QUICK DRYING CASTABLES BASED UPON 80% ALUMINA CEMENTS

Transcription

1 Page : 1/11 QUICK DRYING CASTABLES BASED UPON 80% ALUMINA CEMENTS C. Parr*, C. Alt, J.M. Auvray, H. Fryda, Ch. Wöhrmeyer Kerneos SA, Paris, France Presented at 46 th Symposium on Refractories, Saint Louis, USA, March 2010

2 Page : 2/11 ABSTRACT The successful development of high performance castables relies upon the optimization of both their placing characteristics and their installed properties. This is normally achieved through combinations of suitable aggregate systems together with a deflocculated binder phase. The latter component is commonly comprised of high purity Calcium Aluminate Cements (CAC) in conjunction with reactive fine fillers and additives. As the number of castable placing technologies has developed over recent years, the constraints upon the refractory formulator have multiplied as these reactions have become more and more complicated. Furthermore, in recent years there has been an increased focus on castable dry out with the aim of decreasing downtime for a more rapid turnaround and energy savings. It has been shown that one of the key parameters affecting dry out is the permeability of the binder phase. This paper presents engineered Calcium Aluminate Cements that facilitate the optimization and control of castable systems. SECAR Plenium (SP) is used to create regular and deflocculated castables with lower lime contents, lower water demand and enhanced installed properties. At the same time it is possible to maximize permeability to ensure a rapid turnaround, limited downtime and reduced energy costs. In this paper, different castable oxide systems are investigated and include high purity Alumina and Alumina-Magnesia. Additionally, new formulation concepts for Regular Castable (RC) systems with increased performance profiles are presented. This performance boost is achieved without complicating or changing the formulation and its costs. These new types of RC rely on a modified microstructure which results in significantly higher hot strength, abrasion and penetration resistance when compared to traditional RC.

3 Page : 3/11 1 Introduction 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 [1,2] 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 80% alumina binders such as SECAR 80. The strength improvement via the introduction of SECAR 80 can be seen in Fig. 1 in comparison to SECAR 71 (S71) for different CAC-contents in a generic model regular castable formulation.. CCS (800 C) ,5 3 3,5 4 4,5 5 5,5 CaO in castable S71 S80 Fig. 1. CCS 800 C as function of CaO content in an RC model system At equivalent CaO-content and initial flow, significantly higher strength is achieved with SECAR 80. Conversely equivalent strengths can be maintained at lower CaO levels with SECAR 80 (S80). High purity 80% alumina cements yield regular castables with lower lime contents, lower water demand and consequently lower porosity than achievable with 70% alumina containing calcium aluminate cements (CAC). Using SECAR 80, 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. The micro porosity via the Hg-intrusion method was measured on two paste compositions (Tab. 1), one using S71+ reactive alumina and the other S80, both with equivalent total CaO content. After firing at 1100 C the micro pores remain with diameters below 1 µm in case of S80 while with S71 a significant amount of pores were found above 1 µm (Fig. 2, 3). Table 1. Paste composition for micro pore analyses Size (mm) P-S71 (%) P-S80 (%) Tab. Alumina Tab. Alumina React. Alumina d50 = SECAR SECAR H 2O % 100% The microstructure modification in the matrix of the RC-S80 at 15% S80 can be seen in Fig. 4. While after drying at 110 C relatively coarse hydrates are visible. They have been re-crystallized in very small anhydrous phases at 800 C and have increased their size along with the pore diameter after firing at 1100 C.The microstructure modification in the matrix of the RC-S80 at 15% S80 can be seen in Fig. 4. After drying at 110 C relatively coarse hydrates are visible. They have been re-crystallized in very small anhydrous phases at 800 C and have increased their size along with the pore diameter after firing at 1100 C.

4 Page : 4/11 0,4 0,4 Hg-intrusion (ml/g) 0,3 0,2 0, C 110 C 800 C Hg-intrusion (ml/g) 0,3 0,2 0, C 800 C 110 C 0 0,001 0,01 0, Pore diameter (micron) Fig. 2. Micro pore distribution of P-S71 0 0,001 0,01 0, Pore diameter (micron) Fig. 3. Micro pore distribution of P-S80 Fig. 4. Microstructure of RC-S80 (15%) after treatment at 110 C, 800 C and 1100 C (magnification 15000x) This paper details the drastically increased performance profiles that can be achieved by applying the latest calcium aluminate technology based upon 80% alumina to castable systems. The objective is to improve the strength particularly in the range between 500 and 1100 C. To further improve the castable system, tests with the latest generation of 80% alumina containing calcium aluminate binder, SECAR Plenium (SP) have been conducted in both regular and deflocculated cement additions (as given by calcium oxide contents). Two examples are considered, high cement regular castables and reduced cement, alumina-magnesia ladle castables. This performance boost can be brought without compromising simplicity or moving the system completely out of their actual formulation cost frame. At the same time the permeability values have been optimized using organic fibers and mineral additives to ensure a rapid turnaround, limited downtime and reduced energy costs.

5 Page : 5/11 2 EXPERIMENTAL DETAILS The basic model castables that have been used are shown in table 2 and 3 with basic CAC properties shown in table 4. Table 2. Model castable test recipes High Cement Systems RC RC RC RC Properties S71-15 S80-15 SP-15 SP-8 Tab. Alumina Tab. Alumina Tab. Alumina Tab. Alumina 0.2-0, Tab. Alumina Tab. Alumina Calcined Alumina d50 4 microns 0 7 SECAR cm2/g SECAR cm2/g SECAR Plenium 8000 cm2/g 15 8 Polypropylene fibers 6mm x14 mic H 2O Table 3. Model castable test recipes Alumina Magnesia Systems Properties Ref OMP0.5 OMP1.0 PVA0.1 PVA0.2 PVA0.1O MP Tab. Alumina 2-5mm Tab. Alumina 1-2mm Tab. Alumina Tab. Alumina -0.3mm Tab. Alumina mm Tab. Alumina mm Reactive Alumina 3 m 2 /g UBE 95 MgO mm Silica Fume E971U PVA0.2 OMP Secar Plenium 8000 cm2/g Sod. Poly Acrylate PVA fibers/kurarey 3mm x1.1dtex OMP (Organo Mineral precursor) H 2O The Alumina Magnesia castable uses a simple polyacrylate dispersion system and a 6% addition of SECAR Plenium. The water soluble PVA fibers contain up to 50% of water and are generally added at a higher dose that polypropylene fibers. They are soluble around 50/90 C and so are more effective than polypropylene at low temperatures. The Organo Mineral precursor is a proprietary development product containing around 30% alumina and is designed to augment permeability of cast shapes.

6 Page : 6/11 Table 4. CAC characteristics Secar Secar Secar Property Units Plenium CaO % Al 2O 3 % SiO 2 % <0.35 Fe 2O 3 % <0.2 Na 2O+K 2O % <0.5 <0.7 <0.7 Blaine cm 2 /g 4000 >8000 > Test methods The vibration flow was measured at periodic intervals after casting to determine the model castable flow profile as a function of time. Vibration was for 20 seconds at amplitude of 0,5mm and 51Hz. The same samples were then used to measure the exothermic [3] and ultrasonic profiles [4] and the means to characterize hardening. These were backed up by the measurement of MOR and CCS after casting, drying and firing. Permeabilities were measured by a vacuum decay technique using a VacuPerm developed by the late Dr. R.E. Moore, Dr. J.D. Smith, Todd Sander, Jason Canon and Bill Headrick. The VacuPerm is capable of detecting small changes in permeability and can measure down to millidarcy. The method is based on the work of Schonlin and Hilsdorf, 1988 and uses sample disks (approx 100mm x 25mm diameter by 25mm thick disk). Castables were cured at 20 C and 100% relative humidity for 24h in the mold, followed by drying at 110 C for 24 hours. 4 Experimental results REGULAR CASTABLES EXPLE OF HIGH ALUMINA SYSTEMS The test recipes represent regular castables of a very high purity with a liquidus temperature above 1800 C. A dosage of 15% CAC, S71 contributes 4.4% CaO to the castable. With S80 and SP, the CaO content can be reduced to 2.6%. An initial vibration flow (20s) of 85-95% using an ASTM cone has been achieved by adjusting the water dosage. Water demand was 8.5% for S71, 7.2% for S80, and only 6.4% in case of SP (6.1% with 8% SP). Rheology was in all cases stable during the first 30 minutes. The S80 mix was also evaluated without fibers to evaluate their effect on placing properties. Without the 0.1% addition of fibers, the water demand for a similar initial flow (90%) can be reduced to 7.0%. the flow decay was similar with 73% and 70% respectively after 30 minutes with a final working time around 50 minutes. The castable setting has been studied with the ultrasonic method as previously described [4]. S80 and SP show a two-step hydration mechanism with SP having a longer open time (Fig. 5). A one-step hydration scheme can be observed for S71. After drying at 110 C the compressive strength is outstanding for SP with a 35-50% higher level than for the other cements. This positive trend continuous as well after firing between 800 and 1500 C where the CCS is almost 100% higher compared to S71 and still more than 30% higher compared to S80 for example in the temperature range of 800 to 1100 C (Fig. 6). This is normally the most critical temperature range for RC where cement is already de-hydrated and re-crystallized but strength acquisition through sintering reactions has not fully started yet. Here SP has a clear advantage. Fig. 5. Ultrasound profile of RC CAC=15% with different calcium aluminates Fig. 6. Green MOR after 6h (20 C) and CCS 110 C

7 Page : 7/11 The measured porosities and air permeabilities of samples after 110 C and 200 C are shown in tables 5 and 6. Measurements were performed on the same sample disks. In order to be able to assess the impact of fibers, samples were prepared with and without polypropylene fibers Table 5. Open Porosity after drying at 110 C and 200 C of Regular Castables Treatment temp C Porosity /% RC RC RC RC S71 S80 SP15 SP8 110 C W/O Fibers W Fibers C W/O Fibers W Fibers Table 6. Air Permeabilities after drying at 110 C and 200 C of Regular Castables Treatment temperature C Permeability/ millidarcy RC S71 RC S80 RC SP15 RC SP8 110 C W/O Fibers W Fibers C W/O Fibers W Fibers The impact of polypropylene fibers can be seen on both the porosities and the permeability values. As expected, the increase in porosity is more marked at 200 C than 110 C. In the case of permeability, the increase in permeability with the fiber addition is particularly marked for the 80% alumina cements. The use of fibers brings the permeability values of 80% alumina cements close to the 70% alumina cement reference. The system with reduced SECAR Plenium shows a spectacular increase in permeability which after both 110 C and 200 C is higher than the reference systems based upon either SECAR 71 or 80. It is believed this increase is due to a modified hydration pattern in the presence of fine alumina. This would suggest a facilitated dry out option. Table 7 shows the moisture loss after drying to constant weight at 110 C and 200 C. In the case of the 80% alumina cements a higher percentage of water is removed at a lower temperature than in the case of the 70% alumina cement. The higher values of water removal for the 80% alumina cements can be explained by their composition which is comprised of aluminate clinker and alumina. This results in a lower ratio of hydrated water to free water compared to the 70% alumina product.

8 Page : 8/11 Table 7.Moisture loss as a % of initial moisture Weight loss% RC RC RC RC S71 S80 SP15 SP8 110 C C The impact on porosity after firing with the different CAC can be seen in Fig. 7. Fig. 7. Apparent porosity of RC CAC-15 with different calcium aluminates Fig. 8. CCS evolution of RC CAC-15 with different Calcium Aluminates The apparent porosity can be reduced by about 4%-points by replacing 15% S71 by 15% SP. While the formulation with 15% 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. 9). This coupled with a high air permeability suggests interesting application properties where high strength and abrasion resistance are required coupled with an ease of dry out comparable with regular systems. In this case, although strength is lower after drying, it starts to increase above 800 C and reaches 50 to 100% higher strength after firing at 1350 and 1500 C respectively. Here the strength evolution is now closer to silica fume-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 formulation simplicity and is made without addition of supplementary filler or deflocculants. At the same time permeability can be increased relative to the reference systems. Fig. 9. CCS of RC SP-8 in comparison to RC S71-1

9 Page : 9/11 REDUCED CEMENT CASTABLES EXPLE OF ALUMINA MAGNESIA LADLE CASTABLES Different Alumina Magnesia castables were prepared based around SECAR Plenium at 6% addition level to give a system corresponding to an ULCC. A sodium polyacrylate was used as the dispersing additive and the addition level chosen to optimize placing properties. High early strengths were not specifically targeted for this formulation used for ladle applications where a minimum of 3MPa was required as the de-molding criteria. The water demand was kept at a constant 5.6% for all variations. The placing properties are shown in table 8 and show vibration flow in excess of 100% even with large additions of a PVA fiber. The flow is maintained for 45 minutes in the case of the reference system and this is significantly extended when the OMP additive is employed. At 1.0% addition the working time is almost tripled. However, it is interesting to note that the hardening time is not really impacted. The time to reach 2000 m/s as measured by the ultrasonic equipment is similar for all model systems. The point at a velocity of 2000 m/s corresponding to the midpoint of the initial hardening for these model Al-MgO systems. The OMP additive also provides a significant strength boost after casting/curing and after drying. If the data at 24 hours is compared the OMP additive more than doubles the compressive strength relative to the reference system. Table 8 : Castable placing properties and initial mechanical properties Ref OMP0.5 OMP1.0 PVA0.1 PVA0.2 PVA0.1O MP PVA0.2 OMP T0 vibration flow % T30 vibration flow % Working time mins Time to reach 2000 m/s - U/sound mins CCS 24h MPa CCS 110 C MPa Fig. 10. Impact of permeability as a function of fiber addition Fig. 11. Impact of permeability as a function of OMP addition In contrast, the addition of PVA fibers tends to lower the mechanical strength after casting at 24 hours. The permeability values after 110 C and 200 C are shown in figures 10 and 11. The beneficial effect of fibers can be seen at addition levels above 0.1%. A spectacular increase in permeability is observed with the OMP additive. At additions between 0.5 and 1.0% the permeability increases several fold and exceeds values found with the regular concretes in the previous section.

10 Page : 10/11 In order to limit the quantity of OMP additive for both cost and technical reasons combinations of PVA fiber and OMP were investigated. Fig. 12. Impact of permeability as a function of a fiber addition at 0.5% OMP The permeability of model systems which use a combination of PVA fiber and OMP at 0.5% can be seen in Figure 12. At only 0.1% fiber addition there is a spectacular many fold rise in permeability at both 110 C and 200 C. The permeability increases with increasing fiber addition up to 0.2%. The explanation for the increase in permeability is not completely understood but it is believed to be linked to the formation of an AHx gel which forms within the castable matrix and when drying commences. This gel shrinks and creates a network between the pores increasing the permeability. The use of a combination of fibers and OMP also is effective in limiting the extension to the working time. The limiting factor with respect to the OMP additive is the trend to higher as cast shrinkage. At an addition of 0.5% the shrinkage from as case to dried at 110 C is -0.21% compared to the reference system at <-0.05%. The shrinkage from dry to 200 C is -0.23% compared to with the reference product. Future development will focus around reducing these values. The implementation of the latest CAC-technology into regular castables boosts their performance to a level which was not achievable so far by classical 70% and 80% Al 2 O 3 -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 % compared to SECAR 71 can be achieved with SP throughout the whole temperature profile from 800 to 1500 C at equivalent amount of binder. Excellent abrasion resistance can be expected in particular in the temperature range of 800 to 1100 C for example in petro chemistry applications. The use of SECAR Plenium gives rise to a microstructure with a low porosity and small pore diameters which make them less sensitive to infiltration of liquid metals and slags. Combined with calcined alumina and polypropylene fiber high strength and density can be maintained whilst at the same time increasing permeability compared to the reference systems. SECAR Plenium has been used for many years in silica fume-free (AA and AlSP type castables) and Alumina Magnesia castables. Through the use of a novel organo-mineral precursor the permeability can be increased several fold. It is believed a gel forms that then shrinks on the application of heat and this creates a permeable network within the castable to facilitate dry out. Work will continue to optimize this precursor and reduce drying shrinkage to give a more efficient and viable alternative to organic fibers to enhance permeability. This state-of-the-art calcium aluminate cement represents an all-round binder from which not only reduced lime content dense castables but also regular castables can profit. 4 Conclusion Using the latest 80% alumina 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. Using different techniques the permeability can be enhanced to facilitate quicker dry out.

11 Page : 11/11 5 Acknowledgements The authors would like to thank all the technicians in the Kerneos Research and Development Centers who have contributed to these studies. 6 References [1] 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 08 p. XXXXII-LIII, Kolkata, [2] Parr C, Bier TA, Bunt N, Spreafico E. Calcium Aluminate Cement (CAC) based castables for demanding applications. 1 st Monolithics Conference, Iran, 1997 [3] C. Alt, L. Wong, C. Parr, Measuring castable rheology by exothermic profile, Refractories applications, Vol. 8, No. 2, pp [4] F. Simonin, Ch. Wöhrmeyer, C. Parr, A New Method for Assessing Calcium Aluminate Cements, Unitecr 2005, Orlando, USA, 2005 [5] C. Parr et al, A new insight into the matrix interactions of deflocculated castables which control placing and setting properties, UNITECR 2007, Dresden, Germany, 2007, (2007). [6] 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 60, No. 11, 2008.