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Technical PaperReference : TP-GB-RE-LAF-071

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CALCIUM ALUMINATE CEMENT – WHAT HAPPENS WHEN

THINGS GO WRONG ?

Chris Parr,

Kerneos SA, Paris, France

Presented at IRE annual conference, Rotherham UK, September 2008


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ABSTRACT

Monolithic refractory castables, bonded with calcium aluminate cements, have a rich history ofover 80 years. They have evolved from high cement conventional formulations to the era ofreduced cement castable systems1 that can be placed using a variety of techniques such ascasting, gunning, self flow, pumping and shotcreting. These developments have been madepossible by the versatility of the calcium aluminate cements which play a major role in determiningthe properties of such monolithic refractories.The usage chain of monolithics containing calcium aluminate cement contains several steps suchas mixing, placing and consolidation, commissioning and dry out and finally use in service. Each ofthese steps within the castable placing chain is intimately linked to the hydration process ofcalcium aluminate cement (CAC). Their cementitious properties enable the refectory to be cast inplace using and to set and harden under ambient conditions. As the refractory is heated to hightemperature, the water needed for placing and the formation of hydration products is driven off andthe hot strength is provided by sintering and ceramic bonding. This process of first heating or “dryout” must be carefully managed to avoid undesirable strength loss, spalling or explosion.

This paper reviews some of the undesirable effects that can occur during the use of calciumaluminate based castables either through unwanted effects and reactions or the non respect ofguidelines. These effects will be illustrated through a variety of examples taken from actualexperience. The impact of parameters such as temperature, mineralogy and other formulationcomponents will be discussed and some potential precautionary and preventive measuresdiscussed.


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1 Calcium aluminate phases

The phases present in a given calciumaluminate cement (CAC) depends upon anumber of parameters of which the chemistry

is the most important. The anhydrous CACphases present in a range of commercialproducts are shown in table 1.

Table 1: Chemical composition and mineralogy of three different calcium aluminate cements

% 40% aluminaCAC

50% alumina CAC 70% AluminaCAC

Al2O3 37,5 – 41,5 50,8 – 54,2 68,7 – 70,5CaO 36,5 – 39,5 35,9 – 38,9 28,5 – 30,5SiO2 42, - 5,0 4,0 – 5,5 0,2 – 0,6FeO+Fe2O3 14,0-18,0 1,0 – 2,2 <0,4Ti02 <4,0 <4,0 <0,5CA 47-57 64 - 74 54 – 64CA2 +++A +C12A7 1-5% Trace TraceC2AS ++ ++C4AF ++C2S TraceCT +

Cement chemistry abbreviations: C = CaO, A = Al2O3, S = SiO2, F = Fe2O3,M = MgO, T = TiO2, H = H2O

The mineralogy shown in table 1 isdetermined by quantitative methods using theRietveld approach2. The dominant phase inall cases is CA, calcium monoaluminate. Thereactions of calcium aluminates and theirkinetics will be influenced by the compositionof the mineralogical phases present in a

given cement. A general comparison of thereactivity of the anhydrous phases is shownin table 2 taken from George3. It can be seenthat as the phases become more lime rich,i.e. as the C/A ratio increases, then so doestheir reactivity.

Table 2: The reactivity of the various calcium aluminate phases

C3A C12A7 CA CA2 CA6

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

Reactivity at 20°C Very rapid Fast Slow Very slow None

Table 3. lists some useful properties of thephases found in CAC. Monocalciumaluminate (CaO.Al2O3 or CA) is the mostimportant component of CAC’s and has arelatively high melting point (1600oC), anddevelops the highest strength among thephases listed during the relatively short timeavailable for hydrating refractory concretes. Ittakes some time to start setting, but hardensrapidly after the initial set. Calciumdialuminate (CA2) is a secondary phase inCAC’s and is more refractory than CA but

takes a long time to set due to its lowhydraulic activity although accelerated astemperature increases4. While hydration ofCA is known to be accelerated by thepresence of CA2, the opposite does not holdtrue, and the hydration of CA2 may actuallybe hindered by the presence of CA5. Thestrength of CA2 after three days hydration iscomparable to that of the pure CA. C12A7

hydrates rapidly and has a relatively lowmelting point.


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Table 3 Properties of CAC Mineral Constituents

Mineral Chemical Composition (wt %) Tm Density Crystal

C A F S (oC) (g/cm3) System

C 99.8 2570 3.32 Cubic

C12A7 48.6 51.4 1415-1495 2.69 Cubic

CA 35.4 64.6 1600 2.98 Mon.

CA2 21.7 78.3 1750-1765 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

C2S and C4AF are common in Portlandcement, but can also occur in the high-silicaand iron-rich low-purity CAC’s, respectively.C4AF forms hydrates of calcium aluminateand calcium ferrite or solid solutions of thetwo hydrates. C2AS (gehlenite) shows littletendency to hydrate and is an undesirablecomponent of alumina cement which limitsrefractoriness and hot-strength properties6.CA6 is the only non-hydrating phase in the

pure calcium aluminate system and is often areaction product in alumina castables bondedwith high-purity aluminate cement. It isbelieved that CA6 is most readily formed inalumina castables when using CA2 as aprecursor7. Alumina (A) is sometimespresent as an addition and this is especiallytrue in the case of 80% alumina calciumaluminate cements.

2Calcium aluminate cementhydration

The mechanism of hydration of calciumaluminate is via solution, where an anhydrousphase dissolves and is followed by theprecipitation of the hydrates from solution3.Three distinct phases can be identified;dissolution, nucleation and precipitation. Thehydration process10 is initiated by thehydroxylation of the cement surface followedby dissolution of cement in water and theliberation of calcium and aluminium ions. Asmall amount of gel like hydrates will form atthis point if the solution concentration risesover the super saturation limit level of thehydrates C2AH8 and AH3. The dissolution willcontinue, with a consequent increase in theconcentration of calcium and aluminium ionsuntil a saturation point is reached. This is thepoint which is the equilibrium solubility of theanhydrous phases with the hydroxylatedsurface layers. After the dissolution phase

there follows an induction period during whichnuclei attain a critical size and quantity.Once this is achieved the nucleation phase isfollowed by a rapid and massive precipitationof the hydrates, leading to a drop in solutionconcentration. This is a dynamic processwhich continues to consume anhydrouscement. In a physical sense, it is the growthof these hydrates which interlock and bindtogether to provide mechanical resistance.The driving force of the hydration reactions isthe lower solubility of the hydrates comparedto the anhydrous species. In contrast toPortland cements where the hydrates formedremain broadly similar8 with time andtemperature the hydration of CAC is stronglydependent upon temperature12. The ambienttemperature significantly modifies thehydrates that result. The associatedhydration reactions are shown in table 4.


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Table 4: Hydration scheme for monocalcium aluminate

Temperature Hydration Reaction

< 10°C CA + 10H CAH10

2CA + 11H C2AH8 + AH310 – 27°C

CA + 10H CAH10

>27°C 3CA + 12H C3AH6 + 2AH3

F(t°C + time) 2CAH10 C2AH8 + AH3 + 9H

3C2AH8 2C3AH6 + AH3 + 9H

CAH10 C2AH8

Density = 1720 Kg/m3

Cubic system : Hexagonal



C4AH13 C3AH6




Cubic system : Cubic

Fig. 1: Key properties and Micrographs of the calcium aluminate hydrates


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Below 10°C it is the formation of CAH10 thatdominates and this continues up to around27°C. Between 10 and 27°C CAH10 and C2AH8

are formed together. At higher temperaturesabove 27°C C3AH6 formation occurs early inthe hydration process. It is believed that theformation of C3AH6 often passes through atransient C2AH8 phase8. The other twoimportant reactions are the conversion of themetastable hydrates, CAH10 and C2AH8 to thestable C3AH6 hydrates. These reactions aretime, temperature and humidity dependant.The form of gibbsite (AH3) also changes withtemperature. At lower temperatures it existsas a gel form and becomes increasinglycrystalline as the temperature increases.Analogous hydration reactions can be writtenfor CA2 as well as for C12A7. Other hydrateshave also been reported10,11 and are knownto exist, for example the hydrates C4AH13 andC4A3H3. The later existing mainly underhydrothermal conditions (such as exists inrefractory concrete during the dry out phase).The morphology and some key properties ofeach of the main hydrates are shown in Fig. 1

The de-hydration phase12,13, is illustratedusing a high purity castable with 30% of an80% CAC and 70% of a sintered alumina witha maximum particle of 7mm and with a wateraddition of 8,5%. During the heating, thebond phase undergoes varioustransformations which can be followed eitherby an analysis of the phase composition orindirectly via the mechanical and physicalproperties. These are shown in Fig. 2. Theexample serves to illustrate the intimate linkbetween CAC dehydration reactions,themselves a consequence of the initialhydration, and the installed castableproperties:

- Up to 24 hours after casting, there isincomplete hydration which continuesafter drying at 110°C; the residual CAas well as a large part of the CA2 ishydrated during drying at 110°C. Atthis point the strength is at amaximum reflecting the degree ofhydrate development. AH3 appears asa crystalline phase gibbsite

- Between 100 and 400°C: AH3 andC3AH6 gradually decompose to giveamorphous anhydrous relics andwater vapour. This water vapour hasto escape from the concrete. Theporosity increases from 13 to 17%and consequently the mechanicalstrengths decrease. This two stagerelease of water is an advantage ofCAC based systems as it helps toreduce build-up in vapour pressurecompared to other bond systemswhich tend to release water over anarrow temperature range.

- 400-900°C: During this subsequentdehydration phase the previouslystable hydrates C3AH6 continue todehydrate progressively to C12A7Hand at the same time Gibbsite (AH3) istransformed to Alumina hydrate relics.The measurable mineralogy does notchange significantly during this period.Porosity continues to increase toaround 23% and strengths declinerelative to the strengths at 110°C

- Above 800-1100°C, the relics of thecement phases and elements of thebonding phase re-crystallize first toC12A7 then CA and from 950°C CA2

starts to form progressively. Porosityreaches a maximum of around 25%and the strengths tend to a minima

- 1100 – 1300C° CA2 reaches amaximum of about 30% at about1100°C. Alumina decreases as it isconsumed in the reaction CA + A CA2

3Calcium aluminate de-

hydration


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Dry

24

hr

11

0

30

0

900

95

0

10

00

10

50

110

0

13

50

14

50

al alphaCA2

al betaCA

C12A7

CA6

gibbsite

0

10

20

30

40

50

60

70

80

wt%

ph

ase

s

Temperature /°C

al alpha

CA2

al beta

CA

C12A7

CA6

gibbsite

Fig. 2. The change in phase composition as afunction of temperature for a high purity aluminacastable

- Above 1300°C the reaction continuestowards thermodynamic equilibrium,with CA6 formed from CA2 and Al2O3.The final content of CA6 reachesabout 40% at 1450°C. The porositydecreases as evidence of thesintering reactions taking place andthe strengths increase in parallel withthe development of the ceramicbonding.

4Some examples of whenthings go wrong and why

Three examples of problems linked toinstallation are illustrated. The first is anexample to show the impact water quality onplacing properties. The second and thirdexamples show the unwanted effects that canoccur when raw materials of variable qualityor unknown side effects are used.

The impact of water qualityWater quality is believed to have a greaterimpact on low cement castables than regularcement content castables. This is illustratedin table 5. An andalusite based LC castable(5% cement) was mixed with two differentwater types; type 1 being tap water and type2 water available at an industrial site wherethe castable was to be installed.

The castable cannot be placed at 5% wateraddition using type 2 water. The very highlevel of sulphates can be noted in this watertype. It is for this reason that refractoryproducers normally recommend only to usepotable or drinkable water for usage inconnection with their castables.However, it is not always possible to get suchwater on the job site, and even if this is thecase, the ph-value can either be high or low,with a differing degree of impurity. Sea wateror process water is unfit, as well as water withorganic or chemical contents. Decisionshave to be made as to whether to continuethe installation or not. From a materialperspective there is a limited possibility to useadditives to overcome the intrinsiccharacteristics of water in terms of thedetrimental effect of dissolved salts. This is aclear case where more useful and practicalguidelines as to suitable and tolerable waterquality for low cement castables would bebeneficial. In parallel a simple and robustmeans of assessing water quality should bedeveloped so that potential problems can bedetected prior to the start of casting.

Table 5. The effect of water quality on LC castables

CompoundWater type

1 mg/lWater type

2 mg/l

Calcium as Ca 26,6 372,30

Chloride as Cl 8,7 370,30

Sulphate as SO4 13,8 2628,60

Iron as Fe < 0.07 0,11Magnesium asMg 4,6 422,40

Chloride as Cl 8,7 370,30Electricalconductivity at 25º C 23,6 516,00

pH at 25 º C 8,18 8,04Total suspendedsolids 0,2 57,70% Water :Andalusite LCC 5 5,00

wet out time secs 45 none

Flow t0 % 47 0

Flow t60 % 32 0


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EfflorescenceA second example is linked to thephenomena of efflorescence. This can occurin refractory castables and lead to adestruction of the coherence and bonding inthe castable. It can also be more of anaesthetic problem giving the perception of amaterial quality problem or that something isnot right with the installed quality. There aremany different mechanisms14,15 that causeefflorescence, either as a destructive or nondestructive form and it is probably true to saythey are not all fully understood. The exampleshown in figure 3 is taken from a conventionalcastable based upon a chamotte aggregatewith an addition (20%) of an 80% aluminacalcium aluminate cement. The efflorescencewas detected on the surface of the castblocks some time after casting. Theinstallation company became concerned andan analysis was undertaken prior to thedecision that the concrete could becommissioned and dry out initiated. Sampleswere recovered and the amount of castingwater estimated (12%). Replicate laboratorysize blocks were cast from the same castabledry mix and left to stand. The efflorescenceeffect was re produced in the laboratory afterair curing for 1 week (shown in figure 3)

preceded by an immersion in water for 24hours. Mechanical investigations revealedthat there was no reduction in mechanicalproperties. X-Ray diffraction of the whitepowder shows the presence of, Na2CO3.7H2O, K2CO3 .1.5 H2O and AH3 (gibbsite) asthe major phases.It transpired that a sodium salt based setmodifier had been employed in the dry mixand this contributed to a higher thanexpected (+1% Na2O) sodium content. Theefflorescence is believed to be due to asequence of underlying reactions16 betweenthe sodium, the air and the calcium aluminatehydrates. This is developed in figure 4. Anumber of different hydrated forms of sodiumcarbonate result. Some of which can causerupture or bursting of cast blocks. In theexample illustrated here, no sodiumcarbonate deca hydrate was detected andthe structure remained intact. Clearly, thehigh level of sodium as a soluble salt had adeterminant effect upon the formation ofefflorescence. The relatively high watercontent also favored the efflorescence due toenhanced ion mobility and the final factor waslinked to a lengthy period where the castblocks had been exposed to moisture prior todry out.

Fig. 3. Examples Non destructive efflorescence on the surface of a refractory castable


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Fig. 4. Reactions involved in non destructive efflorescence

The example shows the risks that can occur ifthe level of soluble salts such as sodium isnot controlled within a refractory concrete.

Delayed hardening of castablesAlmost everyone involved in the installation oflow cement castables will have come acrossproblems linked to a delayed setting andhardening after casting. In dramatic casesthis leads to a delay in former removal andcan hold up a large installation. There arenumerous factors that can cause this and arecommonly linked to lower than anticipatedambient temperatures, variable quality rawmaterials, water quality, or even use of oldstocks of dry mixed castables and or rawmaterials. The example shown in figure 5 isof a series of pre-cast blocks based upon abauxite based low cement castable thatdisplayed ‘slow setting’ to the extent that demoulding was longer than 24 hours. Oncedemoulded, the blocks had visible rings orcircles over the entire surface adjacent to themoulds. Testing of physical propertiesrevealed no anomalies; the only explanationthat could be found was that the dry mixcastable had been produced more than 6months previously. It is suspected that an

ageing17 of the dry mix had occurred and thishad translated into a longer than normalsetting. When a low cement castable dry mixages a series of interactions between thedifferent components occurs which in effectdelay the initiation of the hydration of thealuminate cement binder. The rings/circlesare associated with localised setting andthese move progressively outwards creatinga series of hardened spheres within the massof refractory concrete. Eventually the degreeof hardening is continuous throughout theblock and moulds can be safely removed.This heterogeneous setting and hardeningeffect is sometimes termed ‘nodule’ setting orcastable ‘spotting’ An illustration of this canbe found if the pure calcium aluminate binderis considered. The set time is artificiallylengthened by using old cement and or lowertemperatures. The right hand illustration infigure 5 shows such a cement which hasbeen mixed in the pure state withoutaggregates. Just at the point between initialand final set the sample is washed gentlyunder water and a series of nodules can befound illustrating this specific setting pattern.


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Fig. 5. Examples of delayed hardening in castables

The mechanism can be explained by aconsideration of the hydration of calciumaluminates and specifically the nucleationphase. As discussed in section 3 at this pointthere are ions in solution which are notstructured. The precipitation of hydrates isreversible as the small nuclei that form havenot reached a critical size.

Fig. 6. Nodule hardening mechanism

Once this occurs the structure will bestabilised, massive precipitation will continuewith increasing speed and in physical termsthe setting can be seen to occur‘homogeneously’ through the sample at thesame time. If the nucleation phase is slow or

difficult to achieve then the quantity of nucleipossessing the critical size would be low. Inthis case the massive precipitation will occurbut be local and around the nuclei consumingthe water and matrix in the immediateenvironment. These are then are the nodulesthat can sometimes be seen. This is shownschematically in figure 6. Therefore, in theexample considered, a lower reactivitycastable dry mix (due to ageing) induces amore difficult nucleation period which givesrise not only to a localised heterogeneoussetting and the formation of nodules but alsoto a slower overall hardening and demouldingtime.


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5 Summary and conclusions

This review paper has shown the intimate linkbetween CAC hydration and castableproperties. CAC have been used for over 80years in successive generations of refractorycastables which have become increasinglysophisticated as installation techniques andperformance constraints become moredemanding. They have been shown to bereliable and delivered a high degree ofoperational durability. However, there is a riskto the successful usage of castables if certainguidelines and precautions are not adopted.This is particularly relevant in the case ofreduced cement systems where the CAC andits hydration are affected by the presence offine fillers and additives and externalinstallation parameters. The selection ofstable raw materials with known impurities isclearly a critical success factor.With regard to the successful onsiteinstallation of monolithics containing CAC,external installation parameters such asmixing water, mixer type and ambienttemperature all play an important role indictating final installed properties. This isespecially true with respect to the quality ofwater used in terms of impurities arising fromdissolved salts, and organic content. There isa case to be made for all the actors in theusage chain (CAC producer, refractoryproducer, installer and final user) to developand adopt industry wide guidelines for waterquality. Installers and end users would ensurethat the guidelines remain practical and usefulover a wide range of installation environmentsand the CAC producer and refractoryproducers would provide expert materialsknowledge and input. The guidelines wouldneed to be determined on a quantitative basisand a suitable and simple measurementmethod for site use developed in parallel. Thebenefits would surely be a reduction inproblems during installation and lower thantargeted installed properties.

Over 30 years ago, Dr. B. Jackson fromDysons observed in his 1974 presentation toan IRE meeting at Scunthorpe “It’s theChemistry that counts!”citing the need to uselow impurity materials to increase refractorylife. This review paper has shown, albeit in aslightly different context that his words are asrelevant today as they were over 30 yearsago!

A list of references for this paper is availableupon request

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