NEW ALUMINATE BINDERS AND SYSTEMS FOR REFRACTORY MONOLITHICS

Transcription

1 Reference : TP-GB-RE-LAF-8 Page : 1/11 NEW ALUMINATE BINDERS AND SYSTEMS FOR REFRACTORY MONOLITHICS C. Parr*, J.M. Auvray, H. Fryda, Ch. Wöhrmeyer Kerneos SA, Paris, France Presented at 8 th India International Refractories Congress, Kolkata, February 21 Tel : 33 () Fax : 33 ()

2 Reference : TP-GB-RE-LAF-8 Page : 2/11 ABSTRACT Recent developments of Aluminate binders and systems that can be harnessed to deliver castables with improved properties and more predictable behaviour will be presented in this paper. Examples will show how the control of the hydraulic reactivity of different calcium aluminate phases can be advantageously used to deliver castables with improved workability and hardening profiles with a high degree of predictability as well improved installed properties A series of model systems representing low cement castables containing calcium aluminates have been studied and compared to reference systems. Experiments are conducted using, wet mixed, model systems through the phase of initial flow, flow decay and stiffening using parallel measurement techniques. Classical flow measurements are used to evaluate placing properties and these are coupled with ultrasonic measurements and exothermic profiles to assess castable hardening. These experiments are able to describe the flow, workability and hardening relationships and show the improved performance that the new binders can offer. These physical measurements are complemented by ionic conductimetry, analysis of pore solutions during the castable placing steps and differential thermal analysis which provide an insight into the underlying mechanisms and hydration reactions Tel : 33 () Fax : 33 ()

3 Reference : TP-GB-RE-LAF-8 Page : 3/11 1 Introduction The role of calcium aluminates in refractory castables have evolved from that of a fundamental binder largely dictating castable placing properties in regular conventional concretes to more of a chemical reactive in deflocculated castables. Significant progress has been made with a continuous evolution of calcium aluminate binders towards products displaying significantly reduced variation. 1 However, in order to support further progression of monolithics there is a need for castables which have a greater robustness to placing variables such as ambient temperature. In addition, there is the challenge of developing more predictable behaviour in terms of the workability and hardening relationship. Naturally, this has to be achieved whilst maintaining or developing improved installed properties. This paper details aluminate binders that are able to give enhanced properties for both regular as well as reduced lime content castables. The two families of products present different challenges for the binders ; - A key area for improvement in the reliability of deflocculated dense low calcium content castables is the trade off that has to be made between early strength gain (within 6 hours from casting) and the time that a wet mixed castable remains workable. One option is to use the latest aluminate binders with targeted properties such as SECAR Xeniom. An alternative approach applicable to calcium aluminates with 7 alumina content is to modify the workability/hardening relationship relying only on the intrinsic reactivity of the different mineralogical phases that comprise the hydraulic binder. This paper will detail a new technology that can be applied to aluminate binders capable of modifying this workability/hardening relationship. Regular castables are well known for their robustness and tolerance to external placing parameters. The fact that they still represent, on a global basis, a significant part of total castable volumes is a testament to their reliable and simple formulation concept which delivers robust placing properties. A long held ambition is to achieve the reliability of regular castables combined with the performance of reduced calcium systems. A first step in this regard is the use of 8 alumina binders such as SECAR 8. The second part of this paper details the drastically increased performance profiles that can be used by applying the latest calcium aluminate technology to regular castable systems. This performance boost can be brought to RC without compromising on simplicity or moving the system completely out of their actual formulation cost frame. 2 New Technologies for Deflocculated Castables It has been shown 2 that the presence of reactive CAC phases such as C12A7 can have a positive effect on early strength gain and robustness but a detrimental effect upon castable workability and working time. If their negative impact on the placing stage could be managed then their incorporation as part of the phase assemblage of calcium aluminates would be very attractive for deflocculated castables. A new process has been developed which aims to modify the reactivity of this phase and its interaction with the other calcium aluminate phases. Thus, providing a longer working time whilst conserving the ability to develop significant mechanical strengths within 6 hours from casting. Tel : 33 () Fax : 33 ()

4 Reference : TP-GB-RE-LAF-8 Page : 4/11 EXPERIMENTAL DETAILS Three laboratory prepared calcium binders have been used. The basic chemical and mineralogical properties are shown in table 1. Table 1. Composition of Calcium Aluminate Binders dry calcined basis wt. REF CAC QS CAC RC CAC Al 2O CaO SiO Fe 2O MgO Alkalies Na 2O + K 2O CA CA C12A7 < C2AS Others <3. <3. <3. where C= CaO, A= Al 2O 3, S= SiO 2 REF CAC is the reference calcium aluminate cement (CAC) based around a nominal 7 alumina content, with a mineralogy based upon calcium monoaluminate (CA) and calcium dialuminate (CA2). QS CAC is also nominally a 7 Alumina content calcium aluminate but with a modified mineralogy which contains an appreciably amount of Dodeca-calcium hepta-aluminate (C12A7). RC CAC is the reactivity controlled calcium aluminate with a mineralogy analogous to the QS sample. This calcium aluminate cement was prepared in the laboratory using a new experimental process which modifies the reactivity of the different calcium aluminate mineralogical phases compared to normally processed calcium aluminates such as REF and QS CAC. All samples were milled to a nominal 4 cm 2 /g specific surface Blaine value. A model system representing a generic low cement castable (LCC) was used to evaluate the placing and hardening properties of these three calcium aluminates (table 2). A binder phase mortar comprising of the fine part of a model LCC has been derived from the full sized version (table 2). Analytical Methods The vibration flow was measured at periodic intervals after casting to determine the model castable flow profile as a function of time. The same samples were then used to measure the exothermic 3 and ultrasonic profiles 4 as the means to characterise hardening. These were backed up by the measurement of MOR and CCS 6 hours after casting. Conductimetry and pore solution extraction and analysis were performed using the binder phase mortar and the experimental details are as previously described,6 Table 2. Composition of Model Low Cement (LCC) Castable Binder phase Brown Fused Alumina : 6-1-1mm Brown Fused Alumina : mm Fume Silica : Elkem 971U 29.4 Alumina/Alcan P2SB CA: 7 CAC 29.4 Sod. Tripolyphosphate (TPP) +, Water 3, w/c, EXPERIMENTAL RESULTS Placing and hardening properties Figure 1 shows the flow profiles of the three CAC binders. All three binders have a similar flow profile over the first 3 minutes with QS CAC displaying a slightly lower initial flow. Vibration flow () REF CAC QS CAC RC CAC Time (mins) Fig. 1. Vibration flow profiles for three CAC binders The QS CAC has the most rapid flow decay with the working time occurring in just over 4 minutes compared to the reference binder which has a working time around 14 minutes. This is as expected from previous results 2 and Tel : 33 () Fax : 33 ()

5 Reference : TP-GB-RE-LAF-8 Page : /11 linked to the significant presence of reactive C12A7 phase. The reactivity controlled CAC (RC CAC) shows a flow profile much closer to the reference with a working time around 2 hours. The hardening properties are demonstrated in figures 2 and 3 with the ultrasonic and exothermic profiles. There is generally an agreement between the point of initial rise in the ultrasonic velocity and the first exothermic peak 3. The QS shows the quickest rise in ultrasonic velocity and the shortest 1st exothermic peak. Fig. 3. Exothermic profiles for the three CAC binders These data are coherent with the flow profiles for REF and QS CAC in that the shortest flow decay results in the quickest development of structure. RC CAC shows a different behaviour where the initial exothermic peak occurs later than QS yet the rise to the main exothermic peak is very similar. This suggests a different hydration kinetic/mechanism. Fig. 2. Ultrasonic profiles for the three CAC binders The ultrasonic profiles for RC shows that the first development of structure occurs between the REF point at over 2 hours and the QS time at around one hour. The reference CAC displays the longest first exothermic peak time and longest time to the first increase in ultrasonic velocity. The main exothermic peaks occur between 6 and 8 hours with the reference CAC being noticeably longer than the other two binders. The inference, as shown in figure 3, is that the period between the first and main exothermic peaks is shorter for the reactivity controlled cement than either the reference or the QS variation. In practical terms, this suggests a working time closer to the REF and a hardening profile closer to QS. This was verified by the measurement of CCS after 6 hours and the results are compared with the flow at 9 minutes in figure 4. The reactivity controlled (RC) CAC shows the best compromise between maintaining flow at 9 mins and developing almost 3Mpa CCS at 6 hours after casting. The strength development is almost the same as QS CAC but with an appreciable flow at 9 minutes. This suggests a real demoulding time of only a few hours after casting. Tel : 33 () Fax : 33 ()

6 Reference : TP-GB-RE-LAF-8 Page : 6/11 V i b ra tio n F lo w T 9 ( ) H CCS 29.9 Flow T REFCAC RC CAC QS CAC Fig. 4. Trade off between castable flow time and early strength development h o u r C C S ( M p a ) Fig.. Binder phase conductimetry Conductimetry and pore solution analysis In an attempt to understand the mechanisms at play during the workable period, recourse was made to conductimetry and periodic pore solution analysis of the corresponding binder phase of the model LCC. The conductimetry data (figure ) shows an initial increase in conductivity which corresponds to the end of the workable period. It has been show 6 to be linked to an increase in alumina and sodium ion concentrations. The major drop in conductivity, linked to the massive precipitation of hydrates occurs in the same time interval as the main exothermic peak, although it must be noted there is a slight offset between the data in figures 3 and. The reasons for this are not clear and would require further investigation. The conductimetry data confirms the modified reactivity of RC CAC as the rise in conductivity occurs later than QS CAC but the precipitation times are very similar. The clear difference in mineralogy between REF CAC and the other two samples can be seen by the long precipitation time which is preceded by a long nucleation period (flat part of curve). Fig. 6. Ion concentration as a function of time (Calcium and Phosphate) The corresponding pore solution data for the extractions made during the workable phase are found in figures 6 and 7. In these figures the main soluble species are shown namely, calcium, phosphate, alumina and silica. Sodium is not shown as the concentrations, although high at around mm/l, remained stable throughout the workable period, Tel : 33 () Fax : 33 ()

7 Reference : TP-GB-RE-LAF-8 Page : 7/11 coherent with previous data,6 A previous publication has investigated the relationship between calcium and phosphate ions as a function of time during the workable period. There is an initial increase in calcium ion concentration followed by a sharp decrease which coincides with the loss of fluidity. The phosphate concentration decreases continually during the same period. It is believed that there is a progressive formation of calcium tripolyphosphate and the consumption of all the phosphate results in the loss of fluidity and the end of the workable period. The presence of C 12 A 7 in the QS CAC results in a lower phosphate ion concentration in the solution and a more rapid drop in parallel with a drop in calcium ion concentration. The inference is a more rapid formation of calcium tripolyphosphate than the REF CAC System. This would be coherent with the presence of C 12 A 7 which is normally 2 associated with a high reactivity to water. In comparison, the phosphate ion concentration as a function of time for RC CAC follows the reference more closely. A similar effect can be observed for the change in calcium ion concentration with time albeit that it decreases to zero at 12 minutes whereas the REF CAC is slightly longer. This tends to suggest that the reactivity controlled mechanism is effective in suppressing the initial calcium ion reactivity with respect to the phosphate. This then accounts for a longer working time than the mineralogically comparable QS CAC. The ionic concentration changes with time for alumina and silica (figure 7) tend to confirm the data in figure 6. The decrease in silica ion concentration occurs in a similar time interval to the end of working time and a rapid increase in the alumina ion concentration at the same moment. The reactivity controlled CAC, displays ionic concentrations for alumina and silica that are much closer to the reference CAC than its mineralogical analogue of QS CAC. Silica decreases at the same time as the end of working time. This has been interpreted as a result of a precipitation reaction with calcium. Once the calcium, phosphate and silica ion concentrations have reduced to almost zero, the alumina concentration continues to increase (not shown) up to 8 mm/l for both the REF and the RC CAC, and at the respective points in time, massive precipitation occurs. It would appear from the data available that the modification of the hydration kinetics associated with the reactivity controlled CAC is linked to the dissolution phase. Thereafter the hydration of RC CAC proceeds in a manner similar to the QS CAC with an analogous mineralogy. Fig. 7. Ion concentration as a and Alumina) function of time (Silica The advantages with this reactivity controlled calcium aluminate are considered to be the delivery of optimised placing properties and rapid strength gain at the same time. This leaves total formulation flexibility in terms of additives for the user. Tel : 33 () Fax : 33 ()

8 Reference : TP-GB-RE-LAF-8 Page : 8/11 3 Regular castables High purity 8 alumina cements yield conventional castables with lower lime contents, lower water demand and consequently lower porosity than achievable with 7 alumina containing calcium aluminate cements (CAC). CCS (8 C) , 3 3, 4 4,, CaO in castable S71 S8 Fig. 8. CCS 8 C as function of CaO content in RC and type of CAC (recipes see Table. 4) Using for example, SECAR 8, an alumina rich microstructure with a good cohesion between cement hydrates and aggregates and with a micro-porous matrix can be developed which leads to improved properties such as those shown in figure 8 Two paste compositions (Table 3), one based on S71+ reactive alumina and one based on S8, both with equivalent total CaO content, have been chosen for the measurement of the micro porosity via the Hg-intrusion method. After firing at 11 C the micro pores remain at diameters below 1 µm in case of S8 while with S71 a significant amount of pores were found above 1 µm (Fig. 9, 1). Table 3. Paste composition for micro pore analyses mm P-S71 P-S8 Tab. Alumina Tab. Alumina React. Alumina d =. 18 SECAR SECAR 8 H2O Hg-intrusion (ml/g),4,3,2,1,1,1,1 1 1 Pore diameter (micron) Fig. 9. Micro pore distribution of P-S71 Hg-intrusion (ml/g),4,3,2,1 11 C 11 C 8 C,1,1,1 1 1 Pore diameter (micron) 11 C 8 C 11 C Fig. 1. Micro pore distribution of P-S8 The microstructure modification in the matrix of the CC-S8 at S8 can be seen in Fig. 11. While after drying at 11 C relatively coarse hydrates are visible. They have been re-crystallised in very small anhydrous phases at 8 C and have increased their size along with the pore diameter after firing at 11 C. To further improve the regular castable system, tests with the latest generation of 8 alumina containing calcium aluminate binder, SECAR Plenium (SP) have been conducted. The CAC properties and test recipes are shown in Tables 4 and. The objective is to improve the strength particularly in the range between and 11 C. These new types of regular castables rely on a modified microstructure which results in significantly higher hot strength, abrasion and penetration resistance. Tel : 33 () Fax : 33 ()

9 Technical Paper Reference : TP-GB-RE-LAF-8 Page : 9/11 11 C Rheology was in all cases stable during the first 3 minutes. 8 C Table. CAC characteristics S71 Table 4. Regular castable test recipes Tab. Alumina 2- CC S71 32 Tab. Alumina 1-2 Tab. Alumina Tab. Alumina.2-, Tab. Alumina Prop. Tab. Alumina -.4 Reactive Alumina m /g Calcined d 4 Alumina microns 4 SECAR 71 cm2/g 8 SECAR 8 cm2/g SECAR 8 Plenium cm2/g Polyprop 14 fibres microns H2 O CC S8 32 SP 8 32 SP EXPERIMENTAL RESULTS The test recipes represent conventional castables of very high purity with a liquidus temperature above 18 C. At a dosage of CAC, S71 brings 4.4 CaO into the castable. With S8 and SP, the CaO content can be reduced to 2.6. An initial ASTM vibration flow of 8-9 has been achieved by adjusting the water dosage. Water demand was 8. for S71, 7.2 for S8, and only 6.4 in case of SP (6.1 with 8 SP). Tel : 33 () Fax : 33 () Al2O SiO2.4.3 <.3 Fe2O3.2.2 <.2 Na2O+K2O <. <.7 < >8 >8 Blaine Fig. 11. Microstructure of CC-S8 () after treatment at 11 C, 8 C and 11 C (magnification x) SP units cm /g SECAR 8 17 CaO Property 11 C S8 SECAR SECAR Plenium 17 The castable setting has been studied with the ultrasonic method as previously described4 S8 and SP show a two-step hydration mechanism with SP having a longer open time (Fig. 12). A one-step hydration scheme can be observed for S71. Fig. 12. Ultrasound profile of RC CAC- with different calcium aluminates After drying at 11 C the compressive strength is outstanding for SP with a 3- higher level than for the other cements. This positive trend continuous as well after firing between 8 and C where the CCS is almost 1 higher compared to S71 and still more than 3 higher compared to S8 for example in the temperature range of 8 to 11 C (Fig. 14). This is normally the most critical temperature range for RC where cement is already de-hydrated and re-

10 Technical Paper Reference : TP-GB-RE-LAF-8 Page : 1/11 crystallised but strength acquisition through sintering reactions has not fully started yet. Here SP has a clear advantage. The impact on porosity achieved with the different CAC can be seen in Fig.. The apparent porosity can be reduced by about 4-points by replacing S71 by SP. While the formulation with S71 creates the typical strength profile of a RC with decreasing CCS along with increasing temperature this trend can be turned around when a dosage of 8 SP is chosen (Fig. 16). In that case, although strength is lower after drying, it starts to increase above 8 C and reaches to 1 higher strength after firing at 13 and C respectively. Here the strength evolution is now closer to what is known from microsilica-free low cement castables and indeed with an addition of 8 SP, the CaO content is at 1.4 as in a classical LCC. But In the case of RC SP-8 the castable maintains a high formulation simplicity and is made without addition of supplementary filler or deflocculants. CCS (MPa) Fig. 13. Green MOR after 6h (2 C) and CCS 11 C Fig.. Apparent porosity of RC CAC- with different calcium aluminates SP-8 S71-2 (6h) Fig. 14. CCS evolution of RC CAC- with different Calcium Aluminates Tel : 33 () Fax : 33 () (24h) Temperature ( C) Fig. 16. CCS of RC SP-8 in comparison to RC S71-

11 Technical Paper Reference : TP-GB-RE-LAF-8 Page : 11/11 4 Conclusion Using the latest Calcium Aluminate technologies opens up perspectives for significant improvements in both reliability and performance to be given to calcium aluminate based regular castables as well as deflocculated reduced lime content castables. The development of a process that allows an intrinsic control of hydraulic reactivity gives rise to an enabling technology for development of new calcium aluminate binders offering a modified trade off between extending the working time without delaying the acquisition of sufficient mechanical strength for demoulding. The technology can give access to a wider range of potential mineralogical compositions whilst at the same time maintaining optimal placing and hardening properties. Further development will continue and the expected robustness with respect to ambient temperature confirmed. The implementation of the latest CACtechnology into regular castables boosts their performance to a level which was not achievable so far by classical 7 and 8 Al2O3-containing CAC. With SECAR Plenium high strength RC can be easily designed without the need to add filler and deflocculants, although this is a possible route for further improved properties where necessary. A strength increase of -1 compared to SECAR 71 can be achieved with SP throughout the whole temperature profile from 8 to C at equivalent amount of binder. Excellent abrasion resistance can be expected in particular in the temperature range of 8 to 11 C for example in petrochemistry applications. SECAR Plenium builds a microstructure with a low porosity and small pore diameters which make them less sensitive to infiltration of liquid metals and slags. Known already for the high performance in Microsilica-free (SP) and Alumina Magnesia castables this state-of-the- Tel : 33 () Fax : 33 () art calcium aluminate cements represents an all-round binder from which regular castables can profit to a high extent. Acknowledgements The authors would like to thank all the technicians in the Kerneos Research and Development Centres who have contributed to these studies. 6 [1] [2] [3] [4] [] [6] References Parr C, Assis G, Auvray JM, Chong Hu, Fryda H, Wöhrmeyer C. Recent advances in refractories aluminate binders and additives for high performance monolithic castables. IREFCON 8 p. XXXXII-LIII, Kolkata, 28. C.Parr, C. Wohrmeyer, B. Valdelievre, A. Namba, Effect of Formulation Parameters upon the Strength Development of Calcium Aluminate Cement Containing Castables, Taikabutsu Overseas, 23, vol 23, part 4, p C. Alt, L. Wong, C. Parr, Measuring castable rheology by exothermic profile, Refractories applications, Vol. 8, No. 2, pp -18 F. Simonin, Ch. Wöhrmeyer, C. Parr, A New Method for Assessing Calcium Aluminate Cements, Unitecr 2, Orlando, USA, 2 C. Parr et al, A new insight into the matrix interactions of deflocculated castables which control placing and setting properties, UNITECR 27, Dresden, Germany, 27, 4-48 (27). C. Parr, H. Fryda, M.Iiyama, A. Borovsky, Interactions of calcium aluminate cements and other matrix components which control the initial hardening of deflocculated castables, Taikabutsu, Vol 6, No. 11, 28.