76-Microsilica-gel Bond Castables for Rapid Heat-up

8/11/2019 76-Microsilica-gel Bond Castables for Rapid Heat-up

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MICROSILICA-GEL BOND

CASTABLES FOR RAPID HEAT-

UP

,

,


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Microsilica-gel bond castables for rapid heat-up.

Bjrn Myhre*, (Elkem Silicon Materials, Norway)Abstract:Microsilica has traditionally been used as a reactive filler in refractory castables. Recent investigations have however

shown that for some of these applications, microsilica also act as binder, giving the desired setting by a coagulationbond (gelling). Normally that coagulation is effected by reaction between calcium ions from cement and microsilica

and the bonding is strong enough to be practically useable. This paper presents this bonding and gives examples of

results obtained under laboratory conditions. Among the very interesting properties of the gel-bond is the absence of

chemically bonded water, which yields castables that can be heated very quickly once they have been dried at low

temperature. Silica sol was also checked as a complimentary binder. The results show a positive effect on the dried

strength.

Introduction and Background

Figure 1: A view of the microsilica process from thestoking deck of a 27MW silicon furnace producing

Elkem Microsilica 971.

Microsilica:

The word microsilica has become generic for

condensed silica fumes produced as by-products of

silicon and ferro-silicon production. Microsilica, also

known as fumed silica, volatilized silica etc. normally

originates from the loss of SiO(g) which is an

intermediate species in the production of silicon or

ferrosilicon. It is difficult to state exactly when the

term microsilica first appeared, but in the early

1980s Elkem registered Elkem Microsilica as a trade

name for their fumed silica. It is probable that this is

the origin of the generic use of microsilica.

Silicon and ferrosilicon is produced in large electrical

smelting furnaces by the reduction of quartz (SiO2)

with carbon. The raw materials are added from the

top of furnace and form a stack. The actual reduction

to metal occurs in the bottom of the stack. Quarts and

the other raw materials are added as fairly large lumps

which heats up on their voyage down the stack. The

carbon may be added as a range of sources like coal,

coke or charcoal; which one that is used is normally

the result of economic and other considerations.Additional to the carbon, normally wood-chips are

added to improve the texture of the stack.

In the bottom of the furnace, where the temperature

exceeds 2000C, the reduction takes place through a

series of intermediate reactions producing carbon

monoxide together with other gaseous species. One of

these reactions produce the volatile species silicon

monoxide (SiO) that at temperatures above

approximately 1800C has a vapour pressure

exceeding 1 atmosphere. This gas which at that time

is under pressure, ejects upwards together with CO,

most of the SiO gets trapped as it condenses, but

some escapes and gets oxidized when it comes in

contact with the air above the stack. This is the origin

of microsilica. Of course, as a lot of energy (some 2/3

of that of Si-production) is used in the partial

reduction of quartz to silicon monoxide, this

represents a loss for the metal producer. Therefore

most metal producers aim for a minimum silica loss.

Nevertheless, some silicon monoxide will always

escape and give microsilica. Typically 10-15% of the

quarts ends up as microsilica under good furnace

operation.

It is not only silicon monoxide that leaves the furnace.

Also a number of impurities are volatilized. The

source of which are the raw materials. Typical volatile

impurities are oxides of alkalis. One of the moresignificant impurities is carbon of various sources.

The carbon of the microsilica is present as several

forms: coke residues, silicon carbide, some tar related

organics and finally as carbon black.

Above the top of the furnace, which essentially is an

open construction, a hood is placed and with the aid

of huge fans, the volatiles are sucked off and forced

through a filter. Because of the suction and because


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the furnace is top-charged, also larger particles like

wood-chips, coke residue and quarts dust are

collected together with the fumes.

Because of the introduction of foreign matter,

microsilica has to be beneficiated to give a high

quality product. In Figure 2, microsilica production is

schematically shown together with a view of the

Fiskaa plant in Norway: The raw materials are fed

into the top of the furnace (A) this is the stoking

deck. The volatiles are sucked off by a fan (D). To get

rid of the coarse particles, the microsilica is treated in

a pre-collector (C). The gas is then blown into a

baghouse filter and collected as raw microsilica.

Further treatment may be given to achieve desired

properties of the product. It should be mentioned that

other filtering equipment than baghouse filters have

been and are being used.

Figure 2: A view of the Fiskaa plant (Norway)together with a simplified flowsheet of the production

of microsilica. (A) Furnace, (B) Chimney, (C) Pre-

collection, (D) Fan, (E) Baghouse filter.

Properties of MicrosilicaIf we grab a handful of microsilica, the appearance is

a fine powder with variable color from almost black

to slightly off-white. There is a strong correlation

between the color and the carbon content. This

correlation is not straight forward though, since the

carbon may be present as coke residue or as silicon

carbide additional to carbon black which is the

impurity influencing color. A closer examination ofthe microsilica by electron microscopy reveals sub-

micron spheres with an average particle size of

approximately 0.15 micron. The surface area, as

determined by nitrogen adsorption (BET), typically

gives values around 20 m2/g which is equivalent to a

spherical diameter of 0.14 micron, i.e. close to the

results obtained by electron microscopy.

Chemically, microsilica consists of amorphous SiO2

with variable purity. Without going into detail, it may

be stated that there is a close relation between the

furnace operation and the quality of the microsilica.

Hence traditionally, the purity of the microsilica tends

to be rather variable, reflecting the furnace operation

at the time it was collected. As the metal normally is

the main product and microsilica the by-product, a

variation in the performance of the microsilica should

thus be expected. Here it should be mentioned that it

is possible to stabilize microsilica quality, but the

procedures required normally adds to cost (like

lowering Si-yield) thus justifying the higher price for

such controlled products (e.g. 971 from Elkem).

Figure 3: Micrograph of a microsilica cluster showing

spherical appearance of the individual particles.

The crystalinity of microsilica is variable according to

source and beneficiation, but values below 0.3 % by

weight 1 have been reported. Unpublished work on

high purity microsilica did however not show any

traces of crystalline silica. When crystallinity is

observed in microsilica, the source is normally quartzcarry-over from the furnace charging. In some special

cases with extended residence time at high

temperatures (> approx 800C for many hours) some

of the microsilica may crystallise as well. The

crystallisation can be accelerated by impurities,

notably alkalies. Such crystallisation should for high

quality microsilica be regarded as negligible though.

Microsilica higher in impurities e.g. alkali levels

above 1% might have more crystalline matter than

purer types.

As shown in Figure 3, microsilica consists of spheres.

These have an average diameter of about 0.15 micron.

The spheres are the building units of agglomeratesthat has been believed to be bonded together by

material bridges2. Improvements in PSD measuring

equipment have however given PSD curves (Figure 4)

that are in good agreement with the results from SEM

and BET surface measurements and the commonness

of those material bridges becomes questionable.


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Figure 4:Typical particle size distribu

dispersed microsilica as measured by la

Although microsilica is dispersed

spheres as seen in Figure 3 & 4, the no

castable is probably as a mixture of dis

and agglomerates. The more dispersed

gets, the better will the resulting flow

is present in sufficient amounts), partic

flowing castables. In free-flowing

increased amount of superfines (e.g. m

acts as flow-enhancers by reducingfriction between the larger aggregates.

The microsilica surfaceEven though the overall composition o

SiO2, the surface of a microsilica parti

siloxane bonds, but is also partially hy

hydrated. Producers of fumed silic

Evonik Degussa) have been very

characterisation and modification of th

in order to tailor specific properties

and fumed silica.

Figure 5: Visualization of the microsili

and with water.

Based on theoretical and experiment

seems that the maximum density of s

groups on the surface of silica is

4.6nm-2.

The surface of microsilica is also co

silanol groups, although the presence

plays some role, measurements have indensities ranging from approximately 2

microsilica from silicon productio

correlation between theoretical valu

found for microsilica may indicate that

of the microsilica are not smothere

surface of the microsilica particles

discrete particles or dissolved in

combination is probable.

tion of a well

ser diffraction.

as individual

rmal state in a

persed spheres

the microsilica

e (provided it

ularly for free-

castables an

icrosilica) also

inter-particle

f microsilica is

le is not plain

droxylated and

(particularly

active in the

silica surface

of precipitated

ca surface, dry

al results 3 , it

lanol (Si-OH)

approximately

ered by such

of impurities

dicated silanolto 4.5nm-2for

. The good

es and those

the impurities

d out on the

but rather as

the silica. A

The presence of the silanol

microsilica easy to disperse i

Depending on the pH, a fraction

dissociate (to H+ and O-) resul

charged surface. The zeta-potent

and values higher than approxim

or negative) are often taken as a

suspensions. Figure 6 shows a m

zeta potential for Elkem Microsi

that the microsilica has a negat

whole range. Due to dissociation

the negative surface charge incr

approximately pH 7. At higher

potential flattens out and at

microsilica starts to dissolve.

Figure 6: Zeta potential of Elkem

function of pH. A 10% microsili

HCl and with zeta potential be

electro-acoustic techniques (Ac

Dynamics)

It is the negative charge on

microsilica that is the property

bonding possible. Such negative

cation and if it is a polyvalent

two adjacent microsilica particle

three dimensions, a gel of

constructed. Figure 7 visualises

dimensions using Ca2+as the brid

Figure 7: Proposed gellin

microsilica-gel bond. Ca2+

microsilica particles.

-35

-30

-25

-20

-15

-10

-5

0

1 2 3 4 5

pH

Zeta-potential(mV)

O-H

O-HH-O

H-O

H-O

O-

O-

O-

O-

OH

O-

+Ca+Microsilica

+C

+Ca+

+Ca+

+Ca+

+Ca+

O-

O-H

H-O+

Microsilica

groups makes the

aqueous systems.

of the silanol groups

ting in a negatively

ial can be measured

tely 25mV (positive

indication of stable

asurement of such a

ica 971, and we see

ive charge over the

of the silanol groups,

ases with pH up to

pH than 7, the zeta

till higher pH the

Microsilica 971 as a

a slurry titrated with

ing measured using

ustosizer, Collodial

the surface of the

that makes the gel-

sites can react with a

ation it may bridge

s. If this happens in

that microsilica is

this process in two

ging cation.

mechanism for

gelling adjacent

6 7 8 9

O-H

O-H

O-H

O-

O-

O-

icrosilica

a+

+Ca+

a+


5/12

Castables based on this bonding concept was

presented as early as at UNITECR 1995 in Kyoto4,5.

At that occasion, also the accelerating effect of

combining small amounts of cement with Alphabond

was demonstrated (Table 1). It was not clear to the

authors what mechanisms was causing the setting, at

that time; it was just assumed that one was talking

about a hydraulic bond of the cement.

Table 1 Fused alumina based castables. Setting time

(days at 20C) for combinations of Alphabond and

cement. Alphabond content (mass%) in left column

and cement content (mass%) in first row.

Alphabond/Cement 0 0.25 0.50 1.00 2.00

0 10 5

0.50 1 1

1.00 18 1

2.00 13 1 1

3.00 4 1

5.00 2

At UNITECR 20116, also this time in Kyoto, a further

study on basically the same system as in 1995 was

presented. At this occasion the setting had been

studied by simultaneous zeta-potential, conductivity

and pH measurements showing that there is a

simultaneous setting combined with a sequestering of

ions in the liquid phase. Figure 8.

Figure 8, Zeta-potential, conductivity and pH of a

slurry mixture of 8parts microsilica, 0.5parts cement,

0.5parts Alphabond, 0.05parts deflocculant

(Castament FS20) and 4.15parts water as a function

of time.

Based on these results, the setting mechanism

proposed in Figure 7 was formulated. The

accelerating effect that had been seen for Alphabond

additions was explained by the Alphabond serving as

nuclei for precipitation of alumina from the liquid;

thus facilitating increased dissolution of Ca2+from the

cement. This in turn reacts with the negatively

charged microsilica surface causing a gel to form.

To test the effect of the setting on real castables,

some compositions based on white fused alumina

with 8wt% microsilcia in combination with different

combinations of cement a Alphabond, were prepared

with 4.1% water. Top size was 5mm and dispersant

was 0.05% Castament FS20. Results from flow decay

measurement s are shown in Figure 9.

Figure 9. Flow decay of combinations of cement and

Alphabond, WFA based castables with 8 wt%

microsilica and 4.1% water.

Clearly, by adding alphabond to the castable together

with 0.5% cement, setting is accelerated. A similar

effect can also be found by using a fixed amount of

Alphabond with the cement as variable. Increased

amounts of Alphabond tend to lower flow though.

Flow and Strength.It should not be surprising to expect that the result of

a bond consisting of a gelled microsilica should be

fairly low. But just what levels are sufficient for

demoulding and handling?

Braulio et al7indicates that an initial splitting tensile

strength higher than 1 MPa would be enough to avoid

crack generation during demoulding. 1MPa splitting

tensile strength is, depending on citation, equivalent

to between 1.3 to 1.4MPa M.O.R. In other words, any

M.O.R value above 1.5MPa should be sufficiently

strong.

Even though a lot of information (e.g.8and9) was

available on the hot-strength of castables based on themicrosilica-gel bond, we had surprisingly few results

from low temperatures. Therefore it was decided as

part of another investigation to measure flow and

green-strength of a chosen castable, the recipe is

given in Table 2.

As good results also were obtained with lower water

additions10such as 3.5% water, it was chosen to make

the mixes with both the custom 4.1, and with 3.5%

-60

-50

-40

-30

-20

-10

0

10

20

30

0.00 2.00 4.00 6.00 25.00 29.00

time[h]

[mS/cm]/[mV]

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

pH

conductivity [mS/cm]

Zeta-potential [mV]

pH value

0

40

80

120

160

200

0 2 4 6 8 10 12

Time [h]

Flow

-value

%

0.5%Cement + 0.0%Alphabond Free-flow 0.5%Cement + 0.0%Alphabond Vibra-flow




6/12

water. For the flow measurements, the flow cone

(height 50mm) as described in ASTM C 230 was used,

but since the European standard EN 1402-4 prescribes

the use of a taller cone (height 80mm) for self-flow

measurements, both self-flow using the ASTM cone

and the EN-cone was measured.

Table 2: White fused alumina castables withmicrosilica gel bond.

NCC microsilica-gel bond [weight %]Elkem Microsilica 971U : 8CAC cement, Secar 71 : 0,5Alphabond 300 : 0,5White Fused Alumina 3-5mm : 10White Fused Alumina 0,5-3 mm : 32White Fused Alumina 0-0,5mm : 16White Fused Alumina -74 mic : 20Calcined alumina,CT 9FG : 13Dispersant, Castament FS20 : 0.05

Figure 10 shows the flow values of the castable of

Table 2 with 3.5% and 4.1% water. It is remarkable

how high the self flow is even at 3.5% water, yielding

values that normally are found for significantly higher

water additions. Here it should be mentioned that to

obtain such high flow values, the microsilica quality

plays a decisive role. It is of uttermost importance

that the microsilica is easy to disperse and that the

mixture does not contain flocculating contaminants

like easily dissolvable salts, notably polyvalent.

Lowering the water addition creates one problem

connected to extended wet-out time. At very low

water additions the wet-out becomes difficult, if theresults of Fig. 10 are taken as an example, the wetout-

time increased from 20 to 45 seconds when water was

reduced from 4.1 to 3.5%. Even lower water additions

can be used, but then wetout-time becomes a

problematic issue. In such cases different strategies

can be sought like mixing parts of the mix with the

water and afterwards adding the rest of the dry mix

etc.

Some hours after the mixing, the castable sets but will

it be possible to demould the piece? Figure 11 and 12

shows the strength green after 24 hours and after

subsequent drying at 110C.

Figure 10: Flow of the white fused alumina based

NCC base on the microsilica-gel bond. Flow values

measured with 3.5 and 4.1% water.

Figure 11: WFA based microsilica-gel bonded

castables cast with 3.5 and 4.1% water. Strength

measured green (undried) after 24hours and after

subsequent drying at 110C/24h.

The green C-M.O.R. was found to be around 2MPa

for both water additions. Although low, it should

according to Braulio et al. be possible to demould.

Drying increases the strength significantly and herethe lower water pays off yielding significantly higher

strength.

87

128 124124

152

180

0

20

40

60

80

100

120

140

160

180

200

self-flow vibra-flow self-flow

ASTM cone ASTM cone EN cone

Flowvalue[%]

3.5% water

4.1% water

2.0

2.3

7.9

0

1

2

3

4

5

6

7

8

9

Green 20C Dried at 110C

Cold-M.O.R.[MPa]

4.15% water

3.5% water

5.3


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Figure 12: WFA based microsilica-gel bonded

castables cast with 3.5 and 4.1% water. Strength

measured green (undried) after 24hours and after

subsequent drying at 110C/24h.

Figure 12 shows the CCS of the samples again

demonstrating the good effect of drying.

Explosion resistanceAt IREFCON 20128Myhre and Fan presented results

on explosion resistance using the castable

composition of Table 2. They compared the results of

the microsilica-gel bond with results using a similar

LCC composition using 6% CA-cement. The

composition is given in Table 3.

Table 3 Low-cement castable based on white fused

alumina. 4.15% water for casting.

LCC CA-cement bond [weight %]Elkem Microsilica 971U : 8CAC cement, Secar 71 : 6White Fused Alumina 3-5mm : 10White Fused Alumina 0,5-3 mm : 32White Fused Alumina 0-0,5mm : 16White Fused Alumina -74 mic : 16Calcined alumina,CT 9FG : 12Dispersant, Castament FS20 : 0.05

Based on the recipes given in Table 2 and 3, a set of

NCC (microsilica-gel bond) and LCC castables were

cast with 4.15% water. Samples were cast into 50mm

cubes as described in the Chinese standard

YB/T4117-2003.

The standard describes placing green (not dried)

samples (cubes of 50mm) into a furnace heated to a

preset temperature and inspecting the sample after 30

minutes. The temperature at which cracks forms or

explosion occurs is then reported as explosion

resistance.

After 24 hours curing at 100% relative humidity and

20C, the demoulded samples were protected from

drying before being placed into the furnace for 30

minutes. After inspecting the samples for cracks etc.,

the temperature of the furnace was increased and a

new sample inserted into the hot furnace. This was

repeated until cracks or explosions were detected.

In Table 4, the results are shown. It became quickly

clear that the undried NCC exploded into tiny

fragments already at 350C, whilst the LCC was able

to take 600C before it split in two pieces. The low

explosion resistance of the NCC surprised us since we

were convinced that the NCC should take heat-up

very well due to the lack of bonded water. Could it be

that our assumptions of only minute amounts of

bonded water were incorrect? It was quickly

suggested that low strength also could be a reason for

the steam explosion of the NCC when tested

green/undried. To check, samples were dried at 110Cbefore testing. Testing of the dried samples gave a

very different result. The LCC split in two pieces at

600C as it did for the undried sample, but the NCC

was virtually indestructable even at temperatures as

high as 1200C!

Table 4: Results from explosion testing of the

castables shown in Table 2 and 3.

Temp.[C]Before drying

After drying at110C

LCC NCC LCC NCC

200

250

300

350

400

500

550

600

800

1000

1200

:sample passed the test, :sample did not pass.Reducing the casting water from 4.15 to 3.5% gives

benefits beyond those related to strength. It does

increase the explosion temperature. In our testing an

increase from 350 to 500C was experienced. Table

5 shows a comparison between the two.

12.4

39.4

16.8

62.2

0

10

20

30

40

50

60

70

Green 20C Dried at 110C

CCS[MP

a]

4.15% water

3.5% water


8/12

Table 5: Explosion resistance for NCC samples tested

green i.e. directly from the mould. Samples with

4.15 mass% and 3.5 mass% water for casting.

Temp. [C] NCC (microsilica-gel bond)4.15 mass%water

3.50 mass%water

300 350

400

500 :sample passed the test, :sample did not pass.

It is seen that drying at 110C does not affect the

explosion resistance of the LCC indicating that the

bond is not considerably changed during the drying.

The fact that it tolerates temperatures up to 600C

before failing the test is probably an effect of the

good strength of the castable. The improvement of the

NCC by lowering the water may also be similar, not

only is the amount of water less, but the strength is

improved as well.

The excellent results of the dried NCC was attributed

to the low amount of residual water in the bond after

drying. To substantiate this hypothesis, a series of

gravimetric experiments were undertaken and the

following figure, Figure 13, was presented at

IREFCON 20128

Figure 13: Weight loss of microsilica-gel bonded

NCC as a function of thermal treatment. Castable of

Table 2 cast with 4.15% water.

According to these results, some 89% of the water

disappears at 110C, leaving only 10.84% of theoriginal 4.15wt% water (0.45wt%). Somewhat

surprising, it was found that as much as 19% (0.78

out of 4.15%) had disappeared before drying at 110C.

It is assumed that most of this loss came during the

handling and weighing of the samples after

demoulding.

We felt that we should have a correlation with the

LCC in order to explain and verify our hypothesis.

Therefore some additional gravimetric investigations

were performed on the LCC composition using 4.15%

water. Instead of 800C a temperature of 600C was

chosen as firing temperature since firing at 1000C

did not give any further weight loss. Figure 14 shows

the results obtained on the LCC sample.

Figure 14 shows that the residual water was as high as

27% after drying at 110C. If this difference is tied up

in hydrates that both clog the pore structure and

liberates over a narrow temperature range, it may well

result in bursting of the sample at 600C.

Figure 14: Weight loss of LCC as a function of

thermal treatment. Castable of Table 3 cast with 4.15%

water.

Silica sol

After the patents (e.g. 11 ) of Magneco Metrel

concerning use of silica sol as binder in refractorycastables recently started to expire we have seen an

incrteased interest in using silica sol as binder. This

has to some extent puzzled us since basically we are

dealing with a variation of the bond system treated in

this paper, i. e. the microsilica-gel bond. The

difference is that a liquid silica sol (sometimes termed

colloidal silica) is made to gel by some gelling agent.

If used together with microsilica, it becomes very

difficult to distinguish between the effects of the

gelling of the sol or the microsilica. To check for

beneficial effects of silica sol additions, we set up a

program where parts of the microsilica was replaced

by silica sol. The silca sol was a silica of 130m2/g

dissolved in an aqueous liquid at 40weight%, while

compensating for the added liquid, so that silica

sol+microsilica and total water was kept constant at 8

and 4.1% respectively. It was expected that the

addition of the silica sol that was much finer than

microsilica, could complement on particle packing

and thus enhance flow. Alternatively a chance existed

that the IPS (interparticle separation), would become

too small so that the particles started to interact and

18.80%

70.36%

10.84%

Curing 20C Drying 110C Firing 800C

10 %

63 %

27 %

Curing 20C Drying 110C Firing 600C


9/12

thus influence flow negatively.

In Table 6, the experimental set up, the recipes, are

given, the compositions were made so that amount of

liquid and amount of superfine silica should be

constant at 4.1 and 8% respectively.

Table 6: Castables with silica sol. The compositions were made so that amount of liquid and amount of silica should

be constant at 4.1 and 8% respectively.

[weight %]Elkem Microsilica 971U 8 7.8 7.2 6.4 5.6CAC cement, Secar 71 0,5 0,5 0,5 0,5 0,5Alphabond 300 0,5 0,5 0,5 0,5 0,5White Fused Alumina 3-5mm 10 10 10 10 10White Fused Alumina 0,5-3 mm 32 32 32 32 32White Fused Alumina 0-0,5mm 16 16 16 16 16White Fused Alumina -74 micron 20 20 20 20 20Calcined alumina,CT 9FG 13 13 13 13 13Dispersant, Castament FS20 0.05 0.05 0.05 0.05 0.05

Silica sol: BindZil 40/130 % 0 0,5 2 4 6Water 4.1 3.8 2.9 1.7 0.5

Figure 15: Flow (ASTM cone) as a function of silica

sol addition. Castables with a total of 4.1% water and

8% silica according to recipes given in Table 6.

Figure 15 shows the flow values of the castables in

Table 6 as measured with the 50mm tall ASTM cone.

Up to 4% silica sol addition, the flow is unaffected,

from 4% on a decrease becomes obvious. The reason

for this is not obvious, but may be connected to the

high surface of the colloidal silica. At 4% silica sol

the colloidal silica has a surface area similar to 8-10%

microsilica. As a consequence, the IPS decreases,

possibly to a level where interactions start to become

noticeable. At what distance particle-particle

interaction starts to become problematic is difficult to

predict, but distances of 50-70nm has been suggested

in literature12..

If the water in our mix (i.e. that with 4% silica sol)

is evenly distributed on the silica surface, then rough

calculations without taking interparticle porosity into

consideration, yields a layer of approximate 11nm

thickness, - or an average particle-particle distance of

22nm. This is well below the 50-70nm suggested in10.

With addition of more silica surface, it was

considered probable that this could result in better

strength values due to lower IPS and more silanol

groups. Figure 16 shows the cold modulus of ruptureand the cold crushing strength as measured

immediately after demoulding. The samples had been

cured for 24 hours at close to 100% relative humidity

prior to the testing.

Opposite to the expectations, no positive effect of the

silica sol additions were seen in the green state. A

possible explanation could be that the Bindzil is

stabilised in such a way that it does not react with

Ca2+under alkaline conditions. The rather high pH of

9.2 13 of the Bindzil indicate that some sort of pH

adjustment has been performed on it. Such adjustment

could possibly render the silica surface unreactive

under the present coagulation conditions.


10/12

Figure 16:Green strength as a function

addition. Castables with a total of 4.1%

silica according to recipes given in Tab

After drying however, the picture iFigure 17, it is seen that both M

increases with the addition of silica

during the drying at 110C, the silica

bonding. The mechanism behind is diff

but it has been put forward14

that the

on the silica surfaces may condens

siloxane and water. This condensation i

Figure 17.

Figure 17:Dried strength as a function o

addition. Castables with a total of 4.1%

silica according to recipes given in Tab

f silica sol

water and 8%

e 6.

s changed. InR and CCS

ol. Somehow,

sol improves

cult to explain,

silanol groups

e to bonding

s illustrated in

f silica sol

water and 8%

e 6.

Figure18: Suggested12silanol co

drying of microsilica containing

Although probable, we do not h

this mechanism. Nevertheless

strengthening following th

strengthening seems to be depe

of silica sol so even if the sili

green-bodies, it contributes to hig

The overall conclusion may the

in the following way: If it is pos

silica sol additional to the dry-m

in green-strength is not a problegood idea to make castables

microsilica is supplemented with

resistance has not been tested, bu

similar results as the non-silica so

Hot strength

This paper is not intended to pre

hot-properties of this type of

figures taken from another sou

bond systems.

It is an old experience that iftogether with cement, often

refractoriness may suffer severel

comparison between the refrac

(R.U.L.) of three castables based

given in Table 2 and 3 with 8 ma

two LCC differ in the type of c

common 70% CAC the other

clearly seen that the reduction in

leads to improved refract

improvement comes for our

NCC with properties that can

temperatures up to 1800C. All

formation, a topic that has been tearlier 15 publications. Brief

microsilica, in absence of cement

and forms mullite. The situation

cement is present. A liquid is

composition close to 50 mass%

combination with 50 mass% mi

castable with 5 mass% micros

cement is tested, then 10 mas

1500C. The result is a catastr

densation during

LCC and NCC.

ve firm evidence of

we do observe

e drying. This

dent on the amount

a sol gives weaker

her dried strength.

efore be formulated

ible to have a liquid

ix, and the lowering

m, then it may be ahere some of the

silica sol. Explosion

t is expected to yield

l compositions.

sent much about the

astable, just a few

ce10 with the same

microsilica is usedmass% of each,

. Figure 19 shows a

toriness under load

on the compositons

ss% microsilica. The

ment used, one is a

an 84% CAC. It is

CaO (84% cement)

riness. The big

icrosilica-gel bond

ake it applicable at

his is due to mullite

reated extensively iny described, the

, reacts with alumina

is quite different if

formed that has a

70% CA-cement in

rosilica. So if e.g. a

ilica and 5 mass%

s% liquid forms at

ophic failure of the


11/12

castable when temperature reaches 1500C This may

be seen in Figure 9 as the almost vertical drop at

1500C.

Fig. 19: Effect of cement on RUL for WFA based

castables with 8 mass% microsilica. LCC has6 mass% cement.

Compared to microsilica free castables, the

microsilica-gel bond shows similar refractoriness,

Figure 20. When using the microsilica-gel bond it is

possible to obtain a refractoriness equal to or even

above the microsilica-free LCC alternative.

Fig. 20: Comparison of RUL for WFA based

microsilica-gel bond NCC (0.5 mass% cement) with 8

mass% microsilica, LCC (6 mass% cement) with 8mass% microsilica and LCC (6 mass% cement)

without microsilica

Conclusion:

For castables are based on gelation of microsilica as

bond system, several attractive properties have been

identified. These are; very good placing properties

combined with high refractoriness and hot strength.

The green strength is low but sufficient, and the dried

strength is good. Further, the bond contains only

small amounts of bonded water, so once the free

water is removed, the castable tolerates very high

heating rates. If silica sol is used complementary to

the microsilica, improved strength was seen for the

dried samples. The green strength did not benefit

however.

In general, the gel-bond offer attractive possibilities

for advanced refractories, opportunities that may be

difficult to match with alternative bond-systems.

References:

1K. Heggestad, J.L. Holm, K. Lnvik and B.

Sandberg, "Investigations of Elkem Microsilica by

Thermosonimetry", Thermochimica Acta, 72 (1984),

205-2122E.Dingsyr, M. Dstl and C. Wedberg, in "Preprint

of the Fifth European Symposium Particle

Characterization, 24-26 March 1992, Nrnberg,

Germany. publ. by Nrnberg Messe GMBH3 R.K. Iler, The chemistry of silica, John Wiley &

Sons, New York 1979.4B. Myhre and K. Sunde, "Alumina based castables

with very low contents of hydraulic compound. Part I:

The effect of binder and particle-size distribution on

flow and set.", Proc. UNITECR95, Kyoto, Japan,

Nov. 19-22 1995, p. II/309-165B. Myhre and K. Sunde, "Alumina based castables

with very low contents of hydraulic compound. PartII." Strength and High-Temperature Reactions of No-

cement Castables with Hydraulic Alumina and

Microsilica, Proc. UNITECR95, Kyoto, Japan,

Nov. 19-22 1995, p. II/317-246B. Myhre and H. Fan, Gel Bonded CastablesBased on Microsilica as Binder, proc.

UNITECR20117 M. A. L. Braulio*, V. C. Pandolfelli and C. Tontrup,

Colloidal Alumina as a Novel Refractory Castable

Binder proc. 53rd Int. Coll. on Ref. Aachen 2010 p.

111-1148B. Myhre and Aase M. Hundere: Substitution of

Reactive Alumina with Microsilica in Low Cement

and Ultra Low Cement Castables. Part I: Properties

Related to Installation and Demoulding Proc.

UNITECR97, New Orleans, USA, Nov. 4-7 1997, p.

43-529Aase M. Hundere and B. Myhre: Substitution of

Reactive Alumina with Microsilica in Low Cement

and Ultra Low Cement Castables. Part II: The Effect

of Temperature on Hot Properties Proc.

-2

-1

0

1

2

0 500 1000 1500 2000

Temperature [C]

Expansion[%]

LCC (70% CAC)

LCC (84% CAC)

0.5% (70%CAC)

-2

-1

0

1

2

0 500 1000 1500 2000

Temperature [C]

Expansion[%]

0.5% cement 8% MS6% cement 8% MS6% cement , 0% MS


12/12

UNITECR97, New Orleans, USA, Nov. 4-7 1997, p.

91-10010Bjrn Myhre and Haibing Fan, Microsilica-gel

bond for explosion proof castables proc. IREFCON

2012, Kolkata 2012 p 71-7611US pat. 5147830 Composition and method for

manufacturing steel-containment equipment

Subrata Banerjee et al12J.E. Funk and D. R. Dinger:"Particle Size Control

for High-Solids Castable Refractories", Am. Ceram.

Soc. Bull.73[10],66-69, (1994)13ALBIN KLINT, Amphiphilic surface modification

of colloidal silica sols, Master of Science Thesis in

the Master Degree Programme Chemistry and

Bioscience, Department of Chemical and Biological

Engineering, Division of Applied Surface Chemistry

CHALMERS UNIVERSITY OF TECHNOLOGY,

SWEDEN14Li Zaigeng et al., Phase Compositions and Setting

Mechanisms of Low Cement, Ultra-low Cement andCement-free Castables in Proc. 2nd Int. Symp on

Refr. Beijing China, Oct. 30 - Nov. 2 1992, p.

540-54715B. Myhre: Lets Make a Mullite Matrix! Ref.

Appl. and News, vol 13, No 6, 2008

76-Microsilica-gel Bond Castables for Rapid Heat-up

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