New Additive Packages for Self-Flowing High-Alumina and MgO Based Refractory Castables

New Additive Packages for Self-flowing High-alumina and MgO Based Refractory Castables* Hong Peng, Ming Luo, Bjorn Myhre

c.,)elt-tlowing refractory castables with special performance are continuously being developed. New and

improved dispersant systems play an essential role in this development. The objective of this paper is to highlight the benefits of two novel admixture systems that have been tested in various refractory cast-

ables.

sioxX/sioxX-Quick is a specialty additive package developed for use in microsilica containing alumina based castable systems. The main function of SioxX is to control flow properties whereas SioxX-Quick controls setting characteristics. ln combination they will enable the castable producer to control flow and set time accurately. When sioxX replaces sodium hexametaphosphate, self-flow is improved by 20 to 30 % and shelf-life of dry premixes is significantry improved.

SioxX-A/ag is a new additive package being developed for basic refractory castables. lt contributes to better workability and setting behavior of Mgo based castables while hoi-properties remain unchanged. High performance, cement-free Mgo castables based on Mgo-Sior-Hro bond using sioxX-lVlag as dispersant have been tested. using sioxX-Mag together with microsilLa,-slaking causJd by brucite formation is suppressed and crack-free dried samples are made. Another benefit of using this additive package is the improved drying characteristics allowing fast firing of the castable.

tion between MgO fines, microsilica (SiOr)

Í lntroduction

and water

Characterjstrcs of advanced self-flowing refractory castables are ease of installation,

possibility to cast intricate shapes, energy savings and better performance in general. lmportant factors to control in these castables incÍude particle size distribution (P5D), additives (deflocculants, dispersants, accelerators) [1,2] and the raw materials, particu-

larly with respect to the super-fines and cement [3,4]. Numerous recent research pro-

grams have focused on understanding the mechanism of dispersion and optimization

of dispersants íor specific castables [5-8]. No doubt, dispersants continue to play an essential role in further develoijment of advanced reíractories. For state-of-the-art steel-making and clean steel production high-per{ormance basic refractory castables are

of particular import-

in 1 989, Elken Materials started development work on a new binder sysance. Already

tem for basic castables based on the reac-

refractories woRLDFoRUM s

[9]. This bond system was first applied to magnesia, silicon nitride and mag-

Hong Peng, Bjorn Myhre

Elken Silicon Matenals

nesra-carbon castables and required low water addition (5,0-5,5 %) resulting in adequate mechanical properties. An important

4675 Kristiansand Norway

observation was that at 6 % microsilica add-

Ming Luo

ttion no slaking occurred. Since then, signiÍicant research on the use of microsrlica in basic castables has been run 110-131, sel!

ílowlng cement-free MgO_5iO,

Wuhan University of Science and Technology 43008 I Wuhan China

bonded

castables can be produced. About 6 mass-% microsilica seems to be essential to obtain

both good placement and hot properties. A surprising observatron is that the slag resistance to both BOF and EAF slag compared to an alumina spinel castable is improved [1,1]. Recently, many research programs have focused on hydration mechanism of magneíra

with water and additives (dispersants and

Corresponding author: Hong peng E'mail: [email protected]

Keywords: self-flowing, new dispersantl Figh-alumina and basic reÍractory castables, microsilica gel-bonding

Received: 21.11.2012 Accepted: 21.12.2012

retarders) in systems such as MgO-SiOr-HrO,

*

MgO-Al,0r-H,0 and Mg0-SiO,-Al2O3-H2O

CancÚn/MX in Nov' 2012 and received the

|+_ll1' However, self_ílowing magnesia castables have not been widely used until (20j3) I2l

The paper was presented at ALAFAR

in

3,r prize of the ALAFAR Award 20 j 2

99

Tab. 1 Composition of Si€ based LCC [mass-%]

Elkem Microsilica 97I U

ence íor comparison with 5ioxX/SioxX-Quick

Ref-l

sx-l

sxi sx-Q-l

10

8,1

I

mixes.

To investigate the effect of humidity

on

shelí-life, dry-mixed 25 kg samples packed

6

6

6

in closed, plastic lined paper bags

SiC

12

12

1?

stored in a temperature and humidity con-

Calcined alumina

13

12,3

12

Cernenl

SHMP

trolled room ("cllmate room"

were

with

70-80 % relative humidity at 20 "C), These

0,2

SioxX

storage conditions have been used several

2

2

5,50

\qn

times beíore and give an accelerated ageing

SioxX-Qu ick

of the castable. 5,50

Water

2.2 Composition design

SioxX-Mag

-

now. 0ne of the challenges is that during the

Based on our experience from the use of

hydration process brucite is formed which

microsiIica in difÍerent refractory castables,

Concerning SioxX'Mag (prototype), cement

causes volume expansion and subsequent

two types of additive packages have recent-

Íree lVgO_based castable is used to compare

a phenomenon commonly cailed

ly been developed. SioxX/SioxX-Quick is íor

"slaking" IlSl.Although it has been demonstrated that the interaction between Mg0

high-alumina castables and SioxX-lVag for

the effects of various dispersants on flow, Mg0 hydration mechanism and hot proper-

basic castables. For ease of application and

ties. Tab.

and Si0, prevents slaking, the mechanism

improved functionality, high-grade microsil-

castables with an optimized particle size dis-

has not been íully understood.

ica is used as carrier in these products. As

tribution. Commercially available dispersants

For more than 30 years, Elkem's focus has

seen in theJollowing this is compensated íor

A and.B and the new SioxX-Maq (prototype)

in the mix desiqn by reducinq the mícrosilica

from Elkem are used' The dosage Ievel oí the

dosage correspondingly.

dlspersants are optimized, 0,25 % for A and

cracking

-

been on understanding the influence of microsilica on the properties

of

refractory

castables. Microsilica (5i0r) consists of spherical particles oÍ amorphous silicon

d1-

oxide (Si0r) with an average particle size of 0,15 pm (150 nm) The most important benefits of microsilica in castables are:

the

B, and 2 % for SioxX-lVag, The water add-

2. Experimental

ition is kept at 5,5 % 16' 6ll mixes.

2.1 Composition design

SioxX/SioxX-Quick

2 shows the composition of

-

2.3 Measurements 5elf-flow of the freshly mixed castable (after

. .

Increased packing density

SioxX/SioxX-Quick has been tested in a var-

íour minutes wet-mixing) was

lmproved ílowability, i.e. reduction in mix-

iety of refractory castables such as white

using the flow-cone described in ASTM

.

ing water needed for a given flow [19]

fused, mulcoa and bauxlte based systems

Contribution to hightemperature strength

[2

due to íormation oÍ mulIite at temperature

vestigate the key parameters affecting shelf-

4:2003). The self flow value is the %-ln-

above 1300"C in alumina based cast-

llfe of LCC premixes and the impact oí dis-

crease oÍ the diameter of the fresh mix

ables I20l

persants on the properties

Bond formation by interaction with Mg0

set and drying of MgO-based castabies

% cement was used. The Ref-l mix with sodium hexametaphosphate (SHMP) as dispersant is a

crease in the mix. This is often referred to as

Í14_17l'

type oí mix used industrial|y and is the refer-

the working time. Fresh samples were placed

.

fines and avoidance of "slaking" during

1

]. Here, a SiC based LCC is selected to ln-

LCC.

As seen in

oí se|Í-flowing

Tab. 1, 6

measured

C230 (height of 50 mm, not the more recent

cone

of

80 mm described

in EN

1402-

measured 90 s after removing the cone. Set time is normally defined as the time írom

rnixing to rhe f;rst noriced temperarure in-

in insulated boxes and the time to onset of temperature increase recorded.

Tab. 2 Composition of gel-bonded MgO based castable [mass-%]

Nedmaq lVgO

M3

12

12

12

mm

3-l

mm

24

24

24

l-0

mm

21

21

2l

00 mesh

10

10

'10

325 mesh

21

)1

20,5

6

6

4,5

911U A

Dispersants

M2

5-3

1

Elkem Microsilica

M1

B

100

modulus of rupture (HM0R) were measured. The HM0R testing apparatus (lsoheat, GB) is

equipped with a pre-heating chamber such

that 0 samples can be kept at the test temperature. The dried samples (25 mm x 25 mm x 150 mm) were heated '1

at a rate of 300'C/h.

Exploslon resistance of the cement{ree MgO-based castables was tested according

0,2 5

to the Chinese Standard

0,25

5ioxX-Mag (prototype) Wateí

Cold modulus of rupture (CMOR) and hot

YB/T4117 2003.

50 mm cubes were placed into

2

heated to

5,5

a

furnace

a preset temperature. The cubes

were inspected aíter 30 min exposure. The

refractories woRLDFoRUM !

5 (2013) t2l

120

4B:00:00

EFresh

fi9months

100

38;24;00

s80 .l

E

E 28:48;oo

bu

6 o

E

o

= o

40

19:12:oo

9:36:00

20 0 Full

Fig'

l

ÍVI|X

No

N4s

No

cement

0;00:00

No additives

EÍfect of 9 months storage on self_ílow as a Íun€tion oÍ

"missing" ingredient

in the

The conditions

explosive spalling occurs is reported as the

(70-80 % relative humidity

3 Results and discussion 3.1 Interaction between cement

and other ingredients during storage of dry-mix

No

MS

No

cement

No additives

Fig. 2 Set-tinre after 9 months storage as a function of "missing" ingredient

temperature at which crack5 start to íorm or explosion resistance.

Mix

Fuil

climate room at 20'C) are

exposed when the castable is mixed. The hy-

rather harsh and the ageing process will be

drates will accelerate setting by acting as

points are Without hydÍates, and these get

for precipitation oí the

more extensive than would normally be ex-

nucIei

perienced in "real life", However, the test

products. ln

setup should give va1uabIe indications oÍ what is better or worse though. Fig, 1-2

cement, the cement grain surface gets even-

show the se|tflow and set-time of diíferent

of the cement. The result is long set. ln this

mixes before and after nine months storage.

way the seemingly contradictory behaviour

hydration

a premix with well distributed

ly hydrated and this hampers the dissolution

It is not an uncommon problem that cast-

Ail mixes show lower self'flow after storage

of cement ageing may be explained. The low

ables deteriorate during storage. Often set-

than íresh mixes. The sample where the ce-

ílow may be caused by the quick set

time gets 5eriou5ly disturbed' A seiÍ-flowing

ment is the "missing" ingredient stands out

scribed above,

de-

castable based on SiC as agqregate, SHMP

and has a very Iow self-flow combíned with

Overall, when the results in Fig, 1-2 are ex-

as dispersant and with 6 % cement (Retl in

a very short set-time. A plauslble explanation

amined, it is the samples that were stored

Tab. 1) was selected to investiqate possible

might be that cement ages differently when

without additives that show least degrad-

interactions between the individual ingredi-

mixed with microsilica or other fines, Pure

ation. Obviously, the ageing is caused not

ents duríng storage. The test set-up WaS to

cement reacts with humldity resulting

in

only by individual components like cement

dry mix all the ingiedients except for one

lumps, whiie ih a castable dry-mix it is dis-

and water but also by interactions between

specific ingredient each time. The dry-mix

persed and coated i.e. the cement grains are

additives and one or several other ingredi-

and the "missing" ingredient were stored

spaced by the other ingredients so that ce-

ents of the castable. lnfluenced by humidity,

separately in the climate room. Tests were

ment lumps are avoided. Lumpy cement has

the dispersant SHMP possibly reacts with ce-

done on freshly mixed samples and after

a tendency oí fast set, or even flash-set, when used in refractory mixes. The reason

tion oí the cement surface alone'

additive (SH|\/P) was stored in a sealed plas-

may be that in the cenent lumps the surface

To avoid additives from reactinq with the ce-

tic box, i.e. not exposed to humidity.

gets only partially hydrated, the contact

ment they should be kept physically apart or

9 months storage

in a climate room.

The

160

84:00:00 >

140

!o

E

E80 o '60

40

E

48:00:00

I

so,oo,oo

3

z+,oo,oo

Frcsh 1

month

3

months

12:00:00

20

SHIVP

SioxX

0:00:00

Sioxxsioxx-Quick

Fig. 3 Self-flow as a function oÍ additive package and storage time

ref

tr

a

60:00:00

too

0

72 hours

72:00:00

120

i

ment and give longer set time than hydra-

ractories woRLDFoRUM s

(20i3) t2l

SHI\4P

Sioxx

Sioxx/Sioxx-Quick

Fig. 4 Set-time as a function of additive package and storage time

150

eSelf-flow

r20

_.

stVibíation-ftow___

'-

ffi

?

E,O 3

a"" 30

0

DispersantA DispersantB

Sioxx-Mag

Fig. 5 Flow of MgO based castable with different dispersants

additives that

do not react with

.cement

SioxX-Mag. For comparison, mixes with two

rules out the phosphates, or at least most of

placed SHMP and that the addition of SioxX-

other commercially available dispersants are

them. Other organic additives and dispers-

Quick did not influence ílow negatively. As

presented. The composirions are given ;r

ants do probably fulfil that criterion, provid-

shown in Fig. 4 the premix with SHMP as

Tab. 2

ed they are not too hygroscopic. This is ex-

dispersant turns out to be useless since it did

As seen in Fig. 5 both mixes with dispersant

emplified in the self-flowing SiC LCC where

not set:.áven after three days' The addition of

A"and B have a self-flow of *30 % and the

the SHMP is replaced by the novel dispers-

SioxX-Quick not only reduced the set time

vibrationJlow is

ant system, SioxX/SioxX-Quick described

for fresh compositions, but aiso offset the

addition (5,5 %)the mix with SioxX-Mag ex-

ageing somewhat.

hibits dramatic improved ílow properties.

Based on

All samples made for mechanical

in

Quick

I

10 %. At the same water

strength

should be careíul to draw firm conclusions,

and hot properties testing were crackJree

but lt seems fair to say that additive/cement

during set and drying process, as exemplified

ínteractlon are minimised if the phosphate

in

additive package SioxX/SioxX-Quick. The

dispersant is replaced by SioxX and further

and SioxX-lVag were not used, as shown in

improved by the combination of SioxX and

Fig. 7, slakinq was observed after dried at

SioxX-Quick,

]

castable compositions are shown in Tab,

1.

room. Based on previous experience three months in the climate room will have the

I

our limited test program one

^-1

ln this series, SHMP is replaced by the novel

paper bags for up to 3 months in the climate I

ables by using the new additive package

oÍ the fresh sample when SioxX re-

The components were mixed and stored in I

o/o

120

3.2 SiC LCC with SioxX/SioxX-

I

note an increase in self-fiow from 92 to

should be used. The latter option probably

the following.

I

Fig, 6 Photo of cement-free MgO based castable with SioxX-Mag and microsilica

same ageing effect as 6

to 12 months

in

Fig 6.

']0

For the sample in which Microsilica

"C íor 24 h. This indicates that the add-

ition of microsilica and SioxX-Mag may sup-

3.3 MgO based gel-bonded castable using SioxX-Mag as

press the hydration of Mg0 and formation of

brucite and subsequently avoid cracking. The

dispersant

set-mechanism and the interaction between

il

regular (dry) warehouse conditions.

Dispersants play an extremely important role

fine N/gO, microsilica and SioxX-lVag are not

ir

The results (Fig, 3) indicate that independent

in basic castables. ln this study, a microsilica

fully understood yet and further research

of the additive package the flow of all mixes

gel-bonded Mg0-based castable

degrades somewhat during the three months storage period. lt is interesting to

duced to demonstrate the feasibility oÍ mak.

The cold modulus of rupture (CMOR) is plot-

ing high_períormance basic refractory cast-

ted as a function of firíng temperatu'e

I

i i

is

pro-

is

ongoing. in

I

16

ii G

L

=12

I

I'

E

i,

!8

!r

o

4

0 110

1000 1200 1300 1400

1500

Temperature I l[Cl

Fig.7 Photo of cement-free MgO based castable without SioxX-Mag and without rnicrosilica

102

Fig. 8 Cold MoR as a Íunction of firing tempeÍature

refractories woRLDFoRUM s

(2013) t2l

Fig 8. The reason for the drop in strength 18

from 600 to 1 000 "C is not fully understood, but it may be connected to crystallization of

15

an amorphous bond-phase. At higher tem' peíatures strength is regained and this may

;72 ca e Éo o-

be attributed to formation oÍ Íorsterite (M25)

Írom the reaction between MgO and micro-

silica starting at a temperature above ap-

= o-

proximately 1000 "C. However, the strength '1 drop írom 600 to 000

-b

'C seems to depend

on the type of dispersant used. The specimens with SioxX-Mag show highest CMOR

3

of -10 lVPa at 1000 "C, being about 70 % stronger than what is achieved using dis-

0

persant

B.

Apparently the type of dispersant

not only aífects the flowability and settin9 process of gel-bonded MgO castables, but

also impacts the strength at intermediate temperatures.

Fig.9 shows the hot modulus of rupture (Hll40R) as a function of test temperature.

Fig.9 Hot MOR as a Íunction of test teÍnpeÍatures Tab. 3 Explosion resistance according to chinese standard YB/T41 17-2003. r/denotes that the sarnple passed the test; x denotes that the sample failed Temp. ['C]

300

increases and reaches a maximln

400

What is instereting is that,

500

the HM0R for the castable with Sioxx-lvlag is consistently higher than the other cast-

600

Hl\4OR

value at 1300 'C,

abl es.

It may therefore be concluded that the lvlg0

based castable with SioxX-Mag as additive

outperíorms terms

the other two castab1es

oí HMOR. The reason for this is

rently unclear but further research work is ongoing, Tab. 3 summarizes the explosion test results

M2 (Dispersant-B)

Dried

Wet

Dried

M3 (SioxX-Mag) Wet

Dried

X

x

700 800 1

x

000

in

cur-

(Dispersant-A)

Wet

oí H|Vl0R is similar. From 1000" towards 1300 "C, For all mixes the development

Ml

thereíore, íast flring

lí this type of castabIes

For basic castables, which are based on microsilica gel bond system, several attract-

is feasible,

ive properties have been identiíied:

.

4 Conclusions

Using SioxX-N4ag together with Micro-

of both "wet" and "dried" samples of gel-

In this paper, a SiC based selí_Ílowing LCC

silica, slaking caused by brucite formation

bonded MgO-based castables. The samples were cured at 100 % relative humidity at

with SioxX/SioxX-Quick was produced to investigate the interaction between cement

is suppressed and crack-free dried samples

room temperature for 24

and other ingredients during storage of dry-

h beíore de-

moulding. The íreshly de_moulded samples

mix (shelÍ-life).

are labelled "wet" and samples dried

A gel-bonded lVg0 castable with SioxX-l\4ag was selected as example to illustrate how

for further 24 h are called "dried".

1 0 'C All "dried" samples show excellent 1

at

explo-

.

sion resistance and pass the test at 1 000 "C

The good performance is attributed to the low amount oÍ resldual water in the bond

basic castables.

MgO-SiOz bonded castables exhibit high

green strength, e.g CN/0R is above '1

.

1

10 "C.

strength and excellent explosion resista

ln a humid environment not only the cement

2 MPa after drying at

N4gO based castables exhibit very good

placing properties combined with high hot

difÍerent di5persant5 influence the flow_ ability, bond formation and hot propertles oÍ

are made,

nce,

. The bond phase contains only

small

dispersants presumably

amounts of chemically bound water, so

during the firing process. When the "wet" samples were tested, best explosion resist-

enter into reactions with cement and/or other constituents ln the castable premix

castable can be fired at very hlgh heating

SioxX-Mag

during storage'The right choice oí dispersant

This indicaÍes that the dispersants

system is a good way to help control the ageing at "normal" moisture levels This has

phase after drying, and a stable bond phase

ance was achieved with (700'c)

play a role in of the bond phase formation,

hydrates, but the

such as Mg0 hydration and the interaction

been lllustrated in this paper where SHMP

between MgO and microsillca in the pres-

was replaced by.the SioxX/SioxX-Quick dis-

ence of water, The drying characteristics have been improved by using SioxX-Mag,

persant system resultinq in both better flow and improved she|í liÍe'

reÍractorie5 WoRLDFoRUM 5 (2013)

[2]

once the free water is removed, the

.

rates,

Substitution of commercially available dis-

persants with SioxX-Mag not only gave better flowability but also improved the

.

hot properties and explosion resistance.

lmproved drying characteristics allowing fast firing of this type of castables.

103

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