Technical Paper THE USE OF CONDUCTIMETRY TO CHARACTERIZE THE REACTIVITY OF HIGH ALUMINA CEMENTS

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1 Reference : TP-GB-RE-LAF- Page : /8 THE USE OF CONDUCTIMETRY TO CHARACTERIZE THE REACTIVITY OF HIGH ALUMINA CEMENTS by Thomas A. BIER, Alain MATHIEU, Bruno ESPINOSA, Jean-Pierre BAYOUX presented at the UNITECR Congress; 99. Tel : () Fax : ()

2 Reference : TP-GB-RE-LAF- Page : /8 Abstract For the development of LCC formulations the reactivity of the Calcium Aluminate Cement (CAC) used becomes an important parameter besides the behaviour of the other components. A fast, reproducible method which can characterize the cement reliably and with sufficient resolution is needed. This paper presents the conductivity as such a method. Conductivity in this context is the monitoring of the dissolution (anhydrous) and precipitation (hydrates) kinetics of a CAC suspension. The results obtained allow a reliable prediction of macroscopic properties of the cement. It should also be an interesting tool to study the influence of CAC in a more complex system such as a mixture of different binders. This study shows first the influence of experimental parameters and the reproducibility which can be obtained with the method. Correlation with macroscopic properties such as rheology and setting time are given. The results are discussed with respect to mineralogy of the cement used and model systems consisting of pure phases. Ramifications of the method towards a fast, reliable characterization of reactivity even in mixed systems are given. Tel : () Fax : ()

3 Reference : TP-GB-RE-LAF- Page : /8 Introduction Hydraulic and rheological properties of cements are generally characterized by classical methods. That is to say by methods which were developed phenomenologically and which are closely related to the applications. For high duty CAC's these methods were mostly used to characterize the cement to be used in conventional castables (about % cement). Figure shows a sketch of these classical methods and the properties they are measuring. The sketch also shows influences of a characteristic property on several parameters. In this sense reactivity and rheology are both related to the beginning of set, and strength more to the end of set. Nevertheless the results obtained allow a good prediction of the behaviour of the cement in a conventional mix. Reactivity Mechanical strenght Rheology setting time begin end cold crushing strength LCPC ASTM flow Figure - Classical methods for testing of cement With the development of LCC mixes the generic composition of such a refractory has changed as it is shown in Figure. cement ab. 5% fine, reactive fillers aggregates Figure - Main components of a LCC mix admixtures ab. % Three things distinguish a LCC mix from a CC mix what the composition is concerned: reduction of cement content - the cement can even be considered as a chemical reagent; introduction of fine reactive fillers such as silica fume or reactive alumina; addition of admixtures such as deflocculants, accelerators and retarders. All three components are chemically reactive and influence each others' behaviour. As a consequence the result of a classical cement mortar or concrete test does not necessarily predict the behaviour of the whole mixture. Testing has to be done on the final product which can consume quite some time if all possible combinations need to be checked. To facilitate the study of the reactivity of cement and the interactions with other chemicals, the conductivity has been explored. It turned out to be a fast, sensitive and well reproducible method. Results for different cements as well as the influence of admixtures and fine fillers are given and discussed. Conductivity - History The measurement of conductivity as a function of time in an aqueous suspension has been one of the parameters studied with the so-called "chemical reactor" //. Other parameters are temperature, ph, ion concentration in solution and hydrates formed in the precipitate. All these parameters together allow the development of ideas about what is happening upon dissolution of the anhydrous and more important upon subsequent formation of hydrates. It turned out that the measurement of conductivity alone - although not giving the abundance of information provided by the chemical reactor - is in phase with other parameters where evolution in time is concerned. It allows already an accurate and easy characterization of the materials (cements) behaviour. Tel : () Fax : ()

4 Reference : TP-GB-RE-LAF- Page : 4/8 - Description of the method With of-the-shelf equipment the conductivity is measured in a constantly stirred suspension, for example at W/C =, as a function of time. By means of a double walled beaker connected to a cooling bath the suspension is kept at a temperature of C. If the specific surface area of the cement is kept constant and the measurements are carried out according to the same procedures the results obtained are very reproducible with coefficients of variation around -%. A schematic of the experimental set-up is given in Figure and an idealized curve is given in Figure 4. For comparison the Ca-ion concentration and the LOI on the precipitate (bound water) are included in the diagram. The additional parameters come from chemical reactor measurements. Cement Suspension Stirrer Conductivity Probe Arbitrary Units 4 I II Ca Conductivity LOI Figure 4 - Typical conductivity curve as compared to Ca-ion concentration and bound water (LOI) Three different periods which correspond to different phases can be distinguished for all parameters: I- Dissolution of the anhydrous cement II- Induction period (germ formation) III- Massive precipitation of hydrates (setting) III to cooling circuit double walled beaker Conductivity meter Depending on the chemical and mineralogical composition of a cement or a mixture of various reactive ingredients these curves show changed characteristics, which can explain to a certain extent their macroscopic behaviour. from cooling circuit Figure - Experimental set-up for the conductivity measurements Tel : () Fax : ()

5 Reference : TP-GB-RE-LAF- Page : 5/8 Results - Model Systems To see the influence of different mineralogical phases present in a CAC the conductivity has been measured for the pure phases CA and CA7 as well as for mixtures thereof. The resulting curves are given in Figure 5. CA7 CA CA + % CA7 CA + 5% CA7 CA + % CA7 Figure 5 - Conductivity curves for CA, CA7 and their mixtures In these graphs only periods I and II are given. The CA shows basically the same curve as the schematic curve in Figure 4 - the conductivity reaches a plateau after a fairly quick initial dissolution (= initial reactivity). The CA7 starts with a much faster dissolution (= higher initial reactivity) indicated by the almost vertical slope, then the conductivity drops and further regains the plateau. This drop represents the early precipitation of CAH8 from a supersaturated solution and can be observed macroscopically as an early stiffening. Due to their shape these drops in conductivity will be referred to as "ears". After the ear a behaviour similar to that of the pure CA can be observed. If CA7 is mixed to CA a faster dissolution of ions due to the different C/A ratio is achieved. For small amounts this results in curves a little bit above that of the CA. With increasing amounts, however, the ears start to occur - usually at later times, but at the same level of conductivity which corresponds to the over saturation of the solution. - Pure Cements Data on alumina cements of Ciment Fondu type have been published earlier //. The curves in Figure 6 show a comparison between a CAC (7% AlO) and Ciment Fondu. HAC from CA - CA7 clinker HAC from CA - CA clinker FONDU Figure 6 - Comparison between conductivity curves of low (4% AlO) and 7% AlO cements. The two 7% AlO cements show different reactivity due to the different mineralogy. The one with CA7 comes close to the Ciment Fondu type cement, but does not show an ear and dissolves more rapidly during the first minute. The CA - CA cement has a lower level of reactivity. These differences in reactivity as observed by the conductivity curves can also be picked up by macroscopic parameters. Table shows a comparison of three different CAC's characterized by setting time, ASTM flow at minutes, C A7 content and conductivity characterized by the min value. A consistent picture is obtained. The higher the CA7 content, the higher the initial reactivity ( min conductivity value), the shorter the setting time and the lower the ASTM flow at minutes, which represents a tendency towards stiffening. Tel : () Fax : ()

6 Reference : TP-GB-RE-LAF- Page : 6/8 Sample CA7 (%) Cond. min (ms/cm) Set (min) Flow (%) - Cements with other reactive components -- Admixtures LS,,4 9 6 LS 4,8, LS 6,5, 7 5 Table - Comparison of different properties for various CAC's For a larger population of different alumina cements similar relations were observed between the min conductivity and the ASTM flow value. This is demonstrated in Figure 7. ASTM Flow Value (%),5 ASTM ASTM,,5, Conductivity at minutes (ms/cm) Figure 7 - Correlation between workability expressed as ASTM flow and reactivity measured by conductivity It is clear that even without considering the individual chemical and mineralogical compositions of the alumina cements used, the conductivity at minutes shows satisfactory ability to evaluate a cement where its behaviour in an application is concerned. For more complex interactions as they take place between the components of a LCC mix for example, conductivity curves can also help to evaluate the impact on macroscopic behaviour.,5 The influence of admixtures to the Ciment Fondu system such as lithium carbonate and sodium gluconate has been shown in an earlier paper presented at the last UNITECR conference //. For a Secar 7 CAC the influence of a tripolyphosphate (TPP) according to // is demonstrated in Figure 8. 4 CAC +.5% TPP pure CAC CAC +.75% TPP HAC +.4% TPP Figure 8 - Calcium Aluminate Cement with different additions of TPP The addition of TPP, besides the deflocculating effect, retards the dissolution kinetics. Also, the effect of an overdose of TPP and the resulting increase of initial reactivity can be seen quite clearly. -- Fine, reactive fillers Reactive Alumina The influence of two fine alumina with different reactivities is shown for the model system from Figure 5 for the CA - % CA7 mix. The same effects have been observed for the more realistic 5% CA7 mixture but for reasons of clarity the more pronounced effects of the more concentrated system are displayed here in Figure 9. Tel : () Fax : ()

7 Reference : TP-GB-RE-LAF- Page : 7/8 4 CAC + SF CA CA + % CA7 CA +%CA7+ Alumina (BET=m/g) CA + % CA7+Alumina (BET=7,9 m/g) CAC + SF pure CAC 5 Figure 9 - The system CA - % CA7 with different aluminas Compared to the curve without alumina addition, the reactive alumina extends the time until the ears appear. In terms of ions in solution this means that saturation or super saturation is reached later. This is due to the fact that the aluminas can adsorb part of the Ca-ions in the solution, as is show in another paper presented here //. This Ca-ion uptake is proportional to the BET surface area of the corresponding alumina which explains the more pronounced effect for the alumina #. It should be kept in mind, however, that effects of flocculation and deflocculation play an important role in such systems and reactivity measurements have to be complimented by methods which evaluate inter particle forces. Silica fume The influence of silica fume (SF) on hydration behaviour has been presented earlier //, mostly observed by other methods. The same effects can be seen by the conductivity. Figure shows the effect of two different silica fumes on the hydration behaviour of a CAC. A more pronounced retardation as for the TPP in Figure 8 is observed for the initial reactivity (low level plateau). On the other hand the massive precipitation (hydration) is accelerated as compared to the pure cement, an effect measured also by calorimetry //. 5 Figure - Influence of SF additions to Secar 7 on conductivity Silica fume and TPP 4 CAC + SF +.5% TPP CAC + SF +.75% TPP CAC + SF CAC + SF +,4% TPP Figure - Adjustment of conductivity curves for a S7/SF mix with the addition of TPP The effect of a strong retardation in the initial phase, but early hydration afterwards with SF can be controlled by TPP (or other deflocculants) as is usual practice in LCC formulation. The influence of the deflocculation itself is not taken into account. An example for such adjustments is given in Figure. The addition of TPP extends the low level plateau, which would mean a low reactivity and good workability (,4 and.75%). At the same time the beginning of hydration is not to much extended compared to the pure cement. Tel : () Fax : ()

8 Reference : TP-GB-RE-LAF- Page : 8/8 4 Discussions and conclusions The conductivity as described herein is a quick test to evaluate the reactivity of a cement. This reactivity can be evaluated because the conductivity is strongly related to the kinetics of dissolution and precipitation of the ions in solution. With model systems the contribution of the differently hydraulic phases such as CA, CA and CA7 can be understood. The effects of these phases on macroscopic behaviour like ASTM flow and setting time is known. Therefore the conductivity can also be related to macroscopic properties as it has been demonstrated for the case of the ASTM flow. These insights from model systems and pure cement behaviour can, to a certain extend, be transferred to systems made up of several hydraulic, active ingredients. For refractories we find these systems for example in LCC applications. In this sense the conductivity might turn out to be a very helpful tool to study complex binder. It should be pointed out, however, that not all effects can be measured by a simple reactivity test. As already mentioned flocculation and deflocculation phenomena have to be looked at with different methods. Also, it should be kept in mind that the use of the conductivity on a routine basis demands a good knowledge of the systems dealt with. This means that the correlation between conductivity and properties have to be established first. The results obtained are in good correlation with important macroscopic properties; It can also be applied to complex binders. 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 M. S. Sabio who carried out most of the conductivity work and invested his efforts to develop the method as an useful tool. 6 Literature // 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 99. // J.P. Bayoux, C.M. George and J.P. Letourneux; "Theory and practice of fume silica - aluminous cement interactions" - Part II. // A. Mathieu, A. Capmas, J.P. Bayoux and D. Richon; "Calcium aluminates cement and reactive alumina"; UNITECR 99. In conclusion the conductivity can be summarized as follows: It is a fast, reproducible method; It is sensitive and might reveal differences which cannot be picked up by conventional methods; It gives information on the hydration mechanisms; It allows reactivity to be measured in a simple, direct way; Tel : () Fax : ()