Calcium Aluminate Cements for Refractory Gunning Applications

7/28/2019 Calcium Aluminate Cements for Refractory Gunning Applications

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CALCIUM ALUMINATE CEMENTS FOR REFRACTORY GUNNING

APPLICATIONS

by Christopher Parr, Catherine Revais, Thomas A. Bier

presented at The Third International Symposium on Refractories, Beijing, China; 1998.


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Abstract

This paper will present the results of an investigation in the application of CalciumAluminate Cements (CAC) for refractory gunning products to determine the relationshipsbetween cement behaviour and gunning performance.

Three CAC's are evaluated in a variety of model formulations. Properties studied includegunning parameters and characteristics along with the resultant properties of gunnedsamples. A parallel laboratory investigation into the cement characteristics is presentedand conclusions are drawn between cement characteristics and actual gunningperformance.


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1 Introduction

The development of monolithic refractories based

upon calcium aluminate bonding systems can be

considered to have followed two axes in recent

years1,2

. The first being an improvement in

ultimate performance through the application of

lower cement content deflocculated castables.

This however was largely at the expensive of

installation flexibility and often resulted inproducts which had a high degree of installation

sensitivity. The second and more recent

development axe has been the move towards

products with improved installation performance

whilst still retaining the inherent performance

advantages realised by the first evolution.

This development has been related to both

casting installation techniques3,4

as well as

gunning techniques5,6

. More recent developments

with gunning installations have focussed upon

both the wet or semi-wet gunning techniques7

as

well as the more traditional dry gunning

technique8

. It is clear that9

further development

and optimisation of these products will continue.

However the dry gunning technique still offers

many inherent advantages8

such as its simplicity,

quick set up time and ease of use. This is

evidenced today by the relatively large quantities

of product still installed by this technique in a wide

variety of application areas.

The disadvantages of the dry technique are well

known and are primarily related to dusting and

rebound losses. This paper investigates these drygunning mixes and attempts to identify some of

the parameters that influence the gunning

performance of calcium aluminate based

compositions. The objective is to optimise the

levers which lead to mixes with low rebound and

dusting whilst still retaining good physical

properties in the as gunned state.

2 Experimental approach

Materials used

The program was designed to evaluate the

behaviour of three calcium aluminate cement

types along with a selection of additives which

would function as rheology modifiers in model

formulations based upon a chamotte aggregate.

The chemical composition of the main raw

materials used are shown in Table 1. From thesebase materials model formulations were

constructed (Table 2). A small clay addition was

made to these base formulations to provide

sufficient plasticity. Three additive types were

considered, an increased clay addition, Lithium

Carbonate and R1001 a calcium aluminate with a

reactive mineral phase. For each of these base

formulations three different cements were used; a

50% alumina cement Secar 51BTF, a 70%

alumina cement Secar 71 and an 80% alumina

cement Secar 80. The particle size distributions

were optimised using the Dinger and Funk

model10 with a distribution modulus of 0,2. This

value was chosen as previous experimental

evidence had shown this to be optimum for

gunning performance. The actual particle size

distribution achieved compared to the target is

shown in Graph 1.


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Table 1. Chemical composition of raw materials : %

Al2O3 SiO2 Fe2O3 TiO2 CaO MgO LOI

Chamotte 41,00 54,20 1,70 1,70 0,30 0,30 -

Clay 25,00 62,30 1,00 1,40 0,10 0,30 7,40

Secar

51BTF 54,01 4,95 0,75 2,48 36,68 0,36 0,31

Secar

71 70,60 0,18 0,12 - 27,91 0,15 0,10

Secar

80 81,36 0,20 0,08 - 17,00 0,14 1,10

R1001 >41


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Figure 1. Arrangement of gunning equipment

For each test a total of 250 kg of dry material

was gunned on to a vertically suspended board

composed of marine plywood; into which 150 mm

nails ware placed on 200 mm centres to act as

support for the gunned mass. Measured gunning

rates ranged from 2 to 3 t/hr.

Water adjustment at the nozzle was optimised for

each test. After each run the rebound was

collected and weighed. The rebound was

calculated as a percentage of the total mass

gunned with a correction made for water content

in the gunned mass and the non adhered material.

A thermocouple was inserted into the dry mass to

record the exothermic profile. The procedure was

based on the same method already published by

Lafarge11

over 10 years ago. At the same time

the development of hardness was measured

using a penetration needle (Maynadier) whichgave the resistance to penetration (kg) with the

passage of time. After 24 hours the hardened

samples were cored and tested for mechanical

properties. Parallel tests were conducted on

samples of the dry mix cast into samples in the

laboratory.

3 Results

Gunning characteristics

The amount of dust generated during gunning was

very low for all cases and no real distinctions

between the various mixes could be seen. In

contrast significant differences were observed in

terms of gunning behaviour as measured by the

water added and the resultant measured rebound.

The amount of fine material which dripped from

the nozzle (nozzle slop) was also difficult todifferentiate between each formulation type and in

all cases can be considered as low.

The water added at the nozzle was adjusted for

each formulation/cement batch and optimised for

minimal rebound. The base formulations labelled

G1 with Secar 51BTF, Secar 71 and Secar 80

with 2% of clay were not successful. In all cases

the adhesion was insufficient and either during

the gunning test or immediately after the gunned

mass fell to the ground. Therefore the results of

these formulations have not been included.

10

12

14

16

18

20

22

G2 G3 G4 G5

Formulation

Totalwaterdemand%

S51BTF

S71

S80

Graph 2. Total added water

Air inletGunned mass

Marine board

Data logger f(tc)

Pressure gauge

W at er Sca le

MaterialPump

35 mm

300 mm1000 mm


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0

5

10

15

20

25

30

35

G2 G3 G4 G5

Formulation

Rebound%

S51BTF

S71

S80

Graph 3. Total measured rebound

Graphs 2 and 3 show a summary of the results

for the other formulations for each cement type

tested. Each line represents a cement type.

Graph 2 shows the total measured amount of

water added expressed as a percentage of the

total dry mass gunned (including rebound). With

the exception of formulation 2 and Secar 80 the

total added water varied from 12% to 14%. The

amount of rebound generated (Graph 3) varied

according to formulation and cement type with

values ranging from 10% to over 25% being

recorded. The ability of each formulation to

reduce rebound depended upon each cement

type. Generally Secar 71 based formulations

yielded the lowest rebound figures with values

around 10% being recorded. For Secar 51BTF

and Secar 71 the formulation G2 based upon a

clay addition yielded the lowest figures. With

Secar 80 formulations G4 and G5 based upondifferent additions of the reactive mineral phase

yielded lower values but these were higher (20%

vs. 10-15%) than the lowest values for Secar

51BTF and Secar 71. The highly accelerated

formulations based upon an addition of lithium

carbonate showed intermediate behaviour.

Gunned properties

The penetration measurements were not possible

for the highly accelerated formulation (G3) due to

the rapid hardening characteristics.

The limit of measurement (40kg) was reached

with the first test. This was also the case for the

formulations G2 based upon Secar 80.

Formulations G3 and G4 for all cement types

generally gave similar results. Selective results of

the penetration needle tests are presented in

Graph 4 for Secar 51BTF and Secar 71. As

seen in the graph the addition of the reactive

mineral phase (G4) with Secar 51BTF results in

a more rapid and greater development of

penetration resistance than with the formulation

based on a clay addition alone. However the

reverse is true for the formulations based upon

Secar 71. The rebound values are also shown

for each formulation and no apparent correlation

is found between the development of penetration

resistance and rebound generated.

0,0

10,0

20,0

30,0

0 20 40 60 80 100 120 140

Time (mins)

Kg

G2 S51G4 S51G2 S71G4 S71Rebound 16,2%

Rebound 18,5%

Rebound 15,3%

Rebound 11,6%

Graph 4. Penetration resistance of gunned samples

The exothermic profiles were recorded for boththe gunned samples and laboratory prepared

samples. The laboratory samples were

maintained with an initial ambient temperature of

20C and enclosed in an insulated cell whereas

the field samples were subjected to ambient

temperature conditions of 7-12C and no

insulation was possible. As a result the field

samples reached a maximum temperature of

some 10 to 20C lower than the insulated

laboratory samples. There was generally good


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agreement between the two methods for the time

taken to reach maximum temperature. The results

of the field tests are shown in Table 3 with a

graphic illustration for the formulations based

upon Secar 51BTF in Graph 5.


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Table 3. Exothermic profile data

Start of heat

generationTime to max. temp Max temp

Mins Mins C

Secar

51BTF G2 140 300 34,6

Secar

51BTF G3 10 108 36,1

Secar

51BTF G4 110 258 47,5

Secar

51BTF G5 110 258 45,4

Secar

71 G2 140 240 44,5

Secar

71 G3


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0

20

40

60

80

25h 110C 800C 1100C

Treatment temperature C

CCS:Mpa G1

G2

G3

G4

Graph 7. Cold crushing strength for Secar

71

formulations

0,0

20,0

40,0

60,0

80,0

25h 110C 800C 1100C

Treatment temperature C

CCS

:Mpa G1

G2

G3

G4

Graph 8. Cold crushing strength for Secar 80formulations

All samples showed the classic changes in

mechanical strength that result when conventional

castable/gunning systems are fired to 1100C.

That is, a decrease in strength during

dehydration. In most cases the addition of the

reactive mineral phase resulted in improved

mechanical performance, although the differ-

ences after dehydration i.e. at 800 to 1100C are

much less than after drying. The differences were

also much less evident with Secar

80 basedsystems.

The differences between the various cement

types can be related to the differences in CaO%

content in the formulations and the hydraulic

potential of each cement type. It is interesting to

note that both Secar 71 and Secar 80 show a

lower reduction in strength as fired temperature

increases than the Secar 51BTF based

formulations.

The results from the laboratory prepared samples

(not shown) had higher values after 25 hours as a

result of more ideal curing conditions in the

laboratory, otherwise the results were

comparable. The density after 110C of both

sample types were similar, indicating similar

compaction by vibration installation in the

laboratory and gunning application in the site

tests. Measured densities were all very similar

ranging from 2000kg/m3

to 2200 kg/m3

for

samples after drying at 110C. After firing at1100C measured densities ranged from 1900

kg/m3

to 2000-2200 kg/m3

.

The permanent volume changes were measured

from the cored samples to assess whether

changing additive type had a significant effect

upon volume change. The range measured was

from 110C to after firing at 1100C. Previous

results8

had suggested that increasing the clay

addition was detrimental. The results in Graph 9

show that in all cases a volume shrinkage was

recorded but that the increased clay addition

(formulations G2) did not necessarily lead to a

high volume shrinkage. The highest volume

shrinkage was observed for the G3 formulations

with lithium carbonate.

0,0

1,0

2,0

3,0

4,0

5,0

G2 G3 G4 G5 G2 G3 G4 G5 G2 G3 G4 G5Perman

entVolumeshrinkage%

Secar 51BTF Secar 71 Secar 80

Graph 9. Permanent volume change 110C to 1100C


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4 Discussion

The installed performance of gunned materials

depends to a large degree upon the gunning

quality during installation. It has been suggested12

that the gunning quality in turn is affected by both

the gunning technique and the material design.

Factors such as rebound and adherence (Graphs

2 and 3) can be measured after gunning to assesthe overall gunning quality but it is much more

difficult to identify and isolate the individual

factors within the gunning technique and the

material design that contribute to the installed

quality. It must also be remembered that

optimising an installation parameter such as

rebound or adhesion might impact negatively upon

the installed mechanical performance. Also the

degree of adhesion and rebound are not totally

independent.

A model has been developed5

which considers

the gunned mass to be in a constant state of

evolution from a relatively fluid surface to a moreviscous plastic phase behind the gunned surface.

These layers change as the gunned layer

becomes thicker. It is the balance of these layers

that determine rebound and adhesion. Too slow,

and the evolution into the plastic phase and

subsequent stiffening will cause slumping or even

falling. For example, the formulations G1 which

adhered initially but subsequently fell due to

insufficient stiffening within the gunned mass. Too

rapid stiffening will increase rebound as the yield

stress increases and thus incoming particles

would need more kinetic energy to penetrate thegunned surface. This speed of evolution of the

rheology phases is linked to material design and

the total water added.

The causes of rebound have been extensively

studied within the civil engineering sector13

. The

key individual factors have been suggested14

to

be the added water and gunning technique such

as nozzle angle and distance along with material

design factors. Particle size distribution, and

rheology modifying fine components which actupon the yield stress are shown to be important

elements in material design. The mechanism

proposed is that impacting particles have an

energy of rebound which must be compensated

by the adhesion of the particle to the gunned layer

to ensure embedment of the particle. The energy

was shown to be affected by factors such as

water addition and cement content.

Thus it is clear from the above models that the

additional water will play a key role. An example is

shown in Graph 10 for the formulations based on

Secar 51BTF where a range of preliminary tests

were conducted and the effect of water variation

seen.


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R2

= 0,333

0

10

20

30

40

9 10 11 12 13 14

% water added

CCS24hours:Mpa

2R = 0,8115

0

10

20

30

40

8 9 10 11 12 13 14

% water added

Rebound%

Graph 10. Rebound and C.C.S. as a function of added water for Secar

51BTF formulations

The effect of additional water has a more

dramatic effect upon the level of rebound than the

mechanical resistance with the range of water

tested. The type of additive probably has a moresignificant effect upon the mechanical strength

than the water content within the narrow range

tested here.

Within the formulations tested two basic types of

additives were assessed. The first group are the

systems based upon lithium carbonate

(formulations G3) which functions as a strong

accelerator15

. The second system comprising of

the clay and the reactive mineral have actions

closer to yield stress modification. With these

latter systems too high a yield stress will increaserebound as more energy will be required to

penetrate the gunned surface. In consequence

the particle will have more rebound energy and

more adhesion will be required to ensure that it

embeds. The penetration tests conducted

immediately after gunning with the penetration

needle can be considered as a crude measure of

the yield stress. Graph 11 shows this relationship

for the formulations type G2, G4 and G5.

It is evidenced in this graph that a relationship

exists between initial penetration value and the

measured rebound. No similar relationship has

been found for the highly accelerated systemsbased on lithium carbonate and no such model

exists which can fully explain all the rebound

figures shown in Graphs 2 and 3. One of the

problems is certainly related to the speed of

action with these highly accelerated systems as it

is difficult to measure the actual yield stress

values at the point of gunning. Afterwards is too

late as the stiffening has already occurred and no

useful measurements can be made.


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R2 = 0,6331

0

10

20

30

40

10 15 20 25 30

Measured Rebound %

InitialpenetrationresistanceKg

Graph 11. Measured rebound as a function of initialpenetration resistance

The results of the setting time test as measured

by the exothermic profiles showed no correlation

with the gunning performance with the quickest

setting systems not necessarily having the lowest

rebound figures.

It is clear from the results shown in Graphs 2 to

10 that the interactions between each additiveand cement type are not uniform. Therefore no

universal solutions exist and optimum solutions

must be found for each cement type.

For example, from the results presented, Secar51BTF and Secar 71 can be considered to be

the easiest cements to use in gunning

applications with good gunning performances

along with excellent mechanical strengths being

observed. These two cements gave good

performance in simple formulations with only a

single clay addition.

Secar 80 showed the best results when used in

combination with clay and the reactive mineral

product. This is probably related to the fact that

this cement is specifically designed for casting

applications while the other products can be

considered to be more multi-purpose and

therefore easier to apply within gunning

applications.


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5 Conclusions

It appears as if the combination of cement type

and additive along with the resultant water

addition is critical in determining gunning

performance as measured by the material

rebound and adherence characteristics.

It can be seen from the results presented that

conventional gunning formulations can beoptimised to yield low rebound products with good

installed properties.

The exact nature of optimisation will depend upon

the calcium aluminate cement type being used.

The use of a multiple additive could profitably be

employed such as combinations of clay and

lithium carbonate to give low rebound whilst

assuring excellent adhesion characteristics.

The application of Secar 51BTF and Secar 71

in gunning products results in particularly good

installation properties whilst maintaining excellent

installed properties.

Further work will concentrate on identifying the

individual levers responsible for gunning

behaviour as well as investigating the effect of

reducing the cement content formulations.

5 Acknowledgments

The authors would like to thank all the co workers

at who contributed to the studies that led to thispaper.

7 References

1Bier Th; Bunt N.E; Parr C; Calcium Aluminate

bonded castables their advantages and

applications; Alafar proceedings; 1996.

2Valdelivre B, Parr C, Bier Th; Medium

cement castables; Alafar proceedings ; 1997.3

Bier Th, Mathieu A, Parr C; "Calcium

Aluminates in self flowing castables"; Silicate

Society conference; Prague 1996.4

Mathieu A; "Calcium Aluminates in self flowing

castables"; Brazilian Ceramic association

Aguas de Lindoia; Brazil ; June 1995.5

Landman et al; The rehabilitation of gunning

refractories; 2nd

. International conference on

refractories; Japan; 1987.6

Biever et al; The characterisation of reduced

cement, high strength abrasion resistant

gunning castable refractories; Anaheim;

Unitecr 1989.7

Tabata et al; Application of high density

gunning mix for repair of Torpedo Ladle;

p110-p113 Aachen conference proceedings

1988.8

Fisher R.E.; Critical issues for successful

performance; St. Louis section meeting;

American ceramic society, 1996.9

W.G. Allen; "Advanced equipment systems forrefractory placement"; UNITECR proceedings

1997.10

Dinger D and Funk J; Predictive process

control of crowded particulate suspensions

Kluwer academic publishers; USA; 1994.11

Fentiman G; Montgomery R; The heat

evolution test for setting time of cements and

castables; Advances in ceramics; 13 ; 198512

Franz Petio ; Top quality gunning mixes for

BOF; UNITECR proceedings 1997.13

Armelin et al; Rebound in Dry-Mix Shotcrete;

Concrete international; September 1997 p54-p60.14

Armelin et al; Mechanics of aggregate

rebound in Shotcrete (Part 1); Materials and

Structures; Vol. 31, March 1998; p91-p98.15

Th. A. Bier; "Admixtures and their interactions

with high range calcium aluminate cement";

UNITECR 1995.

Calcium Aluminate Cements for Refractory Gunning Applications

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