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

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Self-flowing 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 papar is to higlight the benefits of two novel admixture systems that have been tested in various refractory castables.
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  • 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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    Congr, Mexico 1994

    UnpubIished data. AppIication o SioxX/SioxX-

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