High Purity Calcium Aluminate Cements, Production and Properties

HIGH PURITY CALCIUM ALUMINATE CEMENTS, PRODUCTION AND PROPERTIES

Ingo Stinnessen*, Andreas Buhr*, Rainer Kockegey-Lorenz**, Raymond Racher*** * Alcoa World Chemicals, Franfurt/Main, Germany ** Alcoa World Chemicals, Ludwigshafen, Germany *** Alcoa World Chemicals, Leetsdale, USA

ABSTRACT the 70% calcium aluminate cements which became the dominant cements used in the development of The manufacture and use of high purity calcium low-cement castables in the early 1970’s. New aluminate cements is reviewed. Particular attention is castable technologies for low moisture, self-leveling, given to the influence of these cements on the set and vibratable and wet gunning (shotcreting) installations flow behavior of refractory castables. The use of required increasingly sophisticated formulations, dispersing aluminas together with careful selection of which often contain a wide variety of components. cement properties can be shown to overcome Since that time, the use of high-purity 70% alumina variability in typical low-cement castable raw cement has increased, as the demands of these materials such as microsilica. castables on the cement properties have become more severe. The overall amount of cement has continued to be reduced for the most severe refractory KEYWORDS: Calcium aluminate cements, applications, in order to lower the total level of CaO dispersants, dispersing aluminas, microsilica, in the formulation, which can lead to development of refractory castables, monolithics low-melting point phases in combination with silica containing refractory aggregates. INTRODUCTION The availability of another critical raw material, The history of the development of calcium microsilica (silica fume), was also a major factor in aluminate cements is long and well documented, the development of high-performance low-cement covering over 150 years. Kopanda and MacZura (Ref castables. However, the presence of these raw 1) give a good summary of these developments. materials in the fine fraction of the castable will lead Calcium aluminate cement was originally developed to matrix interaction, as the hydration of the calcium for its chemical resistance, but found rapid growth aluminate cement is affected by such factors as pH because of its high early strength development. There and presence of organic contaminants. The hydration were several early instances of these new cements of the calcium aluminate cement can be significantly being used for refractory applications (e.g. crucibles) affected by the quality of the microsilica. As the type but extensive use of calcium aluminate cements as and amount of available microsilica grades has binders for refractory castables did not occur until the increased, interaction between the microsilica and 1920’s. As calcium aluminate cements became more cement has been shown to cause problems in the popular for refractory applications, the refractory control of monolithic set and flow. performance of traditional 40-50% alumina cements quickly became a limiting factor and, to overcome MANUFACTURING these deficiencies, high purity calcium aluminate cements were developed in the mid-1950’s. The High purity calcium aluminate cements are availability of these high purity calcium aluminate manufactured by sintering (Figure 1), usually in cements significantly increased the rate at which rotary kilns. This is in contrast to the fusion route refractory monolithics were adopted for use in severe generally used to manufacture lower purity calcium wear applications. The primary application for high aluminates. Alcoa World Chemicals manufactures purity cements is still refractory monolithics, high purity calcium aluminate cement in its although there are significant applications in Rotterdam (The Netherlands) plant. In 2001, the construction. decision was taken to concentrate worldwide cement production in this plant to optimize the availability of Both 70% and 80% alumina calcium aluminate consistent calcium aluminate cement for the global cements have been factors in the growth of cement market. The Rotterdam plant has state-of-the- monolithic technology worldwide. However, it was

art production controls, and utilizes the Alcoa QUALITY Production System (APS) to ensure consistent product, delivered to the customer when it is needed, Alcoa manufactures four grades of 70% cement, at the lowest cost. APS utilizes dedicated materials targeted at different applications. CA-14 cements are flow paths, based on pull signals from the traditional 70% cements, whereas CA-270 is a 70% downstream process, with intermediate stores for cement with different mineralogy and particle size management of production to meet customer demand distribution, developed to give low water demand and (Ref 2). high hot strength. The development of CA-270

Sintering: Lime + alumina react to hydratable clinker Sinter process phases

C + A Î C12A7 > CA > CA2 Milling Raw Mix Clinker Cement z Setting behavior of Clinker Phases

ª Fast Setting: C12A7 ª Moderate Setting: CA Fusion process ª Slow Setting: CA C AF Additions 2, 4 ª Minimal or Non-Setting: CA6, α-Al2O3, Figure 1 - Cement Production Process Type C2AS, CT, C F 2 zwith C=CaO, A=Al O ,S=SiO ,F=FeO , T=TiO 70% calcium aluminate cements are usually 2 3 2 2 3 2 manufactured from high-purity raw materials, such as Figure 2 - Calcium Aluminate Cement Phases low-silica lime-bearing compounds and reactive required the ability to precisely control clinker aluminas. It is certainly possible to manufacture 70% production under conditions of great stability. calcium aluminate cement from lower purity raw Transferring the knowledge gained, Alcoa could materials, but the inconsistency of the feedstocks will improve the clinker process to be able to precisely lead to variability in finished product properties control CA-14 cement properties with regard to which will, in turn, make the manufacture of working and setting times. CA-14 is now offered in consistent refractory monolithics more difficult. three tightly controlled grades, differentiated by working time (Figure 3). These grades are designated The production process for making 70% as CA-14S (S = summer grade, cement is relatively alumina calcium aluminate cements is, on the slow-setting), CA-14M (M = Medium) and CA-14W surface, very simple. High purity raw materials are (W = Winter grade, relatively fast setting). The blended and ground together and fed to a rotary kiln. ranges of the tested properties are tighter than The raw materials sinter very readily to yield a previously offered, giving good consistency between cement clinker. The lime and alumina compounds lots of cement. CA-14 properties are controlled react to form a variety of cement phases (Figure 2), without the use of any chemical additives, only by predominantly CA (calcium mono-aluminate) and consistent but different relative ratios of the cement CA2 (calcium di-aluminate). The effect of impurities, phases (Ref 3). old old particularly SiO2, TiO2 and Fe2O3 (S, T, F) can have Product CA-14 W CA-14 M CA-14 S CA-14 M CA-14 S a marked influence on the amount and ratio of these minmaxminmaxminmaxmin max min max phases that can, in turn, lead to inconsistency in Vicat Setting Time 10% H2O 12% H2O cement properties. This is because these impurities Initial Setting [min] 150 230 320 150 290 react preferentially with the calcia and alumina, Final Setting [min] 170 250 250 350 350 480 350 450 producing (generally) non -hydratable compounds Vibration Flow such as calcium titanate, calcium alumino-ferrites and F10 [cm] 15 15 15 15 16 F30 [cm] >13 14 14 12 14 calcium alumino-silicates and thus reducing the F60 [cm] >12 13 13 11 12 amount of available calcium aluminate for proper Figure 3 – CA-14 Setting Time Comparison cement hydration and affecting setting time and strength development.

To illustrate the differences between grades, CA-14 has been tested in three separate standard refractory formulations; Alcoa’s Nortab test grog, an alumino- silicate low-cement vibration castable, and a tabular alumina self-flow castable.

Test Mix NORTAB CA-14 CA-14 CA-14 CAC WMS Testing tem perature 20°C 20°C 20°C corresponds with hardening, to the point of + Mixing W ater % H2O % 10 10 10 Vic at s etting Initial s et min 220 280 330 developing sufficient strength for demolding (Ref 4). Final set min 230 300 350 In order to demonstrate specific raw material Vibration flow F 10 cm 15.7 16.6 17.2 F 30 cm 15.5 16.6 17.4 concepts, Alcoa has developed test mixes that more F 60 cm 15.4 16.2 17.4 closely match formulations common in the Exoth. Reaction Exo start min 227 289 313 Exo +5°C min 278 337 357 refractories industry. The two formulations used here Exo max min 367 416 432 (Mulcoa VIB and SFL 204) are typical of these test Flex. Strength 20°C/24h N/mm² 8.9 9.1 8.0 mixes and are utilized in Alcoa’s Applications 110°C/24h N/mm² 12.5 13.5 11.9 1000°C/5h N/mm² 6.4 5.6 6.1 Laboratories worldwide. Crush. Strength 20°C/24h N/mm² 49.1 43.8 46.3 110°C/24h N/mm² 63.1 70.0 60.4 1000°C/5h N/mm² 38.4 36.9 36.8 Figures 4-6 show that the type of cement used Figure 4 – CA-14, Nortab Test Mix controls the properties of the individual mixes. In general, the properties of the mixes follow the Test Mix SFL-204 properties of the individual cements – i.e., as the CA-14 CA-14 CA-14 CAC W M S cement addition changes from CA-14W (fast) to CA- + Additives ADS-3 % 0.5 0.5 0.5 14S (slow) the properties of the mixes change ADW -1 % 0.5 0.5 0.5 +Mixing W ater % H2O % 4.5 4.5 4.5 similarly. The importance of this control can not be F 10 cm 23.8 23.0 24.2 underestimated when refractories manufacturers have F 30 cm 20.3 23.0 24.4 F 60 cm No flow No flow 18.3 to produce mixes intended for widely varying Exoth. Reaction Exo start min 36 54 59 environmental conditions (high summer heat vs. cold Exo start 2 min 156 197 209 winters). Exo Max min 257 306 311 Crush. Strength 20°C/24h N/mm² 19 19 18 110°C/24h N/mm² 91 89 74 1000°C/5h N/mm² 44 46 44 1500°C/5h N/mm² 293 301 292 Purity of raw materials is extremely important in controlling properties, particularly hot properties Figure 5 – CA-14, Tabular Self-flow Test Mix (creep, hot MoR). CA-14 and CA-270 are designed with the highest purity raw materials consistent with Test Mix Mulcoa CA-14 CA-14 CA-14 CAC W M S Cements:CA-270 CA-14 M CAC 1 CAC 2 CAC 3 CAC 4 16 +Mixing W ater % H2O % 6.0 6.0 6.0 Vibration Flow F 10 cm 18.8 18.4 18.5 15.0 15.4 F 30 cm 18.6 17.5 18.3 12 13.7 F 60 cm 16.3 17.1 17.7 F 90 cm No flow 13.7 17.5 8 8.7 F 120 cm No flow No flow 8.0 Exoth. Reaction Exo start min 51 74 76 HOT MOR [MPa] Exo start 2 min 211 236 227 4 Exo Max min 301 334 328 3.1 Flex. Strength 20°C/24h N/mm 4.1 4.0 4.0 0 0.38% 0.36% 1.09% 0.53% 1.01% 1.34% 110°C/24h N/mm 9.6 9.8 8.9 Crush. Strength 20°C/24h N/mm 25.0 23.8 23.1 Sum of Impurities (SiO2, Fe2O3, Na2O) 110°C/24h N/mm 60 59.4 60.6

Figure 6 – CA-14, Mulcoa Vibration Test Mix Figure 7 – Effect of Impurities on Hot MoR (1500C) of Tabular vibration castable castable, 5% cement

As standardized test methods are often used to maintaining realistic cost. A comparison of CA-14 compare types and grades of ordinary Portland and CA-270 with other available 70% cements shows cements, our Nortab test grog was developed to the dramatic effect of just small amounts (even for a standardize test methods between Alcoa laboratories LCC mix with only 5% CA cement) of impurities on and our customers for evaluation of quality of the hot MoR and, by extension, all high temperature calcium aluminate cements. Alcoa also utilizes the properties (Figure 7). Hot MoR drops by about 80% measurement of castable heat development with increase of impurities by about 1%. (exotherm) to study the evolution of heat from the reactions of the cement during setting and hardening. Two points on the curve of heat evolution vs. time are particularly important. Exo Start is the time at which the temperature of the test castable begins to rise, and corresponds to the end of workability, Exo Max is the time of maximum temperature rise, and

EXOthermic Reaction 20°C : ADW 1 - ADS 3

30 0,8 % ADW 1; 0,2% ADS 3 0,7 % ADW 1; 0,3 % ADS 3 28 0,6 % ADW 1; 0,4 % ADS 3 CASTABLE FORMULATION 0,4 % ADW 1; 0,6 % ADS 3 26

Another aspect of the interactions present in 24 low-cement castable materials is the presence of [°C] temperature strong dispersant systems, which are required for 22 modern low cement castables containing reactive aluminas or microsilica (silica fume). Alcoa has 20 0123456789101112 developed dispersing aluminas designed to optimize time [h] flow and setting behavior under conditions of widely Figure 9 – Effect of Dispersing Aluminas on Exo differing ambient temperature (Figure 8). It is well a very small variety of chemicals – phosphates for known that the reduction of water content improves deflocculation, typically sodium hexa-metaphosphate physical properties of the refractory castable, as the or sodium tri-poly phosphate, water-reducing agents material components become more tightly packed such as citric acid or sodium citrate, and and porosity is minimized. However, under normal (occasionally) an accelerator such as lithium salts to conditions, there is a lower limit of water overcome the retarding effects of the other additives. requirement, below which the castable will not flow. A review of the effects of various additives on Alcoa’s dispersing aluminas decrease water castable performance was given by Banerjee (Ref 5). requirements when compared to “traditional” Such an additive cocktail has the advantage of being dispersant packages, as well as allowing superior well proven. However, the drawback is the very control over flow and setting behavior at varying small amounts of these materials required. Only a temperatures. few tenths of one percent in total are usually needed to significantly affect the set and flow of castables. Short/long workability at different temperature conditions The primary concern then becomes the physical Temperature Range: < 15 °C 15 - 25°C > 25°C dispersion of these additives throughout the dry castable during batching. Because the dispersing Dispersant ADW 1 and ADS 1 ADW 1 and ADS 1 ADW 1 and ADS 3 aluminas are primarily fine reactive alumina with Combinations: ADW 1 and ADS 3 blended organic ingredients, the alumina thus acts as Workability -> SHORT 0,9 % ADW 1 0,5 % ADW 1 0,1 % ADW 1 a carrier, allowing more thorough mixing of the 0,1 % ADS 1 0,5% ADS 1 0,9 % ADS 1 dispersant into the castable. The dispersing alumina or or system requires the addition of a higher amount of 0,8 % ADW 1 0,5 % ADW 1 overall additive and thereby the improved mixing 0,2 % ADS 3 0,5 % ADS 3 will yield significant improvement in consistency. Workability -> LONG 0,5 % ADW 1 0,4 % ADW 1 0,1 % ADW 1 0,5 % ADS 1 0,6 % ADS 3 0,9 % ADS 3 There is a further difference in the efficiency of dispersing aluminas when used in conjunction with Figure 8 – Dispersing Aluminas 70% calcium aluminate cement. A dispersing alumina system is capable of significant reduction in Figure 9 shows the control of setting and water demand over an equivalent traditional hardening, as measured by Exo, of a tabular alumina phosphate/citrate system. Figure 10 shows the based self-flowing castable with a varying ratio of reduction in water demand capable when switching retarding and accelerating dispersing aluminas. ADS between these two systems. Consequently, a 3 is stronger retarding than ADS 1, ADW 1 is considerable improvement in physical properties is accelerating. Pure ADS 3 may result in never setting. gained. One concern of such a dense castable would be the thermal shock resistance. However, TSR tests The traditional additive packages for low- show that such a formulation is quite capable of cement castable have, for many years, been based on surviving 20 cycles without deterioration.

EFFECT OF MICROSILICA PURITY 7/1 VB 173 (7/1)

T-60 up to 6 mm [%] 83 83 It has been clear for some time that the precise CL 370 C [%] 4.5 4.5 setting characteristics required by the current CT 4000 SG-R [%] 6.5 6.5 generation of low-cement castables is significantly CA-14 S [%] 6 6 influenced by the chemical purity of the microsilica Additives [%] Phosphate 0.05 ADS 3 0.4 added (Ref 6). The purity of microsilica, and its Citric acid 0.03 ADW 1 0.6 • Formulation of Mulcoa test-castable: H2O [%] 5.5 4.6 M-60 88% VIB Flow 10 min [cm] 18.8 18.5 Microsilica* 5% *)SiO2 = 98%, 20 °C / 24 hrs 97%, 94% CCS [MPa] 8 31 CAC-cement** 7% **) CA-14 W, CA-14 S

Figure 10 – Comparison of dispersing H2O: 6% aluminas to traditional additives in a Tabular M-ADS 1/ M-ADW 1 addition: 1% in total Test temperature condition: 20 °C vibration test mix

Figure 12 – test castable components The original dispersing aluminas ADS 1, ADS 3, ADW 1 did not work well in systems containing Castable test No. 123456 microsilica. As these systems are important Microsilica 98% 5 5 Microsilica 97% 5 5 formulations for low-, and ultra-low-cement Microsilica 94% 5 5 castables, new dispersing aluminas were developed to Cement CA-14 S [%] 7 7 7 7 7 7 be compatible in microsilica containing systems. The Dispersing Alumina M-ADS 1 [%] 0.5 0.5 0.5 0 0 0 Dispersing Alumina M-ADW 1 [%] 0.5 0.5 0.5 1 1 1 M-ADS 1 and M-ADW 1 dispersants were tested in a Mixing Water [%] 6 6 6 6 6 6 tabular alumina based self-flow castable containing Flow Diameter F10 [mm] 194 186 180 140 186 168 3% microsilica (Fig. 11). A plot of the exotherm Flow Diameter F30 [mm] 185 181 177 no flow 102 161 Flow Diameter F60 [mm] 183 180 172 no flow no flow 146 curves for the mix with varying ratios of accelerating Exo start [min] 77 199 254 17 35 115 and retarding dispersing aluminas shows that a Exo max [min] 261 382 1354 131 151 270 similar control of setting and hardening time is available with these silica compatible products as Figure 13 - Flow and exo in Mulcoa test-castable with the other dispersing aluminas products. with CA-14 S cement

30 1% M-ADW 1 Castable test No. 789101112 0.25% M-ADS 1 / 0.75% M-ADW 1 Microsilica 98% SiO2 5 5 0.50% M-ADS 1 / 0.50% M-ADW 1 Microsilica 97% SiO2 5 5 0.75% M-ADS 1 / 0.25% M-ADW 1 Microsilica 94% SiO2 5 5 28 1% M-ADS 1 Cement CA-14 W [%] 7 7 7 7 7 7 Dispersing Alumina M-ADS 1 [%] 0.5 0.5 0.5 1 1 1 26 Dispersing Alumina M-ADW 1 [%] 0.5 0.5 0.5 0 0 0 Mixing Water [%] 6 6 6 6 6 6 Flow Diameter F10 [mm] 193 186 169 191 183 175 24 Flow Diameter F30 [mm] 134 169 165 191 180 170 Temp. [°C] Flow Diameter F60 [mm] no flow 100 160 128 178 169 Exo start [min] 49 77 187 115 238 428 22 Exo max [min] 216 244 435 544 638 1514

20 0 120 240 360 480 600 720 Figure 14 Flow and exo in Mulcoa test-castable time [min] with CA-14 W cement Figure 11 – Effect of varying amounts of dispersing effect on the control of low-cement castable systems, aluminas on exotherm of a microsilica-containing is also of critical importance in the development of tabular self-flow castable robust castables. The effect of low-purity fumes are primarily seen in flow and set control, as seen in the following figures. Three microsilicas were incorporated into mullite-based formulations (Figure

12) using both fast setting (CA-14W) and slow setting (CA-14S) cements. The significant difference SUMMARY between the three grades of microsilica is in the overall chemical purity, with the total silica content, The production of 70% calcium aluminate ranging from relatively low (94% SiO2) to high cements has been improved to allow close control (>98%). It is important to note that dispersing over the cement clinker phases developed. This aluminas can be adjusted to overcome the deleterious allows close control of cement properties, which is effects on set and flow of low-purity microsilicas, as critically important to the use of these cements in seen in Figures 13-14. Note that even though the current high-performance refractory castables. In decreasing purity of the microsilica can have a particular, the control of set and flow characteristics significant effect on the setting and flow has allowed the development of high-flow castables. characteristics of the mix, this can be overcome with The combination of consistent cement together with the correct choice of cement and use of dispersing dispersing aluminas allows considerable flexibility in aluminas. castable formulation, and may be able to overcome inconsistencies introduced by other raw materials such as low-purity microsilicas.

REFERENCES

1) Kopanda, J. E., MacZura, G.; Production 4) Fentiman, C. H., George, C. M., Montgomery, R. Processes, Properties and Applications for Calcium G. J.; The Heat Evolution Test for Setting Time of Aluminate Cements, Alumina Chemicals Science and Cements and Castables, New Developments In Technology handbook, ed. LeRoy D. Hart, American Monolithic Refractories, ed R. E. Fisher, American Ceramic Society 1990, pp 171-181 Ceramic Society 1985, pp 131-135

2) Post, P. E., van Garsel, D. and Kriechbaum, G. W.; 5) Banerjee, S.; Monolithic Refractories, A ABS - a new way to optimize business and Comprehensive Handbook, World Scientific manufacturing efficiency; 43.International Colloqium Publishing Co., 1998 on Refractories, Aachen 2000, pp 51-56 6) Mosberg, T., Sandberg, B.; Use of Microsilica in 3) Van Garsel, D., van der Heijden, J., Kockegey- Binder Systems for Ultra-low and Cement-free Lorenz, R., Kriechbaum, G. W.; New Developments Castables and Basic “Cement-Free” Castables; in Calcium Aluminate Cements and in Dispersing Advances in Refractories Technology, ed R. E. Aluminas for Microsilica-Containing Castable Fisher, American Ceramic Society 1989 Systems, XIII Conference on Refractories, Prague 2000