1 STUDIES ON FLOW BEHAVIOUR OF LOW CEMENT CASTABLES …

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56th International Colloquium on Refractories 2013 – Refractories for Industrials

STUDIES ON FLOW BEHAVIOUR OF LOW CEMENT CASTABLES IN PRESENCE OF DIFFERENT REACTIVE ALUMINA AND MICROSILICA

Dr. A. K. Samanta, S. Satpathy, Dr. S. Adak & Dr. A. K. ChattopadhyayTRL Krosaki Refractories Limited, Belpahar – 768 218, Odisha (INDIA)

AbstractProper placement of low cement castables is one of the most im-portant aspects while assessing its performance. Quality selection and designing of castables is another aspect which depends main-ly on operational parameters for any application. Both quality and placement are equally important to get the best performance of any castables. During placement three properties are very im-portant i.e. optimum water addition, proper fl ow and initial setting within the specifi ed time. These three properties can be achieved by proper designing of matrix which consists of reactive alunmi-na, micro-silica and binder (i.e. cement). Dispersing agents also play important role on water demand and fl ow of castables. Main-ly two kinds of dispersing agents are used i.e. inorganic and or-ganic depending upon the nature and designing of castables. Sev-eral works have been done to optimise the particle size and its distribution to achieve optimum packing. Designing of matrix of low cement castables control the placement properties; particu-larly water demand and fl ow. Different kinds of reactive alumina are available having different d50 and particle size distribution like mono-modal, bi-modal or poly-modal in nature. Selection of right quality of reactive alumina and its quantity gives best packing in matrix which results lower water demand and good fl owability. Quality of reactive alumina may also give better thermo-mechan-ical properties while forming different ceramic phases like mul-lite, CA6 etc. depending upon the presence of other constituents in matrix. Microsilica is another important constituent to be con-sidered while designing a castable having lower water demand and good fl owability. Apart from fl owability and water demand qual-ity of microsilica has an effect on setting of castables. Micro-sil-ica conyains different impurities like Na2O, K2O, sulphate com-pounds etc, which control its pH. Higher alkali results slow setting of castables by changing the total pH of castable system. There-fore, purity level and presence of alkalies are the controlling pa-rameters while selection of microsilica for designing castables.In this present work, 70% Al2O3 low cement castables have been designed with four different calcined/reactive aluminas to observe the effect of different aluminas on different placement properties as well as other physical properties of castables. Calcined/ reac-tive aluminas are selected on the basis of specifi c surface area, d50 and nature of particle size distribution.

IntroductionMain constituents for matrix of low cement and ultra-low cement castables are cement, micro-silica and calcined/ reactive alumina. These give not only the fl ow of castables but also optimum pack-ing in matrix which results low water demand and improved sev-eral physical properties. Further to achieve the maximum packing, selection of calcined/reactive alumina and its quantity are equally important. Different calcined/reactive alumina are characterized by their chemical analysis i.e. purity level, specifi c surface area, d50 and nature of particle size distribution.Apart from calcined/reactive alumina, micro silica and cement have a strong infl uence on placement behavior of castables and other physical properties. While fully disperse of micro-silica helps to reduce the water demand for placement [1, 2]. On place-ment of castables, some portion of micro-silica reacts with cal-cium aluminate cement and water to form so called C-A-S-H, one sol-like compound. On heating it converts into gel and gives

strong bonding of aggregates and matrix. On the other hand, it re-duces the open porosity of castables at ~ 1000˚C compared to con-ventional dense castables. On fi ring of low and ultra-low cement castables, amorphous micro-silica and calcined/ reactive alumi-na react with each other to form mullite and other ceramic phas-es which improve thermo-mechanical properties [3]. Formation of mullite depends upon the nature of micro-silica and calcined/reac-tive alumina. It has been observed that alumina having lower d50, higher specifi c surface area and multi-modal particle size distribu-tion accelerate the formation of mullite. It also helps to start mul-lite formation at relatively low temperature. Alkalies particularly Na2O+K2O are the impurities present in different calcined/reac-tive alumina which affect the placement properties as well as dif-ferent thermo-mechanical properties also. Therefore, it is essential to select proper quality of calcined/reactive alumina while de-signing castables for different applications. For application where temperature is high and the castable is under thermo-mechanical stress, it is suggested to select reactive alumina having negligible amount of alkalies while designing silica-free low cement casta-ble for some special applications.To get dense matrix either in low cement or ultra-low cement castables, addition of casting water is very important. Low-er amount of casting water is always good to get better physical properties like strength, abrasion resistance. Calcined/reactive alumina plays a role on casting water to get suffi cient vibro-fl ow. With same amount of water, castables having different calcined/reactive alumina give different vibro-fl ow [4]

ExperimentalIn the present work four different formulations of low cement castables (having ~ 70% Al2O3) have been designed in combina-tion with Chinese Bauxite, Silliminite Sand, BFA and Chamotte. In four different formulations, four kinds of calcined/reactive alu-mina of 5% were added keeping micro-silica and cement in the same level. All four batch formulations are given in Ttable-1. Dif-ferent calcined/ reactive aluminas are designated as ALUMINA-1, ALUMINA-2, ALUMINA-3 and ALUMINA-4

Table 1: Different Batch Formulations

Raw Materials (wt%) T1 T2 T3 T4

Chinese Bauxite 15 15 15 15

Chamotte 18 18 18 18

BFA 42 42 42 42

Silliminite Sand 10 10 10 10

ALUMINA-1 5 - - -

ALUMINA-2 - 5 - -

ALUMINA-3 - 5 -

ALUMINA-4 - - - 5

High Alumina Cement 5 5 5 5

Micro-silica 5 5 5 5

Dispersant 0.1 0.1 0.1 0.1

Organic Fiber 0.05 0.05 0.05 0.05

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Castables were made in Hobart Mixer with 5 minutes dry mixing followed by water addition and wet mixing for another 5 minutes. Water addition was limited to 5% for each formulation. Flow de-cay was measured with the help of fl ow meter upto 45 minutes with an interval of 15 minutes. CCS, CMOR BD, AP and PLC were measured from the sample of size 160mm x 40mm x 40mm. Initially all samples were dried at 110°C for 24 hours. Firing was carried out at 800°C, 1100°C, 1400°C and 1500°C with 3 hours soaking. In case of PLC measurement initial length was taken af-ter drying at 110°C for 24 hours. AP (apparent porosity) and BD (bulk density) both were measured as per Archimedes Principle by water displacement method. XRD analysis was carried out with the samples fi red at 1100°C, 1400°C and 1500°C.

Results and DiscussionChemical analysis for different raw materials is given in Table-2. The same for different calcined/reactive alumina is shown in Ta-ble-3 whereas particle size distribution is shown in Fig.1From particle size distribution it is observed that ALUMINA-1 is a bimodal type having two peaks at 4µm and 25µm of volume ra-tio 90:10. ALUMINA-2 and ALUMINA-3are also bimodal hav-ing peaks at 3µm and 5µm of volume ratio 60:40; and 4.5µm and 70µm of volume ratio 80:20 respectively. ALUMINA-4 is mono-modal having peak at 6.5µm.

Table 2: Chemical Analysis of Raw Materials

Raw Material (wt%) Al2O3 Fe2O3 SiO2 CaO Na2O + K2O

Chinese Bxt. 85.40 1.62 7.61 0.18 0.22

Chamotte 40.30 0.88 55.2 0.12 0.45

Silliminite Sand 57.20 0.81 39.1 0.18 0.09

Calcined Alumina 98.71 0.02 0.02 - 0.08

High Alumina Cement 70.20 0.19 0.29 28.3 0.22

BFA 94.8 0.42 0.81 0.22 0.21

Micro-silica 0.38 0.06 95.8 0.26 0.12

Table-3: Properties of different calcined/reactive aluminas

Alumina Alu-mina-1

Alu-mina-2

Alu-mina-3

Alu-mina-4

Al2O3 (%) 99 99.5 99 99

Spec. Surface Area (m2/g)

2.1 3 0.8 0.5

D50 (µm) 2.8 3 5 5.5

Vibrofl ow for all four batches were measured with 5% water by ASTM C233 method with 20 seconds vibration. The results are shown in Fig.2. With same amount of water, T1 gives maximum vibrofl ow followed by T3 and T4. The mimimum vibrafl ow is ob-served in case of T2. It clearly indicates that particle size and par-ticle size distribution of calcined/reactive alumina have an impor-tant role to acheive good fl owability for low cement or ultra-low cement castables.The fl ow decay of different batches is shown in Fig.3. It has been observed that vibrofl ow of >70% was there for batches upto 15 minutes which is suffi cient from the installation point of view. Af-ter 45 minutes, suffi cient vibrofl ow was not there for T2, T3 and T4 but in case of T1 there was still vibrofl ow beyond 45 minutes which is suffi cient for installation.Apparent porosity (AP) was measured after drying the samples at 110°C and after fi ring at 800°C, 1100°C, 1400°C and 1500°C with

3 hrs soaking and resuts are shown in Fig.4. In all cases, there was sharp increase in AP upto 800°C then it was almost constant upto 1100°C. Beyond 1100°C there was decresed in AP with increased fi ring temperature. The sharp increase in AP after drying is due to removal of chemically combined water. This effect was continued

Fig.1: Particle Size Distribution for Different Aluminas

Fig. 3: Flow decay for different batches upo 45 minutes

Fig. 2: Vibrafl ow for different batches with 5% water

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upto 800°C where most of the combined water was removed from the body. A negligible amount of water was left beyond 800°C and it was completed after fi ring above 1100°C. Beyond 1100°C, sin-tering starts in the system which results densifi cation in the ma-trix and reduces the AP. Sintering process was continued with in-crease in fi ring temperature, with continuous reduction in AP. The reverse trend was found while measuring bulk density (BD) after drying and fi ring (Fig.5). BD was decreased upto 1100°C followed by increasing trend with increase in fi ring temperature. Higher BD indicates densifi cation in matrix due to sintering.

phases are mullite and corundum with some minor phase called anorthite (which is a low temperature eutectic phase). It is very interesting to note that mullite formation starts even after fi ring at 1100°C. This may be due to presence of microsilica and very fi ne calcined/reactive alumina in matrix. The peak intensity of mullite increases with increase in fi ring temperature; which in-dicates the completion of reaction between microsilica and cal-cined/ reactive alumina. Formation of anorthite is mainly due to the reaction of calcium oxide (which forms from the dehydro-

There was continuous increse in cold crushing strength (CCS) af-ter drying and after fi ring (Fig.6) which is the common behaviour for low cement and ultra-low cement castables. In case of T1, CCS was always higher upto 1400°C but beyond that there was no si-gnifi cant difference between all four batches. CMOR for all bat-ches show the same behaviour like CCS (Fig.7).PLC for all four batches were measured after fi ring at 1400°C and 1500°C; and results are shown in Fig.8. In all cases there was shrinkage and the fi ring shrinkage was more during fi ring at 1500°C compared to 1400°C. Low shrinkage was observed for T1. Difference in shrinkage were not so signifi cant in other three cases.The identifi cation of different crystalline phases after fi ring the samples at 1100°C, 1400°C and 1500°C were carried out by XRD; and results are shown in the Fig.9 to Fig.12. In all cases, major

Fig. 5: Bulk density after drying and after fi ring

Fig. 4: Apparent porosity after drying and after fi ring

Fig. 6: CCS after drying and after fi ring

Fig. 7: CMOR after drying and after fi ring

Fig. 8: PLC measured after fi ring at 1400°C & 1500°C

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xylation of cement hydrates at relatively low temperature) with alumino silicate.SEM Micrographs of T1 and T4, after fi ring the samples at 15000C, are shown in the Fig.13 and Fig.14, respectively. Pres-ence of needle shaped mullite crystals are clearly visible, spe-cially for T1. SEM analysis was carried out of sample without etching and hence crystalline phases are covered by some glassy phase.

ConclusionsCalcined or reactive alumina is one of the most important con-stituent while designing low and ultra-low cement castables. Dif-ferent placement properties particularly fl ow and fl ow decay of castables with optimum amount of water are essential for fl awless installation. It has been observed that same quantity of calcined alumina gives different fl owabiliy and fl ow decay with constant amount of water. This is due to specifi c surface area and parti-

Fig. 9: XRD for T1 after fi ring at 1100°C, 1400°C & 1500°C

Fig. 10: XRD for T2 after fi ring at 1100°C, 1400°C & 1500°C Fig. 12: XRD for T1 after fi ring at 1100°C, 1400°C & 1500°C

Fig. 11: XRD for T3 after fi ring at 1100°C, 1400°C & 1500°C

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56th International Colloquium on Refractories 2013 – Refractories for Industrials

cle size distribution of calcined/reactive alumina. Higher specifi c surface area and bimodal or multimodal particle size distribution with lower d50 value play the important role to give better fl owa-biliy of castables and hence it is important to select suitable quali-ty of calcined or reactive alumina for designing best quality casta-bles. Calcined or reactive alumina also helps to form mullite and other phases at relatively lower temperature based on their reac-tivity and other physical propertties.

REFERENCES[1] Prost, D. & Panillac, A., “Hydraulically setting refractory

compositions”. France Patent No. 6934405, 1969[2] Myhre, B., “Strength development of bauxite bas ed ultra-

low-cement castables”.Am Ceram. Soc. Bull., Vol. 73. No. 5, 1994.

[3] Evangelista, P. C., Parr, C. & Revails, C., “Control of formu-lation and optimization of self fl ow castable based on calcium aluminate cement”, Refractory Application and News, Vol 7, No-2, 2002

[4] Saxena, M., “Development of shotcreting castablese”, The-sis for the Degree in Master of Technology in Ceramic Engi-neering, NIT, Rourkela, India, 2013.

Fig. 13: Microstructure of T1 after Firing at 1500°C

Fig. 14: Microstructure of T4 after Firing at 1500°C