Impact of CAC Hydration

7/27/2019 Impact of CAC Hydration

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THE IMPACT OF CALCIUM ALUMINATE CEMENT HYDRATION UPON THEPROPERTIES OFREFRACTORYCASTABLES

by Christopher Parr, Fabien Simonin, Bruno Touzo, Christoph Whrmeyer, Benot Valdelivre, AkihikoNamba

Presented at TARJ meeting, Ako, Japan September 2004


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1 Introduction

Monolithic refractory castables, bonded withcalcium aluminate cements, have a rich history ofover 80 years. They have evolved from highcement conventional formulations to the era ofreduced cement castable systems

1that can be

placed using a variety of techniques such ascasting, gunning, self flow, pumping andshotcreting. These developments have beenmade possible by the versatility of the calciumaluminate cements which play a major role indetermining the properties of such monolithicrefractories.The usage chain of monolithics containingcalcium aluminate cement contains several stepssuch as mixing, placing and consolidation,commissioning and dry out and finally use inservice. Each of these steps within the castableplacing chain are intimately linked to thehydration process of calcium aluminate cement(CAC). In order to understand the role of calcium

aluminate cement (CAC), it is first necessary toconsider the reactions that take place within CACduring the hydration process and then to linkthese reactions to the physical aspects of thecastable properties. Their cementitious propertiesenable the refractory to be cast in place and toset and harden under ambient conditions. As therefractory is heated to high temperature, thewater needed for placing and the formation ofhydration products is driven off and the hotstrength is provided by sintering and ceramicbonding. This process of first heating or dry outmust be carefully managed to avoid undesirable

strength loss, spalling or explosion.This review will investigate in detail the hydration

and dehydration reactions that occur from thepoint that water is added at the beginning ofmixing through to the point that the installedrefractory lining is put into service. The effect ofthese reactions on the physical properties of thecastables will be shown in a variety of systems.The impact of parameters such as temperature,mineralogy and other formulation componentswill be also reviewed.

2 Calcium aluminate phases

The phases present in a given calcium aluminatecement (CAC) depend upon a number ofparameters of which the chemistry is the mostimportant. The anhydrous CAC phases present ina range of commercial products are shown intable 1.

Table 1: chemical composition and mineralogy of

three different calcium aluminate cementsCement chemistry abbreviations: C = CaO, A = Al2O3, S = SiO2,F = Fe2O3, M = MgO, T = TiO2, H = H2O

The mineralogy shown in table 1 is determinedby quantitative methods using the Rietveld

approach

2

. The dominant phase in all cases isCA, calcium monoaluminate. The reactions ofcalcium aluminates and their kinetics will beinfluenced by the composition of themineralogical phases present in a given cement.A general comparison of the reactivity of theanhydrous phases is shown in table 2 taken fromGeorge

3. It can be seen that as the phases

become more lime rich, i.e. as the C/A ratio

increases, then so does their reactivity.

40% aluminaCAC

50% aluminaCAC

70% AluminaCAC

Al2O3 37,5 41,5 50,8 54,2 68,7 70,5

CaO 36,5 39,5 35,9 38,9 28,5 30,5

SiO2 42, - 5,0 4,0 5,5 0,2 0,6

FeO+Fe2O3 14,0-18,0 1,0 2,2


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Table 2: The reactivity of the various calcium aluminatephases

C3A C12A7 CA CA2 CA6

C/A 3 1,7 1 0,5 0,2

Reactivity at20C

Veryrapid

Fast Slow Veryslow

None

Table 3 taken from reference 1 lists some usefulproperties of the phases found in CAC.Monocalcium aluminate (CaO.Al2O3 or CA) is themost important component of CACs and has a

relatively high melting point (1600

o

C), anddevelops the highest strength among the phaseslisted during the relatively short time available forhydrating refractory concretes. It takes some timeto start setting, but hardens rapidly after the initialset. Calcium dialuminate (CA2) is a secondaryphase in CACs and is more refractory than CAbut takes a long time to set due to its lowhydraulic activity although accelerated astemperature increases

4. While hydration of CA is

known to be accelerated by the presence of CA2,the opposite does not hold true. The hydration ofCA2 may actually be hindered by the presence of

CA5

. The strength of CA2 after three dayshydration is comparable to that of the pure CA.C12A7 hydrates rapidly and has a relatively lowmelting point. C2S and C4AF are common inPortland cement, but can also occur in the high-silica and iron-rich low-purity CACs, respectively.C4AF forms hydrates of calcium aluminate andcalcium ferrite or solid solutions of the twohydrates. C2AS (gehlenite) shows little tendencyto hydrate and is an undesirable component ofalumina cement which limits refractoriness andhot-strength properties

6.

Table 3: Properties of CAC Mineral ConstituentsMiner

alChemical Composition (wt

%)Tm Density Crystal

C A F S oC g/cm3

C 99.8 2570 3.32 Cubic

C12A7 48.6 51.4 1495 2.69 Cubic

CA 35.4 64.6 1600 2.98 Mon.

CA2 21.7 78.3 1765 2.91 Mon.

C2S 65.1 34.9 2066 3.27 Mon.

C4AF 46.2 20.9 32.9 1415 3.77 Orth.

C2AS 40.9 37.2 21.9 1590 3.04 Tet.

CA6 8.4 91.6 1830 3.38 Hex.

A 99.8 2051 3.98 Rhomb

CA6 is the only non-hydrating phase in the purecalcium aluminate system and is often a reactionproduct in alumina castables bonded with high-purity aluminate cement. It is believed that CA6 ismost readily formed in alumina castables whenusing CA2 as a precursor

7. Alumina (A) is

sometimes present as an addition and this isespecially true in the case of 80% aluminacalcium aluminate cements.

3 Calcium aluminate cement

hydration

In contrast to Portland cements where thehydrates formed remain broadly similar

8with time

and temperature the hydration of CAC is stronglydependant upon temperature. The ambienttemperature significantly modifies the hydratesthat result. The associated hydration reactionsare shown in table 4.

Table 4: Hydration scheme for mono calcium aluminate.

Temperature Hydration Reaction

< 10C CA + 10H CAH10

2CA+ 11H C2AH8+ AH310 27C

CA + 10H CAH10

>27C 3CA + 12H C3AH6+ 2AH3

F(tC + time) 2CAH10 C2AH8+ AH3+ 9H

3C2AH8 2C3AH6+ AH3 + 9H

Below 10C it is the formation of CAH10 thatdominates and this continues up to around 27C.Between 10 and 27C CAH10 and C2AH8 areformed together. At higher temperatures above27C C3AH6 formation occurs early in thehydration process. It is believed that theformation of C3AH6 often passes through atransient C2AH8 phase

8. The other two important

reactions are the conversion of the metastablehydrates, CAH10 and C2AH8 to the stable C3AH6


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hydrates. In addition to the C-A-H hydrates,Alumina tri hydrate (AH3) is also formed as partof the hydration reactions. All these reactions aretime, temperature and humidity dependant.Themorphology of the AH3 that is formed alsochanges with temperature. At lower temperaturesit exists as a gel form and becomes increasinglycrystalline as the temperature increases.Analogous hydration reactions can be written forCA2 as well as for C12A7. Other hydrates havealso been reported

10,11and are known to exist,

for example the hydrates C4AH13 and C4A3H3.The later existing mainly under hydrothermal

conditions (such as exists in refractory concreteduring the dry out phase). The morphology andsome key properties of each of the main hydratesare shown in Fig. 1.

Fig. 1: Key properties and Micrographs of the calciumaluminate hydrates

CAH10

Density = 1720 Kg/m3

Cubic system : Hexagonal

C4AH13



C2AH8



C3AH6


Cubic system : Cubic

4 Mechanisms

The mechanism of hydration of calciumaluminate is via solution, where an anhydrousphase dissolves and is followed by theprecipitation of the hydrates from solution

3. Three

distinct phases can be identified; dissolution,

nucleation and precipitation. The hydrationprocess

10is initiated by the hydroxylation of the

cement surface followed by dissolution of cementin water and the liberation of calcium andaluminium ions. A small amount of gel likehydrates will form at this point if the solutionconcentration rises over the super saturation limitlevel of the hydrates C2AH8 and AH3. Thedissolution will continue, with a consequentincrease in the concentration of calcium andaluminium ions until a saturation point is reached.


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This is the point which is the equilibrium solubilityof the anhydrous phases with the hydroxylatedsurface layers. After the dissolution phase therefollows an induction period during which nucleiattain a critical size and quantity.

Once this is achieved the nucleation phase isfollowed by a rapid and massive precipitation ofthe hydrates, leading to a drop in solutionconcentration. This is a dynamic process whichcontinues to consume anhydrous cement. In aphysical sense, it is the growth of these hydrateswhich interlock and bind together to provide

mechanical resistance. The driving force of thehydration reactions is the lower solubility of thehydrates compared to the anhydrous species.The ambient temperature significantly modifiesthe hydrates that result due to the fact that theirsolubility changes with temperature

12.

As has been explained, hydration is a processthat involves the passage of ions into solution.This can be followed experimentally

13using

conductimetry and is shown schematically in Fig.2. In this technique a quantity of cement is putinto solution in a cell and the change in

conductivity with time is continuously monitored.The water is maintained at constant temperaturewhich is normally 20C. The conductivity of asolution is directly linked to the number of ions insolution. The measured conductivity will increaseas the quantity of ions in solution increases. Theresulting data is collected electronically andpresented in the form of a curve (fig. 2). Threedistinct zones can be identified.

The first zone (I) is characterised by a rapidincrease in conductivity. This corresponds tothe dissolution of the anhydrous cement.

Calcium aluminate cement forms ions in thepresence of water and these ions correspond

to Ca2+

and Al(OH)4-. This phase is

exothermic and heat is generated. There is aslow precipitation of gel like hydrates

14

These primary hydrates form both in solutionand as small colloidal particles surroundingthe anhydrous grains.

The second zone (II) is the plateau with astable conductivity; this corresponds to the

induction period where nuclei or germs areformed preceding precipitation. This is alsoreferred to as the saturation stage wheresaturation, in terms of ionic concentration, isbeing reached. There is no change in ionicconcentration during this period where thereis slow growth of gel like hydrates whichequals their dissolution rate.

The third zone(III)is characterised by a rapiddecrease in conductivity. This corresponds tothe formation of hydrates (~ setting) as thesolution concentration is oversaturated with

respect to the hydrates. The massiveprecipitation causes a decrease inconductivity. The precipitation of the hydratesis responsible for the flocculation of thecalcium Aluminates due to two mechanisms.Firstly, hydrate formation consumes waterand thus workability decreases and secondly,hydrates have a negative surface chargewhich is attracted to the positive surfacecharge on the anhydrous particles. This stepis exothermic and the heat liberated can befollowed as a means of following thehydration process. This step will only occur

when over saturation (with respect to thehydrates) is achieved and when a sufficientnumber of structured nuclei are present. Inthe case of pure CA this may require aconsiderable time.

0 60 120 180 240 300 360

time (mins)

Conductivityms/cm

Dissolution I Massive Precipitation III Nucleation II

T1

- T3

T1T

3

L.O.I.

ions

LOI%

/

Tempera

ture

C

Temp.

W/C=5 / 20C

ions

0 60 120 180 240 300 360

time (mins)

Conductivityms/cm


T1

- T3

T1T

3

L.O.I.

ionsions

LOI%

/

Tempera

ture

C

Temp.

W/C=5 / 20C

ions

Fig. 2: Conductimetry curves for Secar

71


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Also shown in fig. 2 are the comparison curvesfor the increase in loss in ignition. It is clear thatthe decrease in conductivity is coherent withhydrate formation as evidenced by the increasein LOI of the cement material. Similarly theevolution of conductivity follows the determined

concentrations of [Ca2+] ions in solution.

5 Relationship of hydration

reactions to castable proprties

The impact on castable properties due to thesehydration reactions can be measured through avariety of physical techniques. Two castableproperties are of specific interest, as they governthe time scheduling of castable installations; theworkable time that a castable can be placedbefore becoming unworkable or stiff and the timewhen the cast structure has sufficient strengththat the formwork and moulds can be stripped.Fig. 3 shows the link between these aspects ofcastable properties, workability and hardeningtime and the hydration process as followed byconductimetry and exothermic profiles

14. The

period of workability is linked to the formation ofgel like hydrates. These can not be measured byclassical techniques and their detectionnecessitates techniques such as AFM and X raytransmission microscopy

15

Period of flow

Flow decay

Working time

time

Conductivity

ms/cm


L.O.I.

LOI%

/

Temperature

C

Temp.

W/C=5 / 20C

Hardening and strengthdevelopment

Fig. 3: The link between hydration steps andcastable properties

These particles are of nanometric size and leadto a progressive structuring of the material asthey develop in size and quantity.The working time occurs when there is sufficientflocculation and stiffening so that the castable willnot move under vibration and thus cannot beproperly consolidated anymore. The hardeninggenerally starts to occur as the massiveprecipitation occurs as can be seen by theincrease in temperature (exo profiles) anddecrease in ionic activity (conductimetry) in Fig.3.Thus, a quicker time to the massive precipitationwill result in a reduced time to attain a given

strength. The optimisation of castable propertiesoften aims at modifying these antagonisticproperties i.e. prolonging the working time andreducing t he hardening time. Clearly, this canonly be achieved if an understanding of theunderlying hydration reaction kinetics isachieved. The hydration mechanisms andkinetics such as those described above can besignificantly modified

16,17when fine reactive

powders (fillers) and additives are present as inthe case of deflocculated castables. The natureof these reactions depends to a large extent uponeach unique combination of CAC, fillers and

additives. It is possible in these systems todistinguish short-term hydration reaction effects,related to castable placing characteristics, fromlonger-term hydration mechanisms that are linkedto castable hardening. The reactions are complexand occur at all stages through the castableprocessing chain. The identified chemicalreactions occur via solution and are based uponionic dissolution, complexing mechanisms andprecipitation reactions. The effect of the surfacemust also be considered. The impact of thesecomponents as well as the base mineralogy onCAC hydration and the link to several key

castable properties will be discussed in the nextsection.


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6Factors affecting CAChydration and castableproperties

6.1 MineralogyCAC mineralogy has a large impact on both theplacing properties and castable workability aswell as the hardening. As shown in table 2 thehigher the C/ A ratio the greater the reactivity ofthe CAC.

AFNOR mortar flow properties

0

20

40

60

80

100

120

140

0 15 30 45 60

Time (mins)

%

flow

40% Alumina CAC

50% alumina CAC

147

218

169

255

AFNOR Mortar set time (min)

Initial set Final set

Fig. 4: Placing and set properties of two Calciumaluminate cement

However, all commercial products are comprisedof a multi phase assemblages and whenconsidered together the effect of each phase isnot linear. This is illustrated in Fig. 4 which showsthe basic placing properties of 2 different CAC

cements the properties of which are shown intable 1. As can be seen the CAC with a higherC12A7 contents has more rapid flow decay alongwith a quicker set.An explanation for this difference in behaviourcan be found by studying the hydration reactionsof the different pure calcium aluminate phases.C12A7 has a higher C/A ratio (1,7/1) than CA(ratio = 1) and this gives a higher calcium ionconcentration in solution than in the case of CA.This brings the solution into a rapid oversaturation with respect to C2AH8 and it is thisphase that precipitates rapidly. These hydrates

are formed at an early stage in the hydrationprocess. These hydrates serve to reduce the flowvia a flocculation and decrease the nucleationtime leading to a rapid massive precipitation i.e.shorter set. This is illustrated in Fig. 5 whichshows the conductimetry curves for the C12A7phase at different temperatures. If the curves at20C are compared it can be seen that the C 12A7containing cement displays a very rapiddissolution followed by a rapid precipitation with avery short nucleation period. The massiveprecipitation occurs at a much shorter timeinterval than the A phase. The CA phase shows

the classical dissolution, nucleation andprecipitation periods. Fig. 5 taken from reference12 also shows the impact of temperature uponhydration as measured by conductimetry withclear differences between CA and C12A7 phasesbeing evident. In the case of C12A7, theprecipitation of C2AH8 phase

8accelerates

continuously as temperature is increased. Bycontrast, the anomalous setting of CA can beseen around 28C where the precipitation takesplace later as the temperature increases fromaround 20 to 28C. The explanation for thisanomalous behaviour is linked to the increasing

difficulty of nucleating CAH10 as the temperatureincreases. This arises as the solubility of CAH10increases with increasing temperature making i tmore difficult to nucleate. At the same ti me thenucleation of AH3 becomes quicker astemperature increases. Up to 27C the nucleationspeed is controlled by the formation of CAH10(increasing precipitation time with temperature).Above this temperature the nucleation of AH3 ismore rapid than CAH10 and this controls thehydration and precipitation times which nowdecrease with increasing temperature.


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Fig. 5: Conductimetry curves for two differentcalcium aluminate cement phases

This is seen in real cement systems as alengthening of set around 27C. Fig. 6 shows thiseffect as measured by the exothermic profiles ofan AFNOR sand mortar system and a 70%

alumina cement at different temperatures.

5

10

15

20

25

30

35

40

45

50

0,5 1,5 2,5 3,5 4,5 5,5 6,5 7,5 8,5 9,5

TIme hours

TC

8C

12C

20C

25C

28C

32C

35C

42C

Fig. 6. Exothermic profiles at differenttemperatures for a 70% alumina cement.

The anomalous set around 28C can be seen bythe longer time taken (around 8 hours) to reachthe exothermic peak temperature.

6.2 Additives

Chemical additives or admixtures can be used tomodify the placing properties of refractory

castables. This is achieved via two basicmechanisms, which can be active either asunique mechanisms or as a combination of bothtypes:

- Modification of CAC reactivity viachemical interactions with the hydrationreactions and kinetics

- Modification via a physical effect such assteric dispersion or viscosity modificationvia water retention.

The nature of chemical interactions of additives

on calcium aluminate cements is shown in Fig. 7.Depending upon the specific additive thedissolution, nucleation and precipitation reactionsof the CAC can be modified

18,19,20,21,22The

specific nature of interaction is also dependant tosome extent on the starting mineralogy as well asthe other components of a castable system.

Conductimetry curves for C12A7

Conductimetry curves for CA


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Mix ing D is soluti on A ct ion of r et arde rs

Stiffening

Nucleation

Action of accelerators

Set

Precipitation

Strength

Hydrat ion

Time

scale

=

Action of retarders

Tri sodium citrate

Citric acid

Sodium Carbonate

Lithium Carbonate Hydrated lime

Tri sodium citrate

Citric acid Sodium phosphates

Fig. 7: The mode of action of generic additives onCAC hydration.

Two specific examples of two classical additives,lithium carbonate (LC) and tri-sodium citrate(TSC), upon the hydration of a 70% aluminacement are shown in Fig.8.1 and 8.2 These well-known additives are used to accelerate hydrationin the case of LC and retard hydration in the caseof TSC. Conductivity analysis shows themechanism of action for each of these additives.With increasing dosage of LC, the massive

precipitation time is reduced significantly whilethe dissolution time remains constant. In fact, thenucleation period decreases. The inverse is truefor TSC, where the nucleation time increases asthe dose increases.

The actual mechanism can often be deduced bywithdrawing samples of the suspension atspecific intervals and stopping the reaction bywashing the sample with alcohol, followed bysubsequent (ICP, XRD) analysis of the solution.The shortening of the massive precipitation timein the case of LC has been found to be due to an

increased nucleation rate of alumina trihydrate(AH3).

0

200

400

0 0,05 0,1 0,15 0,2

Lithium Carbonate %

(mins)

Dissolution

Precipitation

Fig. 8.1 : The effect of lithium carbonate on thehydration of a 70% alumina cement

0

400

800

1200

1600

2000

0 0,5 1 1,5 2 2,5 3

Tri-sodium citrate %

(mins)

Precipitation

Dissolution

Fig. 8.2: The effect of trisodium citrate on the

hydration of a 70% alumina cement

The retardation of the massive precipitation seenwith TSC is due to a lengthening of the inductionperiod. This is believed to be due, from solutionchemistry analysis, to the formation of a gelcontaining citrate, calcium and aluminium ions.The downside is that this gel can perturb castablerheology and decrease the flow. Theseinteresting results show that the action of theseadditives upon the three stages of CAC hydrationis specific. This opens the possibility to envisagethe use of multiple additives to optimise castablesystems, each acting upon a specific phase ofthe hydration process. Thus, the combination ofthese two additives in a castable would give acastable with a long working time followed by arapid hardening.


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6. 3 Reactive fillers

The two most commonly used fine fillers arefume silica and reactive alumina. Each of thesefillers provokes different interactions with CACand its hydration. The specific properties ofreactive alumina (BET, Soluble sodium) and theirimpact upon CAC hydration have beenreported

23,24in the context of reduced cement

castable systems. The accelerating effect of boththe surface and the soluble sodium content onthe dissolution and precipitation reactions was

demonstrated. This is in contrast to fume silicawhich tends to have a retarding effect upon theCAC dissolution.

The hydration mechanism of a 70% aluminacement is significantly modified in LCC systemscontaining fume silica. This is particularlynoticeable when a dispersing additive (in thiscase sodium tri-polyphosphate TPP) is present.

The initial dissolution is delayed (Fig. 9)compared with the cement only system as shownin figure 3. Also, the dissolution occurs at a much

slower rate with the LCC system. However, theprecipitation occurs during a similar time periodfor both the dispersed system and the cementalone. The precipitation time corresponds wellwith the onset of hardening and the acquisition ofmechanical resistance in castables. The initiallevel of conductivity is derived from the TPPadditive which is added directly into solution. Thisalso offers a possibility of using conductivity as ameans to control the TPP dose in castables. Theinitial conductivity is strongly correlated with theTPP dose.

Time (minutes)

0

0,5

1

1,5

2

2,5

3

0

0,5

1

1,5

2

2,5

3

0 25 75 100 125 175 2000 25 75 100 125 175 200

Conduc

tiv

ityms

/cm

CAC + Fume silica + TPP 0,075

CAC + Fume Silica

Fig. 9. Conductimetry curves for differentcombinations of CAC, fume silica and TPP

The delay in CAC dissolution is the mechanism25

which provides the castable with sufficientworking time. The longer the delay in dissolution,the longer the working time becomes. Thisretarding effect on CAC dissolution is not seenwhen binary combinations of phosphate and CACor fume silica and CAC are analysed. It ispresumed, therefore, that the surface of the fumesilica in the presence of phosphates also plays akey role. It can be considered that the length ofthe working time of low cement castables resultsfrom a series of interactions between theadditives and the fine fillers, together with theCAC. The working time is not simply dependentupon the cement alone as in the case of classicalor conventional castables with a high cementdose. Thus, by studying the duration of thisdelayed dissolution through conductimetry, amethod exists by which castable working timescan be measured and optimised more preciselythan through classical methods.

The practical implications of this are that in areduced cement castable system when onlyalumina is used as the fine filler then provisionmust be made for a retarding mechanismotherwise the flow decay will be extremely rapid.This is further illustrated in the example below.


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A study of numerous low cement binder phasesystems (variations of both fillers and additives)has highlighted

16significant differences in the

exothermic profiles measured by calorimetry.Two exothermic peaks can be identified. The twopeaks have been generically labelled as Pi andPm. The points Pi and Pm are taken as thepoints of maximum heat flow and the time takento reach these peaks is recorded. The peak Pmis believed to correspond to the massiveprecipitation of the hydrates, whereas Pi isassociated with the dissolution phase of the

cement. In terms of castable placingcharacteristics, Pi is associated with the loss ofworkability and castable flow decay. Pmcorresponds to the onset of strengthdevelopment during castable hardening. Thereexists a relationship between working time of thefull castable and the apparition of this peak, butthe relationship depends upon the composition ofeach specific binder phase composition (fumedsilica, alumina etc). Fig. 10 illustrates this effect.The curves represent the peak Pi for two differentsystems of reactive fillers. The first graph showsa system containing 25% of a 70% alumina

cement, 50% of a reactive alumina (CT3000SG)and 25% of fumed silica (Elkem 971U). Theadditive system is a binary system (sodiumpolymethacrylate and citric acid) added at 0,5%by weight of the total castable binder phase. Thesystem in the second graph is identical except fora substitution of the fumed silica by the reactivealumina (i.e. CAC=25% and reactive alumina =75%). The dotted line in both graphs shows themeasured working time of each castable whichcorresponds to the apparition of Pi in both cases.The graphs provide some explanation of the wellknown effect that removing fumed silica from the

system results in a much shorter working time ofthe castable.

25%CAC50 % reactive alumina25 % fume silica

w/g *10 -3

time (mins)

00

2

4

1

3

5

15 30 45 60 75 90 105


w/g *10 -3

time (mins)

00

2

4

1

3

5

15 30 45 60 75 90 105


w/g *10 -3

time (mins)

00

2

4

1

3

5

15 30 45 60 75 90 105

w/g *10

25% CAC75% reactive alumina

-3

time (mins)

00

2

4

1

3

5

15 30 45 60 75 90 105

w/g *10


-3

time (mins)

00

2

4

1

3

5

15 30 45 60 75 90 105

w/g *10


-3

time (mins)

00

2

4

1

3

5

15 30 45 60 75 90 105


-3

time (mins)

00

2

4

1

3

5

15 30 45 60 75 90 105

Fig. 10: The impact of different filler systems uponthe first calorimetric peak.

The heat flow data clearly shows that there is achemical reaction related to the fillers and that,

although they are not hydraulic binders, they actin a chemical sense within low cement castablesystems. In effect, the fume silica provides astrong retarding effect upon the cementdissolution, which gives a usable working time tothe castable. By contrast, the system withoutfume silica does not have this retarding effectupon cement dissolution. As a consequence, ashort working time and rapid flow is expected. Inthe case of an actual castable, this would need tobe corrected through a modification of theadditive system, for example, by using a fluidifier


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which also retards the CAC or via theincorporation of a specific retarder. This appliesspecifically to systems that are controlled by ionicinteractions. Another alternative exists and that isto use a dispersion system that relies on stericeffects

26to overcome the impact of an intrinsic

accelerating effect of reactive alumina.

Table 5: The effect of fume silica properties onCAC and castable characteristics

Element/

Property

%increasing

Effect on CAChydration

Impact on LCcastable propertiesand characteristics

Na2O, K2O,MgO, SO3

Provoke creation ofgels and flocculation

Reduced flow andrapid flow decay

SO3 Retards hydrationkinetics

Increased hardeningtime

pH Acceleratesprecipitation

Decreasedhardening time

BET Acceleratesprecipitation.

Reduced flow andrapid flow decay

LOI/moisturecontent

Retards dissolutionvia hydroxylation ofthe surface of thecement grains

Promotes presence ofearly hydrates whichaffect rheology

Increases ageingand shelf life of drymix

Reduces flow

Carbon Delays precipitationreactions

Reduces mechanicalstrengths

Fume silica displays a natural retarding effect onthe CAC dissolution when used in combinationwith additives such as phosphates that rely onelectrostatic dispersion and so the managementof working time is more straightforward. However,the variable nature of impurities in fume silica canperturb the hydration reactions and lead to

variable working and hardening times in reducedcement castables. The impurities present in fumesilica as well as the basic physical properties canhave a marked effect on castable properties viainteractions with the hydration process. The maineffects are summarised in table 5:The following example illustrates the differencesin castable properties that result as aconsequence of fume silica type. Two fume silicatypes have been evaluated in low cementcastable based upon sintered alumina with 5%

fume silica and 5% of a 70% alumina CAC.Sodium Tripolyphosphate was used as thedispersant at 0,03% (dry wt. basis). The analysisof the fume silica is shown in table 6.

Table 6: Analysis of two fume silica types

FS1 FS2

BET (m2/g) 20 17.6

% C (free) 0.27 0.81

H2 O % 0.789 1.982

SiO2 % 97.64 94.71

Fe2O3 % 0.07 0.1

MgO % 0.22 0.56

SO3 % 0.08 0.14

K2O % 0.28 1.28

Na2 O % 0.09 0.13

pH 6.3 7.9

FS1 contains fewer impurities in terms of alkalis,free carbon and has a more acidic pH and lowersulphate content. Based on the above discussion

it would be expected that this fume silica yieldsbetter placing properties in terms of flow, flowdecay and castable working time.

0

20

40

60

80

100

120

0 50 100 150 200

Time (mins)

Vibra

tion

flow

%

FS2

FS1

Working

time = 140

100 mins


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Pi PmPi PmPi Pm

Pi PmPi Pm

dQ/dt[J/g*h]

Time [h]

Time [h]

dQ/dt[J/g*h]

FS2

FS1Pi PmPi PmPi Pm

Pi PmPi Pm

dQ/dt[J/g*h]

Time [h]

Time [h]

dQ/dt[J/g*h]

FS2

FS1

Fig. 11: The effect of two fume silica types on

castable placing properties and CAC hydration

Fig. 11 shows the experimental data whichconfirms this hypothesis. In addition a study of asimplified binder system (CAC + FS) shows usingcalorimetry reveals the effect upon CAChydration. The intermediate peak Pi occurs laterwith FS1 and the induction period (differencebetween Pi and Pm) is also longer.

The exact mechanism of interaction around thisintermediate peak is unclear but an explanationhas been proposed

27whereby a primary layer of

CASH develops on the surface of the cementgrain this then converts into an alumina poorCASH or CSH type layer. In so doing a largequantity of [Al(OH)4]

-is liberated into the solution.

This renders the solution oversaturated andmetastable with a likelihood of AH3 precipitationand flocculation i.e. the end of castable workingtime. A clearer understanding of thesemechanisms that describe the variousobservations is still needed and is of importance

if the complete control of such systems is to beensured.

7 Initial drying and the impact on

CAC hydrates

Once the installed castable is hard, and themoulds removed drying can commence. Duringheating the microstructure of the bond phaseundergoes various transformations.

A marked difference in mechanical propertiesexists between the as cast state and after dryingat 110C depending upon the starting mineralogyof the CAC. Figure 12 demonstrates this.

- In the case of the 50% alumina cement,on heating from ambient temperature to100C, the cement hydrates that areinitially formed are converted to thestables hydrates AH3, and C3AH6, with arelease of free water, increase in densityof the hydrates, increased porosity and areduction in the measured mechanicalstrength

- In the case of the 70% alumina cementwhich contains appreciable quantities ofCA2 an increase in strength is observedafter drying. As in the example aboveinitial heating leads to conversion and therelease of free water. In addition, theexcess water used for casting is alsoliberated as steam. Some of the waterreleased by the conversion of metastableto stable hydrates leads to furtherhydration of the CA2 and there is an

increase in strength, as discussed in aprevious study28


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0

10

20

30

40

50

60

70

CCS

MPa

24hr 110C 24hr 110C

50% alumina C AC 70% alumina C AC

- 38% + 30%

Fig. 12 Comparison of as cast and dried strengthsof two different CAC types

8 Calcium aluminate cement de-

hydration

The de-hydration phase is illustrated using a highpurity castable with 30% of an 80% CAC and70% of a sintered alumina with a maximumparticle of 7mm and with a water addition of8,5%. During the heating, the bond phaseundergoes various transformations which can befollowed either by an analysis of the phasecomposition or indirectly via the mechanical andphysical properties. These are shown in Figs. 13and 14. The example serves to illustrate theintimate link between CAC dehydration reactions,themselves a consequence of the initialhydration, and the installed castable properties:

- Up to 24 hours after casting, there isincomplete hydration which continues afterdrying at 110C; the residual CA as well as alarge part of the CA2is hydrated during dryingat 110C. At this point the strength is at amaximum reflecting the degree of hydratedevelopment. AH3 appears as a crystallinephase gibbsite

- Between 100 and 400C: AH3 and C3AH6gradually decompose to give amorphousanhydrous relics and water vapour. Thiswater vapour has to escape from theconcrete. The porosity increases from 13 to17% and consequently the mechanicalstrengths decrease. This two stage release ofwater is an advantage of CAC based systemsas it helps to reduce build-up in vapourpressure compared to other bond systemswhich tend to release water over a narrowtemperature range.

- 4 00-900C: During this subsequentdehydration phase the previously stablehydrates C3AH6 continue to dehydrateprogressively to C12A7H and at the same timeGibbsite (AH3) is transformed to Aluminahydrate relics. The measurable mineralogydoes not change significantly during thisperiod. Porosity continues to increase toaround 23% and strengths decline relative tothe strengths at 110C

- Above 800-1100C, the relics of the cementphases and elements of the bonding phase

re-crystallize first to C12A7 then CA and from950C CA2 starts to form progressively.Porosity reaches a maximum of around 25%and the strengths tend to a minima

Dry

110

900

1000

1100

1450

al alpha

al beta

C12A7

gibbsite

0

10

20

30

40

50

60

70

80

wt%

phases

Temperature /C

al alpha

CA2

al beta

CA

C12A7

CA6

gibbsite

Fig. 13 The change in phase composition as afunction of temperature for a high purity aluminacastable


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- 1100 1300C CA2 reaches a maximum ofabout 30% at about 1100C. Aluminadecreases as it is consumed in the reactionCA + A CA2

- Above 1300C the reaction continues towardsthermodynamic equilibrium, with CA6 formedfrom CA2 and Al2O3. The final content of CA6reaches about 40% at 1450C. The porositydecreases as evidence of the sinteringreactions taking place and the strengthsincrease in parallel with the development of

the ceramic bonding.

0

5

10

15

20

25

30

0100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

Temperature C

Porosity%

+

MOR

MPa

0

10

20

30

40

50

60

70

80

90

100

CCS

MPa

MOR Porosity CCS

Fig. 14 Change in Mechanical properties andporosity of a high purity castable as a function offiring temperature.

9 Summary and conclusions

This review paper has shown the intimate linkbetween CAC hydration and castable properties.Improvements in castable technology can only besuccessfully achieved via a greater understanding of this link.

This is especially true in the case of reducedcement systems where the CAC and its hydrationare affected by the presence of fine fillers andadditives. Hence, the notion exists that the bindersystem in reduced cement castable systems is

comprised of an interdependent system betweenthree key components, CAC, fillers and additives.It is the versatility of the CAC in terms of thepossibility to modify the hydration reactions thathas made the development of such castablesystems possible.

It is likely that the future generations of castableswill become increasingly sophisticated asinstallation techniques and performanceconstraints become more demanding. Thedevelopment of these castables will only besuccessful if they are derived from an

understanding of the underlying mechanisms andinteractions. Thus, the continued development oftechniques able to study these complexinteractions is forecast to be a key success factorin the future.

10 References

1) W.E. Lee, W. Vieira, S. Zhang, K. Ghanbari-Ahari, H. Sarppoolaky, C. Parr, Castablerefractory concretes, Intl. Mat. Reviews, Vol46, No.3, 2001, 145- 167

2) T. Fllman, G. W alenta, T. Bier, B. Espinosa,K.L. Scrivener, Quantative Rietveld analysisof calcium aluminatec cements, WorldCement Research, June 1999, 91 -96

3) C.M. George, Aspects of Calcium AluminateCement (CAC), Hydration Proceedings ofthe Refractories Symposium, AmericanCeramic Society, St. Louis Section, St. Louis,1994.

4) M. Ohta, T. Kimma and H. Yanagida, The

early stage hydration of CaO.2Al2O3, J.Ceram. Soc. Jap. Int. 96, 36-40 (1988)

5) A.Capmas S. Mhmel, W. Gener, D. Mller,The Behaviour of CA/CA2 Cements DuringHydration and Thermal Treatment, Proc.UNITECR 97 (New Orleans, USA, 1997)1273-1282.

6) Th. Bier, N.E. Bunt, C. Parr, CalciumAluminate Bonded Castables TheirAdvantages and Applications, Alafar


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proceedings, Bariloche, Argentina, Vol. 2,1996, pp. 73-84.

7) E. Criado, D. A. Estrada, and S. De Aza,Dilatometric Study of the Formation of CA2and CA6 in Cements and RefractoryConcretes, Bull. Sp. Ceram. Glass Soc. 15[5] 319-21 (1976).

8) K. L Scrivener and A. Capmas, CalciumAluminate Cements; Chapter 13 in LeasChemistry of Cement and Concrete, 4

thed.

Edited by Peter C. Hewlett, John W iley and

Sons New York, 1998.

9) S. Rashid, P. Barnes, X. Turrillas, The rapidconversion of calcium aluminate cementhydrates as revealed by synchrotron energydiffraction, Adv. Cem. Res 4 [14] 61-7(1991)

10) F. Sorrentino, D. Sorrentino, C.M. George,Mechanisms of Hydration of CalciumAluminate Cements, in Scalny J.P. ed.Materials Science of Concrete, vol IV.Amercian Ceramic Society 1995; 41-90.

11) A.J. Majumdar, R. Roy, The system CaO-Al2O3 H2O, J. Am Ceram Soc. Vol 39, nb.12, 1956, 434-442

12) A.Capmas, D. Mntrier-Sorrentino, D.Damidot, Effect of Temperature on SettingTime of Calcium Aluminate Cements, inCalcium Aluminate Cements, ed. R.J.Mangabhai, Chapman and Hall, 1990.

13) Th. A. Bier, A. Mathieu, B. Espinosa and J.P. Bayoux, "The Use of Conductimetry to

Characterise the Reactivity of CalciumAluminate Cements", UNITECR, Sao Paulo,Brazil, 1993, pp.705-716.

14) C. Fentiman, C. M. George, R. Montgomery,The Heat Evolution Test for Setting Time ofCements and Castables, Advances inCeramics, Amercian Ceramic Society, 13;1985.

15) V.H.R Lamour, P.J.M. Monteiro, Microscopicstudies of the early hydration of calcium

aluminate cements, Proc. Intl. Conf. OnCalcium Aluminate Cements, R.J.Mangabhai,ed, F.P. Glasser, ed, IOMcommunications, Edinburgh, 2001, pp169-180

16) H. Fryda, K. Scrivener, Th. Bier, B. Espinosa,Relation Between Setting Properties of LowCement Castables and Interactions Withinthe Binder System (CAC-Fillers-Additives-Water), Vol. 3 UNITECR 97 Proceedings,USA, 1997, pp.1315-1323.

17) J.P. Bayoux, J.P. Letourneux, C. M. George,

Fumed Silica and Cement Interactions Parts1-3, UNITECR 89, Anaheim, USA, 1989.

18) Chappuis et Al., "Admixture for calciumaluminates cements in refractory concretes"Presented at the American Society meeting,Indianapolis, April 1989.

19) S.A. Rodger, D.D. Double, The chemistry ofhydration of high Alumina cement in thepresence of accelerating and retardingadmixtures, Cement and ConcreteResearch, Vol.14, 1984, pp 73-82

20) D. Sorrentino, J.P. Bayoux,R. Montgomery,A.Mathieu, A. Capmas, The Effect of SodiumGluconate and Lithium Carbonate on theProperties of Calcium Aluminate Cements,UNITECR 91, Germany 1991, pp. 530-533.

21) Th. A. Bier, A. Mathieu, B. Espinosa and C.Marcelon ; "Admixtures and Their Interactions with High Range CalciumAluminate Cement." ; UNITECRProceedings, Kyoto, Japan, Vol. 1,1995, pp.357-364.

22) Th. A. Bier, Ch. Parr ; "Admixtures withCalcium Aluminate Cements and CAC BasedCastables" presented at the 28th Annual SACeramic Society Symposium atJohannesburg, RSA 1996.

23) A. Mathieu, A. Capmas, J.P. Bayoux and D.Richon; Calcium aluminate cement andreactive alumina; UNITECR 93, Brazil, 1993


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24) C. Parr, C; Whrmeyer, B. Valdelivre, A.Namba, Effect of formulation parametersupon the strength development of calciumaluminate cement containing castables, J.Tec. Assoc. refractories Japan, 23 [4] 231 238 (2003)

25) A. Mathieu, B. Espinosa, ElectricalConductimetry to Control LCCs,Proceedings of Alafar Conference, Mexico,1994, pp. 139-156.

26) C. Parr, B. Valdelivre, A. Namba, The

effect of ambient temperature conditions onthe placing properties of deflocculatedrefractory castables, Taikabutsu Overseas,21,[3], 148-154, (2000)

27) S. Moehmel, W. Gessner, Th. Bier, C. Parr,The influence of microsilica on the course ofhydration of monocalcium aluminate, Proc.Intl. Conf. On Calcium Aluminate Cements,R.J. Mangabhai, ed, F.P. Glasser, ed, IOMcommunications, Edinburgh, 2001, pp 319-329

28) E. Nonnet, N. Lequeux, P. Boch, Elasticproperties of high alumina cement castablesfrom room temperature to 1600C Journal ofthe European Ceramic Society, 19, (1999)p1575-158

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