New Additive Packages for Self-flowing High-alumina andMgO Based Refractory Castables*
Hong Peng, Ming Luo, Bjorn Myhre
c.,-)elt-tlowing refractory castables with special performance are continuously being developed. New andimproved dispersant systems play an essential role in this development. The objective of this paper is tohighlight 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 aluminabased castable systems. The main function of SioxX is to control flow properties whereas SioxX-Quickcontrols setting characteristics. ln combination they will enable the castable producer to control flow andset time accurately. When sioxX replaces sodium hexametaphosphate, self-flow is improved by 20 to30 % 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 bet-ter 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 dis-persant have been tested. using sioxX-Mag together with microsilLa,-slaking causJd by brucite forma-tion is suppressed and crack-free dried samples are made. Another benefit of using this additive packageis the improved drying characteristics allowing fast firing of the castable.
lntroductionCharacterjstrcs of advanced self-flowing re-fractory castables are ease of installation,possibility to cast intricate shapes, energysavings and better performance in general.lmportant factors to control in these cast-ables incude particle size distribution (P5D),
additives (deflocculants, dispersants, accel-erators) [1,2] and the raw materials, particu-larly with respect to the super-fines and ce-ment [3,4]. Numerous recent research pro-grams have focused on understanding themechanism of dispersion and optimizationof dispersants or specific castables [5-8].No doubt, dispersants continue to play anessential role in further develoijment of ad-vanced reractories.
For state-of-the-art steel-making and cleansteel production high-per{ormance basic re-fractory castables are of particular import-ance. Already in 1 989, Elken Materials start-ed development work on a new binder sys-tem for basic castables based on the reac-
tion between MgO fines, microsilica (SiOr)and water [9]. This bond system was first ap-plied to magnesia, silicon nitride and mag-nesra-carbon castables and required lowwater addition (5,0-5,5 %) resulting in ad-equate mechanical properties. An importantobservation was that at 6 % microsilica add-ttion no slaking occurred. Since then, sig-niicant research on the use of microsrlica inbasic castables has been run 110-131, sel!lowlng cement-free MgO_5iO, bondedcastables can be produced. About 6 mass-%microsilica seems to be essential to obtainboth good placement and hot properties. Asurprising observatron is that the slag resist-ance to both BOF and EAF slag compared toan alumina spinel castable is improved [1,1].Recently, many research programs have fo-cused on hydration mechanism of magnerawith water and additives (dispersants andretarders) in systems such as MgO-SiOr-HrO,MgO-Al,0r-H,0 and Mg0-SiO,-Al2O3-H2O|+_ll1' However, self_lowing magnesiacastables have not been widely used until
Hong Peng, Bjorn Myhre
Elken Silicon Matenals
4675 Kristiansand
Norway
Ming Luo
Wuhan University of Science and Technology
43008 I Wuhan
China
Corresponding author: Hong peng
E'mail: [email protected]
Keywords: self-flowing, new dispersantlFigh-alumina and basic reractory
castables, microsilica gel-bonding
Received: 21.11.2012
Accepted: 21.12.2012
* The paper was presented at ALAFAR inCancn/MX in Nov' 2012 and received the
3,r prize of the ALAFAR Award 20 j 2
refractories woRLDFoRUM s (20j3) I2l 99
Tab. 1 Composition of Si based LCC [mass-%]
Ref-l sx-l sxi sx-Q-l
Elkem Microsilica 97I U 10 8,1 I
Cernenl 6 6 6
SiC 12 12 1?
Calcined alumina 13 12,3 12
SHMP 0,2
SioxX 2 2
SioxX-Qu ick
Water 5,50 5,50 \qn
now. 0ne of the challenges is that during the
hydration process brucite is formed which
causes volume expansion and subsequent
cracking - a phenomenon commonly cailed"slaking" IlSl.Although it has been demon-
strated that the interaction between Mg0
and Si0, prevents slaking, the mechanismhas not been ully understood.
For more than 30 years, Elkem's focus has
been on understanding the influence ofmicrosilica on the properties of refractorycastables. Microsilica (5i0r) consists ofspherical particles o amorphous silicon d1-
oxide (Si0r) with an average particle size of
0,15 pm (150 nm) The most important ben-
efits of microsilica in castables are:. Increased packing density. lmproved lowability, i.e. reduction in mix-
ing water needed for a given flow [19]. Contribution to hightemperature strength
due to ormation o mulIite at temperature
above 1300"C in alumina based cast-ables I20l
. Bond formation by interaction with Mg0fines and avoidance of "slaking" duringset and drying of MgO-based castabies14_17l'
Based on our experience from the use of
microsiIica in diferent refractory castables,
two types of additive packages have recent-
ly been developed. SioxX/SioxX-Quick is or
high-alumina castables and SioxX-lVag for
basic castables. For ease of application and
improved functionality, high-grade microsil-
ica is used as carrier in these products. As
seen in theJollowing this is compensated or
in the mix desiqn by reducinq the mcrosilica
dosage correspondingly.
2. Experimental
2.1 Composition design -SioxX/SioxX-Quick
SioxX/SioxX-Quick has been tested in a var-
iety of refractory castables such as whitefused, mulcoa and bauxlte based systems
[2 1 ]. Here, a SiC based LCC is selected to ln-
vestigate the key parameters affecting shelf-
llfe of LCC premixes and the impact o dis-persants on the properties o se|-flowingLCC. As seen in Tab. 1, 6 % cement wasused. The Ref-l mix with sodium hexa-metaphosphate (SHMP) as dispersant is atype o mix used industrial|y and is the refer-
ence or comparison with 5ioxX/SioxX-Quick
mixes.
To investigate the effect of humidity onshel-life, dry-mixed 25 kg samples packed
in closed, plastic lined paper bags werestored in a temperature and humidity con-trolled room ("cllmate room" with70-80 % relative humidity at 20 "C), Thesestorage conditions have been used several
times beore and give an accelerated ageing
of the castable.
2.2 Composition design -SioxX-Mag
Concerning SioxX'Mag (prototype), cement
ree lVgO_based castable is used to compare
the effects of various dispersants on flow,Mg0 hydration mechanism and hot proper-
ties. Tab. 2 shows the composition of thecastables with an optimized particle size dis-
tribution. Commercially available dispersants
A and.B and the new SioxX-Maq (prototype)
from Elkem are used' The dosage Ievel o the
dlspersants are optimized, 0,25 % for A and
B, and 2 % for SioxX-lVag, The water add-
ition is kept at 5,5 % 16' 6ll mixes.
2.3 Measurements
5elf-flow of the freshly mixed castable (after
our minutes wet-mixing) was measuredusing the flow-cone described in ASTMC230 (height of 50 mm, not the more recent
cone of 80 mm described in EN 1402-4:2003). The self flow value is the %-ln-crease o the diameter of the fresh mixmeasured 90 s after removing the cone.
Set time is normally defined as the time rom
rnixing to rhe f;rst noriced temperarure in-
crease in the mix. This is often referred to as
the working time. Fresh samples were placed
in insulated boxes and the time to onset of
temperature increase recorded.
Cold modulus of rupture (CMOR) and hotmodulus of rupture (HM0R) were measured.
The HM0R testing apparatus (lsoheat, GB) is
equipped with a pre-heating chamber suchthat '1 0 samples can be kept at thetest temperature. The dried samples(25 mm x 25 mm x 150 mm) were heatedat a rate of 300'C/h.Exploslon resistance of the cement{reeMgO-based castables was tested according
to the Chinese Standard YB/T4117 2003.50 mm cubes were placed into a furnaceheated to a preset temperature. The cubeswere inspected ater 30 min exposure. The
refractories woRLDFoRUM 5 (2013) t2l
!
Tab. 2 Composition of gel-bonded MgO based castable [mass-%]
M1 M2 M3
Nedmaq lVgO
5-3 mm 12 12 12
3-l mm 24 24 24
l-0 mm 21 21 2l
1 00 mesh 10 10 '10
325 mesh 21 )1 20,5
Elkem Microsilica 911U 6 6 4,5
Dispersants
A 0,2 5
B 0,25
5ioxX-Mag (prototype) 2
Wate 5,5
100
120
100
s80
.l bu6o
40
20
0Full VI|X No N4s No cement No additives
EFresh fi9months4B:00:00
38;24;00
EE 28:48;oo
E
= 19:12:oo
oo
9:36:00
0;00:00Fuil Mix No MS No cement No additives
Fig' l Efect of 9 months storage on self_low as a untion o"missing" ingredient
temperature at which crack5 start to orm or
explosive spalling occurs is reported as the
explosion resistance.
3 Results and discussion
3.1 Interaction between cementand other ingredients duringstorage of dry-mix
It is not an uncommon problem that cast-ables deteriorate during storage. Often set-
time gets 5eriou5ly disturbed' A sei-flowingcastable based on SiC as agqregate, SHMP
as dispersant and with 6 % cement (Retl inTab. 1) was selected to investiqate possible
interactions between the individual ingredi-
ents durng storage. The test set-up WaS to
dry mix all the ingiedients except for onespecific ingredient each time. The dry-mix
and the "missing" ingredient were storedseparately in the climate room. Tests were
done on freshly mixed samples and after9 months storage in a climate room. Theadditive (SH|\/P) was stored in a sealed plas-
tic box, i.e. not exposed to humidity.
Fig. 2 Set-tinre after 9 months storage as a function of"missing" ingredient
The conditions in the climate room(70-80 % relative humidity at 20'C) arerather harsh and the ageing process will be
more extensive than would normally be ex-
perienced in "real life", However, the testsetup should give va1uabIe indications owhat is better or worse though. Fig, 1-2show the se|tflow and set-time of diferent
mixes before and after nine months storage.
Ail mixes show lower self'flow after storage
than resh mixes. The sample where the ce-
ment is the "missing" ingredient stands out
and has a very Iow self-flow combned with
a very short set-time. A plauslble explanation
might be that cement ages differently when
mixed with microsilica or other fines, Pure
cement reacts with humldity resulting inlumps, whiie ih a castable dry-mix it is dis-
persed and coated i.e. the cement grains are
spaced by the other ingredients so that ce-
ment lumps are avoided. Lumpy cement has
a tendency o fast set, or even flash-set,when used in refractory mixes. The reason
may be that in the cenent lumps the surface
gets only partially hydrated, the contact
points are Without hydates, and these get
exposed when the castable is mixed. The hy-
drates will accelerate setting by acting asnucIei for precipitation o the hydrationproducts. ln a premix with well distributedcement, the cement grain surface gets even-
ly hydrated and this hampers the dissolution
of the cement. The result is long set. ln this
way the seemingly contradictory behaviour
of cement ageing may be explained. The low
low may be caused by the quick set de-scribed above,
Overall, when the results in Fig, 1-2 are ex-amined, it is the samples that were storedwithout additives that show least degrad-
ation. Obviously, the ageing is caused not
only by individual components like cement
and water but also by interactions between
additives and one or several other ingredi-
ents of the castable. lnfluenced by humidity,
the dispersant SHMP possibly reacts with ce-
ment and give longer set time than hydra-
tion o the cement surface alone'
To avoid additives from reactinq with the ce-
ment they should be kept physically apart or
Fig. 3 Self-flow as a function o additive packageand storage time
ref ractories woRLDFoRUM s (20i3) t2l
Fig. 4 Set-time as a function of additive package
and storage time
160
140
120
i too!oE80o'60
40
20
0SHIVP SioxX Sioxxsioxx-Quick
84:00:00
72:00:00
60:00:00
EE 48:00:00
I so,oo,oo
3 z+,oo,oo
12:00:00
0:00:00
> 72 hourstr Frcsh
a 1 month
3 months
SHI\4P Sioxx Sioxx/Sioxx-Quick
150
r20
?
E,O3a""
30
0
eSelf-flow
_. stVibation-ftow___ '- ffi
DispersantA DispersantB Sioxx-Mag
Fig. 5 Flow of MgO based castable with different dispersants Fig, 6 Photo of cement-free MgO based castable with SioxX-Magand microsilica
I
I
I
I
I
I
il
irI
i
i
I
ii
I
I'
i,!r
additives that do not react with .cementshould be used. The latter option probably
rules out the phosphates, or at least most of
them. Other organic additives and dispers-
ants do probably fulfil that criterion, provid-
ed they are not too hygroscopic. This is ex-
emplified in the self-flowing SiC LCC where
the SHMP is replaced by the novel dispers-
ant system, SioxX/SioxX-Quick described in
the following.
3.2 SiC LCC with SioxX/SioxX-Quick
ln this series, SHMP is replaced by the novel
additive package SioxX/SioxX-Quick. The
castable compositions are shown in Tab, 1.
The components were mixed and stored in
paper bags for up to 3 months in the climate
room. Based on previous experience three
months in the climate room will have thesame ageing effect as 6 to 12 months inregular (dry) warehouse conditions.
The results (Fig, 3) indicate that independent
of the additive package the flow of all mixes
degrades somewhat during the threemonths storage period. lt is interesting to
note an increase in self-fiow from 92 to120 o/o o the fresh sample when SioxX re-
placed SHMP and that the addition of SioxX-
Quick did not influence low negatively. As
shown in Fig. 4 the premix with SHMP asdispersant turns out to be useless since it did
not set:.ven after three days' The addition of
SioxX-Quick not only reduced the set time
for fresh compositions, but aiso offset the
ageing somewhat.
Based on our limited test program oneshould be careul to draw firm conclusions,
but lt seems fair to say that additive/cement
nteractlon are minimised if the phosphate
dispersant is replaced by SioxX and further
improved by the combination of SioxX and
SioxX-Quick,
3.3 MgO based gel-bondedcastable using SioxX-Mag asdispersant
Dispersants play an extremely important role
in basic castables. ln this study, a microsilica
gel-bonded Mg0-based castable is pro-duced to demonstrate the feasibility o mak.
ing high_perormance basic refractory cast-
ables by using the new additive package
SioxX-Mag. For comparison, mixes with two
other commercially available dispersants are
presented. The composirions are given ;r
Tab. 2
As seen in Fig. 5 both mixes with dispersant
A"and B have a self-flow of *30 % and thevibrationJlow is ^-1 10 %. At the same water
addition (5,5 %)the mix with SioxX-Mag ex-
hibits dramatic improved low properties.
All samples made for mechanical strengthand hot properties testing were crackJreeduring set and drying process, as exemplified
in Fig 6. For the sample in which Microsilica
and SioxX-lVag were not used, as shown in
Fig. 7, slakinq was observed after dried at
] ']0 "C or 24 h. This indicates that the add-
ition of microsilica and SioxX-Mag may sup-
press the hydration of Mg0 and formation of
brucite and subsequently avoid cracking. The
set-mechanism and the interaction between
fine N/gO, microsilica and SioxX-lVag are not
fully understood yet and further research is
ongoing.
The cold modulus of rupture (CMOR) is plot-
ted as a function of firng temperatu'e in
Fig.7 Photo of cement-free MgO based castablewithout SioxX-Mag and without rnicrosilica
102
16
GL
=12
E!8o
4
0
110 1000 1200 1300 1400 1500Temperature I l[Cl
Fig. 8 Cold MoR as a unction of firing tempeature
refractories woRLDFoRUM s (2013) t2l
18
15
;72aceoo-=o--b
3
0
Fig 8. The reason for the drop in strength
from 600 to 1 000 "C is not fully understood,
but it may be connected to crystallization of
an amorphous bond-phase. At higher tem'peatures strength is regained and this may
be attributed to formation o orsterite (M25)
rom the reaction between MgO and micro-
silica starting at a temperature above ap-proximately 1000 "C. However, the strength
drop rom 600 to '1 000 'C seems to depend
on the type of dispersant used. The speci-
mens with SioxX-Mag show highest CMOR
of -10 lVPa at 1000 "C, being about 70 %
stronger than what is achieved using dis-persant B. Apparently the type of dispersant
not only afects 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.
For all mixes the development o H|Vl0R is
similar. From 1000" towards 1300 "C,Hl\4OR increases and reaches a maximlnvalue at 1300 'C, What is instereting is that,
the HM0R for the castable with Sioxx-lvlag
is consistently higher than the other cast-
abl es.
It may therefore be concluded that the lvlg0
based castable with SioxX-Mag as additive
outperorms the other two castab1es interms o HMOR. The reason for this is cur-
rently unclear but further research work is
ongoing,
Tab. 3 summarizes the explosion test results
of both "wet" and "dried" samples of gel-
bonded MgO-based castables. The samples
were cured at 100 % relative humidity atroom temperature for 24 h beore de-moulding. The reshly de_moulded samples
are labelled "wet" and samples dried at
1 1 0 'C for further 24 h are called "dried".
All "dried" samples show excellent explo-sion resistance and pass the test at 1 000 "C
The good performance is attributed to the
low amount o resldual water in the bond
phase after drying, and a stable bond phase
during the firing process. When the "wet"
samples were tested, best explosion resist-
ance was achieved with SioxX-Mag(700'c) This indicaes that the dispersantsplay a role in of the bond phase formation,
such as Mg0 hydration and the interaction
between MgO and microsillca in the pres-
ence of water, The drying characteristicshave been improved by using SioxX-Mag,
Fig.9 Hot MOR as a unction of test tenpeatures
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
thereore, ast flring l this type of castabIesis feasible,
4 Conclusions
In this paper, a SiC based sel_lowing LCC
with SioxX/SioxX-Quick was produced to in-
vestigate the interaction between cement
and other ingredients during storage of dry-
mix (shel-life).
A gel-bonded lVg0 castable with SioxX-l\4ag
was selected as example to illustrate how
diferent di5persant5 influence the flow_
ability, bond formation and hot propertles o
basic castables.
ln a humid environment not only the cement
hydrates, but the dispersants presumably
enter into reactions with cement and/or
other constituents ln the castable premix
during storage'The right choice o dispersant
system is a good way to help control the
ageing at "normal" moisture levels This has
been lllustrated in this paper where SHMP
was replaced by.the SioxX/SioxX-Quick dis-
persant system resultinq in both better flow
and improved she| lie'
For basic castables, which are based on
microsilica gel bond system, several attract-
ive properties have been identiied:
. Using SioxX-N4ag together with Micro-silica, slaking caused by brucite formation
is suppressed and crack-free dried samples
are made,. MgO-SiOz bonded castables exhibit high
green strength, e.g CN/0R is above'1 2 MPa after drying at 1 10 "C.
. N4gO based castables exhibit very goodplacing properties combined with high hot
strength and excellent explosion resist-
a nce,
. The bond phase contains only smallamounts of chemically bound water, so
once the free water is removed, thecastable can be fired at very hlgh heating
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 allowingfast firing of this type of castables.
Temp. ['C] Ml (Dispersant-A) M2 (Dispersant-B) M3 (SioxX-Mag)
Wet Dried Wet Dried Wet Dried
300
400
500 X
600
700 x
800
1 000 x
reractorie5 WoRLDFoRUM 5 (2013) [2] 103
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UnpubIished data. AppIication o SioxX/SioxX-
Quick in various reractory castables' lnternal
report, Elkem, 201 1
ref ractories woRLDFoRUM 5 (2013) t2l
!
[1 5]t8l
t16ltel
l1 7lt1 0l
t11l
tl el
{1 2l
i1 3l
t20l
121l
I1 4l
t18l
104
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