Technical Paper CHEMICAL INTERACTIONS IN CALCIUM ALUMINATE CEMENT (CAC) BASED CASTABLES CONTAINING MAGNESIA

Transcription

1 Page : 1/7 CHEMICAL INTERACTIONS IN CALCIUM ALUMINATE CEMENT (CAC) BASED CASTABLES CONTAINING MAGNESIA by Thomas A. Bier, Christopher Parr, Catherine Revais, Hervé Fryda presented at the UNITECR congress; 1997.

2 Page : 2/7 Abstract Castables based on Alumina and CAC which contain Magnesia form Al2O3. spinels which have the potential to enhance performance. The principal applications are within Steel Ladles in areas such as impact zones. Numerous papers have been published which show the potential of these castables via investigations into phase formation during heating, expansion and strength development. It is these properties that will largely determine the performance of these castables within the environment of the steel ladle. This paper investigates the role of admixtures and amorphous silica with respect to their chemical interactions with the reactive. Correlations to rheological behavior will be shown. A simple model of interactions between reactive components in such systems will be proposed.

3 Page : 3/7 1 Introduction The development to use castables in ladle lining has led to refractory concrete which contain in the fired state spinel in order to resist the severe attacks in a ladle thus being competitive to bricks. This can be achieved by two approaches : (a) including synthetic spinel as aggregate in an alumina castable 1 or (b) including to the binder phase of the castable, which will form during heat up with the reactive alumina present, Al2O3. spinel. The second approach seems to give more resistant concretes in specific applications 2 and is certainly less costly than the spinel aggregate solution. Numerous papers 3,4,5 have been published in recent years relating to the resistance to slag attack, expansion behaviour and thermodynamic stability of spinel containing castables. There are however few published studies relating to the rheological behaviour and hardening kinetics of such systems. It is well known that for spinel forming castables the inclusion of disturbs the workability of a concrete. First results on workability have been presented in 6 focusing on flow decay curves for a containing ULCC laboratory formulation. Some results comparing workability of spinel containing versus containing spinel-forming castables have been shown in 7, 8. In the following chapters results are shown on chemical interactions in spinel forming systems. Figure 1 shows a sketch of the active ingredients when looking at spinel forming castables. Calcium Aluminate Cement (CAC) Admixtures Fine, reactive alumina Normally we have interactions between four mineral phases controlled by admixtures to give the desired placing properties and green strengths. For spinel forming and the performance of the resulting concrete only fine alumina, and their respective physico-chemical properties are relevant. Silica fume plays according to most refractory researchers a twofold role : - Influence on reactivity; anti-slaking agent. - Mitigation of volume expansion due to spinel formation. 2 Experimental approach Silica Fume Figure 1. Active ingredients for spinel forming castables Formulations investigated The experiments conducted are based on a simple formulation of a ULCC castable containing Magnesia () in order to form Al2O3. spinels upon firing. Some comparative tests have been carried out on the formulation without addition. This formulation has been optimised where particle size distribution and spinel forming properties are concerned. The binder phase of the formulation is given in Table I. Figure 1 shows the particle size distribution used for the tabular alumina aggregates and active fine components.

4 Page : 4/7 Figure 2. Particle size distribution of formulation Admixture packages and fineness have %Passing 1 1 been varied in order to influence setting behaviour (Working Time). The impact of these variations, as well as the presence of, have been studied with various methods. Two different admixtures or admixture packages have been used: Sodium polyphosphate (TPP) and a combination of Darvan 7S, citric acid (CT) and sodium carbonate. The fineness of the has been varied by combining the 5% of in the formula out of two differently ground 's as shown in Table II. Table II. compositions for formulations tested use for castable (%) Raw material % Coarse Medium Fine N <,4 mm N <,2 mm 1 q =, Particle size (microns) 5 2, 5 2,5 q =,39 Table I. Binder composition Active (fine) material (%) Type of formulation Elkem 971U Alumina 152SV Secar 71 with without For the formulation with the medium the admixtures were adjusted in such a way as to obtain a working of 6 minutes. These admixture additions were then also applied with the coarse and fine formulations. 5 Water 5,5 5,5 3 Experimental methods used Investigations on Concretes Working time and flow v alue hav e been determined with the ASTM cone (1 mm base Ø, 7 mm top Ø and 8 mm height) on a vibrating table. The flow values have been determined for a 2 seconds vibration time directly after mixing and after 15, 3, 45 and 6 min. They are expressed in mm and represent the diameter of the cake reached after vibration of the cone. The working time represents the time at the end of which no flow on the vibrating table can be achieved. Heat profiles The heat produced by exothermic reactions within the concrete has been measured as a function of time. The concrete has been placed in insulated (styro-foam) beaker and a thermocouple stuck into the concrete. Investigations on binder phase Calorimetry and rheological profiles have been measured for the binder phases, consisting of the composition given in Table I and the -325 mesh part of the tabular alumina aggregate. A detailed description of the method and its application on castables is being presented in 9. Calorimetry After mixing, the binder is placed into the cell of a calorimeter, which has been specially developed to measure the heat after mixing. Heat flow is associated with a chemical reaction occurring within the binder. Heat flow has been reported by others workers to be representative of castables properties 1. Rheometry After mixing, the binder is placed into a rheometer. The rheometer (Contrives, Rheomat 11 5A) is normally composed of two coaxial cylinders, the binder being placed within the narrow space between the cylinders. In the present experiment, a special blade replaced the inner cylinder. The binder is placed within the outer cylinder after mixing. The propeller is rotated every 15 minutes at 5 rpm during 1 seconds, and the momentum necessary is monitored. The result of the

5 Page : 5/7 experiment is a value of the momentum every 15 minutes. An increase of the momentum is associated with a stiffening of the binder. 4 Results Tests on concretes Results for flow decay and working times obtained are given in Table III for five castables. Table III. Flow values and working times measured on concretes Fine Coarse Formulation min Flow value in mm min 15 min 3 after min 45 min 6 Working time -,6% TPP package ,6% TPP package Without -,6% TPP Package =,5% Darvan 7S /,1% Citric Acid, 1 H2 /,5% Na2CO3 min The values obtained for the castables containing are very close to results reported in 6, where also results on the formulations with the medium grading are shown. In order to better compare only formulations with either coarse or five are treated in this paper. The major effects shown here are : - Fine reduces flow value and working time. - An additive combination allows optimizing flow behavior with respect to a monoadditive formula. In order to demonstrate the influence of and decreased silica fume the last line shows results from the ULCC formulation. The flow value and working time for the formulation is higher and especially the working time with 42 minutes is much longer for the ULCC formulation without. Tests on binder phase of castables Figure 3 shows rheometry curves for the binder phases of the formulations shown above. The momentum is given as a function of time for a measurement every 15 minutes Time (min) Coarse - TPP Coarse - Package F i n e M g O - T P P Fine - Package ULCC w/o Figure 3. Rheology: Momentum as a function of time for the five different binder phases

6 Page : 6/7 Low momentum indicates that the material is still workable that is to say that no structuring due to hydration has occurred yet. Once hydration or even partial hydration starts, the material acquires a certain structure and stiffens, represented by an increase in momentum. The curves shown are in good agreement with the working times measured on the concretes, representing the whole formulation. These working times are given in the diagram for the corresponding formulations. Also, the ULCC formulation without does not show any structuring at all within the time scale looked at in the diagram. Figure 4 shows calorimetry curves for the five binder phases investigated. The heat flow is given as a function of time. Similar effects have been measured by monitoring heat profiles on the concretes. Figure 4. Calorimetry curves for the binder phases of the five formulations mw/g 1,8,6,4,2 -,2 -,4 -,6 -,8 Package, pi TPP, pi Fine ULCC w/o massive precipitation Coarse min The system without shows a curve with a behavior close to a pure cement hydration. Three phases can be distinguished: dissolution or wetting, dormant period and massive precipitation which are classical for the hydration of a cementitious material. When looking at the systems containing an additional peak pi can be observed. This peak is not present in the ULCC formulation without or cannot be detected. This peak, occurring also in other refractory LCC formulations is being described in more detail in 9. It indicates by its exothermic character that a chemical reactionoccurs even if it might be, quantitatively, a weak reaction. This reaction seems to be influenced by admixtures, active fillers and CAC. For the ULCC formulation investigated the addition of seems to provoke or strengthen this peak. The time the peak occurs seems to be controlled by the admixtures used because we can clearly distinguish two sets of peaks representing the formulations with TPP and with the admixture package, respectively. The admixture package causes an earlier peak pi than the use of TPP in the formulation. A similar effect has also been seen in earlier investigations where conductimetry has been used to characterize the effect of admixtures in refractory castables11,1 2. TPP shows also as a very strong retardation of the dissolution step. This strong retardation of dissolution would be in accordance with a later appearance of pi with respect to the use of the admixture package. For the massive precipitation the effect is rev ersed. The massiv e precipitation for formulations with the admixture package occurs at about 2 minutes and is not given in the diagram. TPP as an admixture allows an earlier massive precipitation than the use of the admixture package. This is confirmed by higher green strengths after 36 hours. The castables with fine and TPP exhibits 8,7 MPa as compared to 4,6 MPa for the use of the admixture package. However, the fineness of the does not play any role on the time when the peak Pi occurs. On the other hand the flow decay curves observed in 6 and also presented in Table III and the rheometry (Figure 3) show an influence of fineness and admixtures. This means also that despite the potential of a disturbance (decay) of flow by an early reaction we do not find any general correlation between the time of the peak Pi and a rheological parameter such as working time or increase in momentum. In 9 correlation are shown but only within a given, well defined system.

7 Page : 7/7 These results combined (calorimetry and rheometry) clearly demonstrate that we have to investigate further concentrating in distinguishing and monitoring at least two different chemical mechanisms: - Interactions (formation of hydrates) which can be detected by calorimetry, conductivity or LOI. - Topochemical interactions modifying surface charges that influence the forces of defloculation, hence the structuring of the paste and eventually the rheological behavior. 5 Conclusions Spinel forming castables contain and are difficult to formulate not only what high temperature properties are concerned but also with respect to master placing properties. Results from our study show that : - The presence of disturbs working time. - The fineness of influences working time; fine yields shorter working times than coarse. - Admixtures are necessary to master rheological properties of containing formulations. - Admixture combinations are more efficient than a mono-admixture approach. - reacts with admixtures and fillers, modifies the thermal profile and hence the hydration. - The exact nature of these reactions and their impact on rheology are not known yet. 6 References 1 T. Kanatani, Y. Imaiida, "Application of an alumina-spinel castable to the teeming ladle for stainless steelmaking", UNITECR '93. 2 H. Naaby, O. Abildgaard, G. Stallmann, C. Wöhrmeyer und J. Meidell, "Refractory wear mechanisms and influence on metallurgy and steel quality as a result of the conversion to endless lining at Det Danske Stalvalsevaerk", XXXVII Refractory Colloquium, October 1994, Aachen, Germany. 3 A. Sainz, A. Mazzoni, E. Aglietti and A. Caballero, "Thermochemical formation and stability of spinel (.Al2O3 ) under strongly reduction conditions", UNITECR '95. 4 Zenbe-e Nakagawa, Naoya Enomoto, In-Suk Yi, and Keisuke Asano, "Effect of Corundum/periclase sizes on expansion behavior during synthesis of spinel.", UNITECR '95. 5 M. Rigaud, S. Palco, Ning Wang, "Spinel formation in the -Al2O3 system relevant to basic castables", UNITECR '95. 6 Th. A. Bier et al, "Workability of Calcium Aluminates Cement based castables containing magnesia", ALAFAR Ch. Alt, T.A. Bier, N.E. Bunt and C. Parr, "Comparison of the effects of workability on spinel containing and spinel forming castables", American Ceramic Society, Th.A. Bier, C. Parr, C. Revais, "Formulation logics in Spinel Castables", ALAFAR H. Fryda et al, "Relation between setting properties of Low Cement Castables and interactions within the binder system (CAC-Fillers- Admixtures-Water)", UN ITECR '97. 1 R. Krebs, "Demands on unshaped refractories in the next decade", UNITECR '95, 2, pp , T.A. Bier, A. Mathieu, B. Espinosa, J.P. Bayoux, "The use of conductimetry to characterize the reactivity of high alumina cements", UNITECR ' T.A. Bier, C. Parr, "Admixtures with calcium aluminate cements and CAC based castables", 28th Annual SA Ceramic Society Symposium, Johannesburg, Republic of South Africa, 1996.