quartzite grains. This substitution was carried out a few times and as aresuit seven different mixtures were obtained, of which the composition of the raw materials is shown in TABLE 2 and

represented graphically in FIG. 2.

As shown in TABLE 2, the mixtures numbered 1 and 7, 2 and 6, and

3 and 5 respectively have the same rnineralogical composition.

TABLE 2

Composition of the raw materials in the test mixtures

Mixture Clay

I

Quartzite Chamotte Corundum

no. (% wt) (% wt) (% wt) (% wt) 1 16 24.36 42 17.64 2 16 16.24 56 11.76 3 16 8.12 70 5.88 4 16 - 84

-5 16 8.12 70 5.88 6 16 16.24 56 11.76 7 16 24.36 42 17.64

As regards grain size, the distribution of the rninerals over the different grain si ze fractions is not identical, there being a great difference both

6 7

- bondclay ~. quartzite

specimen number

o

grog _ corundum

Fig. 2. Distribution of the raw material grains over the different sieve divisions.

-2800

I

+2000 -; N -2000 ·or +1200 ~ -1200 ~

+ 420

150 75 75 20 150

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between the groups of mixtures having the same composition of the raw materials and between the mixtures belonging to the same group, as is shown in TABLE 3.

TABLE 3

ParticIe size distribution of the raw materials in the mixtures

Mixture no. Raw material Sieve division 1 2 3 4

5

I

6 7 (% wt) (%wt) (% wt) (% wt) (% wt) (% wt) (% wt) -2800+2000 !Lm - 2.24 8.12 14.00 5.88 - --2000+1200

..

10.36 14.00 14.00 14.00 14.00 11.76 3.64 kaolinite -1200+ 420

..

14.00 14.00 14.00 14.00 14.00 14.00 14.00 chamotte - 420+ 150

..

14.00 14.00 14.00 14.00 14.00 14.00 14.00 - 150+ 75

..

3.64 11.76 14.00 14.00 14.00 14.00 10.36 - 75+ 20

..

- - 5.88 14.00 8.12 2.24 --2800+2000 !Lm 14.00 11.76 5.88 -

-

- --2000+1200

..

3.64 - - - -corundum -1200+ 420

..

- -

-

- - - -- 420+ 150

..

- - - -

_-

- -- 150+ 75

..

- - - 3.64 - 75+ 20

..

- - - - 5.88 11.76 14.00 -2800+2000 !Lm - - - - 8.12 14.00 14.00 -2000+1200

..

- - - 2.24 10.36 quartzite -1200+ 420

..

- - - -- 420+ 150

..

- - - -- 150+ 75

..

10.36 2.24 - - - - -- 75+ 20

..

14.00 14.00 8.12 - -

-

-clay

-

150 !Lm 16.00 16.00 16.00 16.00 16.00 16.00 16.00

TABLE 3 shows that all the mixtures contain 28% non-plastic raw materials and 16% clay with a particle diameter of minus 150 microns.

The percent composition of the raw materials of the gra~ns with a

diameter of minus 150 microns was calculated for all the different mix-tures. The result is shown in TABLE 4.

TABLE 4 shows that going from mixture no. 1 to mixture no. 7, the petrographical character of the fine grains becomes less and less acid.

By keeping the ratio of non-plastic to plastic material constant, by adding an equal amount of liquid binder to the mixture, by mixing the materials in the same way and by pressing in the same way pieces of the same shape and size, it is possible to obtain a reproducible

distri-bution and coherence of the non-plastic, plastic and gaseous constituents (= pores) in the unfired compacts. As there is a difference in the specific

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TABLE 4

Composition of the raw materials of the grains with a diameter of

minus 150 microns in the test mixtures

Mixture Clay Quartzite

I

Chamotte Corundum no. (% wt) (%wt) (% wt) (% wt) 1 36.36 55.36 8.28 -2 36.36 36.91 26.73 -3 36.36 18.46 45.18 -4 36.36 - 63.64 -5 36.36

-

50.27 13.37 6 36.36 - 36.91 26.73 7 36.36 - 23.54 40.10

gravity of the non-plastic grains in the mixtures, the number of grains in the corresponding fractions of the different mixtures will vary. Further there is also a difference in the partic1e shape of the grains of the mate-rials used.

These two facts may cause a difference in the packing density of the compacts. This, however, was accepted knowingly, as it was a prime requisite to keep the average chemical composition constant. Never-theless, TABLE 10 on page 21 shows that in spite of the mentioned

differ-ence, the porosity of the unfired test cylinders is virtually identical.

2.3 Mean chemical composition

Taken into account that the chemical composition of the c1ay given in TABLE 1, was determined after calcining at 1100

oe

and that the weight losses of the c1ay due to drying and calcining were approx. 9%, the mean chemical composition of the test mixtures was calculated with the aid of the figures shown in TABLE 1 and TABLE 2.

The re sult of these calculations is shown in TABLE 5.

TABLE 5

Mean chemical composition of the test mixtures

Mixture Si02 Al203+Ti02 Al20.+ Ti02+ SiO. Fe.03 CaO + MgO K.O+Na.O

no. (% wt) (% wt) (% wt) (% wt) (% wt) (% wt) 1 57.1 40.6 97.7 0.7 0.3 1.2 2 57.0 40.3 97.3 0.8 0.4 1.4 3 57.0 40.0 97.0 0.9 0.4 1.7 4 57.0 39.7 96.7 1.0 0.5 2.0 5 57.0 40.0 97.0 0.9 0.4 1.7 6 57.0 40.3 97.3 0.8 0.4 1.4 7 57.1 40.6 97.7 0.7 0.3 1.2

17

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In agreement with the plan of this investigation, the mean chemical composition of all the test mixtures appears to be practically identical.

2.4 Chemical composition of the minus 150 microns grains in the mixtures Similarly the chemical composition of the fine grain fraction (minus 150 microns) in the mixtures was calculated, the re sult of which is given in TABLE 6.

TABLE 6

Chemical composition of the fines (minus 150 microns) in the mixtures

Mixture SiO. Al.Os+TiO. Al.Os + TiO. +SiO. Fe.Os CaO+MgO K.O+Na.O

no. (% wt) (% wt) (% wt) (% wt) (% wt) (% wt) 1 81.6 15.8 97.4 0.9 0.5 0.8 2 73.9 23.2 97.1 1.0 0.6 1.2 3 66.1 30.6 96.7 1.1 0.6 1.5 4 58.4 38.0 96.4 1.1 0.6 1.8 5 50.7 46.1 96.8 1.0 0.5 1.5 6 43.1 54.2 97.3 0.9 0.5 1.2 7 35.4 62.3 97.7 0.8 0.4 0.9

TABLE 6 shows that proceeding from test mixture no. 1 to no. 7, the A1203+ Ti02 percentage increases from 16 to 63%, while the Si02 percentage ofthe fine grains decreases from 82 to 35%. The Si02+Al20 3

+ Ti02 percentage is practically constant, namely 97

±

0,7%.

2.5 Seger cone number of the different mixtures and of the grains with a diameter smaller than 150 microns

At higher temperatures as occuring during the Seger cone test carried out in accordance with the Dutch standard N. 412, the refractoriness of products of the alumino-silicate type depends almost entirely on the high percentage of the liquid phase, then present therein. The refrac-toriness, expressed in the Seger cone number, of all the previously powdered mixtures proved to be 34/35. However, the Seger cone num-ber of the grains having a diameter of minus 150 microns in the mixtures of which the chemical composition is shown in TABLE 6, varied from 29 for mixture no. 1 to 36/37 for mixture no. 7. In other words, the Seger cone refractoriness varies from the lower limits of what is still called refractory to the high refractoriness of a so-called sillimanite product, as shown in TABLE 7.

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TABLE 7

Seger cone number of the fines (minus 150 microns) in the test mixtures

Mixture no. 1 2 3 4 5 6 7

Seger cone number 29 30 31/32 33/34 34+ 35/36 36/37

2.6 Mineralogical composition of the test mixtures

The mineralogical composition of the raw materials was determined

by X-ray analysis. 1) The mineralogical determination by X-rays is based

on the comparison of the intensity of one or more characteristic inter-ference lines with those of calibrating materials which contained varying quantities of the same mineral constituents as the materials under test.

On the strength of the data obtained by X-rays the remainder win

always be considered as amorphous phase.2) 3)

The results of the mineralogical analysis of the different raw materials are given in TABLE 8.

TABLE 8

Mineralogical composition of the raw materials

Minerals Chamotte Quartzite

I

Corundum Clay (% wt) (% wt) (% wt) (% wt) quartz < 3 95 - 25 corundum - - 100

-mullite 51') - -

-kaolinite - - - 50 illite - 3 - 10 orthoclase < 2 - - 10 amorph. phase 44 - -

-rutile - < 2 - < 2 Fe.03·nH•O -

-

- _<2 calcite - - - _<2 dolomite -

-

- < 2

1) All the X-ray investigations necessary for this thesis were performed by the "Institut

für Gesteinshüttenkunde der Rhein-Westf. Technischen Hochschule" at Aix-la-Chapelle

which is directed by Prof. Dr. H. E. SC'HWIETE. The investigations were carried out under the

direct supervision of Dr. H. MÜLLER-HESSE for whose valued co-operation I wish to express

my special thanks.

2) MÜLLER HESSE H. - BOSE, A. K. - SCHWIETE, H. E.: Arch. Eisenhüttenwes. 27 (1956)

665/671 - 28 (1957) 667/670.

3) MÜLLER HESSE, H. -GELSDORF, G. - SCHWIETE, H. E.: Arch. Eisenhüttenwes. 27 (1956)

807/811 -29 (1958) 513/519.

4) Dr. H. MÜLLER HESSE established that the mullite in the original chamotte as weil as

that formed in the test pieces during firing contained approximately 1.3% Fe,O. and 0.4%

TiO •.

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The mineralogical composition of the chamotte shown in TABLE 8, was

determined on the chamotte grains with a diameter of minus 150 microns. As shown in TABLE 8, the fine chamotte grain fraction contains a small

percentage of quartz and orthoclase. According to the supplier, these impurities entered the fine grained chamotte material during their grinding, the coarser grains having a composition of about 54

%

mullite and 46% amorphous phase.

With the aid of the data given in TABLE 2 and TABLE 8 the corundum,

quartz, mullite and amorphous phase percentages in the unfired com-pacts were calculated. At the same time the weight losses of the clay due to drying and calcining, and the stated small differences in the mineral-ogical composition of the chamotte grains with a diameter of minus 150 microns and plus 150 microns, were taken into account. Further it was assumed that the quartz percentage was the same in the calcined and non-calcined clay.

The re sult of these calculations is shown in TABLE 9.

TABLE 9

Mineralogical composition of the unfired specimen

Mixture Corundum Quartz Mullite Amorph.phase

no. (% wt) (% wt) (% wt) (% wt) 1 17,9 27,3 22.9 19.5 2 11.9 19.7 30.3 25.9 3 6.0 12.1 37.7 32.3 4 - 4.5 45.2 38.6 5 6.0 12.2 37.7 32.2 6 11.9 19.8 30.2 25.8 7 17.9 27.5 22.7 19.4

2.7 Manufacture of the test cylinders

The composition of the test mixtures is shown in TABLE 3 on page 16.

The different raw materials were first blended dry by hand and then -also by hand - intensively mixed with 5 vol.

%

of water and 3 vol.

%

of sulfite lye.

Using a hydraulic press, cylinders were pressed having a diameter of 35 mm and a height of approx. 50 mmo The pressure exerted on these cylinders was 500 kgfcm2

•

Solid cylinders were made for the investigation of the refractoriness-under-Ioad at high temperatures. Cylinders having a central hole of 8 mm diameter were made for the investigation of the change in length during the firing.

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The cylinders were dried for some days in a drying closet at approx.

80°C and then at 110°C for one night.

Some of the dried test cylinders were broken up. Examination showed, that the different raw material grains were homogeneously distributed in the compacts.

2.8 Density and porosity of the unfired test cylinders 1)

Of the various raw materials from which the test cylinders were made,

the true density (St) was determined in accordance with the Dutch

standard N 413. While water was used as the immers ion liquid for the quartzite, corundum and kaolinite chamotte, benzol was used for the bonding clay in order to exclude the influence of "swelling" .

The following average values of true density were found

quartzite 2.65 gfcm3

corundum 3.865 gfcm3

kaolinite chamotte 2.645 gfcm3

bonding clay 3.635 gfcm3

As the weight percentages of the raw materials added to the 7 different mixtures are known (see TABLE 2 on page 15), the true density (St) of these mixtures could be calculated.

Of two of each of the 7 types of unfired test cylinders, the bulk density

(Sa) was determined according to the mentioned Dutch standard, benzol

again being used as the immersion liquid as otherwise the cylinders would have decomposed during the tests.

With the aid of these determined and calculated values the true and apparent porosity (Pt and Pa resp.) were calculated also according to the

Dutch standard. These different values are shown in TABLE 10.

TABLE 10

Density and porosity of the unfired test cylinders

Test True Bulk True Apparent (True-Apparent)

cylinder density density porosity porosity porosity

no. (g/cm') (g/cm') (%) (%) (%) 1 2.86 2.23 22.0 18.4 3.6 2 2.79 2.18 21.9 19.5 2.4 3 2.72 2.13 21.7 19.7 2.0 4 2.65 2.07 21.9 20.2 1.7 5 2.72 2.12 22.1 19.0 3.1 6 2.79 2.20 21.2 18.5 2.7 7 2.86 2.28 20.3 17.1 3.2

1) For definitions of these properties refer to page 49 and 50.

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This TABLE shows in the first place that in spite of the possible

differ-ences mentioned in PARAGRAPH 2.2, the proposed objective of an identical

porosity in the unfired test cylinders has been attained reasonably weIl.

The true porosity is 21 ±1 %. Further the apparent porosity, being

18.7±1.5% on the average, does not show considerable variation, while the average percentage of closed pores - (true minus apparent porosity)-is 2.7±1 %.

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CHAPTER 3

FIRING PROCESS

3.1 Introduction

During the firing process the topology of the test cylinders will change and as a result of this also their properties and behaviour. One of these properties, the change in length, was determined during the firing process as a function of temperature and time. Although it does not come under the scope of this investigation, we availed ourselves of this opportunity to establish the infiuence of the topology on the change in length during heating by starting from unfired cylinders. The results show that in spite of identical chemicalor mineralogical composition, there can be great individual differences in the change in length during the heating up process. On the basis of the topology of the unfired materials and with the aid of the information found in literature concerning the volume change of minerals as a function of temperature and time, it is possible to explain qualitatively the observed differences in the length change of the test cylinders when subjected to a certain thermal treatment. The topology of the fired products will also be affected by the change in length after firing. These data will be used later and an explanation will be given when more knowledge has been gained regarding the topology of these fired cylinders.

3.2 Method of firing

The test cylinders were divided into 3 groups which were fired at 1350°, 1400 ° and 1450°C respectively, these being the temperatures at which materials of this kind will also be fired in practice. Apart from the difference in the maximum temperature, the firing process was identical for all the cylinders. The heating up rate was invariably 4 °C per minute. When the required temperature had been reached, a soak followed for 4 hours. Heating-up and soaking time were in the first place determined by the practical consideration that the firing process should not last longer than approx. 10 hours, the cooling time not inc1uded. The test cylinders were cooled down to ambient temperature in about twenty-four hours. The firing of the test pieces took place in a gas furnace, all possible precautions thereby being taken to enable the above-mentioned firing

23

(22)

dial gauge,---{~",=,(j====:F=I

furna(:e----I~.

Fig, 3. Arrangement for measur;ng the change in leng th of the test cylinders during firing.

cycle to be observed as closely as possible. In addition cylinders with a 8 rnrn diameter central hole were fired in avertical electric tubular fur-nace. The changes in length occurring during the firing process in this furnace, were measured by means of the arrangement shown in FIG. 3.

In this way 21 different curves were obtained representing the change in length of the different test cylinders as a function of temperature and time (so-called firing curves).

3.3 Change in length during firing

FIGs.

4, 5 and 6 show the above mentioned curves, representing the three firing temperatures of each of the 7 different test cylinders.

In general the curves could be reproduced reasonably weIl.

a. Heating-up range

200

to 900°C: The heating-up range gives cause to the following remarks. The substitution of chamotte by fine quartzite and coarse

(23)

o 40 SO 120 160 200 240

1.2 r _ -.-7 - -.... I ' , " , ,

1 ( 6 - - , .... I

g • , ... =--""1- -.:::., ... soaklng time at max. temperature rml~1

c l /I ~~~'. I o O.S. // ---:...-i~~-... ~... I

~

l

/-//~

"

"""'

3 ""

....

>",

.·

\'~l-_

.

______

.

__

.

_._

· -

· _-

· -7

~ O.4l~/'/ 4 "'. \ .. \ 1-_________ ~ ____________ 6 . l ij'/'" - ... \ "\ I c:~ . "" "'1 .= 0.01 .... ~ . '\~I""""'''''''''''''''''''''''''''''''''''''''''' ... . . ... 5 ~

l

\.

Ë O.4l t~:\,...

- - - -____

4 c:

I

1 \ ' , ••••• _, B . ! , ,I ', . c:

I

100 300 500 700 900 1100 1300 '0. O,Sl" '~" __ g ,temper.tur.

r:9

___

~=:,::.::::.':.::::..":':.::::.~~-=-~-=.::: 2 . ... 3 1

Fig. 4. Length changes during firing and soaking at the max. firing temperature of 1350 °C. 1.2 ~ O.S c: 0 .~ ~ 0. _0.4 x " C 0.0 c: 0.4 0 -~ c: O.S 8 c: g 1.2 Fig. 5. 1.2 g _O_._S c: 0 .~ c: (t 0.4 x " c: 0.0 0.4 O.S c 0 ';; v

S

1.2 c: 8 . ~ 1.6 ~ 2.0 o

I--·:t:::~,

·...

....

:

/ ","-'--1 -,,""- ... :

,

_~~_~

~._"'

.,'

_.',

~

_

.. ,.

__ '

..

/

.

/-./

.

I%~c:r-:~:~,~~5~,~L--

.

....

>:::\~!

'

_

________________________

--

6 f-"=---~.\..: . ...., ... . sa 160 200. _.- 1

_.-'

-

'

-, 1 \\1 1'1

\1

:f'··"

..

1:

\

··

..

·

...

-

... I """ 200 400 600 SOO 1000 1200 1400 "-:-'::'::":::::'::.:.::.::.:.::,:.:::::::::-:::=::,r •... _,3

---4

. ... -3

Length changes during firing and soaking at the max. firing temperature of 1400 °C.

o I, I',

i\

\\, I 1"·1 11"1 40 _, ...

V'"

~\.... . SO _, 120 160 _, 200 _, ~-'---'~~~-~~~-~~-~~--'-~~-' \ ... ", ... ,.-soa 1000 1200 1400\ " 4 \., ... .,...

---...

---.... __ . __ . _ _ . - - - -'2 - ·----1 200 4DO 600 ···,···, ·"",···,····,····_···3

(24)

Z +1.2 c:

°

.u; ~ +1.0 x " +0.5 I I I I I I I I

.

, , , ·2 4 5 6 7 specimen number

Fig. 7. Linear expansion at 900

o

e.

corundum (nos. 4 -+ 1) or by coarse quartzite and fine corundum (nos.

4 -+ 7) causes great differences in the thermal expansion. Particularly

when the substitution of the chamotte by quartzite and corundum grains is slight, as in the mixtures 3 and 5 which both have an identical mineral-ogical composition, this difference is evidenced by the extent of the

f3

-+ a quartz dilatation at the transformation temperature.1₎ _{Striking is}

also the fact that up to approx. 900°C the firing curves of the mixtures nos. 2 and 5, 1 and 6 respectively, almost coincide. This shows that the presence of coarse quartzite grains in the matrix has a far greater effect on the expansion curve than that of fine quartzite grains.

In FIG. 7 the expansion of the test cylinders at 900°C is represented

graphically. The test points are connected by asolid line. This will also be done in further graphs where the properties of the test materials are plotted versus the test number. The difference in the change in length which occurs in spite of identical mineralogical compositions, can be explained as follows. At 575°C convers ion takes place from

f3

to a quartz, this being associated with an increase in volume which is large in com-parison with the thermal expansion of the minerals with which the quartz is in contact in alumino-silicate compacts. 2) 3)

Further a quartzite grain is composed of a great number of minute quartz fragments, which in addition are oriented in different ways. The

1) SOSMAN, R. B.: The properties of Silica, New York 1927, 117/124.

2) WINKLER, H. G. F.: Struktur und Eigenschaften der KristalIe, Berlin/ Göttingen/Hei-delberg. 1950, 98.

(25)

thermal expansion of quartz

..l

to the crystallographical

e

axis is dif-ferent from that // to this

e

axis. As a result of this such great stresses may arise in the coarse quartzite grains during the heating up process that they are cracked (PHOTO 7.185.2 in APPENDIX), an effect which

is associated with an extra increase in volume.1

) When {3 quartz is

converted into a quartz, such great stresses will occur in the material bridging the coarse quartzite grains, that fracture takes. place and the bulk of the compacted system is increased. With fine quartzite grains this cracking effect occurs to a much lesser degree, while in addition these grains are found in a relatively porous mass (= matrix) between the coarser grains. Hence in the conversion from (3 to a quarrtz their increase in volume will take place partly at the co st of the porosity of the matrix. It will therefore not produce a corresponding equal increase in volume of the entire system.

900 °G-max. temperature: All curves show a maximum expansion at approximately 900

o

e.

In the FIGS. 4, 5 and 6 the shrinkage occurring at

3 o.o0 , . - -- - -- - - , - - - - -c

°

.;:;

5

c ° v '" " c -2.0 I I I I I I I I ~----[1350·C)

/1'- - -- -

--[1400'C) I J-. _.-I ,. ·_ · - · _·-[1450'C) I . I I

.I

/

I / I . I

.I

I

.I

,,/ .I

_....

/ //

/

i

/

/ 2 5 6 7 specimen number Fig. 8. Change in length during firing from 900 °C up to

indicated temperatures.

(26)

~ c:

°

.;;; :;; Q. X '" L ~ '" c: c

°

. ~ c: 8 L ~ '" c: +1.00 +0.50 0.00 -0.5 -1.0 ! I [1350 CJ / [1400 CJ / / / [1450 CJ / / / / I ' 1 / 1 /

1/

I . I

1/

)I.

I

i

/

/ l

\ I /: " , , / / . / I I

-

/

, .... _ .1 2

/

.I

5 6 7 specimen number

Fig. 9. Change in length after firing to indicated temperatures.

higher temperatures shows the same picture of curves with steep ends for the test pieces 1, 2 and 3, the shrinkage of the other test pieces being

much smaller (see also FIG. 8). The decrease in length commencing af ter

approx. 900

oe

is due to the fact that between 850 and 950

o

e

mullite is

formed from the clay minerals which are then reduced in volume.I₎ _The

relatively high degree of shrinkage occurring in the test pieces 1, 2 and 3 af ter reaching approx. 1150

oe,

is due to the fact that so much liquid phase is being formed in the matrix that it shrinks together under the in-fluence of capillary forces. This effect is associated with a decrease in volume which will be very strongly manifested in the volume behaviour of the test cylinders. The formation of liquid phases is much Ie ss in the test pieces 5, 6 and 7 as there is no free quartz in the matrix, amineral which highly promotes the formation of liquid phases in alumino-silicate

products at high temperatures.

The curves of FIG. 8 show clearly that the substitution of chamotte

by coarse quartzite and fine corundum (nos. 4 ~ 7) has practically no ') NORTON, F. A.: Elements of ceramics, Cambridge 42 Mass U.S.A., 1952, 129.

(27)

influence on the shrinkage of the different test cylinders. Inversely the shrinkage will increase both with increasing substitution and increasing temperature.

20 ° C-max. temperature: The overall percent change in length is given in FIG. 9. In the first place we see that upon reaching their respective maximum firing temperatures the materials 5, 6 and 7 show an expansion which increases as the used quantities of coarse quartzite and fine corun-dum increase and the firing temperature is lower. Compared with the

ex-pansion of the chamotte test piece no. 4 the numbers 5, 6 and 7 appear to

have an expansion which is almost independent of the firing temperature. All the other test materials appear to have shrunk upon reaching the maximum firing temperature. When the substitution of chamotte by fine quartzite and coarse corundum increases, a minimum shrinkage is reached, as the shrinkage of no. 2 is again less than that of no. 1, an effect which increases with decreasing firing temperature. Unlike the shrinkage of no. 4, that of nos. 1,2 and 3 proves to be dependent on thefiringtem-perature. The explanation for this variation in the curves is implied in that al ready given for the variation of the curves of the temperature

ranges 20 ° -7 900°C and 900°C -7 max. temperature. The expansion

of the test pieces 5, 6 and 7 as well as the decrease in shrinkage of material no. las compared with no. 2 and no. 3, are due to the

f3

-7 a trans-formation of the quartz at 575

oe.

b. Soaking temperature range

In spite of the identical chemical composition of all the test materials, the two test series nos. 4 -7 1 and 4 -7 7 can also be clearly distinguished by this part of the curves of the FIGS. 4, 5 and 6. From the change in length af ter 4 hours soaking at the maximum firing temperature, it is evident that the expansion of the numbers 6 and 7 (see also FIG. 10) increases with the soaking temperature. This is due to the fact that the rate of transformation of the added quartzite to cristobalite increases with rising temperature, which is associated with an increase in volume. I) In test cylinder 5 (FIG. 10) a small shrinkage is observed. Apparently the expansion due to the conversion of quartz into cristobalite is com-pensated by a shrinkage of the material present in the matrix, which took place during firing. When chamotte is replaced by fine quartzite and coarse corundum (nos. 4 - 7 1), shrinkage invariably occurs. The increase

in volume which also occurs here owing to the conversion of a quartz

into a cristobalite, is not concentrated locally as is the case when coarse 1) SoSMAN, R. B.: The properties of Silica, New York, 1927,66/72.

(28)

g c 0 ,;; ~ Cl. )(

.,

~ '" ., .:§ c 0 oB t c 0 u ~ '"

.,

c ~. +1.00 +0.50 1[1450°C]

/

I

/

I

i

/ / ;l1400°C] / 0.0 lol---+---f-~:::::::__[1350

i

I °C]

I

·1 .I

I1

":'0.5 -1.00 - I/I / ....

,.,

.

/

11

/ /_.~ Ij I · 'V

1.'/

1 I~ I 1 . 1 1

/;

I

2 4 5 6 specimen number

Fig. 10. Change in length during soaking for 4 hours at indicated temperatures.

+1.50 _I_. /[14SOOq ~ I / c I / .~ +1.00 I ~ 1 I ,[1400°C] Cl. 1 _/_[1350_°_C] x / " ~ 1 +0.50 _I

/

'" I " c 1 _// 1

_7/

0.00' c I. 0

//

.;:; -0.50 ~ _c

I/j

".-ri

0 u ~ -1.00 ./' I· '"

_{/' /1'}

" .:!:: ,~ / / / . 1 " . 1 -1.50 .... / /'

.,.

/

I

. 1 . / 1 -2.00 / 1 ... I I I I 1 2 4 6 7 specimen number

(29)

quartz is added, but distributed evenly over the matrix. Further, owing to its more open texture the matrix is capable of disposing of the expan-sion internally. From no. 4 to no. 1 the shrinkage increases regularly at 1350 °C. Soaked at 1450 °C the shrinkage of no. 4 is considerable, and decreases fr om no. 4 to no. 3 and increases again from no. 3 to no. l.

The shrinkage of no. lis almast independent of the soaking temperature. c. Overall change in length

Shown in FIG. 11 is the change in length of the test cylinders af ter

having reached the maximum firing temperature and being soaked for 4 hours at that temperature. From the analysis in a and b of the change in length occurring during the heating up process and the subsequent soak, it is of course also possible to explain the curves of this figure. Remarkable is the fact that, almast independent of the firing temperature, the length of test cylinder no. 5 is practically identical to that of the unfired cylinder. c: .~ :;; "-x " c:

°

';:;

g

c: 8 +1.00 0.00 -1.00 -2.00 -3.00 I I I I I : 'I

I

I I / I .

--{

/

/ ....

1/

/ I.

//

_

.

.J1

;' . / ' I ... "" / I . I / I / ' I --- I 2 I I 4 5 I

i

/[1450oq

/

.

1

/ ...r1400o[1350oq q 6 7 specimen number

Fig. 12. Final change in length af ter firing up to and soaking at

indicated temperatures and cooling down to ambient temperature.

(30)

3.4 Change in length af ter cooling

Represented graphically in FIG. 12 are the percent permanent changes

in length which occur af ter the firing process. Later (in PARAGRAPH 4.4)

these permanent changes in length will be related to the change In

porosity and mineralogical composition due to the firing process.

(31)

CHAPTER 4

TOPOLOGY OF THE FlRED TEST CYLINDERS

4.1 Introduction

According to the topo-ceramic conception, which is the basis of this investigation, the important properties and behaviour of ceramic refrac-tories are primarily determined by the topology of their matrix. A pre-viously agreed arrangement is necessary as to what is considered to be part of the matrix and what not. It is now determined somewhat arbi-trarily that the following components will be considered to be part of the matrix:

a. crystalline phase

Not only the crystalline phases which were added to the original mixtures as grains with a diameter of less than 150 microns, but also the crystalline phases formed from these grains during the firing process, - for instance, the cristobalite out of the fine quartzite grains and the mullite formed from the fine corundum and Si02 - , will be considered to

be part of the matrix. The following will show that the quartzite grains with a diameter of more than 150 microns are cracked during the firing process. However, in this treatise these small quartzite fragments will not be considered to be part of the matrix as in general they still form coherent compacts.

b. amorphous phase

Considered to be part of the matrix are all the amorphous phases formed mainly from the day during the firing, the reaction products between the day and the quartzite grains, and also the amorphous phase in the chamotte grains with a diameter of less than 150 microns which were worked into the mixtures.

c. pores

In addition to the phases mentioned in a. and b., all the pores in the test mixtures are considered to belong to the matrix, except those in the test cylinders numbered 5 to 7 which are mentioned below. Thus, it is taken that the porosity of the grains with a diameter of more than 150 microns may be neglected as compared with the porosity of the interstitial mass. In view of the cracking of the coarse quartzite grains in the test pieces

(32)

TABLE 11

Mineralogical compositi

Firing temperature: 1350 °C Firing temperatu

Test

cylinder Corundum Quartz Cristo- Mullite Amorph. Corundum Quartz

no. balite phase (% wt) (% wt) (% wt) (% wt) (% wt) (% wt) (% wt) 1 a 17.9 27.3 - 22.9 19.5 17.9 27.3 b 15.5 15.0 2.5 24.0 43.0 14.0 13.0 c - 2.4 -12.3 + 2.5 +1.1 +23.5 - 3.9 -14.3 2 a 11.9 19.7

-

30.3 25.9 11.9 19.7 b 10.0 10.0 1.5 31.0 47.5 9.5 8.0 c - 1.9 - 9.7 +1.5 + 0.7 +21.6 - 2.4 -11.7 3 a 6.0 12.1 - 37.7 32.3 6.0 12.1 b 5.5 5.0 0.5 37.0 52.0 5.2 4.0 c - 0.5 - 7.1 + 0.5 - 0.7 +19.7 - 0.8 - 8.1 4 a - 4.5 - 45.2 38.6 - 4.5 b - - - 45.0 55.0 - -c - - 4.5 - - 0.2 +16.4 - - 4.5 I 5 a 6.0 12.2 - 37.7 32.2 6.0 12.2 _I b 4.5 6.0 1.0 40.0 48.5 4.3 5.1 c - 1.5 - 6.2 + 1.0 + 2.3 +16.3 - 1.7 - 7.1 6 a 11.9 19.8 - 30.2 25.8 11.9 19.8 b 9.0 13.0 1.5 35.0 41.5 8.0 12.0 c - 2.9 - 6.8 + 1.5 + 4.8 +15.7 - 3.9 - 7.8 7 a 17.9 27.5 - 22.7 19.4 17.9 27.5 b 13.0 22.0 1.5 27.5 36.0 11.0 20.5 c - 4.9 - 5.5 +1.5 + 4.8 +16.6 - 6.9 - 7.0

5 to 7 as mentioned in a., the above verdict concerning this series does not entirely hold good without any restictions.

4.2 Mineralogical composition of the fired test cylinders

The mineralogical composition of the test pieces as determined by X-ray analysis is given in the lines marked b of TABLE 11. In the lines

marked a the mineralogical composition is given of the unfired speci-men as shown in TABLE 9. Ey firing these products, chemical reactions

and changes in the mineralogical composition occur. The changes in the mineralogical composition of the test cylinders due to firing, are calcu-lated by comparing the values given in the lines marked a and b, and the results shown in the lines marked c.

(33)

~f the test pieces

i

400°C Firing temperature: 1450 °C

Cristo- Mullite Amorph. Corundum Quartz Cristo- Mullite Amorph.

balite phase balite phase

(% wt) (% wt) (% wt) (% wt) (% wt) (% wt) (% wt) (% wt) - 22.9 19.5 17.9 27.3 - 22.9 19.5 3.0 24.5 45.5 12.0 8.3 4.2 25.0 50.5 + 3.0 + 1.6 +26.0 - 5.9 --19.0 + 4.2 + 2.1 +31.0 - 30.3 25.9 11.9 19.7 - 30.3 25.9 1.7 30.8 50.0 8.2 4.0 2.0 31.0 54.8 + 1.7 + 0.5 +24.1 - 3.7 -15.7 + 2.0 + 0.7 +28.9

-

37.7 32.3 6.0 12.1 - 37.7 32.3 0.8 36.0 54.0 5.0 1.7 0.8 36.0 56.5 + 0.8 -1.7 +21.7 - 1.0 -10.4 + 0.8 - 1.7 +24.2 - 45.2 38.6 - 4.5 - 45.2 38.6 - 44.5 55.5 - - - 44.0 56.0 - - 0.7 +16.9 - - 4.5

-

- 1.2 +17.4 - 37.7 32.2 6.0 12.2 - 37.7 32.2 1.0 40.3 49.3 3.6 4.0 1.8 40.3 50.3 + 1.0 + 2.6 +17.1 - 2.4 - 8.2 + 1.8 + 2.6 +18.1 - 30.2 25.8 11.9 19.8 - 30.2 25.8 1.8 36.0 42.2 7.0 9.0 3.8 36.9 43.4 + 1.8 + 5.8 +16.4 - 4.9 -10.8 + 3.8 + 6.6 +17.6 - 22.7 19.4 17.9 27.5 - 22.7 19.4 2.5 29.0 37.0 10.0 16.0 5.5 30.5 38.0 + 2.5 + 6.3 +17.6 - 7.9 -11.5 + 5.5 + 7.8 +18.6

When the percentage of a certain mineral phase is higher in the fired than in the unfired state of the test piece, the difference in line c is given as plus, if the above percentage is lower the difference is given as minus. The weight percentages of each of the mineral constituents given in line c of TABLE 11, are represented graphically in the FIGS. 13.1, 13.2, 13.3,

13.4 and 13.5 as a function of the test number, the firing temperature being the parameter. The experimental results will be compared with the calculated mineralogical composition of the unfired products In

order to obtain information on the change of each of the individual constituents as a function of the original composition characterised by the test number and as a function of the firing temperature (PARAGRAPH 4.2.1). The change in each of the mineral constituents will be compared in order to obtain information on the various reactions that took place

(34)

during the firing process (PARAGRAPH 4.2.2). The mineralogical

compo-sition of the matrix will finally be calculated from the available data

(PARAGRAPH 4.2.3).

4.2.1 Change in the mineralogical composition due to firing

1. Quartz

The data on the percentages of quartz given in TABLE 11 have been

collected in TABLE 12.

TABLE 12

Weight percentage of quartz in the test materials

Test Unfired cylinder no. a 1 27.3 2 19.7 3 12.1 4 4.5 5 12.2 6 19.8 7 27.5

\

\.

\

r I 2 3 Fig. 13.1. 1350 o

e

b c 15.0 -12.3 10.0 - 9.7 5.0 - 7.1 0.0 - 4.5 6.0 - 6.2 13.0 - 6.8 22.0 - 5.5 4 5 6 7 specimen number Decrease in quartz. Firing temperature b 13.0 8.0 4.0 0.0 5.1 12.0 20.5 1400 o

e

1450°C c b c -14.3 8.3 -19.0 -11.7 4.0 -15.7 - 8.1 1.7 -10.4 - 4.5 0.0 - 4.5 - 7.1 4.0 - 8.2 - 7.8 9.0 -10.8 - 7.0 16.0 -11.5

The values c, being the dif-ference between the values a and b in TABLE 12 are

re-presented graphically in FIG.

13.1 as a function ofthe test number, the firing tempera-ture being the parameter.

From FIG. 13.1 we see

that due to firing the de-crease in quartz content is greater if:

1. more quartzite was used in the unfired test mate-rials;

2. this quartzite was added as fine grain (4 -+ 1); 3. the firing temperature

(35)

2. Cristobalite

T ABLE 11 showed that

in the unfired test materials no cristobalite occurred, hence this was formed du-ring the fidu-ring.

In FIG. 13.2 the

percen-tage of cristobalite is plotted as a function of the test number, the firing tempe-rature being the parameter.

"

E

_c: '" ::! '" 0.. 2 Fig. 13.2. .4 5 6 7 specimen number. Formation of cristobalite.

From FIG. 13.2 we see that more cristobalite was formed if:

1. more quartzite was used in the unfired products;

2. the quartzite was added as coarse grains and at the same time the firing temperature was higher (1450 0c). At lower firing temperatures the grain size of the added quartzite appears to be of little or no influence; 3. the firing temperature was higher.

3. Corundum

The data on the percentages of corundum given in T ABLE 11 are again shown in TABLE 13.

TABLE 13

Weight percentage of corundum in the test materials Test Firing temperature Unfired cylinder 1350

o

e

1400

o

e

1450

o

e

no. a b c b c b c 1 17.9 15.5 -2,4 14.0 -3.9 12.0 -5,9 2 11.9 10.0 -1.9 9.5 -2.4 8.2 -3.7 3 6.0 5.5 -0.5 5.2 -0.8 5.0 -1.0 4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5 6.0 4,5 -l,S 4.3 -1.7 3.6 -2.4 6 11.9 9.0 -2.9 8.0 -3.9 7.0 -4.9 7 17.9 13.0 -4.9 11.0 -6.9 10.0 -7.9

The difference between the values a and b of TABLE 13 are plotted in FIG. 13.3 as a function of the number of the test materiais, the firing

temperature being the parameter.

From FIG. 13.3 we see that due to firing the decrease in corundum content is greater if:

(36)

o 2 4. Mullite 1[1450°C] / ,[1400°C] . 1

/1

/ / [1350°C]

//

/~ 6 7 specimen number

Fig. 13.3. Decrease in corundum.

1. more corundum was used in the original mixtures; 2. this corundum was added

as fine grains (4 ~ 7); 3. the firing temperature

was higher.

Collected lil TABLE 14 are the data on mullite glVen lil TABLE 10.

TABLE 14

Weight percentage mullite in the test materials

Test Unfired cylinder 1350°C no. a b 1 22.9 24.0 2 30.3 31.0 3 37.7 37.0 4 45.2 45.0 5 37.7 40.0 6 30.2 35.0 7 22.7 27.5

The difference between the values a and b of TABLE 14 is plotted in FIG. 13.4 the firing temperature being the parameter.

From FIG. 13.4 we see that in no. 4 (which is the material containing only chamotte as non-plastic grain) the mullite content

Fig. 13.4. Increase in mullite. c

+1.1

+0.7 -0,7 -0.2 +2.3 +4.8 +4.8 Firing temperature 1400°C 1450°C b c b c 24.5 +1.6 25.0 +2.1 30.8 +0.5 31.0 +0.7 36.0 -1.7 36.0 -1.7 44.5 -0.7 44.0 -1.2 40.3 +2.6 40.3 +2.6 36.0 +5.8 36.8 +6.6 29.0 +6.3 30.5 +7.8 specimen number

(37)

has decreased very little af ter firing at 1350

oe,

the rate of the decrease being greater if the firing temperature is higher. The same tendency can be observed in no. 3 (which is a material where only a small per-centage of chamotte of the basis mixture was substituted by fine quartzite and coarse corundum. At higher firing temperature a decrease of the mullite is observed. As regards nos. 2 and 5 the firing temperature ap-pears to have had practically no influence on the mullite content.

In the other numbers we see that the mullite content increases if: 1. more corundum was used in the original mixtures;

2. the corundum was added as fine grains (4 -+ 7);

3. the firing temperature was higher. 5. Amorphous phase

The relevant. data of TABLE 11 are again shown together in TABLE 15. TABLE 15

W eight percentage amorphous phase in the test materials

Test Unfired Firing temperature cylinder 1350

o

e

1400

o

e

1450

o

e

no. a b c b c b c 1 19.5 43.0 +23.5 45.5 +26.0 50.5 +31.0 2 25.9 47,5 +21.6 50.0 +24.1 54.8 +29.8 3 32.3 52.0 +19.7 54.0 +21.7 56.5 +24.2 4 38.6 55.0 +16.4 55.5 +16.9 56.0 +17.4 5 32.2 48.5 +16.3 49.3 +17.1 50.3 +18.1 6 25.8 41.5 +15.7 42.2 +16.4 43.4 +17.6 7 19.4 36.0 +16.6 37.0 +17.6 38.0 +18.6

Plotted in FIG. 13.5 are the values a and b of TABLE 15, with the firing temperature being the parameter, and also the percent change due to firing (column marked c of TABLE 15).

From FIG. 13.5 we see that more amorphous phase was developed af ter firing if:

1. more fine quartzite was added to the original mixtures (4 -+ 1);

2. the firing temperature was higher.

In the test pieces 4 -+ 7 where apart from chamotte only coarse

quartz-ite and fine corundum was used in the mixtures, the percentage of formed amorphous phase appears to be independent of the mineralog-ical composition and to increase only little when the firing temperature

(38)

ti 6 ~ I 1 1 c " ~ ti a... _.-.~ / . ",,-- ~ / , / ' 1I '\~ ~

/

/ ~\\ / \~ fired 5 [1450°C]/ :'\ / \. /

"

/ ~~ / :-\. [1400;C] \~ / '\~ [1350·C] "~ \.

~

_,

/1\

/ \ / \ / \ / , / \ 2 [1450·C] / \ unfjred ... / ' 3 "' / \ ." I/ ' \ [1400·C] )\ \

...

.

/

.

\

\ [1350·è):'....

_,

\ \ ....

.

\ / .... , \ J \ / ' ,. I \ / \ \ '\..' develope~ ~t-:::::==.==

__ ;

I I I

,

J I I I 1 I I 2 4 5 6 7 specimen number

Fig. 13.5. Amorphous phase in the unfired specimen, in the fired specimen and developed during firing.

of the crystalline phases can be obtained both by X-ray and chemical analysis and by microscopie analysis of the test materials (the results

of the latter being discussed in PARAGRAPH 4.4.5); however, knowledge

of the amorphous phase cannot be gained in this way.

The "X-ray amorphous" phase will not only comprise the oxide

silicate glassy phase but also badly crystallized cristobalite and mullite.

This "X-ray amorphous" phase will therefore have a mineralogical and

chemical composition varying from place to place. The composition of

the formed glassy phase will be different depending on the fact whether

the melt was in direct contact with the various crystalline phases or not. The glassy phase of the original chamotte grains will again have a

dif-ferent chemical composition. The physical properties of the amorphous

(39)

nat-urally the above mentioned badly crystallized cristobalite and mullite crystals also having an influence. Further the physical properties will vary with the forces which bind the amorphous phase to a certain place in the mass. It is not only the chemical composition but more so the physical properties of this amorphous phase which are important to the behaviour of the refractories. To a certain extent the physical properties are deter-mined by the chemical composition. Although in principle it is possible to calculate by approximation the mean chemical composition of the total amorphous phase from the available data, it will, in view of the above, be clear that such a calculation offers little advantage.

Therefore in our further investigations we shall not take into account the physical properties of the amorphous phase of the test cylinders.

4.2.2 Relation between the mineral phases that were formed and those that went into reaction during firing

From the obtained information it is evident that the weight percen-tages of quartz and corundum decrease by firing. However, it is not known sufficiently into what mineral phases these minerals were transformed.

During the firing process quartz may be dissolved into the amorphous meIt - in which meIt mullite may dissolve or react with corundum to

~

..

. .c 0.. " o .c

e-o E

..

2 '3 03 .2 2 .1

_4--

₆ 10 '01 15 20 decrease In quartz [%] Fig. 14.1. • - 1350°C o - 1400°C • - 1450°C

41

(40)

!J

...

o

..

E J2 76 decrease in quartz rio] Fig. 14.2.

form mullite - or transform into the crystalline high temperature form of Si02, i.e. Cristobalite.

Corundum may dissolve into the liquid amorphous phase, as weU as form mullite with it.

In order to obtain more information about the influence of the firing temperature and the size of the quartzite and corundum grains on the course of these reactions and the mentioned modification change, the decrease in quartz af ter firing is plotted against the increase in amorphous phase (FIG. 14.1), the increase in muUite (FIG. 14.3) and the percentage of formed cristobalite (FIG. 14.2). The decrease in quartz is plotted against the decrease in corundum in FIG. 14.4. The decrease in corundum

is plotted against the increase in amorphous phase (FIG. 14.5) and the

increase in mullite in FIG. 14.6.

It may be gathered from the above that more quartz and corundum will be dissolved in the liquid amorphous phase, if the quartzite is added to the mixtures as small grains and the corundum as coarse grains, all

g 1 .~ " E .= ~

..

~ u .=

₇

5 10 15 20·

decrease in quartz [Yo]

Fig. 14.3.

(41)

3. !~_50 4' .5 o ~ E " "'0 r::; è 0 v .= ~

5

'" ~ _v

..

"'0 5 10 Fig. 14.4. • 5

•

7 .• 6 70 ~ 06 '7 5 10 decrease in corundum r/,] Fig. 14.5. 15 20 decrease in quartz r/,] 6 • 06 .6 .7

55/

o 6 .5 01 1 .1 2' 0 62 4. ' . / 2 40 . / 4 .. 3 o CA 33 5 10 decrease in corundum r/,] Fig. 14.6.

the more if the firing temperature is higher. If the quartzite grains are added to the mixtures as small grains and the corundum as coarse grains, less quartz will transform into cristobalite or react with corundum and form mullite, all the less if the firing temperature is lower.

4.2.3 Mineralogical composition of the matrix

The ca1culated mineralogical composition of the matrix of the fired test cylinders is shown in TABLE 16.

Topological considerations on some properties of refractory products