admixtures and their interaction with High Alumina Cement

Technical PaperReference : TP-GB-RE-LAF-003

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ADMIXTURES AND THEIR INTERACTIONS WITH HIGH RANGE

CALCIUM ALUMINATE CEMENT

by Thomas A. BIER, Alain MATHIEU, Bruno ESPINOSA, Christophe MARCELON

presented at the UNITECR congress, Japan; 1995.


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Abstract

Besides a careful choice of mineral raw materials, sophisticated applications of calcium

aluminate cements (CAC's) cannot be achieved without the use of admixtures.

Admixtures serve to control mostly the rheological behavior and setting (hardening)

characteristics.

In this paper, different concepts of admixtures are presented in order to influence given

properties such as flocculation, coulability and hardening.

The interactions of these admixtures with the CAC are shown by chemical as well as

rheological measurements. The results are discussed with respect to practical

applications.


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1 Introduction

As with all hydraulic binders, the chemical andmineral composition and the fineness of a CACdetermine the rheology and reactivity of thesystem. This means that setting time and flowbehavior of a refractory concrete or mortardepend for a given water cement ratio only fromthe constituents other than the cement. Theseother constituents influence the rheological andhardening properties of a concrete or mortarmore or less. Aggregates are normallyconsidered as inert, a larger influence beingattributed to reactive fine fillers. And the largestinfluence is played nowadays by admixtureswhich are on purpose added to tailor the abovementioned properties. The raw material togetherinfluence each other and constitute finally theperformance of the concrete. These interactionshave in the past very often been represented by atriangle of interactions as shown in Figure 1.

CAC

Reactivefillers

Additives

AggregateGeneral characteristics

(porosity)Granulometry

Reactive phase

Figure 1 - Components influencing the properties in arefractory concrete.

Amongst these components this paper deals withthe admixtures. Admixtures are definedaccording to ASTM C125-88 as"Materials other than water, aggregates, hydrauliccement and fibre reinforcement used asingredients of concrete or mortar and added tothe batch immediately before or during itsmixing." and in the DBV - Manual :"Are chemical substances added to freshconcrete or mortar in order to influence certainproperties of the fresh and/or hardened concrete.The quantities are so small that they don't haveto be taken into account for the calculation of themix design."

An overview about possible admixtures withCAC's is given in /1/.

The major admixtures used in this study anddiscussed in the paper are: Accelerators Retarders Plasticisers - Water reducing agents

2 Experimental approach

Conductivity of stirred suspensionsThis technique, presented in recent publications/2,3/, consists of measuring the electricalconductivity of a stirred suspension of cement inwater at 20° C.

Conductivity is a very good picture of the ionicconcentration in water. The conductivity of thesuspension gives information on the dissolution /nucleation / precipitation steps of the hydrationreaction of cement in water. A schematic pictureof conductivity and related properties is shown inFigure 2, the above mentioned hydration stepsbeing represented as stages I, II and III.

3002001000

0

1

2

3

4

Conductivity

Ca

LOI

Time (min)

Arb

itra

ryU

nits

I IIIII

Figure 2 - Sketch of a conductivity curve for a calciumalumina cement.


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Self Flow ValueThe self flow has been determined for pastes ofpure cement and mixtures of cement andreactive alumina.

The paste is mixed and then filled into a cylinder(diameter = 30 mm, height = 50 mm) which sitson a glass plate. The cylinder is then withdrawn,the paste starts to flow under its own weight anda "cake" with a much larger diameter is obtained.The size of this diameter, expressed in mm, is ameasure of how well the paste flows. As anindication : a flow of 80 mm represents a castinggrade consistency.

Strength after 6 hoursSetting time and strength after six hoursmeasured on 40 x 40 x 160 mm mortar prismscan be used to evaluated the impact ofaccelerators or retarders. Tests are carried onmortars containing quartz sand (0-2 mm)according to NF P 15 - 315.

3 Results

AcceleratorsThe most common, most effective accelerator foralumina cements is lithium carbonate. Lithiumhydroxide acts more violent and is thereforedifficult to dose. However, for winter concreting itmight be appropriate.

Curves of the dissolution and precipitationbehavior of Ciment Fondu

®and Li2CO3 mixes

have been presented and commented in the past/2/. Figure 3 shows the influence of differentconcentrations of lithium carbonate on theprecipitation behavior of Ciment Fondu

®.

20016012080400

1,0

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8 0,025% Li2CO30,005% Li2CO30,1% Li2CO3

Time (min)

Figure 3 - Conductivity curves of Ciment Fondu®

withdifferent Li2CO3 concentrations.

With increasing Li2CO3 content the massiveprecipitation (hydration) starts earlier and hencethe setting time starts earlier and the differencebetween begin and end of set becomes smaller(steeper decrease in conductivity upon massiveprecipitation).

This translates in earlier setting times and afaster hardening (smaller difference ? betweenbegin and end of set) as can be seen from thefollowing table I depicting mechanical propertiesfor accelerated Ciment Fondu

®systems.

Table 1 - Comparison of macroscopic properties for

differently accelerated Ciment Fondu®

pastes.

Sample Setting Time (min) CompressiveStrength (MPa)

Initial Final ? Set + 2 h 6 h

Ciment Fondu®

200 230 30 0 37,6

Ciment Fondu®

+0,005 Li2CO3

110 135 25 26,8 53,2

Ciment Fondu®

+0,01% Li2CO3

80 95 15 35,8 54,5


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RetardersThe action of a retarder for the same system asin the paragraph above is shown in Figure 4 forthe use of tri-sodium citrate (CT).

350300250200150100500

1,0

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8

0,02% CT

0,2% CT0,1% CT

Time (min)

Figure 4 - Influence of tri-sodium citrate on

conductivity curves of Ciment Fondu®.

The contrary to Li2CO3 is observed : the time ofmassive precipitation becomes longer the higherthe tri-sodium citrate content. Therefore thesetting times are prolonged.

But a second effect, not present with theacceleration, can be observed. The dissolution ofCa2+ and Al(OH)4- ions is much slower withincreasing citrate content. The apparition of an'ear' (early precipitation of C2AH8) is evensuppressed with high CT concentrations. Thisentrains an improved workability as we will seelater.

Retarders with acceleratorsThe combination of retarders and accelerators ispossible and recommended, because the twoeffects do not necessarily cancel each other outbut intervene at different stages of the hydration.Figure 5 gives an example for the use of Li2CO3with CT.

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1,0

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8 0,1% Li2CO3

0,1% Li2C03 + 0,1% CT

0,1% Li2CO3 + 0,2% CT

0,1% CT

Time (min)

Conductivity

(mS

/cm

)

Figure 5 - Conductivity curves for an accelerated

Ciment Fondu®

system with different CT contents.

The Li2CO3 accelerates in this case thehardening (steep massive precipitation) butshortens the setting time to much. The additionaluse of CT adjusts the setting times withoutinterfering strongly with the hardening. As acomparison the curve for the unique use of CT isplotted into the same diagram.

PlasticisersCements pastesDifferent industrially available plasticisers havebeen tested. Figure 6 shows a comparison of CT,trisodiumpolyphosphate (TPP), Darvan 7 S andMelment F 10 in a Secar® 71 cement paste. Theaction of the plasticisers is shown by the self flowvalue.

43210

0

100

200

DARVAN 7STPPMelment F10CT

Plasticiser Addition (%)

Figure 6 - Self flow values for a Secar® 71 cementpaste with different plasticiser additions.


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Different effects can be observed :

Darvan 7S and Melment F 10 act in a similarway : fairly high amounts are necessary (0,5%) to obtain a plasticising effect. The self flowvalue, however, which can be reached ishigher and less sensitive to dosage as for TPPand CT.

CT acts much faster (around 0,1 %) but has avery narrow range of dosage in order to obtainoptimum performance. This fluidifying effect isa result of the slow down of ion dissolution asseen in paragraph IV. 2.

TPP is between CT and the organicplasticisers. It can be more easily dosed thanCT and good flow properties are reached withreasonable amounts of TPP addition (around0,3 to 0,5 %).

For both CT and TPP an over dosage leads toa degradation of flow properties which hasbeen shown for the case of TPP in Secar® 71 /silica fume systems earlier /3,4/.

The positive effects of Melment F10 (high flowand little degradation) and CT (early flow) can becombined by using both plasticisers. This isshown in Figure 7 where the plasticisersMelment, Darvan and Mighty are all used with a0,2 % CT addition. For comparison the pure CTflow curve is also given.

0,80,60,40,20,0

0

100

200

Darvan 7S + 0.20% CitrateMelment F10 + 0.20% CitrateMighty 100 + 0.20% CitrateCitrate


Figure 7 - Self flow values for Secar® 71 cementpastes with 0,2 % CT and plasticisers.

The following remarks become obvious :

With all combinations of CT + plasticisers anearly self flow can be obtained.

The high sensitivity of CT to the amountadded does not exist for the combinations (nodegradation of flow with over dosage).

With certain plasticisers even higher early selfflow values can be obtained as compared toCT or the plasticiser alone.

Cements / reactive alumina pastesInteractions of fine reactive fillers like silica fumeor reactive alumina with CAC and their influenceon hydration behavior have been shown earlier/4,5/. Plasticisers also react differently with sucha system as compared to a pure paste. Figure 8shows self flow values for the case of TPP andCT for a Secar® 71/reactive alumina system.

0,80,60,40,20,0

0

100

200

CitrateCitrate - CAC/AluminaTPPTPP - CAC/Alumina


Figure 8 - Comparison of pure Secar® 71 cementpaste and cement /alumina paste for TPP and CTadditions.

Even without plasticiser addition the self flow isenhanced for a cement/alumina systems. This isin part due to an optimization of the particle sizedistribution - an important parameter whendesigning self or free flow castables /6/. Thisgood self flow can be enhanced, however, by theaddition of plasticisers like CT and TPP. Anadditional positive effect is a decreasedsensibility to the amounts of plasticiser added.

The above mentioned beneficial effect of finefillers on granulometric curves and therefore flowbehavior is not the only factor. There is alsochemical interactions which contribute to theplasticising effect : Figure 9 shows this for the


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use of Darvan 7S with and without the addition ofreactive alumina.

3002001000

1,0

1,2

1,4

1,6

1,8

2,0

2,2

2,4

2,6

2,8

Darvan 7SDarvan 7S + Alumina

Time (min)

Conductivity

(mS

/cm

)

Figure 9 - Conductivity curves for Secar® 71 cementpastes with Darvan 7S and reactive alumina.

The addition of reactive alumina decreases thedissolution of ions in the beginning which meansa better flow a fact demonstrated in Figure 8 forCT and TPP.

4 Conclusions

The following conclusions can be drawn :

In order to optimize sophisticated refractoryconcretes like LCC or SFC differentadmixtures have to be combined besides thecareful choice of mineral raw materials.These different admixtures are employed totailor various characteristics.

It is important that the active binder system istaken into account. This means that theadmixtures have to be adapted to cementplus fine reactive fillers and not only to thecement.

Setting time and hardening are controlled bya retarder/accelerator couple. The use ofboth, seemingly contradictory admixtures,assures a more comfortable control forhydration kinetics.

The workability or flow - especially importantfor SFC application - are best controlled by acouple of CT and a plasticiser or super-

plasticiser of the type TPP, Melment, Darvan,Mighty or others.

It should be kept in mind that the control offluidity retards the systems which means thata classical retarder like CT will play a doublerole as well in the retarder/accelerator as inthe CT/plasticiser couple.

5 Acknowledgements

The authors would like to thank all co-workers atLafarge Fondu International and Lafarge CoppéeRecherche who contributed to the studies whichled to this paper. Special thanks to Ms F. Kebliand Mrs D. Gauthier who carried out most of thework on self-flow and conductimetry.

6 Literature

/1/ J.D. Cox and J.H. Sharp ; "The use ofadmixtures with calcium aluminate cements".

/2/ D. Sorrentino, J.P. Bayoux, R. Montgomery,A. Mathieu and A. Capmas ; "The effect ofsodium gluconate and lithium carbonate onthe properties of calcium aluminate cements";UNITECR 1991.

/3/ Th. A. Bier, A. Mathieu, B. Espinosa andJ.P. Bayoux ; "The use of conductimetry tocharacterize the reactivity of calciumaluminate cements" ; UNITECR 1993.

/4/ J.P. Bayoux, C.M. George and J.P.Letourneux ; "Theory and practice of fumesilica - aluminous cement interactions" - PartII.

/5/ A. Mathieu, A. Capmas, J.P. Bayoux andD. Richon ; "Calcium aluminate cement andreactive alumina" ; UNITECR 1993.

/6/ A. Mathieu ; "Calcium aluminates in selfflowing castables" ; Bresilian Ceramic Asso-ciation Aguas de Lindoia, Brazil, June 1995.

admixtures and their interaction with High Alumina Cement

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