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High Purity Calcium Aluminate Cements, Production and Properties

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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, C2F 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.
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