Technical Paper CALCIUM ALUMINATE CEMENTS (CAC) FOR MONOLITHIC REFRACTORIES. by Christopher Parr, Eduardo Spreafico, Thomas A. Bier, Alain Mathieu

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1 Page : 1/11 CALCIUM ALUMINATE CEMENTS (CAC) FOR MONOLITHIC REFRACTORIES by Christopher Parr, Eduardo Spreafico, Thomas A. Bier, Alain Mathieu presented at the 1st Monolithics Conference, Tehran, Iran; 1997.

2 Page : 2/11 Abstract This review paper will present basic elements relating to CAC use and will include topics such as hydration, dehydration during dryout, interactions with other materials; for example, additives and fine fillers such as alumina and silica. The importance of control during the formulation of refractory castables and application is emphasised.

3 Page : 3/11 1 Introduction Developments in refractory castables, over the last 25 years 2,4 has placed increasingly severe demands upon Calcium Aluminate Cements hereunder called CAC s. With the advent of the low cement castable family of products the role of CACs has changed considerably. 7 Figure 1 illustrates this evolution in terms of the role of CACs. It can be seen that the role has changed from being a fundamental binder to more of a chemical reactive. In this latter case regularity of performance is paramount, as is the ability of calcium aluminate cements to tolerate a wide range of additive types and dosage rates. In order to understand the nature of the interactions that take place within these sophisticated castable systems, it has been necessary to undertake fundamental studies. Within the context of cements these studies have focused upon calcium alum inate hydration and the development of methods to study this process. It is by virtue of these studies that the CACs produced, today, are able to respond to the stringent quality requirements demanded by refractory formulators. Figure 1. Types of Refractory castables 2 CAC Cement hydration In order to understand the role of CACs in modern refractory castable technology it is first necessary to consider the reactions that take place with CACs during the hydration process. The classical approach to investigate calcium alum inate hydration has been to use the wellestablished techniques of mortar based physical testing as shown in Figure 2. Here cement reactivity is measured through setting times, strengths measured mechanically and rheological properties measured through the use flow values at different time intervals. Whilst this programme of testing is well adapted for measuring quality and assessing industrial production, it is not as useful for evaluating the hydration process. It can be considered that the effect is being measured rather than the cause. In the case of CACs it is necessary to investigate the reactions that occur from the point that water is added at the beginning of mixing. These reactions will be influenced by the composition of the cement phases present in a given cement which will react more or less quickly and give different rheological and mechanical characteristics. Temperature will also play a part in determining the rapidity of the reaction. A general comparison of the reactivity of cement phases is shown in Figure 3. It can be seen that as the phases become more lime rich, or the C/A ratio increases, then so does their reactivity. Conventional castables Gunning products Insulation products Low Cement type Placing characteristics Aptitude for gunning and/or casting Low water demand and tolerance to water dosage and temperature Hydraulic potential : Progressive use Rapid use Controlled reactivity Additive compatibility Stable behaviour with time Defined refractory properties The hydration process is initiated by the dissolution of cement in water. The more the calcium aluminate is soluble then, greater will be the concentration of ions in solution. Once the solution becomes saturated with ions then there are two possible outcomes as illustrated in Figure 4. In the case of cements, they are seen to follow Le Chatelier law.

4 Page : 4/11 Therefore once saturation is reached the next stage is precipitation of hydrates. New ions will then form due to further dissolution of cement until saturation is once again reached. This cycle then continues until all the cement is consumed as hydrates. It is this consumption of water and formation of hydrate phases that provide the solidification. In a physical sense it is the growth of the hydrate crystals which interlock and bind together to give mechanical resistance. Figure 2. Classical testing programme for CAC 3 CAC Hydration As discussed in the previous section there will be different reactions for each type of cement depending upon its composition. The types of hydration reactions are different for each cement phase. Figure 5 demonstrates some of the reaction sequences for the different cement phases and it can clearly be seen that the quantity of water consumed is not constant. With the passage of time and or temperature the hydrates formed can then convert into different forms. Figure 4. Le Chatelier principle Standard AFNOR testing: Setting time, ASTM flow and mechanical strengths These are so called hydraulic properties measures on mixtures of cement, sand and water Lafarge Aluminates has chosen the following standard mortars: Saturated solution of ions Two possibilities FONDU SECAR 51 SECAR 71 SECAR 80 Sand 1350g 1350g 1350g 1350g Cement g g g 500g W a t e r g g g 180g,, Initial and final set with Vicat needle EN196,, Flow with ASTM shock table ASTM230,, Strength on 4 x 4 x 16cm prisms EN196 Ions recombine in a form different from that of the initial solid Reaction driven by the formation of a solid which has a lower solubility than that of the original solid New solid continues to be formed until concentration of remaining ions falls to solubility of formed solid Chatelier's law Time and temperature dependent Ions cannot combine in another form ~ Solution remains stable Figure 3. Hydraulic activity of cement phases Figure 5. Hydration reaction schemes for CAC Comparison of the hydraulic activity of Alumina cements and Portland cements Portland cement Calcium Aluminate Cement Phase CS C2S C3S C C3A C12A7 CA CA2 Ratio C / S C/A ,7 1 0,5 C A H 1 CA 10H T ~ 20 C 2CA 11H C12A7 51H 6C 2AH 8 AH 3 C2AH8 AH 3 Reactivity Inert Slow Rapid Instant Very rapid Rapid Slow Lime is considered "basic" Alumina and Silica are considered to be "acidic" As the ratio basic/acid increases so does reactivity 3CA 12H 3CAH 10 C3AH6 2AH 3 C3AH6 2AH 3 18H T C increases and or duration increases The stability of these hydrates is then very specific to each type and the degree of water combined will yield different physical properties (Figure 6). The density increases with increasing thermo dynamic stability and the most stable phase is also the densest.

5 Page : 5/11 Hydrate CAH 10 State C 2 AH 8 AH 3 C 3 A H 6 hexagonal SOLID cubic Least Density 1,72 1,98 2,4 2,52 Solubility Stability Transformation Least METASTABLE STABLE The transformation will occur to thermo-dynamically stable hydrates given sufficient time and or temperature The conversion of metastable hydrates to stable hydrates occurs via the solution Figure 6. Stability of Calcium Aluminate Hydrates Therefore it can be seen that the hydration process is somewhat complicated and that the realisation of a CAC that behaves in a regular manner is a far from an easy achievement. In order to understand more fully the impact of the different cement types upon hydration it is clear that suitable experimental techniques are required. As has been explained hydration is a process that involves the passage of ions into solution therefore the study of hydration should be possible by investigating the quantity of ions in solution at a given time. H Three distinct zones can be identified : - The first zone (I) is characterised by a rapid increase in conductivity. This corresponds to the dissolution of the anhydrous cement. Calcium aluminate cement forms ions in the presence of water and these ions correspond to Ca 2 and Al(OH) The second zone (II) is the plateau with a stable conductivity, this corresponds to the induction period where nuclei or germs are formed preceding precipitation. This is also referred to as to the saturation stage where saturation is being reached. - The third zone (III) is characterised by a rapid decrease in conductivity. This corresponds to the formation of hydrates (~setting). The massive precipitation causes a decrease in conductivity. The precipitation of the hydrates is responsible for the flocculation of the calcium alum inates due to two mechanisms. Firstly, hydrate formation consumes water and thus workability decreases and secondly, hydrates have a negative surface charge that is attracted to the positive surface charge on the anhydrous particles. Also shown in Figure 7 are the comparison curves for the increase in loss in ignition and Ca 2 concentration. It is clear that the decrease in conductivity is coherent with hydrate formation as evidenced by the increase in LOI of the cement material. Similarly the evolution of conductivity follows the determined concentrations of [Ca 2 ] ions in solution. A technique has been developed by Lafarge Alum inates that allows the monitoring of the hydration process by measuring the conductivity of the solution.6 In this technique a quantity of cement is poured into two litres of water and the change in conductivity with time is continuously monitored. The water is maintained at constant temperature that is normally 20 C. The conductivity of a solution is directly linked to the number of ions in solution. The greater the quantity of ions the greater the concentration. The resulting data is collected electronically and presented in the form of a curve as shown in Figure 7. Arbirary Units Dissolution Nucleation Massive precipitation 4 I II III 3 Ca Conductivity 2 1 LOI Time (min) ions ions Figure 7. Conductimetry curves for CAC

6 Page : 6/11 Above 27 C there is a decrease in the induction period and a concurrent decrease in the time for massive precipitation. This shows the well-known phenomena of anomalous setting around C. With C12A7 (higher initial reactivity) at 20 C, it is immediately apparent that there is a much faster dissolution of cement followed by a rapid reduction in conductivity. This large initial rise in conductivity is often seen as early stiffening. The strong dependence upon temperature can also be seen. Figure 8. The effect of temperature upon the phase CA Therefore conductimetry is extremely useful for characterising cement types. This technique was applied to two laboratory cements to illustrate the differences in their behaviour. Furthermore the analysis was performed at different temperatures to see the effect upon hydration. From these two examples it is evident that the intrinsic reactivities of the two phases are different as is their temperature dependence. In applications, within refractory castable formulations, where CAC are used as chemical reactives it is clear that the combinations and quantity of each phase is important. Furthermore, in order to have consistent performance, the control of these elements becomes essential. The use of conductimetry allows the study of hydration through the study of ionic activity and this technique shows clear differences between various cement types. This can then explain the different behaviours when such cements are then applied to refractory castables. 4 The impact of fine fillers upon CAC Figure 9. The effect of temperature upon the phase C 12 A 7 Two laboratory cements used were based upon the phase CA and the phase C12A7. The results are presented in Figures 8 and 9. CA at 20 C shows basically the same curve as shown in the previous schematic. The conductivity reaches a stable level after a fairly quick initial dissolution. However when the temperature increases there is a gradual lengthening of the induction period at all temperatures up to 27 C. The advent of low cement castables 4 has placed further requirements upon the demands for CACs. These systems 1 normally rely on mixtures of cement, alumina, fume silica and additives. The fume silica and alumina acting as fine fillers to reduce porosity whilst the additives are needed to disperse these fine materials to allow placing of the castables at low water requirements. In fact these components can be considered to constitute "the binder phase" and are somewhat interdependent.

7 Page : 7/11 That is to say, that changes to one or more of these components will impact upon the other components and that the net effect will be a change in the behaviour of the binder phase. Therefore refractory castable formulations must take this interdependence into account and that substitution of alumina or fume silica types must be made with care. Similarly CAC choice must be made with care to ensure additive compatibility, hardening characteristics and reliability. Figure 10. The impact of fine fillers upon low castable behaviour The role of the cement is by definition somewhat Fume silica Reactive alumina Characteristic ph Free carbon BET F ree wat e Characteristic BET Soluble Alkalis Particle size Consequences Delay setting, Additive complexity Mechanical properties Water demand and ease of deflocculation De activation of cement and additives, modification of castable working time Consequences Water demand and ease of deflocculation Modification of the Na concentration in solution - flocculation Water demand and flow different in these castables than in conventional castable products. Problems that are often attributed to cement in these systems such as delayed or accelerated hardening and poor flow can in fact be often due to the other components within this binder phase. This is evidenced in Figure 10. Here it can be seen that there are a number of characteristic properties that can affect overall castable behaviour. One of the key parameters being the ability to disperse or defloculate these fine fillers. It is this dispersion that contributes to the castable flow and any changes to the ease of degree of dispersion will directly impact upon castable flow. The elements in the figure only cover the castable placing characteristics and that there are other parameters that govern final refractory properties. For example, the crystal size of the fine alumina will govern to a large extent the hot strengths by their thermal reactivity. As fume silica is often a by-product from silicon based metallurgy there exists considerable scope for the product to be variable. Some of the most significant variations in castable behaviour can often be attributed to variable silica fume quality. Characteristics such as ph, BET and free carbon levels can change quite markedly and this would give rise to both castable placing problems and low mechanical strengths. Table I. Characteristics of four selected fume silica's FSI FSII FSIII FSIV Density 2,28 2,33 2,25 2,29 BET 24,90 13,95 23,65 20,25 SiO2 96,9 92,2 91,5 96,1 ZrO2-2, Al2O3 0,15 3,10 0,30 0,20 Fe2O3 0,05 0,15 0,15 0,10 CaO 1,20 0,40 1,00 1,00 SO3 0,20 <0,05 0,30 0,20 K2O 0,36 0,01 0,52 0,44 Na2O 0,08 0,06 0,08 0,16 Loss on ignition 1,10 1,15 6,1 1,70 % Carbon 0,13 0,06 3,51 0,41 ph w/s = 10 6,95 3,48 6,29 6,81 In order to show these effect more clearly four fume silica's have been selected (Table I) and their effect upon castable rheology was evaluated. 5 The four fume-silica's were selected to have a range of different characteristics relating to both chemical and physical properties. The systems were studied in a 50/50 mortar of fume silica and cement. The amount of additive, in this case Sodium Tri-polyphosphate, was also varied and the effect upon water demand/workability and hydration of calcium alum inates evaluated.

8 Page : 8/11 Figure 11 (left-hand graph) shows the influence of sodium Tri-polyphosphate (TPP) and of fume silica type upon the water/solids ratio to obtain a constant workability. It can be stated that FSII offers the lowest water demand at constant workability and is usable over a wide range of TPP dosage rates. By contrast FSIII and FSIV require more water and are only effective over a narrow range of dispersant addition. Thus the amount of water to obtain a constant workability will depend upon calcium alum inate type (in this example only one type is shown), fume silica type (BET and particle size effects) and the TPP content. The result is believed to be the precipitation of insoluble calcium phosphates. Thus as the phosphate is consumed as a calcium phosphate precipitate then the dispersing effect decreases and in consequence workability decreases. However at the optimum TPP content (lowest W/S) a short working time will result due to consumption of the TPP. It is normal to add a slight excess of TPP in order to obtain both a satisfactory fluidity and sufficient working time. Secondly, fume silica can be seen to modify the hydration of calcium aluminate cements (Figure 11). The time for hydration as measured by calorimetry shows that in general there is a modification of the hydration. It is postulated that this is due to surface interactions with the CAC. Due to these interactions with CACs in both the placing phase (working time and fluidity) and during hardening of the castable (hydration), it is clear that the CAC must behave in a regular manner and have a wide tolerance to additive types and dosage. This is clearly an advantage with Secar products that are produced within strict limits and offer a wide range of flexibility with additives. The range of calcined and reactive alum ina's types available commercially is vast and is a direct result of ability of the suppliers to produce multiple grades through different calcining and milling operations. Each type of alumina will have different characteristics in terms of mineralogy, particle morphology, impurity levels etc. The impact of the most important parameters, in terms of interactions with CAC is shown in Figure 10. Figure 11. Examples of the interactions between CAC, Fume silica and additives The general model can be considered in terms of chemical interactions and ions in solution. The working time is dependant upon the presence of phosphate ions remaining in solution and causing a dispersing effect of the fume silica. CA cement releases Ca 2 ions into solution. As evidenced in this figure the key properties of soluble soda, BET and particle size can have a significant effect upon castable rheology. A specific example 8 is also shown in Figure 12 that shows how the presence of two fine alumina types modifies the hydration of pure calcium aluminate cement.

9 Page : 9/11 The hydration is measured by micro calorimetry and the peaks shown represent the evolution of heat due to hydrate formation. As shown the presence of fine alumina tends to accelerate the hydration. The essential difference between the two alumina's being the BET, 23m 2 /g for AR45 and 8m 2 /g for P772SB. Figure 12. Examples of the interactions between CAC, alumina and additives These factors must be taken into account when considering the dry out of calcium aluminate bonded monolithic refractories. The physical dehydration temperatures must be considered in conjunction with other physical parameters such as the monolithic permeability and strength, all of which combine together to dictate the likelihood of explosive spalling during drying. - Calorimetry Reactive alumina modifies the hydration kinetics Dehydration Temperature C CAH10 C2AH8 AH3 C3AH More water combined at low temperature Curing Temperature C <20 C C >35 C Increased curing temperature = increased dehydration temperature Figure 13. The dehydration of calcium aluminate bonded castables I. Pure CAC 1g E/C 0,5 II. 55% CAC 45% alumina P772SB 2g E/C 0,5 III. 55% CAC 45% alumina AR45 2g E/C 0,5 At low temperatures some of the phases formed e.g. AH3 are not very well crystallised and as such are often present in the form of gels. These gels also have very low permeability that will increase the sensitivity to explosive spalling. The rate of heat evolution varies according alumina type Evolved heat C12A7 C A CA2 Al2O The dehydration of CAC The dehydration of CACs is shown schematically in Figure 13. The dehydration characteristics will be specific to each cement depending upon the relative amount of each cement hydrate present. It is further complicated by the fact that there is a strong dependence on the curing temperature. In essence this means that with higher curing temperatures the dehydration temperature also increases Temperature ( C) 1500 Figure 14. The development of phases during dehydration and heating of a cement paste

10 Page : 10/11 Forming Heating C Heating C Heating 1200 C Hydration A ggregates Cem ent W ater Dehydration Water Recrystalization Cement phase changes Ceramic bonding Aggregate and matrix reactions Aggregates Matrix of hydrates Aggregates Matrix of anhydrates Aggregates Matrix of recrystalized cement Aggregates Matrix of refractory phases Combinations of : CA H < 20 C C A (- H) 1 0 C 3 AH (- H) C Hexagonal AH 3 (- H) A 7 > 35 C C 2 AH 8 AH 3 Hexagonal C 3 AH 6 2 A - H Unco mb ined w ater is removed first above 100 C followed by hydrate water between 100 C and 400 C. C 1 2 A 7 A CA CA 2 A C1 2 A 7 Aggregate A X S Y CA C A 2 C A 6 A 3 S 2 CAS 2 C 2 AS AH 3 Figure 15. The hydration/dehydration process for LCC type castables The changes that occur within a castable during initial heating have been investigated. 7 The results in Figure 14 are based on a pure cement paste and show the relative quantity of each type of calcium aluminate phases that develop during the hydrate dehydration process. As shown the amount of C12A7 increases dramatically up to 800 C, after which it is slowly consumed as CA and CA2 become more prevalent with higher temperature. Thus as the CA hydrates dehydrate, a recrystallisation takes place which first forms C12A7 followed by stable anhydrous phases. This is typical of the mechanisms that take place in conventional castables and explains why there is a loss of strength at intermediate temperatures before ceramic reactions start to occur at high temperature. With defloculated systems such as those that form the basis of low cement castables the changes that occur during dehydration are somewhat different. Figure 15 shows the dehydration process schematically. It has been suggested 4 that pseudo-zeolithic type bond structures can be formed. These phases release water gradually over a wide temperature range and thus the dry out requirements are somewhat different for this type of castable. However these castables have a more compact structure. This translates into higher density, lower porosity and normally lower permeability which might the escape of steam during dry-out more difficult. Therefore caution is required more as a result of the physical structure rather than the dehydration of calcium alum inate phases is the case with conventional castables.

11 Page : 11/12 The continuing progression of monolithic refractories has been able to occur due to the flexibility and adaptability of the Calcium Alum inate bonding system. 6 Summary This review paper has shown some of the key formulation and application considerations that are necessary for the successful application of Calcium Alum inate Cements. It is by virtue of numerous fundamental studies that we have today a somewhat clearer vision as to the key interactions of Calcium Alum inate Cement and other materials in sophisticated castable formulations. Analytical tools developed in these studies have become essential aids in assessing the impact of formulation changes upon castable placing characteristics. Whatever the type of refractory castable under consideration it is clear that they must conform to a series of minimum requirements during the placing phase. These are considered to be : - easy mixing, - minimum water demand, - workability, - defined working time, - defined setting/hardening time, - rapid hardening. 7 Ackowledgements The authors would like to thank all co-workers at Lafarge Aluminates who contributed to work included in this paper. 8 References 1 A. Math ieu, "Aluminous cement with high alumina content and chemical binders", Conf. IRE South Africa, Soc. J&A. Pavin de Lafarge, French patent applications , , L.Prost, A.Pauilliac, French Patent Application No , B.Clavaud, J.P.Kiehl, R.D.Schmidt-Whitley, "15 Years of Low Cement Castable in Steelmaking", 1st International Conference on Refractories, Japan, J.P. Bayoux, J. P. Letourneux and M. George, "Fume silica and cement interactions", Parts 1, 2 and 3, Lafarge Aluminates. 6 T.Bier, A.Mathieu, B.Espinosa, and J.P.Bayoux "The use of conductimetry to characterize the react ivity of calcium aluminat e cements", UNITECR '93 Proceedings, Sao Paulo Brazil, A.Capmas, Th.Bier, "Possibilities of special cements in ceramic applications", Lafarge Aluminates. 8 A. Mathieu, A. Capmas, D. Richon, "Calcium aluminate cements and reactive alumina", Lafarge Aluminates, UNITECR '93 Proceedings, Sao Paulo Brazil, The control of these characteristics depends essentially upon the totality of the raw materials selected. It is evident that unless all raw materials are selected with care the potential performance benefits, available with toady's high quality CACs, will not be realised.