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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 calciumaluminate 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 givenproperties such as flocculation, coulability and hardening.
The interactions of these admixtures with the CAC are shown by chemical as well asrheological measurements. The results are discussed with respect to practicalapplications.
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An overview about possible admixtures withCAC's is given in /1/.
The major admixtures used in this study and As with all hydraulic binders, the chemical and mineral composition and the fineness of a CAC determine the rheology and reactivity of the system. This means that setting time and flow behavior of a refractory concrete or mortardepend for a given water cement ratio only fromthe constituents other than the cement. These other constituents influence the rheological andhardening properties of a concrete or mortarmore Conductivity of stirred suspensions
considered as inert, a larger influence being This technique, presented in recent publications attributed to reactive fine fillers. And the largest /2,3/, consists of measuring the electrical influence is played nowadays by admixtures conductivity of a stirred suspension of cement in which are on purpose added to tailor the above mentioned properties. The raw material togetherinfluence each other and constitute finally the Conductivity is a very good picture of the ionic performance of the concrete. These interactions concentration in water. The conductivity of the have in the past very often been represented by a suspension gives information on the dissolution / triangle of interactions as shown in Figure 1.
nucleation / precipitation steps of the hydrationreaction of cement in water. A schematic picture of conductivity and related properties is shown inFigure 2, the above mentioned hydration steps being represented as stages I, II and III.
Figure 1 - Components influencing the properties in arefractory concrete.
Amongst these components this paper deals with according to ASTM C125-88 as"Materials other than water, aggregates, hydraulic ingredients of concrete or mortar and added to the batch immediately before or during itsmixing." and in the DBV - Manual : Figure 2 - Sketch of a conductivity curve for a calcium "Are chemical substances added to fresh concrete 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." 8 rue des Graviers – 92521 Neuilly sur Seine Cedex
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Self Flow Value
The self flow has been determined for pastes of The paste is mixed and then filled into a cylinder (diameter = 30 mm, height = 50 mm) which sits on a glass plate. The cylinder is then withdrawn, the paste starts to flow under its own weight and a "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 casting Figure 3 - Conductivity curves of Ciment Fondu® withdifferent Li2CO3 concentrations.
Strength after 6 hours
With increasing Li2CO3 content the massive Setting time and strength after six hours precipitation (hydration) starts earlier and hence measured on 40 x 40 x 160 mm mortar prisms the setting time starts earlier and the difference between begin and end of set becomes smaller accelerators or retarders. Tests are carried on (steeper decrease in conductivity upon massive This translates in earlier setting times and afaster hardening (smaller difference ? between begin and end of set) as can be seen from thefollowing table I depicting mechanical propertiesfor accelerated Ciment Fondu® systems.
Accelerators
The most common, most effective accelerator for
alumina cements is lithium carbonate. Lithium
hydroxide acts more violent and is therefore
Setting Time (min)
Compressive
Strength (MPa)
difficult to dose. However, for winter concreting it 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 different concentrations of lithium carbonate on the precipitation behavior of Ciment Fondu®.
Table 1 - Comparison of macroscopic properties fordifferently accelerated Ciment Fondu® pastes.
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Retarders
The action of a retarder for the same system as
in the paragraph above is shown in Figure 4 for Figure 5 - Conductivity curves for an accelerated Ciment Fondu® system with different CT contents.
The Li2CO3 accelerates in this case thehardening (steep massive precipitation) but Figure 4 - Influence of tri-sodium citrate on shortens the setting time to much. The additional conductivity curves of Ciment Fondu®.
use of CT adjusts the setting times withoutinterfering strongly with the hardening. As a The contrary to Li2CO3 is observed : the time of comparison the curve for the unique use of CT is massive precipitation becomes longer the higher the tri-sodium citrate content. Therefore thesetting times are prolonged.
Plasticisers
Cements pastes
But a second effect, not present with the Different industrially available plasticisers have acceleration, can be observed. The dissolution of been tested. Figure 6 shows a comparison of CT, Ca2+ and Al(OH)4- ions is much slower with trisodiumpolyphosphate (TPP), Darvan 7 S and increasing citrate content. The apparition of an Melment F 10 in a Secar® 71 cement paste. The 'ear' (early precipitation of C2AH8) is even action of the plasticisers is shown by the self flow suppressed with high CT concentrations. This entrains an improved workability as we will seelater.
Retarders with accelerators
The combination of retarders and accelerators is
possible and recommended, because the two effects do not necessarily cancel each other out but intervene at different stages of the hydration.
Figure 5 gives an example for the use of Li2CO3with CT.
Figure 6 - Self flow values for a Secar® 71 cementpaste with different plasticiser additions.
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Darvan 7S and Melment F 10 act in a similar With all combinations of CT + plasticisers an way : fairly high amounts are necessary (0,5 %) to obtain a plasticising effect. The self flow added does not exist for the combinations (no higher and less sensitive to dosage as for TPP degradation of flow with over dosage).
With certain plasticisers even higher early self flow values can be obtained as compared to CT acts much faster (around 0,1 %) but has avery narrow range of dosage in order to obtain optimum performance. This fluidifying effect is a result of the slow down of ion dissolution as Interactions of fine reactive fillers like silica fume or reactive alumina with CAC and their influence on hydration behavior have been shown earlier plasticisers. It can be more easily dosed than /4,5/. Plasticisers also react differently with such CT and good flow properties are reached with a system as compared to a pure paste. Figure 8 reasonable amounts of TPP addition (around shows self flow values for the case of TPP and CT for a Secar® 71/reactive alumina system.
For both CT and TPP an over dosage leads toa degradation of flow properties which has been 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 be combined by using both plasticisers. This is Melment, Darvan and Mighty are all used with a 0,2 % CT addition. For comparison the pure CT Figure 8 - Comparison of pure Secar® 71 cementpaste and cement /alumina paste for TPP and CTadditions.
Even without plasticiser addition the self flow is enhanced for a cement/alumina systems. This is in part due to an optimization of the particle size distribution - an important parameter when designing self or free flow castables /6/. Thisgood self flow can be enhanced, however, by the addition of plasticisers like CT and TPP. An sensibility to the amounts of plasticiser added.
Figure 7 - Self flow values for Secar® 71 cement The above mentioned beneficial effect of fine pastes with 0,2 % CT and plasticisers.
fillers 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 8 rue des Graviers – 92521 Neuilly sur Seine Cedex
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use of Darvan 7S with and without the addition of plasticiser of the type TPP, Melment, Darvan, It should be kept in mind that the control of fluidity retards the systems which means that a classical retarder like CT will play a double role as well in the retarder/accelerator as in The authors would like to thank all co-workers at Figure 9 - Conductivity curves for Secar® 71 cement Lafarge Fondu International and Lafarge Coppée pastes with Darvan 7S and reactive alumina.
Recherche who contributed to the studies which The addition of reactive alumina decreases the led to this paper. Special thanks to Ms F. Kebli dissolution of ions in the beginning which means and Mrs D. Gauthier who carried out most of the a better flow a fact demonstrated in Figure 8 for work on self-flow and conductimetry.
/1/ J.D. Cox and J.H. Sharp ; "The use of admixtures with calcium aluminate cements".
/2/ D. Sorrentino, J.P. Bayoux, R. Montgomery, In order to optimize sophisticated refractory A. Mathieu and A. Capmas ; "The effect of admixtures have to be combined besides the sodium gluconate and lithium carbonate on careful choice of mineral raw materials.
the properties of calcium aluminate cements"; These different admixtures are employed to /3/ Th. A. Bier, A. Mathieu, B. Espinosa and J.P. Bayoux ; "The use of conductimetry to It is important that the active binder system is aluminate cements" ; UNITECR 1993.
plus fine reactive fillers and not only to the Letourneux ; "Theory and practice of fume silica - aluminous cement interactions" - PartII.
Setting time and hardening are controlled by /5/ A. Mathieu, A. Capmas, J.P. Bayoux and a retarder/accelerator couple. The use of D. Richon ; "Calcium aluminate cement and both, seemingly contradictory admixtures, reactive alumina" ; UNITECR 1993.
/6/ A. Mathieu ; "Calcium aluminates in self flowing castables" ; Bresilian Ceramic Asso-ciation Aguas de Lindoia, Brazil, June 1995.
The workability or flow - especially important for SFC application - are best controlled by acouple of CT and a plasticiser or super- 8 rue des Graviers – 92521 Neuilly sur Seine Cedex
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