Technical Paper ADMIXTURES AND THEIR INTERACTIONS WITH HIGH RANGE CALCIUM ALUMINATE CEMENT

Transcription

1 Reference : TP-GB-RE-LAF-3 Page : 1/7 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; Tel : 33 () Fax : 33 ()

2 Reference : TP-GB-RE-LAF-3 Page : 2/7 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. Tel : 33 () Fax : 33 ()

3 Reference : TP-GB-RE-LAF-3 Page : 3/7 1 Introduction 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 mortar depend for a given water cement ratio only from the constituents other than the cement. These other constituents influence the rheological and hardening properties of a concrete or mortar more or less. Aggregates are normally considered as inert, a larger influence being attributed to reactive fine fillers. And the largest influence is played nowadays by admixtures which are on purpose added to tailor the above mentioned properties. The raw material together influence each other and constitute finally the performance of the concrete. These interactions have in the past very often been represented by a triangle of interactions as shown in Figure 1. Reactive fillers CAC Reactive phase Aggregate General characteristics (porosity) Granulometry Additives Figure 1 - Components influencing the properties in a refractory concrete. Amongst these components this paper deals with the admixtures. Admixtures are defined according to ASTM C as "Materials other than water, aggregates, hydraulic cement and fibre reinforcement used as ingredients of concrete or mortar and added to the batch immediately before or during its mixing." and in the DBV - Manual : "Are chemical substances added to fresh concrete or mortar in order to influence certain properties of the fresh and/or hardened concrete. The quantities are so small that they don't have to be taken into account for the calculation of the mix design." An overview about possible admixtures with CAC's is given in /1/. The major admixtures used in this study and discussed in the paper are: Accelerators Retarders Plasticisers - Water reducing agents 2 Experimental approach Conductivity of stirred suspensions This technique, presented in recent publications /2,3/, consists of measuring the electrical conductivity of a stirred suspension of cement in water at 2 C. Conductivity is a very good picture of the ionic concentration in water. The conductivity of the suspension gives information on the dissolution / nucleation / precipitation steps of the hydration reaction of cement in water. A schematic picture of conductivity and related properties is shown in Figure 2, the above mentioned hydration steps being represented as stages I, II and III. Arbitrary Units I II Ca Conductivity LOI Figure 2 - Sketch of a conductivity curve for a calcium alumina cement. III Tel : 33 () Fax : 33 ()

4 Reference : TP-GB-RE-LAF-3 Page : 4/7 Self Flow Value The self flow has been determined for pastes of pure cement and mixtures of cement and reactive alumina. The paste is mixed and then filled into a cylinder (diameter = 3 mm, height = 5 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 a measure of how well the paste flows. As an indication : a flow of 8 mm represents a casting grade consistency. Strength after 6 hours Setting time and strength after six hours measured on 4 x 4 x 16 mm mortar prisms can be used to evaluated the impact of accelerators or retarders. Tests are carried on mortars containing quartz sand (-2 mm) according to NF P Results Accelerators The most common, most effective accelerator for alumina cements is lithium carbonate. Lithium hydroxide acts more violent and is therefore difficult to dose. However, for winter concreting it might be appropriate. Curves of the dissolution and precipitation behavior 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. 2,8 2, 1, ,25% Li2CO3,5% Li2CO3,1% Li2CO Figure 3 - Conductivity curves of Ciment Fondu with different Li2CO3 concentrations. With increasing Li2CO3 content the massive precipitation (hydration) starts earlier and hence the setting time starts earlier and the difference between begin and end of set becomes smaller (steeper decrease in conductivity upon massive precipitation). This translates in earlier setting times and a faster hardening (smaller difference? between begin and end of set) as can be seen from the following table I depicting mechanical properties for accelerated Ciment Fondu systems. Sample Setting Compressive Strength (MPa) Initial Final? Set + 2 h 6 h Ciment Fondu ,6 Ciment Fondu +,5 Li2CO3 Ciment Fondu +,1% Li2CO ,8 53, ,8 54,5 Table 1 - Comparison of macroscopic properties for differently accelerated Ciment Fondu pastes. Tel : 33 () Fax : 33 ()

5 Reference : TP-GB-RE-LAF-3 Page : 5/7 Retarders The action of a retarder for the same system as in the paragraph above is shown in Figure 4 for the use of tri-sodium citrate (CT). 2,8 2, 1, 5 1,2% CT,1% CT,2% CT Figure 4 - Influence of tri-sodium citrate on conductivity curves of Ciment Fondu. The contrary to Li2CO3 is observed : the time of massive precipitation becomes longer the higher the tri-sodium citrate content. Therefore the setting times are prolonged. But a second effect, not present with the acceleration, can be observed. The dissolution of Ca2+ and Al(OH)4- ions is much slower with increasing citrate content. The apparition of an 'ear' (early precipitation of C2AH8) is even suppressed with high CT concentrations. This entrains an improved workability as we will see later. Conductivity (ms/cm) 2,8,1% Li2CO3 2, 1, ,1% CT,1% Li2CO3 +,2% CT,1% Li2C3 +,1% CT 16 2 Figure 5 - Conductivity curves for an accelerated Ciment Fondu system with different CT contents. The Li2CO3 accelerates in this case the hardening (steep massive precipitation) but shortens the setting time to much. The additional use of CT adjusts the setting times without interfering strongly with the hardening. As a comparison the curve for the unique use of CT is plotted into the same diagram. Plasticisers Cements pastes Different industrially available plasticisers have been tested. Figure 6 shows a comparison of CT, trisodiumpolyphosphate (TPP), Darvan 7 S and Melment F 1 in a Secar 71 cement paste. The action of the plasticisers is shown by the self flow value. 2 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 Li2CO3 with CT. 1 DARVAN 7S TPP Melment F1 CT 1 2 Plasticiser Addition (%) 3 4 Figure 6 - Self flow values for a Secar 71 cement paste with different plasticiser additions. Tel : 33 () Fax : 33 ()

6 Reference : TP-GB-RE-LAF-3 Page : 6/7 Different effects can be observed : Darvan 7S and Melment F 1 act in a similar way : fairly high amounts are necessary (,5 %) to obtain a plasticising effect. The self flow value, however, which can be reached is higher and less sensitive to dosage as for TPP and CT. CT acts much faster (around,1 %) but has a very narrow range of dosage in order to obtain optimum performance. This fluidifying effect is a result of the slow down of ion dissolution as seen in paragraph IV. 2. TPP is between CT and the organic plasticisers. It can be more easily dosed than CT and good flow properties are reached with reasonable amounts of TPP addition (around,3 to,5 %). For both CT and TPP an over dosage leads to a 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 F1 (high flow and little degradation) and CT (early flow) can be combined by using both plasticisers. This is shown in Figure 7 where the plasticisers Melment, Darvan and Mighty are all used with a,2 % CT addition. For comparison the pure CT flow curve is also given. 2 1, Darvan 7S +.2% Citrate Melment F1 +.2% Citrate Mighty 1 +.2% Citrate Citrate,2,4,6 Plasticiser Addition (%) Figure 7 - Self flow values for Secar 71 cement pastes with,2 % CT and plasticisers.,8 The following remarks become obvious : With all combinations of CT + plasticisers an early self flow can be obtained. The high sensitivity of CT to the amount added does not exist for the combinations (no degradation of flow with over dosage). With certain plasticisers even higher early self flow values can be obtained as compared to CT or the plasticiser alone. Cements / reactive alumina pastes Interactions of fine reactive fillers like silica fume or reactive alumina with CAC and their influence on hydration behavior have been shown earlier /4,5/. Plasticisers also react differently with such a system as compared to a pure paste. Figure 8 shows self flow values for the case of TPP and CT for a Secar 71/reactive alumina system. 2 1, Citrate Citrate - CAC/Alumina TPP TPP - CAC/Alumina,2,4,6 Plasticiser Addition (%) Figure 8 - Comparison of pure Secar 71 cement paste and cement /alumina paste for TPP and CT additions. 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/. This good self flow can be enhanced, however, by the addition of plasticisers like CT and TPP. An additional positive effect is a decreased sensibility to the amounts of plasticiser added. The above mentioned beneficial effect of fine fillers on granulometric curves and therefore flow behavior is not the only factor. There is also chemical interactions which contribute to the plasticising effect : Figure 9 shows this for the,8 Tel : 33 () Fax : 33 ()

7 Reference : TP-GB-RE-LAF-3 Page : 7/7 use of Darvan 7S with and without the addition of reactive alumina. Conductivity (ms/cm) 2,8 2, 1, Darvan 7S Darvan 7S + Alumina Figure 9 - Conductivity curves for Secar 71 cement pastes with Darvan 7S and reactive alumina. The addition of reactive alumina decreases the dissolution of ions in the beginning which means a better flow a fact demonstrated in Figure 8 for CT and TPP. 4 Conclusions The workability or flow - especially important for SFC application - are best controlled by a couple of CT and a plasticiser or superplasticiser of the type TPP, Melment, Darvan, Mighty or others. 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 CT/plasticiser couple. 5 Acknowledgements The authors would like to thank all co-workers at Lafarge Fondu International and Lafarge Coppée Recherche who contributed to the studies which led to this paper. Special thanks to Ms F. Kebli and Mrs D. Gauthier who carried out most of the work on self-flow and conductimetry. 6 Literature The following conclusions can be drawn : In order to optimize sophisticated refractory concretes like LCC or SFC different admixtures have to be combined besides the careful choice of mineral raw materials. These different admixtures are employed to tailor various characteristics. It is important that the active binder system is taken into account. This means that the admixtures have to be adapted to cement plus fine reactive fillers and not only to the cement. Setting time and hardening are controlled by a retarder/accelerator couple. The use of both, seemingly contradictory admixtures, assures a more comfortable control for hydration kinetics. /1/ J.D. Cox and J.H. Sharp ; "The use of admixtures with calcium aluminate cements". /2/ D. Sorrentino, J.P. Bayoux, R. Montgomery, A. Mathieu and A. Capmas ; "The effect of sodium gluconate and lithium carbonate on the properties of calcium aluminate cements"; UNITECR /3/ Th. A. Bier, A. Mathieu, B. Espinosa and J.P. Bayoux ; "The use of conductimetry to characterize the reactivity of calcium aluminate cements" ; UNITECR /4/ J.P. Bayoux, C.M. George and J.P. Letourneux ; "Theory and practice of fume silica - aluminous cement interactions" - Part II. /5/ A. Mathieu, A. Capmas, J.P. Bayoux and D. Richon ; "Calcium aluminate cement and reactive alumina" ; UNITECR /6/ A. Mathieu ; "Calcium aluminates in self flowing castables" ; Bresilian Ceramic Association Aguas de Lindoia, Brazil, June Tel : 33 () Fax : 33 ()