Effects of Lithium Carbonate Additives in High Alumina Castable

June 9, 2018 | Author: Ian Aja | Category: Refractory, Program Optimization, Cement, Concrete, Solubility


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Tehran International Conference on Refractories, 4-6 May 2004NEW DEVELOPMENTS IN CALCIUM ALUMINATE CEMENT CONTAINING CASTABLES FOR STEEL MAKING APPLICATIONS C. Parr, R. Roesky and C. Wöhrmeyer Lafarge Aluminates, France Abstract: During the last 30 years the progress of monolithic refractories containing calcium aluminate cements (CAC) in steel applications has continued unabated. It has also been help by the change in steel making technology. For example, the increase in the continuous casting ratio has driven the move towards higher quality refractories for tundish permanent and safety linings. This has favoured the use of calcium aluminate based monolithic castables for tundish permanent linings. In order to successfully develop such monolithic castable applications an understanding of the various formulation parameters and their interactions is necessary. This includes the effect of different cement contents, reactive fillers and additives. In addition external parameters such as ambient temperature, mixing energy and time as well as practical water additions must be considered.This paper details an investigation into reduced cement castables for steel plant applications. A variety of low cement castable systems castables in the Al2O3-SiO2 system are explored. In particular, the matrix component or binder phase of each castable type is studied in order to develop an understanding of the role of each of the binder phase components. Additive systems are optimised for each specific matrix composition. Placing properties as well as the final installed characteristics are evaluated for each castable system. In addition, rheology, calorimetry, and conductivity profiles of selected systems have been employed to investigate both their physical and chemical attributes and provide evidence of the interactions that occur during castable placing and use. From the observed data, important physical, chemical and structural changes in the castables can be seen to occur throughout the placing chain depending upon the specific choice of fine fillers and additives within these castable systems. The impact of these formulation parameters upon final durability is also assessed. Each phase of CAC hydration is linked to castable placing properties as well as the installed properties. Mechanisms are proposed to account for the different characteristics of each specific system. Conclusions are drawn as to the underlying mechanisms and key interactions between the different components and how such castable systems can be optimised to ensure optimal placing characteristics as well a longer campaign life. 1. INTRODUCTION The control and optimisation of the various steps within the castable placing chain necessitates a trade off between opposing material properties. In particular a compromise must be reached between placing properties and castable hardening. Considerable attention has been focused in recent years upon both the optimisation and the control placing properties of deflocculated castable systems. In general this has led to castables with higher flow and longer workability periods that facilitate the achievement of target placed properties. Typical examples are self flow products that can be placed without vibration. However, a frequent consequence of this optimisation is delayed hardening and strength development leading to extended demoulding times. In other words the trade off between placing, workability and hardening has been pushed towards giving priority to placing properties. The resulting demoulding times have been accepted as a necessary constraint. Castables can be further developed to yield lower installation and downtime costs, if simultaneous optimisation of both castable placing properties and their strength development would be achieved. These steps within the castable placing chain are intimately linked to the hydration process of calcium aluminate cement (CAC). Thus, a control of the trade off between workability and hardening presupposes the need to be able to control the various steps of CAC hydration. Previous studies [1] have shown how the three distinct steps of CAC hydration (dissolution of the anhydrous phases, 121 C. Parr, R. Roesky and C. Wöhrmeyer nucleation and precipitation of hydrates from solution) can be linked to the physical properties such as rheology, flow, working time and developed strengths. The nucleation period is of particular interest in the search to optimise hardening [2] and strength development while maintaining placing properties. During the nucleation or induction period, nuclei attain a critical size and quantity. Once this is attained, the nucleation phase is followed by a rapid and massive precipitation of the hydrates leading to a drop in solution concentration. In a physical sense it is the growth of these hydrates, which interlock and bind together to provide mechanical resistance. If the nucleation period, for a given castable system, can be reduced then the strength development will occur earlier and at a given point in time the strengths will be higher. This is illustrated schematically in Fig. 1 from both a chemical and physical perspective. CAC hydration measured by Strength development conductimetry/calorimetry Nucleation period A Youngs Modulus B Nucleation period Conductivity Dissolution of B A anhydrous phases Temperature Exothermic peak Time tp1 tp2 = precipitation time Time Fig. 1. The benefits of a reduction in nucleation time In the case of deflocculated castable systems, the other components of the active binder phase, additives, reactive fine fillers impact upon CAC hydration kinetics of CAC. In so doing they play a role in determining the hardening rate and eventual strength development of the castable system. The following investigation examines how interactions between the different formulation parameters (CAC, fine reactive fillers and additives) can be advantageously used to control the strength development of deflocculated castables. Two base castable systems have been studied; one a high purity system without fume silica, the other a standard high purity LCC castable with fume silica. Two possible formulation logics have been used, one based upon a 70% Alumina cement and the other based upon an 80% Alumina cement. 2. EXPERIMENTAL 2.1. Castable Systems Model castable systems based on two different types of filler systems are used together with a variety of additives. The particle size was estimated using a distribution modulus of 0,25 in a modified Andreassen model. The study of fundamental reactions necessitated the use of simplified mortar systems based upon a binary mixture of CAC and Alumina. These model systems are exhibited in Table 1. 122 Tehran International Conference on Refractories, 4-6 May 2004 Table 1. Model castable systems Raw Material LCC-FS LCC-A LCC-A 80 Mortars Sintered Alumina -7mm to 0 80 61 61 - Alumina Spinel -3mm - 22 22 - Reactive Alumina RA1 (3,3 m2/g) 10 11 7 35-50 RA2 (6,5 m2/g) or 11 Fume Silica 971U 5 - - - CAC 70% CAC 5 6 - 50-65 CAC 80% CAC - - 10 50-65 Additives Q2 /Q3 Q2/Q3 None Q2/Q3 Table 2. Additives used Additive systems Code Addition based upon 100% castable dry mass Sodium Tri-polyphosphate TPP +0,03 to +0,2% Polycarboxylate ether PC-E +0,2 to 0,3% Sodium Hexa-metaphosphate Q2 +0,06 Boric acid +0,01 Sodium methacrylate Q3 +0,05 Boric acid +0,022 Sodium Carbonate +0,001 2.2. Experimental Procedure Properties for each system were characterised through the water demand, flow profile, working time, exothermic profile, conductivity, calorimetry and the mechanical resistance (transverse and compression). Flow value: The flow value has been determined using a cone with 100 mm base diameter, 50 mm high and 70 mm top diameter. Flow measurements on the resulting patty were carried out either after 20s of vibration or without vibration in the case of self-flowing castables. The measured flow value is expressed as a % of the original diameter Working time: The time after mixing at which the initial cone will not flow at all under vibration is called the working time. Flow decay: Flow values measured as a function of time and displayed as a curve. The slope shows at what speed an initial flow decays, or is maintained. Exothermic profiles: The exothermic profiles were determined at 20°C with samples placed in insulated chambers. A thermocouple, imbedded in a cast sample, was linked to a data capture system and the temperature recorded as a function of the time. The time taken to reach the maximum temperature was recorded (Pm) as well as the first peak associated with CAC dissolution (Pi) [ 3] Conductivity: The total ionic activity of a cementitious system was followed as a function of time in a dilute (w/c=5) solution. These technique has been applied equally to classical refractory systems as well as deflocculated castables.[2] In addition samples of the solution were extracted, the hydration stopped [4] and the solutions analysed by ICP. Cold Modulus of Rupture and Cold crushing strength: MOR and C.C.S values were measured on 4x4x16 cm prisms cured or dried to 110°C for 24 hours. High Temperature testing: The Hot Modulus of Rupture and Refractoriness under load values were determined using the ISO 2477 and ISO 1893 procedures. Sample size was 150x25x25mm with 125mm spacing between the loading points. Tests were conducted in an atmosphere of air. Samples were dried at 110°C before testing without any pre-firing to develop specific microstructures. 123 C. Parr, R. Roesky and C. Wöhrmeyer 3. RESULTS 3.1. The Effect of 70% Cac Type The LCC-FS was used to investigate the behaviour of two different 70% Alumina cements; CAC A and B. The basic properties of the cements are apparently similar and differ only in the chemistry and mineralogical composition (Table 3). Both CAC A and B were tested using the LCC-FS as the base castable with TPP as the sole additive at 5% water addition in both cases. The TPP addition was varied from 0 up to 0,14%. Table 3. Mineralogical and chemical composition Phase Unit CAC A CAC B CA % 47 69 CA2 % 46 37 C12A7 % 3 <1 Al2O3 % 70,9 72,1 CaO % 28,4 27,0 SiO2 % < 0,2 <0,2 Surface area/ Blaine cm2/g 4000 4000 The results can be seen in Fig. 2.1 and 2.2. At low TPP additions large differences are seen in the placing properties between the two cements with CAC A always showing shorter working times. A similar trend (not shown) was observed for the flow values. As the TPP addition increases the working time values for the two cements tend to converge together. 160 10 Time to exothermic peak (h) 140 9 CAC/A 8 Working time (min) 120 CAC/B 7 100 6 80 5 60 4 3 40 2 20 1 0 0 0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 0 0,02 0,04 0,06 0,08 0,1 0,12 0,14 TPP (%) TPP (%) Fig. 2.1. The effect of TPP on working time Fig. 2.2. The effect of TPP on exothermic peak time The same trend is seen for the exothermic profiles, with CAC B, which takes longer to reach the exothermic maximum. At lower TPP additions, CAC B has extremely good placing characteristics with long working times in excess of two hours. CAC A on the other hand displays extremely short working times that would severely limit the placeability of this castable. However the period to the maximum exothermic peak is shorter and therefore the strength development would occur at an earlier time interval. In order to investigate this further a conductimetry analysis was run (w/c = 5) for each cement type. The results are shown in Table 4. 124 Tehran International Conference on Refractories, 4-6 May 2004 Table 4. Conductimetry results Minutes Dissolution Induction Precipitation CAC A 13-20 92-122 112-134 CAC B 30-32 132-192 162-230 The induction period is significantly shorter for CAC A. The decrease comes from a shortening of the precipitation time as well as a more rapid dissolution. However, it is the more rapid precipitation that has the larger impact. This shows that intrinsically the CAC A is more reactive and displays quicker hydration kinetics compared to CAC B. In order to understand the differences between CAC A and B more clearly, samples of the solutions were extracted at differing time intervals. The Loss on Ignition (LOI) and XRD were measured for evidence of hydrates along with a visual examination by SEM. LOI increases significantly once the initial precipitation has occurred which is indicative of crystalline hydrate formation (Fig.. 3). This was confirmed by XRD which was qualitatively able to detect peak CAH10 and C2AH8 hydrates once the LOI values were greater than 4%. As can be seen the combination of LOI measurements with conductimetry provides a complete picture of the reactivity of a CAC. In this sense the greater reactivity of CAC A is evident. 30 3 25 2,5 Conductivity (mScm^-1) 20 2 LOI (%) 15 1,5 10 CAC/A_LOI 1 CAC/B_LOI 5 CAC/A_Conduct 0,5 CAC/B_Conduct 0 0 0 50 100 150 200 250 300 350 400 Time (min) Fig. 3a. Conductivity profiles for CAC A and B Formation of C2AH8 1 µm Large growth of C2AH8 1 µm Fig. 3b. AC A after 15 minutes Fig. 3c. CAC A at end of nucleation 125 C. Parr, R. Roesky and C. Wöhrmeyer However, XRD was unable to detect any crystalline hydrates for either CAC A or CAC B until at least 50 minutes after the precipitation times. Consequently SEM images have been used to investigate the nucleation period. For CAC A, small C2AH8 crystals can be seen via SEM around the grains of anhydrous cement as soon as 15 minutes into the reaction (inset photo Fig. 3b) i.e. early in the nucleation process. At the end of the induction period the C2AH8 crystals are more numerous and bigger in size (inset photo Fig. 3c). At the point of precipitation, hall the cement grains are covered in a layer of CAH10 and AH3. This is in contrast to CAC B where no evidence of crystals of C2AH8 was found during the induction period. It appears that the early formation of hydrates with CAC A effectively reduces the nucleation time. Clearly, CAC A in the LCC-FS system is far from an optimum in terms of the trade off between placing properties and hardening. CAC A has a more rapid precipitation than CAC B which leads to more rapid strength acquisition but it presents less adapted placing properties which are difficult to optimise based upon a single additive. CAC B shows different characteristics in that the placing properties are excellent and the strength acquisition is only marginally slower than CAC A. 3.2. The Effect of Alumina Type The hydration of CAC can be significantly [5] modified in the presence of alumina. So it is reasonable to imagine that reactive and calcined Alumina can act as one of the key formulation parameters in the search to optimise the strength development of deflocculated castables. In order to study the effect of Alumina a simplified mortar based on a 50:50 mix of CAC and Alumina has been used. A number of different alumina types were selected having different surface areas but similar sodium contents. Fig. 4.1 shows the calorimetry results. The larger surface area alumina accelerates the precipitation, which is due to a reduction in the nucleation period. XRD analysis of the solutions at the moment of precipitation shows CAH10 in the case of low surface area alumina. C2AH8 and AH3 were detected in the case of the high surface area alumina. Only CAH10 was detected in the control sample without alumina. The degree of modification of the hydrates that are formed as a function of the added alumina type can be seen by studying the ratio of CAH10 to C2AH8. The ratio decreases, as more C2AH8 is present. After 26 hours the ratio of CAH10/C2AH8 was 2,25 in the case of the highest surface area Alumina and 5 in the case of the Alumina with the lowest surface area. The control sample had a ratio of 2,5. Additionally, Infrared analysis confirmed the presence of Al-OH groups on the surface of the Alumina. Thus the higher the surface area of the Alumina, the greater the modification of the CAC hydrates that are formed. With low surface area Alumina the hydration is not significantly modified. The impact of sodium and more specifically soluble sodium was investigated in a similar manner. In order to compare the real effect of sodium each alumina type was first washed to remove all soluble sodium. Subsequently NaOH was dosed into the mortar solution (w/c =5) in a range equivalent to a sodium addition of up to 0,4%. The resulting nucleation times are show in Fig. 4.2 for two different alumina types along with a control sample. A non-monotone effect on the exothermic peak times is seen as the specific surface of the Alumina increases. At high concentrations of sodium, a large reduction in nucleation time is seen irrespective of specific surface. At low concentrations of sodium, the reduction in nucleation time is much smaller for the lower surface alumina. The effect of surface appears to be the dominant effect on nucleation time In order to verify this behaviour of the two different alumina, RA1 (with a BET of 3,3m2/g plus 4,5ppm of soluble sodium) and RA2 (with 6,5m2/g BET plus 25ppm of soluble sodium) were tested in the LCC-A castable with two different fluidification systems. The results in Table 5 show that RA2 yields the shortest time exothermic peak and the greatest strengths after 6 hours irrespective of additive system used. By 24 hours the strengths are similar within each additive system. In the case of RA2, high surface area alumina, the working time is similar irrespective of additive system (Table 5). With the lower surface area alumina the working time is more dependant upon the additive system used. Significant differences are found between additive system Q2 (Table 2) and additive system Q3. System Q3 contains boric acid, which is a strong retarder, and this extends the working time 126 Tehran International Conference on Refractories, 4-6 May 2004 significantly between castables based upon RA1 and RA2. The addition of retarder tends to magnify the effect. 20 2000 (110) (min)= nucleation time CA + Alumina (0,6) Specific surface (m^2/g) CA + Alumina (9,2) Nucleation time (min) 15 1500 CA 10 1000 (190) 5 500 (380) (800) (880) 0 0 0 5 10 15 20 25 0 5 10 15 20 Tmax (h) (m /g) max. surface area 2 NaOH (mM/l) Fig. 4.1. The effect of Alumina surface on the Fig. 4. 2. The effect of sodium content on the exothermic peak times induction period Table 5. Castable properties for LCC-A with two alumina types Q2 Q3 Characteristic Units RA1 RA2 RA1 RA2 Vibration flow t30 % 57 45 67 49 Working time Min 67 45 125 43 Pi Min 35 15 15 <10 Pm h 13 8 11 8,3 CCS – 6hr MPa 3,62 7,46 2,93 5,2 CCS – 24hr MPa 60,3 60,4 43,2 44,5 3.3. The Effect of Different Additive Systems The primary role of additives in deflocculated castable systems is to provide sufficient flow for placing at low water additions, necessary to conserve a compact structure. However, they also impact on the hydration of CAC, often retarding the precipitation reactions. This is particularly true in the case of highly efficient dispersing additives, necessary for high flow or prolonged workability, which utilise electrosteric dispersion. In such cases it is often found that the additives used have an intrinsic disadvantage [6] in that the strength acquisition time is prolonged. If the TPP in the LCC- FS is replaced by a PC-E type superplasticiser at 0,2% addition, the flow remains more stable up to +90 minutes but the green compressive strength after 24 hours is reduced by 77%. In order to understand this effect, the system was simplified to contain just alumina and CAC. Both additives were tested in mortar based on 65% CAC and 35% Alumina, type RA2. The resulting exothermic peaks (Fig. 5.1) showed a strong retardation for the PC-E type additive compared to the reference system and with just TPP. The effect is to retard the strength acquisition (Fig. 5.2) in the initial period. However, eventually both systems reach comparable ultimate strengths. It is just the hydration kinetics that differs. Conductivity analysis revealed that the nucleation period with PC-E is prolonged compared to TPP and that this only occurs after +20 hours. In order to profit from the high efficiency of dispersing additives such as PC-E it is necessary to use additional additives to prevent the lengthening of the nucleation period. Lithium Carbonate was used as a possible additive to correct this prolongation of strength acquisition. The effect with low alumina cements (eg. Fondu) has been well documented [7] but much less data is available for high purity high alumina CAC. The effect of lithium carbonate on a pure cement system 127 C. Parr, R. Roesky and C. Wöhrmeyer was first investigated using conductivity. Addition levels were kept below 0,01% to avoid any impact on long-term strength development. In a simple system Lithium Carbonate is effective in reducing the 80 CAC B + RA2 W/c = 0,30 100 60 90 CAC B % of Max. Strength MPa ( 78 ) 40 80 20 70 dQ/dt (J/g*h) ( 88 ) ( 99 ) 0 60 50 80 40 TPP W/c = 0,20 60 30 PC-E +TPP 20 40 + PC-E CAC B 10 20 0 0 1 3 7 28 0 10 20 30 40 50 Time (Days) Time (h) (MPa) max. strength Fig. 5.1. The effect of additives on the Fig. 5.2. The strength development as a function of exothermic peak time time with different additives 250 3 12 Dissolution 200 Induction 2,5 10 Conductivity (mScm^-1) 8 Time (min) 2 [Li+] mM/l 150 1,5 6 100 1 4 50 CAC B + LiC 0,5 2 0,003% [Li] mM/l 0 0 0 0 0,002 0,004 0,006 0,008 0,01 0 15 30 45 60 75 90 105 120 Li2O3 (%) Li2O3 (%) Fig. 6. 1. The effect of Lithium Carbonate on Fig. 6. 2. [Li+] ion concentration during hydration nucleation time nucleation time without perturbing the dissolution period (Fig. 6.1). A complementary conductimetry analysis (Fig. 6.2) measured the [Li+] concentration as a function of time. Lithium ions are initially consumed during the induction period and therefore it is presumed that they play a role in reducing the nucleation time. Solution Analysis of the filtered and dried solutions shows that as the Lithium Carbonate addition increases then the quantity of C2AH8 that is formed also increases. At the point of massive precipitation, C2AH8 is found in the form of agglomerated balls of hexagonal C2AH8 crystals. CAH10 is found mainly around the surface of the anhydrous grains. The ratio of CAH10/C2AH8 decreases to less than 1 at the highest levels of Lithium Carbonate. Lithium Carbonate additions were then made to the LCC–A castable based upon a PC-E dispersing system (Table 6). The reduction in time to the exothermic peak reduces with increasing Lithium Carbonate addition in a similar relationship to that seen with the neat paste. It follows that the strength at a fixed time period after casting will increase. Flow values are generally conserved. At Lithium carbonate additions of between 0,003-0,005% short exothermic peak times are seen yet useful working times are maintained. 128 Tehran International Conference on Refractories, 4-6 May 2004 Through an adapted addition of Lithium Carbonate together with an efficient fluidifying additive an optimum in terms of flow, workability and strength acquisition can be maintained. Table 6. The effect of Lithium carbonate on LCC –A with additive PC-E Lithium Flow t=0 Working time Exothermic peak CCS :MPa Carbonate % % (min) (Pm) (h) (24 h) 0,0075 130 30 1 47,2 0,0065 130 40 2 43,7 0,005 145 45 2,25 45,6 0,003 140 90 3,5 46,4 0,0015 120 180 7,8 27,3 0 130 300 15 7,2 3.4. An Integrated Solution 3.4.1. Formulation Logic The previous examples have shown how the system constraints have become more stringent with the current generation of refractory castables. Optimisation can only be effectively achieved through a simultaneous consideration of all variables. Consequently reduced cement castables have become more complex. There is also a secondary issue related to the reliability of such castables. Small changes in any of the individual components or the accuracy of combination can lead to modification of the castable properties. Additionally, the passage from classical castables containing 20% or more CAC to Low cement castables (LCC) and Ultra low cement castables (ULCC) containing 5% and less CAC has rendered modern castables more sensitive [8]. Tolerance to placing property variation (water demand, mixing time etc.) becomes much lower as the cement content decreases. A possible solution to these concerns is to use a CAC with a low CaO content, which has been specifically developed for use in these dense castable systems. Thus, alleviating the need for extremely low CAC additions and highly complex additive systems. The key advantages for using this cement are reduced formulation complexity together with optimised performance. However, sufficient formulation flexibility is still maintained to allow each user to further develop and customise their system as desired. This is illustrated in the section below where a new 80% Alumina cement, Secar Plenium, has been used to develop simple yet robust systems with respect to the trade off between hardening and placing properties. The 80% alumina CAC used is a new development, designed for use in reduced cement systems; the basic properties are shown in Table 7. The setting times were measured using an AFNOR mortar with the water cement ratio adjusted to 0,32 compared to the normally used 0,36. This was to allow easy measurement of flow values given the inherent fluidity of the cement. The long working time can be clearly seen yet the final set follows quickly after the initial set. This has been specifically engineered to aid rapid demoulding once the castable has stiffened. Table 7. Properties of the 80% Alumina cement Property Unit Value Al2O3 % 80,1 CaO % 17,6 SiO2 % 0,3 Surface area m2/g 6-7 D50 Particle size Micron 4-5 Initial set Mins 180 Final set Mins 190 Initial flow t=0 % 85 Flow t= 60 % 71 129 C. Parr, R. Roesky and C. Wöhrmeyer This cement was evaluated in a LCC A type system without fume silica i.e. in a typical system that would be used for steel contact applications. The formulation logic for the LCC – A80 system used is shown in Table 1 and unlike the corresponding system with a 70% Alumina CAC no additional additives were used. The water addition in both cases was held at a constant 5% water. The additions for LCC –A and LCC-A80 have been adjusted to give a constant CaO content in the final product. The Alumina Spinel systems conform to an LCC castable with an equivalent lime content of around 2%. The interest of using an 80% Alumina CAC is in the ability to maximise the cement content whilst still maintaining a low lime content in the final product. The higher cement contents8, lead to inherently more robust systems which are less sensitive to variations in external placing conditions such as mixing energy, ambient temperature and water additions. 3.4.2. Placing Properties A comparison of placing properties between the system containing 70% Alumina cement and the LCC A80 system with the 80% Alumina cement is shown in Table 5. As can be seen from the data the initial flow properties at a constant water demand are very similar. The 80% Alumina cement is more able to maintain the flow properties as a function of time than the LCC system with the ternary Q3 additive system. After 60 minutes the 80% Alumina cement system is still fluid whilst the LCC A system has stiffened to an extent that flow properties cannot be measured. This longer working time is also confirmed by the longer time to the first exothermic peak Pi in the case of system LCC A80. However, the hardening profile is not adversely affected by these better flow characteristics and longer working time. In fact the reverse is true as witnessed by the 6-hour strengths as well as the time to the main exothermic peak Pm. This occurs more rapidly in the case of the 80% Alumina cement containing system. Table 5. Castable properties for LCC-A with two alumina types LCC A/Q3 LCC A80 Characteristic Units 70% CAC 80%CAC Vibration flow t0 135 120 Vibration flow t30 % 67 116 Vibration flow t60 - 75 Working time Min 125 100 Pi Min 15 106 Pm H 11 7 CCS – 6hr MPa 2,93 5,5 CCS – 24hr MPa 43,2 40,5 3.4.3. Installed Performance The optimised placing properties of the system containing the 80% Alumina cement do not have any negative impact on the downstream installed properties. Table 6 compares the mechanical strengths after treatment at various temperatures along with the hot modulus of rupture values. Generally equivalent properties can be seen for both castable systems, although the hot strengths are noticeably higher in the case of LCC A80 after 1200°C. Table 6. Castable properties for LCC-A with two alumina types LCC A/Q3 LCC A80 Characteristic Units 70% CAC 80%CAC CCS – 110°C MPa 104 109 CCS – 1100°C MPa 119 131 CCS – 1500°C MPa >160 >160 HMOR 1200°C MPa 7,3 14,7 HMOR 1500°C MPa 21,5 24,2 130 Tehran International Conference on Refractories, 4-6 May 2004 Thus, the LCC A80 system using a specially designed 80% alumina cement, Secar Plenium, has optimised placing properties at low water demands without any negative impact on installed durability. At the same time the formulation complexity is significantly reduced compared to 70% Alumina cement based systems, as additional additives are not necessary. 4. DISCUSSION The results show the role that different formulation parameters play in controlling the point of strength acquisition of castables as well as the likely strength development at periods of 6 and 24 hours after casting. The underlying mechanism is the reduction in the nucleation period of the CAC. A study of CAC hydration with soft X rays [9] revealed the presence of amorphous primary hydrates (AHx alumina gels) being formed during the induction period. They form both in solution and around the anhydrous grains. These primary hydrates grow (with slow diffusion into solution) through the induction period until oversaturation of the CAC hydrates in solution occurs along with formation of crystalline hydrates. At this point they dissolve along followed by a rapid dissolution of the anhydrous grains and the dissolution-precipitation cycle occurs. The quantity and effect of these primary hydrates on the induction period being somewhat different depending upon the starting mineralogy of the CAC. The process is shown schematically in Fig. 8. In the case of CA, AHx type gels are found. The induction period is long, as there is no zone of instant precipitation for CAH10 and AH3 in the range of typical solution concentrations. Heterogeneous nucleation on the primary hydrates occurs with difficulty. The development of the primary hydrates doesn’t affect the castable rheology as seen with CAC B. In the presence of C12A7, C2AH8 and AH3 can form instantly once over saturation is reached. The hydrates would have a lower solubility than the amorphous primary hydrates so they continue to grow and reduce calcium and aluminate ion concentration in solution. In turn, the primary hydrates dissolve allowing rapid dissolution-precipitation to occur. Thus the induction period is much shorter, precipitation more rapid and strength acquisition occurs earlier. The instant precipitation of these primary hydrates has a negative effect on the rheology leading to rapid flow decay and short working time such as seen with CAC A. Thus, simply increasing the quantity of C12A7 is not a real solution to achieve rapid strength acquisition. A more effective option is to favour the onset of nucleation through the use of the other binder phase components In the case of reactive alumina, an increased surface accelerates the nucleation. In addition the hydrate CAC • Dissolution of of primary hydrates and formation of crystalline hydrates in solution. Ç In case of pure CA nucleation is difficult as • Primary hydrates form around the grain there is no instant precipitation due to and in solution during nucleation period solubilities of CAH 10 and AH 3. • Slow growth of primary hydrates Ç With C 12A 7, C 2AH 8 and AH 3 crystals are formed immediately which act as seeds for nucleation which then proceeds rapidly. Fig. 8. Schematic representation of nucleation period 131 C. Parr, R. Roesky and C. Wöhrmeyer formation is modified. As confirmed by the conductimetry analysis the nucleation of AH3 appears to be favoured and a more rapid precipitation occurs due to the presence of C2AH8. The presence of soluble sodium is also favourable for a more rapid nucleation although the effect is less pronounced that the surface effect. It is supposed that the presence of sodium ions in solution moves the zones of oversaturation, which favours the precipitation reactions. However, as previously noted this effect is somewhat dependent upon the specific combination of alumina and additive system utilised. The working times (Table 5) of castables containing the lower surface alumina showing a greater dependence upon additive type than the higher surface area alumina type RA2. In the later case working time is similar irrespective of additive system. The use of effective dispersing additives entrains a significant retardation to the onset of strength development. The reasons for this retardation are not fully understood. From the experimental data evidence is seen that it is the nucleation period which is prolonged rather than the dissolution phase. It is suggested that the presence of ionic groups such as COO- provide a complexing effect with the CAC, which affects the nucleation. This can be overcome through the use of Lithium ions. Based on work with Fondu cement7 the effect is explained via the formation of Lithium Aluminate during the nucleation period. Lithium Aluminate promotes heterogeneous nucleation of AH3 and C2AH8. This would be consistent with the conductimetry data presented. Thus, Lithium Carbonate presents a useful means to optimise the strength development whilst maintaining the excellent rheology that is conferred through the use of superplasticisers. The optimisation of castables can be achieved through an understanding of CAC hydration. The complimentary use of conductimetry and calorimetry techniques has provided a means by which an insight into the underlying mechanisms can be gained. Further analysis would be necessary before definitive conclusions can be drawn regarding the nature of the exact modification of CAC hydration in the presence of additives and fillers. An alternative route to formulate essentially simple robust systems is via the use of an 80% alumina cement which has been specifically engineered for such systems. This offers an in built optimisation of the three key parameters, cement clinker, alumina and additives. 5. CONCLUSIONS The development of strength within the castable systems studies is impacted by all the components of the active binder phase; CAC, Alumina and additives. It is possible to optimise the rheology/strength couple via the modification of the CAC hydration for each system; • Favour nucleation with larger surface reactive alumina and or higher soluble sodium • Addition of Lithium ions which promote AH3 and facilitate nucleation. The onset of strength development can be hastened without a major impact on the rheology. This leads to higher strengths at a given time, eg. 24 hours, after casting. The use of multiple additive systems is necessary to control each step of the CAC hydration process along with a careful choice of alumina type for each castable system. Unfortunately, this implies the development of increasingly complicated formulations by the refractory producer. A future challenge would be to deliver complete binder phase systems which offer the refractory producer an even greater robustness and simplicity that doesn’t exist today. A first step in this direction is the development of an 80% alumina cement which has been specifically designed for reduced cement castable systems. ACKNOWLEDGEMENTS The authors would like to thank all the co-workers at Lafarge Aluminates who contributed to the preparation of this paper and who performed the experimental analyses. In addition the work of Prof. Gessner and Dr. Moehmel, Institute of Applied Chemistry, Berlin is gratefully acknowledged. 132 Tehran International Conference on Refractories, 4-6 May 2004 REFERENCES 1. C. 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