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Hydration mechanisms of magnesia-based refractory bricks Zhou, Shuxin 2004

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H Y D R A T I O N M E C H A N I S M S OF M A G N E S I A - B A S E D R E F R A C T O R Y B R I C K S by SHUXIN ZHOU B. Sc., Tianjin University, China 1986 M. Sc., Shanghai Institute of Ceramics, China 1989 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE m THE FACULTY OF GRADUATE STUDIES (Metals & Materials Engineering) THE UNIVERSITY OF BRITISH COLUMBIA December 2004 Shuxin Zhou, 2004 A B S T R A C T Hydration of magnesia-based refractory bricks could occur during storage, during drying after installation, or in service, and the hydration would cause damage to refractory bricks and furnace linings. In order to understand the hydration mechanisms of magnesia-based refractories, three types of bricks were chosen: magnesia, magnesia-spinel and magnesia-chrome bricks, and hydration tests were performed at 60 to 130°C in 98% relative humidity, water, and steam. The variation of the modulus of elasticity (MOE), determined by the impulse excitation technique (IET), as well as apparent porosity, air permeability, pore size and pore size distribution were correlated with the hydration data. The phase compositions and microstructure modifications were also studied on selected specimens by XRD and SEM/EDS. Based on the experimental results, a hydration model of "cylindrical pore model" was established and a hydration mechanism was suggested. The hydration takes place in three stages. In the first stage, which is controlled by chemical reaction, a film of brucite forms and M O E quickly increases. During the second stage, which is controlled by diffusion, the M O E gradually reaches a maximum value followed by a slow, decrease due to the formation of cracks on the film and weakening of grain boundaries until the M O E reaches the initial value. At this point, the third stage, corresponding to "dusting", starts to take place until the brick disintegrates. The results also indicate that the hydration rate increases with rising temperature and the CaO/Si02 ratio. The variations in permeability and porosity are opposite to that in MOE. A nondestructive method - IET to assess the hydration degree of magnesia-based refractory bricks was proposed. ii T A B L E OF CONTENTS ABSTRACT .' ii TABLE OF CONTENTS iii LIST OF TABLES v LIST OF FIGURES vi LIST OF ABBREVIATIONS AND SYMBOLS ix ACKNOWLEDGMENTS xiv 1. INTRODUCTION 1 2. LITERATURE REVIEW 4 2.1. Hydration of Magnesia-Based Refractories 6 2.2. Hydration of Polycrystalline Magnesia 7 2.3. Hydration of Pure Magnesia in Liquid Water 9 2.4. Hydration of Magnesia in Water Vapor 10 2.5. Kinetics of Hydration 12 2.6. Hydration of Refractory Grade Magnesia 17 2.6.1. Effect of the Physical Properties of Magnesia 17 2.6.2. Effect of Impurities 18 2.6.2.1. CaO 18 2.6.2.2. Si0 2 19 2.6.2.3. CaO/Si02 Ratio 20 2.6.2.4. A1203 20 2.6.2.5. Iron Oxides 20 2.6.2.6. B2O3 22 2.6.2.7. Cr 20 3 22 2.6.2.8. Other Oxides 22 3. OBJECTIVES 23 4. EXPERIMENT PROCEDURES AND METHODOLOGY 25 4.1. Experimental Materials 25 4.2. Samples Preparation Procedures 27 iii 4.3. Experimental Methods ..28 4.4. Hydration Studies 29 4.4.1. Hydration Test Conditions 29 4.4.2. Hydration Equipment and Procedures 29 4.4.3. Calculations 30 4.5. Modulus of Elasticity 33 4.6. Permeability 36 4.7. Pore Size and Pore Size Distribution 38 4.8. Mineralogy Studies 40 4.9. Microstructure Studies 41 5. RESULTS AND DISCUSSION 42 5.1. Degree of Hydration under Different Testing Conditions 42 5.1.1. Hydration of Magnesia-Based Bricks 42 5.1.2. Magnesia-Chrome Bricks 54 5.2. Modulus of Elasticity (MOE) versus Hydration Degree 61 5.3. Influence of Brucite on MOE Variation 68 5.4. Influence of Hydration on Pore Size Distribution 76 5.5. Influence of Hydration on Permeability 78 5.6. A Simplified Kinetic Analysis 80 5.7. The Mechanism of Hydration of Magnesia-Based Refractory Bricks 90 6. CONCLUSIONS 92 7. FUTURE WORK 95 8. REFERENCES 96 iv LIST OF T A B L E S Table 4-1. Properties of the experimental magnesia and magnesia- spinel bricks 26 Table 4-2. The characteristics of experimental magnesia-chrome bricks 26 Table 4-3. A summary of hydration tests 31 Table 4-4. Relations between porosity P and Young's modulus E [41] 35 Table 5-1-1. The compositions of binding phases for sample MS2, M l , and M2 from EDS analysis •. 51 Table 5-1-2. EDS results of the binding phase in the MCI and MC2 samples, wt% 58 Table 5-3-1. The compositions of different grains in MC3 shown in Fig. 5-3-2 70 Table 5-3-2. Apparent porosity of MC3 after hydration in steam at 109°C 76 Table 5-6-1. The reaction rate constants, K, of the M l samples 87 Table 5-6-2. The reaction rate constants, K*\0, of samples MSI, MS2 and M2 in steam and humidity at different temperatures 88 Table 5-6-3. The rate constants, K*\0, for the magnesia-chrome bricks in steam 88 Table 5-6-4. Activation energies, E (kJ/mol), for the hydration of all the experimental bricks in steam and humidity 90 Table 5-6-5. Activation energy for the hydration of magnesia 90 v LIST OF FIGURES Figurel-l. Hydrated magnesia brick 2 Figure 2-1. Crystal structure of periclase 5 Figure 2-2. Crystal structure of brucite 6 Figure 2-3. Hydration curves for the magnesia single crystals, where a is the hydration degree of magnesia 8 Figure 2-4. Hydration model for polycrystalline magnesia 8 Figure 2-5. Brucite participated at 60°C in (a) fast nucleation process at pH > 13 and (b) slow nucleation process at pH ~ 10 10 Figure 2-6. Phase diagram of system CaO-MgO 19 Figure 2-7. Phase diagram of system FeO-MgO 21 Figure 4-1. Schematic representation of the specimen preparation from a brick, a) for bars used in hydration studies and M O E measurement and (b) for cylinders for permeability and pore size and pore size distribution 26 Figure 4-2. The impulse excitation technique apparatus 34 Figure 4-3. The VacuPerm Decay Permeameter (a) schematic diagram and (b) instrument 37 Figure 4-4. The operation principle for measuring pore diameters 40 Figure 5-1-1. Weight increases versus time for hydration in steam at 121°C 44 Figure 5-1-2. Weight increases versus time for hydration in steam at 109°C 44 Figure 5-1-3. Weight increases versus time for hydration in steam at 100°C 45 Figure 5-1-4. Weight increases versus time for hydration in 98% humidity at 80°C 45 Figure 5-1-5. Weight increases versus time for hydration in 98% humidity at 60°C 46 Figure 5-1-6. Weight increases versus time for hydration in water at 100°C 46 Figure 5-1-7. Weight increases versus time for hydration in water at 80°C 47 vi Figure 5-1-8. Weight increases versus time for hydration in water at 60°C 47 Figure 5-1-9. Backscatter electron micrograph of the MSI sample 48 Figure 5-1-10. EDS results of the bonding phase in the MSI matrix 49 Figure 5-1-1.1. The XRD diffractograms for the unused MSI and MS2 samples (P = pericalse and S = spinel) 50 Figure 5-1-12. Backscatter electron micrograph of the MS2 sample 51 Figure 5-1-13. Backscatter electron micrograph of brick M l , binding phase (light gray) and periclase (dark gray) 52 Figure 5-1-14. The XRD diffractograms for the unused M l and M2 (P = pericalse) 53 Figure 5-1-15. Backscatter electron micrograph of the M2 brick structure 53 Figure 5-1-16. Weight increases versus time for hydration in steam at 109°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI, MC4 and MC6 55 Figure 5-1-17. Weight increases versus time for hydration in steam at 121°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI , MC4 and MC6 56 Figure 5-1-18. Weight increases versus time for hydration in steam at 130°C 57 Figure 5-1-19. XRD diffractogra for M C I - the unused sample. P = pericalse (MgO) and C = complex spinel (Mg, Fe)(Cr, Al , Fe)204 58 Figure 5-1-20. Hydrated MgO versus CaO/Si02 ratio for the hydration of the magnesia-spinel and magnesia bricks in water at 60°C 60 Figure 5-1-21. Hydrated MgO versus CaO/Si02 ratio for the hydration of the magnesia-chrome bricks in steam at 109°C 60 Figure 5-2-1. M O E variation versus time for hydration in steam at 121°C for magnesia-spinel and magnesia bricks 61 Figure 5-2-2. M O E variation versus time for hydration in steam at 109°C for magnesia-spinel and magnesia bricks 62 Figure 5-2-3. M O E variation versus time for hydration in steam at 100°C for magnesia-spinel and magnesia bricks 63 Figure 5-2-4. M O E variation versus time for hydration in 98% humidity at 80°C for magnesia-spinel and magnesia bricks 64 Figure 5-2-5. M O E variation versus time for hydration in 98% humidity at 60°C for magnesia-spinel and magnesia bricks 64 Figure 5-2-6. M O E variation versus time for hydration in steam at 109°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI , MC4 and MC6 65 Figure 5-2-7. M O E variation versus time for hydration in steam at 121°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI, MC4 and MC6 66 Figure 5-2-8. M O E variation versus time for hydration in steam at 130°C for the magnesia-chrome bricks MCI , MC2, MC3, MC5 and MC6 : 67 Figure 5-3-1. Weight increase and M O E variation versus the time for hydration in steam at 109°C for brick MC3 69 Figure 5-3-2. SEM image for the fracture section of the non-hydrated MC3 sample....70 Figure 5-3-3. The film of brucite on the wall of a pore after hydration for 8 hours 71 Figure 5-3-4. The fractograph of the MC3 sample after hydration for 24 hours 74 Figure 5-3-5. SEM images of the sample MC3 after hydration for 38 hours in steam at 109°C, (a) the fracture surface and (b) the polished sample 75 Figure 5-4-1. The change in the PSD for different hydration periods in steam at 109°C for brick MC3. (a) smaller than 1 um pores and (b) larger than 1 um pores 77 Figure 5-5-1. Permeability versus time after hydration of brick MSI, MS2, M l , and M2 in 98% relative humidity at 80°C 79 Figure 5-5-2. Permeability versus time after hydration of brick M C I , MC3, MC4 and MC6 in steam at 109°C 80 Figure 5-6-1. Hydration model-"cylinder pore model" 81 Figure 5-6-2. Hydration rate equations, R)(a) and R2(ot), versus time, t, for M l samples, (a) the first stage hydration in steam at 121, 109, and 100°C, (b) the first stage hydration in humidity at 100, 80, and 60°C, (c) the second stage hydration in steam at 121, 109, and 100°C, (d) the second stage hydration in humidity at 100, 80, and 60°C 86 Figure 5-6-3. The plots of ln K versus 1/T for sample M l 89 vin LIST OF A B B R E V I A T I O N S A N D S Y M B O L S Abbreviations ASTM American Society for Testing and Materials BSE Backscatter Electron C2S Di-calcium Silicate EDS Energy Dispersive Spectroscopy FCC Face Centered Cubic IET Impulse Excitation Technique MOE Modulus of Elasticity PSD Pore Size Distribution SEM Scanning Electron Microscopy XRD X-Ray Diffraction S y m b o l s a fraction of reacted magnesia A cross-section area A frequency factor b width of a bar specimen CMgo chemical content of MgO in a brick D diameter of a pore d0 median pore diameter dD range of pore sizes DT diffusion coefficient at temperature T E activation energy E Young's modulus Eo modulus of elasticity before hydration modulus of elasticity after hydration for n hours fd flow rates through dry samples ff fundamental resonant frequency in flexure fw flow rates through wet samples h thickness of a brucite film K rate constant k ratio of reaction constant to particle density in Eq.(2 k permeability k = pbrick M^gc/pbrucite MH20 K' constant in Eq. (2-9) ko constant in Eq. (2-7) k' = P brick K 2 = PMSQ 'MMg(OH)2 P brucite ' M MgO L length of a bar specimen L length of a sample in the macroscopic flow direction in Eq. (4-5) Lo original length of cylindrical pores m mass of a bar specimen m reacted MgO M/-/20 molecular mass of H 2 O MMg(OH)2 molar mass of Mg(OH)2 MMgo molecular mass of MgO n constant in Eq. (2-10) N constant in Eq. (2-12) n number of hours for hydration N number of open pores in a sample p water vapor pressure P apparent porosity p* 0.3 times saturation pressure po saturation vapor pressure ps saturation vapor pressure Q volumetric flow rate xi R gas constant r cylindrical pore radius r0 mean original radius of magnesia particles in Eq. (2-12) r0 original radius of cylindrical pores R,(a) = (l+kia),/2 -1 R2(a) = [(1 +k,a),/2 - ((1 +k,a)- (l-k2) + k2f2]2 S reaction area t time T absolute temperature t thickness of a bar specimen Ti correction factor in Eq. (4-3) u reaction rate per unit area v reaction rate V0 = Ttr02L0 VSampie sample volume in Eq. (5-8) w average MgO mass around each pore W0 sample weight before hydration W„ sample weight after hydration for n hours y surface tension of wetting liquid 8 thickness of brucite layer AE variation of modulus of elasticity AP pressure difference between both ends of a sample AP differential pressure required to remove wetting liquid from a pore xii A W weight increase rj fluid viscosity 6 contact angle of a wetting liquid on a sample material pbrick brick density Pbncite brucite density PMgO magnesia density A C K N O W L E D G E M E N T S This thesis could not have been written without my supervisors, Dr. Tom Troczynski and Dr. George Oprea. Throughout my study and the research work, they provided invaluable guidance and support. I would like to express my sincere gratitude to them. I would also like to thank the members of UBCeram group of UBC: Carmen Oprea, Guotian Ye, and Dr. Ahmad Monshi for their assistance. I want to convey my special thank to Ms. Mary Mager for her assistance with SEM/EDS analysis. I am grateful to all those who have provided help during the period of my study and the research. I would like to thank all the industrial sponsors of the research consortium on refractories at UBCeram and the Natural Sciences and Engineering Research Council of Canada for their financial support. Last but not least, I would like to thank my parents for their support and encouragement and my wife and daughter for their love and sacrifice through my study in UBC. xiv 1. INTRODUCTION Magnesia (magnesium oxide) is an important raw material for the refractory industry because magnesia possesses a high corrosion resistance to basic slags and molten metals. Magnesia-based refractory bricks have been widely used in metallurgical and cement furnaces, however, as a main component of the magnesia-based refractory bricks, magnesia reacts with water (both liquid and vapor) very easily. This reaction of hydration produces magnesium hydroxide (brucite) according to the following reaction: MgO + H 2 0 (liquid or vapor) = Mg(OH)2 (1 -1) Due to the difference in density between magnesia (3.58 g/cm3) and brucite (2.36 g/cm3), when magnesia converts to brucite, the volume increases by factor of 2.2, with an accompanying 45% weight increase. This expansion will degrade the refractory bricks and destroy furnace linings; therefore, the hydration of magnesia-based refractory bricks has been of great concern for users of these materials, where the risk of this reaction taking place existed. The hydration of magnesia-based refractories can occur during storage, during drying after installation, and during service, and generally in any environment where water exists or is generated. For example, in non-ferrous smelting furnaces, water-cooling assists the performance of bricks in service, and water condensation around the cooling pipes or leakage from the cooling equipment often occurs and produces an environment with high water vapor pressure inside of the high temperature refractory linings. This can cause a serious destruction of the refractory linings due to hydration. 1 Figure 1-1 shows a hydrated burned magnesite brick, which was in service in an electrical smelting furnace only for one month. Due to the hydration of magnesia, extensive cracking occurred in the bricks [1]. Fig. 1-1. Hydrated magnesite brick [1]. There are qualitative and quantitative methods to assess the hydration of magnesia-based refractories. The American Society for Testing and Materials (ASTM) standard method for the Hydration Resistance of Basic Bricks and Shapes is a qualitative method to evaluate the hydration resistance of basic bricks by visual inspection after autoclave 2 treatment, using a scale of four rates. In addition to the visual inspection, weight increase is a quantitative method. The hydration of pure magnesium oxide has been extensively studied, but these studies mainly focused on pure caustic magnesias obtained by calcination between 400 and 1200°C of Mg(OH )2 or MgCCb and main topics were: thermodynamic calculations for Mg(OH )2 and MgO, influences of the preparation process of magnesium oxide on hydration, and the hydration kinetics. Hydration of magnesia clinkers was also studied, especially the effects of different chemical compositions on the hydration rate. However, there is scarce literature about hydration and its effects on complex magnesia-based refractory bricks. The present work focuses on the study of the hydration behavior for magnesia-based refractory bricks, in correlation with other properties, such as pore size, pore size distribution, permeability, and modulus of elasticity measured by the impulse excitation technique. Furthermore, the hydration kinetics is discussed and the hydration mechanisms are suggested. 3 2. LITERATURE REVIEW It is known (Eq.(l-l)) that magnesium oxide can react with water (liquid and vapor) to produce magnesium hydroxide (Mg(OH)2). At 25°C and under atmospheric pressure, the enthalpy and Gibbs free energy of the reaction (1-1) are A H 0 = -9.09 kcal/mol and AG°= -6.68 kcal/mol, respectively. When water is in the gaseous state at 100°C and one atmosphere pressure, AH° = -19.43 kcal/mol and AG° = -6.01 kcal/mol [2]. Periclase, which is the only crystalline form of magnesium oxide, has a NaCI type (cubic) structure (Fig. 2-1), consisting of two interpenetrating face-centered cubic (FCC) lattices of magnesium and oxygen. Both cations and anions are in octahedral sites, with a coordination of six. The unit cell lattice of the cubic structure is a = 4.21 lA. The theoretical density and Young's modulus are 3.585 g/cm and 162.7 GPa, respectively [3]. Brucite, which is the crystalline form of magnesium hydroxide, is isostructural with the layered compound Cdl 2 (hexagonal). In brucite, the hydroxyl ions form a hexagonal close-packed stack. The Mg ions are in octahedral sites between alternate pairs of hydroxyl planes (Fig. 2-2). The cell parameters are a = b = 3.142 A and c = 4.766 A; the theoretical density is 2.377 g/cm3 [4]. According to the thermodynamic data [2], brucite begins to decompose into MgO and H2O above 246°C. However, at this temperature the decomposition is very slow. Even at 4 300°C under vacuum, only about 95% of the magnesium hydroxide is decomposed after ten days [5], and above this temperature the dissociation rate increases rapidly [6]. Fig. 2-1. Crystal structure of periclase [3]. 5 Fig. 2-2. Crystal structure of brucite [7]. 2.1 Hydration of Magnesia-Based Refractories Only one published technical paper [8] related to the hydration o f magnesia-based refractory bricks could be found. The authors reported that the commercial magnesia-chrome bricks that were heated to 500°C underwent a significant decrease in the modulus-of-rupture value. Such strength loss is due to the decomposition of the hydroxide in the brick. The thermal decomposition of Mg (OH ) 2 in magnesia-chrome bricks has the following steps: (i) recrystallization of M g O from a defective layer of hexagonal structure to cubic structure, (ii) the precursor (brucite) crystal cracking into small fragments, (iii) gradual desorption of water, (iv) shrinkage of the M g O crystal lattice. As a result, the produced MgO on the surface o f periclase grains has a very high apparent porosity. About 8 0 % strength loss in a magnesia-chrome brick of 5 9 % M g O was due to the hydration of 5.8% of the original content o f M g O during storage [8]. 6 Due to scarce literature on hydration of magnesia-based refractory bricks, the hydration of polycrystalline magnesia and pure magnesia is introduced. 2. 2 Hydration of Polycrystalline Magnesia The hydration process for pure polycrystalline magnesia clinkers under saturated water vapor at temperature range from 135°C to 200°C are different from that for single crystals [9]. For single crystal magnesia, there is a slow stage in the early hydration, especially at the low temperature and with hydration, a Mg(OH )2 layer is gradually formed. For polycrystalline magnesia, at the beginning of the hydration, there is an accelerated period, and the cracks caused by the brucite produced at grain boundaries gradually lead to the destruction of grain boundaries. For the magnesia polycrystals (2.00 - 3.36 mm in diameter), when the hydration degree is 15-20%, the grains start disintegrating until single crystals (about 50 um) are formed. According to Kitamura [9], this phenomenon is called "dusting". During the "dusting" process the hydration rate becomes higher, due to the formation of new surfaces available for hydration. This model of hydration for polycrystalline magnesias is shown in Fig . 2-4. 7 0 10 20 30 40 50 60 70 80 90 100 t(h) t(h) Fig. 2-3. Hydration curves for the magnesia single crystals, where a is the hydration degree of magnesia [9]. Fig. 2-4. Hydration model for polycrystalline magnesia [9]. 2.3. Hydration of Pure Magnesia in Liquid Water Hydration of pure magnesia in liquid water is a process of dissolution of magnesia particles followed by Mg(OH)2 precipitation [6,7,10,11]. This process is controlled by the dissolution of magnesia particles, beginning with the formation of a surface layer of MgOH + around a magnesia particle, via the reaction: MgO s u rrace + H + -> MgOH surface (2-1) In acidic solution (pH < 5), the dissolution of the M g O H + surface occurs through the following reaction: M g O H + s u r f a c e + H + ^ M g 2 + + H 2 0 (2-2) At a pH = 5, the dissolution of MgO depends on the diffusion and pH in the solution. When the pH value is greater than 7, a higher local pH value is produced on the surface of the magnesia particles than that in the bulk solution. When the pH reaches saturation point (>10.7 at 28°C) [12] during the dissolution of magnesia, the Mg(OH) 2 is precipitated: M g O H + s u r f a c e + OH" ^ MgOH + 'OH" s u r f a c e M g 2 + + 20H" (2-3) M g 2 + + 2 OH" -> Mg(OH) 2 (2-4) Various particle shapes of the brucite are obtained in different solutions [13]. The high concentration of OH" caused in the solution (pH > 13) results in a fast nucleation process, generating not-well-defined nuclei, which form globular "cauliflower-like" 9 agglomerations (Fig. 2-5a). In a lower concentration of OH" (pH ~ 10), a platelet-like morphology is attained, resulting from the intergrowth of the hydrate particles (Fig. 2-5b). a) b) Fig. 2-5. Brucite participated at 60°C in (a) fast nucleation process at pH > 13 and (b) slow nucleation process at pH ~ 10 [13]. 2.4. Hydration of Magnesia in Water Vapor According to literature [14], the hydration of magnesia at room temperature with water vapor proceeds in four steps: (i) Formation of a layer of chemically and physically adsorbed water. (ii) Diffusion of M g 2 + and OH" ions in the layer of physically adsorbed water. (iii) Nucleation of brucite. (iv) Crystal growth of Mg(OH)2. 10 The process of the hydration of polycrystalline forms of magnesia depends on the raw material from which it was prepared and the preparation conditions. The hydration rate is also influenced by the specific surface and pore volume of the oxide particles [14]. In studying the hydration of magnesias (specific surface 10 - 180 m2g"') with water vapor at 22°C [15], it was found that before the chemical reaction takes place to produce magnesium hydroxide, the physical adsorption of water vapor on magnesia surface occurs. For magnesia powders with high specific surface areas, the hydration rate is higher than for those with low specific surface areas. The magnesia with a specific 2 1 surface area of 10.5 m g" would hydrate slowly and the large original crystallites would split into a number of small single crystallites. At 35°C little or no adsorption of water vapor on the magnesias with low surface areas occurs until the water vapor pressure, p, reaches 30-60% po ipo is saturation vapor pressure). Therefore for a low specific surface area, the required p/po value is high. The work shown in [16] also indicated that a vapor pressure greater than 0.3 po is necessary for hydration, because the water vapor above this pressure condenses as a film on magnesia particles to complete the hydration reaction. The Van der Waals forces cause the adsorption of water on the particle surface in the initial stage of hydration, followed by the formation of a water film and the slow reaction 2+ between Mg and OH". The thin platelets of Mg(OH)2 grow on the cubic face of periclase grains in different directions .and produce a layer [14]. Hydration studies on caustic magnesias in [17, 18] showed that the capillary condensation of water in the pores of the magnesia particles occurred in the hydration reaction. The process of hydration of caustic magnesias starts with the water 11 condensation in the small pores, followed by the reaction on the pore walls to form hydroxide. 2.5. Kinetics of Hydration The hydration reaction of magnesium oxide with liquid water or water vapor is a heterogeneous reaction and very dependent on the characteristics and physical properties of magnesia, such as reactivity and surface area. These properties are strongly influenced by the preparation of magnesia, such as the chemical form from which MgO is prepared (Mg(OH)2, MgC03, etc.), the calcination temperature and holding time, and the preparation atmosphere (vacuum, air, etc.). Both the temperatures and the vapor pressures affect the hydration rates for vapor phase hydration [16]. For uniformly sized spherical particles the classic "shrinking core model" gives the reaction rate equation for processes controlled by chemical reactions [19]. l - ( l - a ) 1 / 3 =K>t - (2-5) where a - the fraction of reacted magnesia, K = rate constant, sec"1, / = reaction time, sec. For a heterogeneous non-catalytic reaction, the reaction rate is commonly either diffusion controlled or chemical reaction controlled [12]. The two processes can be distinguished by the activation energies. For example, the activation energy for the diffusion in solution 12 reaction is of the order of 5 kcal/mole (21 kJ/mole) and the chemical reaction controlled is usually between 10 and 25 kcal/mole (42 to 105 kJ/mole) [20]. The hydration of magnesia powders (specific surface: 12 to 80 m2/g) and particles (particle size: 45 to 75 [im) under turbulent conditions at different temperatures was investigated [12, 21]. Equation (2-5) could not be applied, due to the non-uniform sizes of the magnesia powders and particles, while equation (2-6) was found suitable. For each particle size fraction of the powders, the individual rate curves can meet equation (2-5), and equation (2-6) would be the weighted average of the rate curves for each individual fraction. - \n (\ - a) = K*t (2-6) where a = fraction of reacted MgO, K = apparent rate constant, directly proportional to the weighted average of the constants for the single particle fractions, sec"1. Under these conditions, the obtained hydration rates are directly proportional to the surface area at the surface of the particles, including pores. The calculated activation energies are 14.1 ± 0.2 kcal /mole (59.0 ± 0.8 kJ/mol) and 45 ± 0.5 kJ/mole (10.7 ± 0.1 kcal/mol). The hydration was controlled by the chemical reaction, because in the stirred solution the layer of brucite is not a stable surface layer and subsequently dissolves allowing further hydration of the magnesia [12, 21]. The hydration reaction of magnesia powders exposed to water vapor pressures of 260 -570 mm Hg (0.342 - 0.75 atm.) and temperatures of 78 - 98°C is governed by the 13 chemical reaction at the interface [16]. At a constant temperature, the reaction rate varies with the vapor pressure almost linearly with an excess pressure over p*, which is 0.3 times saturation pressure. The activation energy of the reaction E = 16,100 cal/mol. The overall equation can be expressed by: v = k0[(p/p *) - 1 ] exp(-E/RT) (2-7) where v = the reaction rate, g'cm'^sec"1, p = water vapor pressure, mmHg, p* « 0.3 times saturation pressure, mmHg, E = activation energy, cal'mol"1, R = gas constant, 1.986 cal'K'^mol"1, . T= absolute temperature, K, 2 1 kg = a constant, g*cm~ »sec~ . The term [(p/p*) - 1] expresses the excess pressure over p* and governs the formation of the water film on the magnesia surface and affects the rate of the hydration [16]. After studying this system, it was found [22] that when the relative humidity is below about 60%, the nucleation of magnesium hydroxide is slow, and this retards the hydration reaction. At a low humidity, the reaction rate is greatly reduced, probably because a layer of water on the surface of magnesia particles forms only partially. At a high relative humidity, the reaction is as follows: MgO(s) + H 20(g) = M g O ( H 2 0 ) a d s o r b e d -*Mg(OH) 2 (s) (2-8) 14 The reaction process is governed by the interface reaction. The activation energy is 17.6 kcal/mole. The rate equation is [22]: \-(\-ay =K-t = K<- exp(- SL) • { P ' P s \ • t (2-9) RT \-(p/ps) where a = fraction of reacted MgO, ps = saturation vapor pressure at temperature T, mmHg, K' = a constant, sec"1, K = rate constant and K = K'—-jLjLL sec-' !-(/>//>,) t = reaction time, sec. Jerofeev's equation (2-10) was used to describe the hydration process of caustic magnesium oxide in aqueous suspensions [23]. This is an empirically established kinetic equation for isothermal heterogeneous processes. ln [1/(1 -a)] = {Kt)n (2-10) where a = fraction of reacted MgO, t - time, sec, ,K = rate constant, sec"1, n = a constant of the empirical equation. By defining the half-time to.5 of the given process as the time when the reacted MgO attains the value of 50%, Eq. (2-10) is changed to the following form: 15 (2-11) For caustic magnesia prepared by calcining magnesium carbonate in air at 900°C for 60 min, the constant, n, of equation (2-10), has the value of 2/3 and the activation energy is E = 54.7 kJ/mole [23]. By incorporating a term that represents the influence of the hydration product deposited on magnesia particles, a modified shrinkage core model was developed [9], where the hydration reaction is controlled by the chemical reaction process. The reaction equation for vapor phase hydration is expressed by the following equation: logr 0 [ l -(1 -a) l / 3] = ( l / A 0 1 o g ( ? - ^ ) + ( l / A 0 1 o g M (2-12) where ro = the mean original radius of magnesia particles, mm, a = fraction of MgO reacted, t = the reaction time, hour, to = the induction time, hour, k = the ratio of the reaction constant to the particle density, mm/hour, N = a constant, expressing the contribution of the thickness of reaction layer to the diffusion. (N = 1, the reaction is controlled by chemical reaction, N = 2, the reaction is controlled by diffusion.). For the vapor phase hydration of single crystals of magnesia at temperatures between 138 and 200°C, Eq. (2-12) fits the reaction well. The activation energy at the beginning of hydration was 65.5 kJ/mole (-15.7 kcal/mole). The values of N are between 1.5 and 1.6, 16 indicating that the reaction is not fully controlled by diffusion or chemical reaction, but by a combination of both [9]. 2.6. Hydration of Refractory Grade Magnesia Generally, magnesia used in refractory is either of dead burned or fused type, which is produced from natural magnesite (MgCO^) or extracted from seawater or inland brines [24]. Refractory grade magnesia, typically dead burned around 1650°C or higher, is of high density (3.2-3.4 g/cm3) and low porosity (5-11%) and may contain different level of impurities, such as Al 203, SiC>2, Fe203, CaO, B2O3 [24]. These compositions can exist as crystalline phases, like spinel (MgAl204), forsterite (Mg2Si0 4 ) , monticellite (CaMgSi04), etc. A complex spinel (Mg, Fe)(Cr, Al , Fe)204 may also form when MgO, A1 20 3 , Fe20 3 , FeO and Q 2 O 3 exist [25]. The CaO, Si02 and B2O3 may form a glass phase. Due to the existence of these phases, the physical and chemical properties of magnesia-based refractories are more complicated than those of pure magnesias, and the hydration is likely affected by them. 2.6.1. Effect of Physical Properties of Magnesia on Hydration Since hydration starts from the surface, the specific surface area of magnesia is particularly important for the hydration process. Fine magnesia particles (i.e. with large specific surface area) have high hydration rates. The surface properties of magnesia, such as specific surface area and reactivity, are determined by the temperature of the calcination process and starting materials [15, 26, 27]. The surface area reaches a maximum and then decreases with increasing the calcination temperature. For example, 17 the surface area of magnesia produced by dehydrating Mg(OH) 2 changes from about 320 2 2 m /g at 350°C to 70 m /g at 700°C [27]; however, the surface area of magnesia obtained by decomposition of magnesite (MgCCh) increases with the temperature up to 900°C [28]. The open porosity of magnesia particles is another important property that influences hydration. Solid MgO reacts slowly with water vapor [29]. For porous magnesia, the reaction proceeds between MgO and water condensed in the micro-pores and absorbed on the surfaces of magnesia particles from water vapor. A sintered magnesia clinker has a higher hydration rate than a fused clinker, because typically the sintered magnesia is higher in porosity (8-11%) than the fused magnesia (porosity 3-5%) [24]. Also, the fused magnesia has larger crystals, which give a lower surface area; consequently the resistance to hydration is higher than for sintered magnesia. In other words, well-fused magnesia of high density, low porosity, and large crystal size has high hydration resistance. 2.6.2. Effect of Impurities 2.6.2.1. CaO From the phase diagram of the CaO-MgO system (Fig. 2-6. [30]), up to 5 wt% of CaO can be soluble in MgO. The CaO content in the MgO solid solution decreases as temperature is reduced. During cooling after the calcination of magnesia, the CaO of the high content region is separated from the high temperature solid solution of CaO-MgO grains and distributed in and around the periclase (MgO) grains. Below the solubility limit, CaO has been shown to preferentially segregate into the grain boundary [31, 32]. 18 The hydration activity of calcium oxide is greater than that of magnesium oxide by approximately 8 times [33]. 2.6.2.2. SiO? Silica can form compounds with MgO. Magnesia forms forsterite (Mg2Si04) at elevated temperatures, which remains along the grain boundaries of periclase crystals and works as a coating to restrict contact with water, improving the hydration resistance of periclase [34]. Due to the different crystal structures between forsterite and periclase, the coating effect of silica is inferior to A I 2 O 3 , but the silica is easily to form silicate glass phase along periclase grain boundaries and the silicate glass is an effective hydration inhibitor. M g O C a O Fig. 2-6. Phase diagram of system CaO-MgO [30]. 19 2.6.2.3. CaO/SiO? Ratio Calcium oxide and silica can form a secondary phase around magnesia grains. With an increase in the CaO /S i0 2 ratio, the secondary phases may be monticellite (CaMgSiO^), and mervinite (Ca3MgSi20s), dicalcium silicate (Ca2SiC>4), or a CaO-rich silicate glass. Phases with a higher content of CaO, like dicalcium silicate (Ca2Si04), are more sensitive to hydration than periclase, resulting in an increase of the hydration rate of magnesia [33]. Therefore, magnesia refractories with a high CaO /S i02 are more sensitive to hydration. A low C a O / S i 0 2 ratio (< 0.5) results in a high hydration resistance, due to the existence of a silicate glass or forsterite (Mg2Si04) along the magnesia grain boundaries. 2.6.2.4. A I 2 O 3 Alumina may form a solid solution in M g O and magnesia-spinel ( M g A ^ O ^ with MgO at high temperatures. When alumina is present in the magnesia during burning, spinel forms and is then precipitated along magnesia grain boundaries during cooling. The spinel has the effect of a coating on periclase grains and enhances the hydration resistance of magnesia. This coating effect is most evident when the content of A I 2 O 3 is between 2 and 4 w t% [34]. When the content of A1 2 0 3 exceeds 4 wt% , the magnesia resistance to hydration is lower, due to the increase in the porosity of the magnesia clinker, caused by the volume expansion of the spinel. 2.6.2.5. Iron Oxides According to the FeO-MgO phase diagram (Fig. 2-7. [35]), ferrous oxide is completely soluble in magnesia above 1200°C. The difference of ionic radii of M g 2 + (0.65A) and 20 Fe 2 + (0.80A) causes the large expansion and weakening of the ionic bond in magnesia when M g 2 + is replaced by Fe 2 + , therefore, the hydration rate of MgO-FeO solid solution is higher than that of magnesia. However, due to the partial oxidation of Fe 2 + to Fe 3 + , the hydration rate of magnesia with 5 atom% Fe 2 + is lower than that of magnesia [36]. The ionic radius of Fe 3 + (0.64A) is essentially the same as of Mg2+(0.65A). A 0.5 atom% substitution of Fe 3 + (in the form of Fe203) for M g 2 + in magnesia produces cation vacancies, which increases the surface energy and promote sinterability. In effect, the density of magnesia increases and the hydration rate decreases [37]. The upper limit of the amount of Fe 3 + to improve sintering is 3 atom% [38]. ?600 ZAOO eooo 1600 1200 1 1 1 1 1 I I I Liquid y y / y y y / / y / y y y y y y y y y y y y / / Magnesio-wu5tite I I I I I I 1 1 L 0 FeO 20 40 60 80 100 MgO Fig. 2-7. Phase diagram of system FeO-MgO [35]. 2.6.2.6. B9O3 Small amounts of B2O3 usually improve the hydration resistance of magnesia in the presence of CaO and Si0 2 [39], by forming a glass phase along the magnesia grain 21 boundaries. The B2O3 apparently increases the solubility of dicalcium silicate (C2S) in the glass phase, even at concentrations of B2O3 as low as 0.1%. Because it reduces the hot strength and corrosion resistance of MgO refractories, therefore, the amount of B2O3 in magnesia-based refractories must be restricted. 2.6.2.7. C r ? Q 3 The effect of C r 2 0 3 on the magnesia sintering and hydration behavior is similar to Fe203 [38] [40]. There is also a 0.1 atom % limit for promoting sintering. The excess C r 2 0 3 forms MgO-Cr203 spinel that does not hydrate. 2.6.2.8. Other Oxides With the addition of oxides such as Zr02 and T i 0 2 to magnesium oxide, the magnesia resistance to hydration is improved, owing to high density and low porosity. Zirconium and titanium ions, both of 4+ valences, produce vacancies in periclase crystals [37] and the increased surface energy. These oxides promote the sintering of magneisa and produce magnesia of low porosity and large crystal sizes. 22 3. OBJECTIVES The present work is a part of an ongoing collaborative research project - "Refractories for Non-ferrous Metals Smelting", in which one of the objectives is to enhance the fundamental knowledge in hydration of magnesia-based refractories and to offer practical solutions that would enable non-ferrous metal producers to extend the life in service of such refractories and make a proper selection of bricks to be used for their specific industrial environments. The objectives of this work are to establish the hydration mechanisms of magnesia-based refractory bricks and to find correlations between the hydration behaviors of various composition bricks and their physical, chemical and micro structural characteristics. In order to achieve these objectives, the following research work has been conducted: 1. Hydration studies in different hydrating conditions. Various brands of magnesia-based bricks were studied: magnesite, magnesia-spinel, and magnesia-chrome bricks. The hydration conditions were slightly different for the various bricks, according to their mineralogical compositions: a) Magnesia bricks, spinel-added and spinel-bonded magnesia bricks: in water at 60, 80, and 100°C, in water vapor at 98% relative humidity at 60 and 80°C, and in steam at 100, 109, and 121°C. b) Magnesia-chrome bricks: in steam at 109, 121, and 130°C. 23 2. Investigation of the correlations of the hydration behaviors and the modulus of elasticity by a non-destructive method. Modulus of elasticity (MOE) was measured by the Impulse Excitation Technique (IET). The hydration of a magnesia brick alters its internal structure, which triggers changes in MOE. If a reliable correlation between variation of the M O E and the hydration degree could be established, the degradation caused by hydration in a refractory brick could be evaluated by IET. 3. Investigation of correlation of the hydration and texture-related properties such as pore size, pore size distribution, permeability, mineralogical phases and their distribution. The following parameters are of particular interest: a) Pore size and pore size distribution, using a Porometer apparatus and kerosene as an impregnating medium. b) Permeability, using a VacuPerm Decay Permeameter. c) Mineralogical compositions, using the X-ray Diffractometry method. d) Microstructure, using a Scanning Electron Microscope (SEM) with an Energy Dispersive Spectrometer (EDS). The results from this work will serve the collaborative research project to develop a selection method for hydration resistant refractories. 24 4. EXPERIMENTAL PROCEDURES AND METHODOLOGY 4.1. Experimental Materials Magnesia-based refractory bricks include magnesite, magnesia-spinel, and magnesia-chrome bricks. In terms of raw materials, there are two types of magnesia-spinel bricks (magnesia-spinel and spinel bonded magnesia bricks)* and three types of magnesia-chrome bricks (burned, direct bonded and rebonded fused grain bricks)**. The raw materials for the manufacturing of the first two types of magnesia-chrome bricks are magnesia and chrome ore (complex solid solution of spinels ((Mg,Fe)(Cr,Al)204)), while the starting materials for the third type are fused grains. In the present work, different types of experimental refractory materials, with various chemical compositions and ratios of CaO /Si02 were chosen, in order to study how these factors would affect the hydration behavior. All three groups of magnesia-based refractory bricks were selected for the experimental work: magnesite with MgO greater than 87%, magnesia-spinel with over 93% MgO (chemical and physical properties of these refractories are presented in Table 4-1) and magnesia-chrome, "rebonded fused grains", "direct bonded" and "burned" bricks (their properties are presented in Table 4-2). * Magnesia-spinel refractory brick - a fired refractory made predominantly from a mix of refractory-grade magnesia and refractory-grade spinel. Spinel bonded magnesia brick - a fired magnesia brick with M g O A l 2 0 3 spinel as the matrix phase. * * Burned magnesia-chrome brick - a fired brick made from a mix o f refractoy-grade burned chrome ore and refractory-grade burned magnesia. Direct bonded basic brick - a fired refractory in which the grains are joined predominantly by a solid state diffusion mechanism. Rebonded fused grain brick - a fired brick made predominantly or entirely from fused grains. 25 Table 4-1. Properties of the experimental magnesia and magnesia- spinel bricks*** Experimental Bricks M l M2 MSI MS2 Chemical composition, % Si0 2 2.40 0.85 0.20 0.60 A1 2 0 3 0.60 0.15 10.6 6.00 Fe 2 0 3 1.40 0.30 0.50 0.20 CaO 1.60 2.15 1.40 1.00 MgO 93.0 96.5 87.3 92.2 B 2 0 3 0.50 - -CaO:Si0 2 0.71 2.71 7.50 1.79 Apparent Porosity, % 14.0 14.0 15.0 15.5 Bulk Density, g/cm3 2.92 2.96 3.00 2.96 Modulus of Rupture, MPa 26.2 17.2 5.60 9.00 Cold Crushing Strength, MPa 103 60.0 60.0 _ Modulus of Elasticity, GPa 111 61.4 9.80 16.8 Type B B B,M+S B, M-S *B = burned, M+S = magnesite-spinel brick, M-S = spinel bonded magnesite brick. Table 4-2. Characteristics of the experimental magnesia-chrome bricks*** Experimental Bricks MCI MC2 MC3 MC4 MC5 MC6 Chemical composition, % Si0 2 4.90 0.90 0.60 1.00 2.00 1.60 A1 2 0 3 21.5 9.80 6.30 6.00 11.3 7.10 Fe 2 0 3 11.8 16.3 12.0 11.0 8.00 12.0 CaO 0.80 0.60 0.80 1.00 0.70 1.10 MgO 34.6 42.0 62.0 63.0 62.5 60.0 C r 2 0 3 26.0 30.4 18.0 17.0 15.5 19.0 CaO:Si0 2 0.17 0.71 1.43 1.07 0.37 0.74 Apparent Porosity, % 20.6 16.4 13.0 14.5 18.0 18.0 Bulk Density, g/cm3 3.06 3.33 3.30 3.25 3.06 3.14 Modulus of Rupture, MPa 7.00 7.00 10.3 10.0 7.60 7.00 Cold Crushing Strength, MPa 26.0 24.0 82.7 90.0 49.0 50.0 Modulus of Elasticity, GPa 31.5 24.4 35.3 36.1 12.7 6.20 Type B B RFG RFG DB DB B = "burned", DB = "direct bonded", RFG = "rebonded fused grain". The data were provided by the brick producers, except for the modulus of elasticity. 4.2. Sample Preparation Procedures The brick samples were first cross-sectioned by dry saw-cutting into 25 mm thick slices across the length and along their original pressing direction and then further cut into 75 x 25 x 25 mm bars, also along their original pressing direction for the hydration tests and the M O E measurements (Fig. 4-la). For determining pore size and permeability, specimens of 45 mm in diameter and 35 mm in height were core-drilled from bricks along their original pressing direction by using a drill machine that was refrigerated with kerosene (Fig.4-lb). After cutting and drill ing, all specimens were dried for 20 hours at 110°C, and stored in desiccators to avoid the contact with moisture. Before the hydration test, they were fired at 800°C for 1 hour, to eliminate the effect of any potential organic hydration inhibitors, and then returned to desiccators and kept there until testing time. 27 45mm in diameter A The original pressing direction b) Fig. 4-1. Schematic representation of the specimen preparation from a brick, (a) for bars used in hydration studies and M O E measurement and (b)-for cylinders used to measure permeability and pore size and pore size distribution. 4.3. Experimental Methods The specimens as prepared above were subjected to hydration cycles under different conditions. After each cycle, they were dried and visually inspected, and the weight, modulus of elasticity (MOE) by impulse excitation technique (IET), and gas permeability were measured. The hydration cycling continued until disintegration occurred or no readings for M O E measurements could be taken. The mineralogical and microstructural studies by X R D and S E M studies were also performed on the as-received specimens and after hydration test; a representative material was studied by S E M after certain hydration cycles. 28 4.4. Hydration Studies In this work the weight increase incurred by specimens during the hydration tests was used for assessing the extent of hydration. 4.4.1. Hydration Test Conditions For magnesite and magnesite-spinel bricks: • In steam at 100, 109 and 121 °C • In humid air (98% relative humidity) at 60 and 80°C • In water at 60, 80 and 100°C At temperatures higher than 100°C, the samples were exposed to steam between two and 12-hour cycles. At 60°C and 80°C, the first cycle of hydration was for 6 hours and increased to 3 days at the end of test because of low hydration rates. For magnesia-chrome bricks: • In steam at 109°C (34.5 kPa), 121°C (103.4 kPa) and 130°C (172.4 kPa). The specimens were exposed to steam for 2-hour cycles at 121 °C and 130°C and up to 12 hours at 109°C, because of expected low hydration rates in these conditions. 4.4.2. Hydration Equipment and Procedures For pressures above 25 psi (172.4 kPa), a Cenco-Menzel autoclave (by Central Scientific Co.) was used, as recommended in A S T M (the American Society for Testing and 29 Materials) C 456-93 "Standard Test Method for Hydration Resistance of Basic Bricks and Shapes." For the low steam pressures (< 103.4 kPa), a model No. 915 low-pressure vessel (by Wisconsin Aluminum Foundry Co. Inc.) was used. At temperatures lower than 100°C, a humidity chamber was used and specimens were individually set in beakers. For hydration in water, the beakers with specimens were immersed in water. For the hydration in relative humidity values at different temperatures, the beakers were set on a sample holder in the chamber and kept away from the water. The hydration time was recorded after the testing temperature was reached. A digital hygrometer was used to measure the relative humidity inside the chamber. The specimens were weighed with ±0.00lg accuracy and placed in glass beakers covered with aluminum foil to protect them against the dripping water from condensation. After treatment in an autoclave, the specimens were dried for 4 hours at 65°C followed by drying at 110°C for 16 hours in a dryer with forced air circulation. They were immediately placed in desiccators to cool down to room temperature and returned to the next hydration cycle after measuring the weight and MOE. The weight increase and MOE were determined on three specimens with the average value taken. 4.4.3. Calculations The extent of hydration for each sample was calculated as (1) weight increase and (2) fraction of hydrated MgO. The following equations were used: (1) Weight increase, A W, %: 30 (4-1) where W„ = the sample weight after hydration for n hours, g, Wo = the sample weight before hydration, g, n = the hydration time, hours. (2) Fraction of reacted MgO, a: MM0 AW 40.3 AW AW a = = • =2.24 M H20 C M G 0 18 CMGQ C-MgO where MM8O = the molecular mass of MgO, MH20 ~ the molecular mass of H 2 O , CMSO ~ the chemical content of MgO in the brick. A summary of the hydration tests is presented in Table 4-3. Table 4-3. A summary of hydration tests Brick Sample Testing Conditions Hydration Hours (Cycles) for M O E Measurement Total Hours (Cycles) of Samples Keeping Integrity AW,% Hours (Cycles) M l Steam, 121°C 0.20 ± 0 . 0 4 24(12) 24(12) > 24 (12) Steam, 109°C 0.23 ± 0.02 48(11) 48 (11) >48 (11) Steam, 100°C 0.21 ± 0 . 0 3 60(12) 60(12) > 60 (12) Humidity, 80°C 0.37 ± 0 . 0 2 96(13) 96(13) > 96 (13) Humidity, 60°C. 0.13 ± 0 . 0 1 192 (9) 192 (9) > 192(9) Water, 100°C 0.16 ± 0 . 0 6 54(13) - -31 Table 4-3 cont'd Water, 80°C 0.14 ±0.02 84(12) -Water, 60°C 0.07 ±0.01 192 (9) - -M2 Steam, 121°C 2.04 ±0.15 4(4) 3(3) 3(3) Steam, 109°C 1.49 ±0.18 6(3) 6(3) 6(3) Steam, 100°C 1.57 ± 0.12 16(6) 12(5) 12(5) Humidity, 80°C 2.35 ±0.12 24 (3) 12(2) 12(2) Humidity, 60°C 1.23 ±0.06 144 (8) 144 (8) 144 (8) Water, 100°C 3.34 ±0.06 16(7) - -Water, 80°C 2.89 ±0.10 32(4) - -Water, 60°C 3.23 ±0.05 192 (9) - -MSI Steam, 121°C 1.35 ±0.11 10(7) 10(7) 10(7) Steam, 109°C 1.76 ±0.09 26(8) 20 (7) 20 (7) Steam, 100°C 1.49 ±0.14 36(10) 30(9) 30(9) Humidity, 80°C 0.90 ±0.05 48 (7) 36(5) 48 (7) Humidity, 60°C 0.30 ±0.07 192 (9) 192 (9) > 192(9) Water, 100°C 1.00 ±0.02 32(10) - -Water, 80°C 0.98 ±0.09 48 (7) - -Water, 60°C 0.77 ±0.08 192 (9) - -MS2 Steam, 121°C 1.04 ±0.08 2(2) K D . 1(1) Steam, 109°C 1.13 ±0.11 4(2) 2(1) 2(1) Steam, 100°C 1.45 ±0.11 6(3) 6(3) 6(3) Humidity, 80°C 1.91 ±0.10 24 (3) 12(2) 12(2) Humidity, 60°C 0.38 ±0.04 . 72 (6) 48 (5) 48 (5) Water, 100°C • 1.61 ±0.08 8(4) -Water, 80°C 0.67 ±0.14 12(2) - -Water, 60°C 0.95 ±0.15 96 (7) -MCI Steam, 130°C 0.14 ±0.01 8(4) 8(4) 8(4) Steam, 121°C 0.37 ±0.02 18(9) 10(5) 16(8) Steam, 109°C 0.19 ±0.01 84 (22) 84 (22) > 84 (22) MC2 Steam, 130°C 0.14 ± 0.01 8(4) 6(3) 6(3) Steam, 121°C 0.22 ±0.01 16(8) 14(7) 14(7) Steam, 109°C 0.27 ±0.01 84 (22) 72 (21) >72 (21) MC3 Steam, 130°C 0.59 ±0.03 12(6) 12(6) > 12 (6) Steam, 121°C 1.28 ±0.06 18(9) 14(7) 16(8) Steam, 109°C 1.14 ±0.05 84 (22) 72 (21) 72 (21) MC4 Steam, 130°C - - - -Steam, 121°C 0.72 ±0.04 18(9) 16(8) 18(9) Steam, 109°C 0.50 ±0.02 84 (22) 84 (22) > 84 (22) MC5 Steam, 130°C 0.49 ±0.02 16(8) 16(6) > 16(8) Steam, 121°C 0.52 ±0.03 18(9) 18(9) >18 (9) Steam, 109°C 0.40 ± 0.02 84 (22) 84 (22) > 84 (22) MC6 Steam, 130°C 0.46 ±0.02 8(4) 8(4) 8(4) Steam, 121°C 0.56 ±0.03 8(4) 8(4) >8(4) Steam, 109°C 0.59 ±0.03 84 (22) 84 (22) > 84 (22) 32 4.5. Modulus of Elasticity The modulus of elasticity (MOE) was determined on as-received samples (not subjected to hydration testing) and after each hydration cycle, using a GrindoSonic MK5 "Industrial" instrument (J. W. Lemmans Inc.), according to the A S T M C1259 -01 the standard test method for "Dynamic Young's Modulus (E), Shear Modulus (G), and Poisson's Ratio for Advanced Ceramics by Impulse Excitation of Vibration." The impulse excitation technique (IET) is a non-destructive method for determining the elastic constant. The instrument consists of an impulser, a transducer, an electronic system (signal amplifier and signal analyzer), a frequency read-out device, and supports as shown in Fig. 4-2. The instrument measures the fundamental resonant frequency of the specimens of suitable geometry, by exciting them mechanically with a light strike of the impulser (a small steel ball attached to an elastic handle). A transducer picks up the vibrations and transforms them into electric signals, processed by an analyzer, which provides a numerical reading that is either the frequency or the period of the specimen's vibration. The appropriate fundamental resonant frequencies and the dimensions and mass of the specimen are used to calculate the dynamic Young's modulus, the dynamic shear modulus, and Poisson's ratio. Soft foam supports, which were used to allow for a free vibration of the specimens, were located at the fundamental nodal points (0.224 of the length of the specimen from each end). The transducer was placed in contact with the test specimen to pick up the vibration 33 and the reading was recorded for the calculation of Young's modulus by using provided software. Signal amplifier Frequency analyzer Read out device Specimen Transducer Supports Impulser Fig. 4-2. The impulse excitation technique apparatus. Young's modulus was expressed the following equation [41] [42]: E = 0.9465 • mf2f j} b t (4-3) where E = Young's modulus, Pa, m = mass of the bar specimen, g, 34 b, L, t- width, length, and thickness of the bar specimen, respectively, mm, ff= measured fundamental resonant frequency in flexure, Hz , Ti = correction factor for fundamental flexural mode to account for finite thickness of the sample bar specimen. Fro a single crystal, the intrinsic elastic extension corresponds to extending the atom separation uniformly. The stronger the atomic bond the greater the stress required to increase the interatomic spacing and the greater the value of E. The characteristic elastic properties of ceramic materials depend primarily on composition, porosity, cracking, and mineralogical phases. The shape and distribution of pores and the interconnections between pores also affect the elastic constants. Many equations have been proposed for relating E to porosity and there is no universal relation. A selection of equations is listed in Table 4-4 [43] Table 4-4 Relations between porosity P and Young's modulus E [43], E = E0(l-aP) E = E0(1 -aP + bP2) E = E0{l-a P)b E = E0\1 +aP/(l-(a+l)P)] E = E0 exp (-a P) E = E0 exp \-(a P + b P2)] where Eo = the elastic modulus of a fully dense ceramic, and P = porosity, a, b = constants. The variations of the modulus of elasticity for hydrated samples were calculated using the following equation: 35 AE = E„ - E, o xlOO% E, (4-4) Where EN = M O E after hydration for n hours, EQ = M O E before hydration, n = the hydration time, hours. 4.6. Permeability Permeability measurements were conducted using a VacuPerm Decay Permeameter (University of Missouri-Rolla, USA ) and the permeating fluid was air. Figure 4-3 shows the schematic and instrument of the VacuPerm Decay Permeameter. By vacuuming one side of a sealed sample, the apparatus produces a difference of pressures between the two sides of the sample and when reaching 7 5 % vacuum, it allows air to flow through the sample. The air permeability is determined by measuring the vacuum decay versus time. The permeability, k, is defined by Darcy's law [44], For sufficiently slow, unidirectional and steady flow: Q = (lcA/n)(AP/L) (4-5) Where k = permeability, Darcy, Q = the volumetric flow rate, cm 3 sec -i A = the cross-section area of the sample, cm 2 , L = the length of the sample in the macroscopic flow direction, cm, AP = ¥\ - ?2, pressure difference between both ends of the sample, atm, rj = the fluid viscosity, cP. 36 Specimen Computer a) 37 A practical unit of permeability is Darcy. A porous material has permeability equal to 1 Darcy if a pressure difference of 1 atm will produce a flow rate of 1 cm3/s of a fluid with 1 cP viscosity through a 1 cm cube, [44]. Thus, 1 Darcy = ^ ' s>^cP) = 0 M 7 ^ ( 4 . 6 ) \{cm ) • \{atm I cm) For low permeability materials, the unit milliDarcy = 0.001 Darcy is used. After the cylindrical sample was dried and cooled down at the end of each hydration cycle, air permeability was measured and the value was the average of three measurements. After that the sample was subjected to the next hydration cycle. 4.7. Pore Size and Pore Size Distribution Pore size and pore size distribution were measured with a Capillary Flow Porometer (Porous Materials Inc.). The apparatus consists of a regulator, valves, barometers, flow meters, and a sample chamber. When compressed air flows through the regulator and valves, which are controlled by a computer, to reach the sample chamber with a sealed sample, pressure differences between both ends of samples and flow rates are recorded, and pore size and pore size distribution can be calculated. When the pores of a sample are spontaneously filled with a wetting liquid by displacing the gas present in the pores, application of differential pressure of a non-reacting gas on the sample is required to remove the wetting liquid from pores (Fig. 4-4). The differential pressure, AP, required to remove the wetting liquid from a pore of diameter D, is given by [45]: 38 AP = 4y cosd/D (4-7) Where AP = differential pressure required to remove a wetting liquid from a pore, Pa, y = surface tension of the wetting liquid, J/m , D = diameter of a pore, m, 6 = contact angle of the wetting liquid on the sample material, degree. The sample filled with a wetting liquid is installed in a sample holder, and the gas pressure is increased on one side of the sample. At a certain pressure (expressed by Eq.(4-8)), the largest pore is emptied and the gas flow starts. With an increase of pressure, air goes through smaller pores and flow rate increases. From the flow rates through wet and dry samples at the different gas pressures, the pore diameters and pore distribution are calculated using equations (4-7) and (4-8), respectively [46]. f=[-d(fJfd)/dD] (4-8) where fw and fd are flow rates through wet and dry samples respectively, nrVsec, and dD is the range of pore sizes corresponding to two differential pressures (APs), m. Kerosene (surface tension = 28 dynes/cm i.e. 0.0028 N/m) was used as the impregnating fluid. Pore size distribution was measured for all of the brick samples before the hydration tests. One selected sample - MC3 was tested only after every four- cycle hydration and the specimen was not returned back to the hydration cycles. 39 Fig. 4-4. The operation principle for measuring pore diameters [47]. 4.8. Mineralogy Studies Mineralogical compositions were analyzed by an X-ray diffractometer ( X R D ) (Siemens D5000 X-ray Powder Diffractometer) using CuKa (X = 0.154 nm) radiation. Samples were prepared as powder and the diffraction patterns were taken between 15 and 70 degree two-theta. The analyses on phase compositions were done for all as-received samples and after-hydration specimens. Because there are low content of brucite (less than 2%) in specimens during hydration cycles, brucite is difficult to be detected for the samples during the hydration. 40 4.9. Microstructure Studies Microstructures were observed with SEM (Hitachi S-3000 Scanning Electron Microscope) on as received specimens and the selected sample MC3 after each four cycles hydration. To avoid hydration of magnesia during preparing specimens, kerosene was used for lubricant with a Buehler polishing machine. Attempt was made to use polished specimens for after-hydration bricks, but it did not succeed because brucite is very soft and it is difficult to keep it on specimens during polishing, even using low viscosity resin. In order to be able to observe brucite in specimens, some SEM/EDS analyses were performed on fresh fractures of samples. 41 5. RESULTS AND DISCUSSION 5.1. Degree of Hydration under Different Testing Conditions 5.1.1. Magnesite and Magnesia-Spinel Bricks For these two types of bricks, hydration tests were conducted under the following conditions: in steam at 121, 109 and 100°C, in 98% relative humidity at 80 and 60°C, and in water at 100, 80 and 60°C. The weight increased for each sample with the hydration time. Figures 5-1-1 to 5-1-8 show the results of the hydration tests. Under the same conditions, different brick samples hydrated differently. The MSI brick had a moderate hydration rate, lower than MS2 and M2, but greater than M l . For the hydration tests conducted above 100°C, the curves of weight increase for MSI showed three stages (Figures 5-1-1 to 5-1-3, 5-1-6): i) a fast increase in weight followed by ii) a slow increase and finally iii) another fast increase, until the sample disintegrated. The last stage was not clear for the hydration of MSI in water or humidity at 80 and 60°C (Figures 5-1-4, 5-1-5 and 5-1-7, 5-1-8). Figures 5-1-1 to 5-1-8 show that brick MS2 is the most vulnerable to hydration. It took the shortest time for its samples to fail, even though the weight increase of the MS2 sample was lower than MSI and M2 in most cases. The hydration was accelerated after the first few cycles. The M l specimens showed the highest hydration resistance: the weight increase for all hydration conditions was very low (less than 0.3%), no disintegration occurred in any tests and no cracking was observed in specimens. 42 The M2 samples were less affected by hydration than the MS2 samples. The weight increase, in some cases, was higher than that of MS2, but the time for the M2 samples to fail was longer than that for MS2. The plots of weight increase versus time for M2 showed three stages, similar to the MSI specimens. For all experimental bricks, the weight increase was higher at higher temperatures. If disintegration time is the total hydration time until a specimen disintegrates, then the disintegration time was longer at low temperatures than at high temperatures. This indicated that high temperature accelerates the hydration of magnesia bricks. The MSI brick, containing 10.6% AI2O3, is a burned magnesia-spinel brick. Spinel was added as aggregate and fine powders, so MSI sample contains large aggregates of magnesia and spinel and the matrix also contains both spinel and periclase. A bonding spinel phase around fine magnesia particles in the matrix was found (Fig. 5-1-9). CaO exists in the binding phase by EDS (Fig. 5-1-10). The XRD diffractogram shows that the mineralogical phases in MSI are periclase and spinel (MgO»Al 2 03) (Fig. 5-1-11). The content of MgO was only about 87.3%, while the 10.6% A l 2 0 3 combines about 4.2% MgO in MgO»Al 2 03 spinel. This was the lowest content of MgO in the four experimental bricks. One of the reasons why the MSI sample had a relative low hydration rate was probably that the spinel binding phase around the periclase grains worked as a protecting layer against hydration. 43 0 4 8 12 16 20 24 Hydration Time, hrs Fig. 5-1-1. Weight increase versus time for hydration in steam at 121°C. Hydration Time, hrs Fig. 5-1-2. Weight increase versus time for hydration in steam at 109°C. 2 -i Hydration Time, hrs Fig. 5-1-3. Weight increase versus time for hydration in steam at 100°C. 2 -i - • - M S I 1.6 - - • -MS2 - A - M l 'ease 1.2 - ^ - M 2 ight Ii 0.8 -0.4 -, t- - * — - » - - * — * * 0 i - * — * — V ™ 1 i 1 1 1 i i 0 12 24 36 48 60 72 84 Hydration Time, hrs Fig. 5-1-4. Weight increase versus time for hydration in 98% relative humidity at 80°C. 45 Fig. 5-1-5. Weight increase versus time for hydration in 98% relative humidity at 60°C. + - M S 1 * - M S 2 0 10 20 30 40 50 60 Hydration Time, hrs Fig. 5-1-6. Weight increase versus time for hydration in water at 100°C. 46 Fig. 5-1-7. Weight increase versus time for hydration in water at 80°C. MSI * - M S 2 0 40 80 120 160 20(J Hydration Time, hrs Fig. 5-1-8. Weight increase versus time for hydration in water at 60°C. Fig. 5-1-9. Backscatter electron micrograph of the as-received MSI sample. 48 Al O Ca Fe Fe — i — keV Oxides MgO A I 2 O 3 Si0 2 CaO Fe203 wt% 18.45 59.12 2.70 18.27 1.45 Fig. 5-1-10. EDS results of the bonding phase in the MSI matrix. The MS2 brick is a spinel bonded magnesite brick, in which the aggregates of coarse and intermediate size are magnesia and the matrix contains a spinel phase as a bond (Fig. 5-1-12). The XRD diffractogram (Fig. 5-1-11) shows that there are two phases in MS2: periclase and MgO»Al203 spinel. Comparing the XRD pattern, the intensity of periclase phase in MS2 is higher than in MSI, i.e. the content of periclase phase is higher in MS2 49 than in MSI. The spinel phase assembles together and does not form a protecting film on periclase (Fig. 5-1-12). Comparing Figures 5-1-12 and 5-1-9, it can be seen that the magnesia particles in MSI and MS2 are different. The magnesia particles in the'MSI sample have irregular shapes, and there are more pores in the MS2 sample than in the MSI sample. The periclase phase in the MS2 sample contains more grain boundaries and their sizes are smaller than in sample MSI. These kinds of periclase grains can more easily produce new surfaces for hydration and tend to disintegrate faster. This was confirmed by the weight increase of the MS2 sample, which was lower at the beginning of hydration, but later increased faster and had a shorter disintegration time than the MSI sample. MSI S S^ P } v S B 4> MS2 S s,p . P S 1 s 15 20 25 30 35 40 45 50 55 60 65 70 2 Theta, Deg. Fig. 5-1-11. XRD diffractograms for the unused MSI and MS2 samples (P = periclase and S = spinel) 50 Fig. 5-1-12. Backscatter electron micrograph of the MS2 sample. Table 5-1-1. The compositions of binding phases for sample MS2, M l , and M2 from EDS analysis. wt% MgO A1 2 0 3 S i 0 2 CaO Fe 2 0 3 MS2 28.12 71.54 0.34 - _ M l 27.03 0.20 38.12 34.36 0.29 M2 98.32 0.21 0.42 0.79 0.25 The M l sample is a burned magnesite brick with a relatively compact texture. The binding phase around the magnesia particles has the ratio of CaO : MgO : S i 0 2 of 1:1:1 51 (Table 5-1-1), which is equivalent to that of monticellite (CaMgSi0 4). The fact that XRD did not detect any silicate compounds in the M l sample indicates that the binding phase is a glass phase (Fig. 5-1-13 to 5-1-14). The silicate glass around periclase and the lowest ratio of CaO/SiCh (= 0.7) of the sample would be the reasons why the hydration rate was the lowest out of the four chrome-free bricks. The graphs of the weight increase of the M l samples showed that there was a fast increase in weight during the first few cycles of hydration, but the second stage of the hydration seemed not complete. Fig. 5-1-13. Backscatter electron micrograph of brick M l : binding phase (light gray) and periclase (dark gray). 52 p M l P C M2 1 15 20 25 30 35 40 45 50 55 60 65 70 2 Theta, Deg. Fig..5-1-14. The X R D diffractograms for the unused M l and M2 samples (P = periclase). Fig. 5-1-15. Backscatter electron micrograph of the M2 brick structure. 53 Sample M2 is also a burned magnesite brick containing the highest content of MgO and has the most compact texture of all four bricks (Fig. 5-1-15). There are some cracks on the magnesia particles, but there was no second phase around magnesia particles (Table 5-1-1). The EDS results showed that there were very small amounts of impurities. From Figures 5-1-1 to 5-1-8, the hydration rate for the M2 samples is higher than for MSI and Ml in terms of the weight increase, and the disintegration time is shorter than for these two samples. Microcracks in the magnesia particles are assumed to have a greater contribution to the high hydration rates. The microcracks probably formed during the cooling process of the brick and that increased surfaces to react with water. 5.1.2. Magnesia-Chrome Bricks For magnesia-chrome bricks, hydration tests were conducted under the following conditions: in steam at 130°C, 121°C and 109°C. The hydration results are shown in Figures 5-1-16 to 5-1-18. The increase of hydration rates with the temperature indicates that the rising temperature accelerates the hydration process. Similar to the hydration of magnesia-spinel and magnesia bricks, the samples were disintegrated in a shorter time for hydration at a higher temperature. At 109°C, all six magnesia-chrome samples maintained their shapes after 84 hours of treatment, but at 121°C the hydration time before disintegration was less than 20 hours. At 130°C, the shortest disintegration time was 8 hours, i. e. MCI and MC2 (Fig. 5-1-18). At 121°C, the three stages of the hydration processes were clearly shown in Fig. 5-1-17 for all six MC bricks, but at 109°C the last stage did not appear for most samples because of lower hydration rates. 54 A M C 2 0 12 24 36 48 60 72 84 Hydration Time, hrs a) b) Fig. 5-1-16. Weight increases versus time for hydration in steam at 109°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI, MC4 and MC6. 55 0.8 -, 0 4 8 12 16 20 Hydration Time, hrs b) Fig. 5-1-17. Weight increases versus time for hydration in steam at 121°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI, MC4 and MC6. 56 0.8 i 0.6 ^ u B DD I 0.4 0.2 • M C I - * - M C 2 - * - M C 3 - • - M C 5 ->—MC6 8 12 Hydration Time, hrs 16 20 Fig. 5-1-18. Weight increases versus time for hydration in steam at 130°C. In terms of weight increase, the MCI sample had the lowest hydration rate of the six magnesia-chrome bricks under all experimental conditions. Brick MCI is of "burned" type brick. The XRD analysis indicates the main phases in MCI are complex spinel (Mg, Fe)(Cr, Al, Fe)204 and periclase (Fig. 5-1-19). Generally, silicate-bonded magnesia-chrome bricks contain - 3 - 1 0 wt% Si02, whereas "direct bonded" bricks contain < 3 wt% Si02 [48]. Brick MCI, containing 4.9% Si02, is a silicate-bonded magnesia-chrome brick with a silicate glass binding phase, which are not detected by XRD analysis. Brick MCI is of the lowest content of MgO (34.6%) and the lowest ratio of CaO/Si02 (= 0.2) of all six magnesia-chrome bricks. The characteristics of the structures and compositions could be the explanation of the lowest hydration rate for the MCI sample. 57 c c pi 15 20 25 30 35 40 45 50 55 60 65 70 Fig. 5-1-19. XRD diffractogram for M C I - the unused sample. P = periclase (MgO) and C = complex spinel (Mg, Fe)(Cr, Al , Fe) 20 4. Table 5-1-2. EDS results of the binding phasee in the MCI and MC2 samples, wt%. Oxide MgO A1 2 0 3 Si0 2 CaO ; C r 2 0 3 Fe 2 0 3 MCI 40.86 8.44 13.66 2.57 9.60 24.86 MC2 39.40 11.29 - - 13.15 : 36.15 MC2 is of the same type as MCI and of similar mineralogical composition, i.e. complex spinel (Mg, Fe) (Cr, Al , Fe)20.4 and periclase (MgO), but the grain boundary does not indicate the presence of silicate-containing phases (Table 5-1.-2). MC2, with the higher MgO (42.0%) and CaO/Si02 ratio (0.7) than MCI , showed higher hydration rates than sample MCI. 58 Both the MC3 and MC4 samples are of "rebonded fused grain" type and have similar properties. Both bricks had higher hydration rates than MCI and MC2. These two bricks also showed that they had higher contents of MgO (62.0% and 63.0%> respectively) and higher CaO/Si0 2 ratio (1.4 and 1.1 respectively) than those of MCI and MC2. Comparing MC3 and MC4, they have almost the same chemical compositions and physical properties, except for a higher CaO/Si0 2 ratio for MC3. The higher hydration rates are assumed to relate to the higher CaO/Si0 2 ratio. Brick MC5 and MC6 are of "direct bonded" type, and their MgO contents, CaO/Si0 2 ratios and hydration rates are between the groups of "burned" and "rebonded fused grain". The MC6 brick showed higher hydration rates and higher CaO/Si0 2 ratio than brick MC5. From the discussion above, the CaO/Si0 2 ratio plays an important role in the hydration of magnesia-based bricks. The phases containing CaO and Si0 2 with a low CaO/Si0 2 ratio (less than 1) is not easy to react with water and can protect the magnesia from water if they surround the pericalse grains. With the increase in the CaO/Si0 2 ratio, especially when greater than 2, the phases containing CaO and Si0 2 can be 2CaO- Si0 2 (C2S), and other compounds with high content of CaO. These CaO-rich phases are very sensitive to hydration. Wherever they are distributed in bricks, they will increase the hydration rate of magnesia-based bricks. For the experimental bricks, the relationship between CaO/Si0 2 ratio and hydrated MgO is shown in Figures 5-1-20 and 5-1-21. For all experimental bricks, the amount of hydrated MgO linearly increases with the CaO/Si0 2 ratio and with the hydration time, the effect is more remarkable. 59 Fig. 5-1-20. Hydrated MgO versus CaO/Si0 2 ratio for the hydration of the magnesia-spinel and magnesia bricks in water at 60°C. 0.6 0.9 Ca0/Si02 Ratio Fig. 5-1-21. Hydrated MgO versus CaO/Si0 2 ratio for the hydration of the magnesia-chrome bricks in steam at 109°C. 60 5.2. Modulus of Elasticity (MOE) versus Hydration Degree The MOE determination was conducted with the impulse excitation technique (IET). Figures 5-2-1 to 5-2-8 show the M O E variation versus the hydration time for all the experimental samples, under different hydration conditions. For MSI, during the first cycle of hydration (the first hour) in steam at 121°C, both MOE and the weight gain increased rapidly, followed by slower variations (Fig. 5-2-1 and Fig. 5-1-1). After the M O E reached a maximum, the weight continued to increase (Fig. 5-1-1), but the M O E decreased to the initial value. M O E values below the initial value corresponded to the final rapid increase in weight. -20 J Hydration Time, hrs Fig. 5-2-1. M O E variation versus time for hydration in steam at 121°C for magnesia-spinel and magnesia bricks (refer to Table 4-1 for the initial value of MOE). 61 In steam at 109 and 100°C and in humidity at 80°C, the changes in M O E have the same profiles as those at 121°C. During the first few cycles, the M O E quickly increased to the maximum values, and after that, the M O E slowly decreased until reduced below the initial values, and finally the samples disintegrated (Figs. 5-2-2 and 5-2-3). In humidity at 60°C, the M O E remained at the maximum level after hydration for 200 hours, and this could be related to the slow hydration. For sample MS2, the M O E variation was different from that for MSI. The M O E decreased just after one cycle of hydration in steam at 121 and 109°C. Actually, the MOE maximum should have occurred during the first cycle, but could not be observed, owing to the high hydration rate. The maximum MOE was shown during the hydration in steam at a lower temperature (100°C) and in humidity at 80 and 60°C (Figures 5-2-3 to5-2-5). Hydration Time, hrs Fig. 5-2-2. M O E variation versus time for hydration in steam at 109°C for magnesia-spinel and magnesia bricks (refer to Table 4-1 for the initial value of MOE). 62 The M l brick had the lowest hydration rate among the chrome-free bricks. For all experimental conditions, the weight increase was slow and the M O E continued to increase slowly. The maximum value of M O E was not reached even after hydration for 200 hours. The patterns of the M O E variation for the M2.brick were similar tb those for MSI. The MOE reached its maximum after one cycle of hydration in steam at 121°C, and during the second cycle of hydration the M O E reduced to the initial value. Further hydration caused the disintegration of the samples. At low temperatures, the time for the MOE to reach the maximum and for the sample to disintegrate became longer than at high temperatures (Figs. 5-2-1 to 5-2-5). Hydration Time, hrs Fig. 5-2-3. M O E variation versus time for hydration in steam at 100°C for magnesia-spinel and magnesia bricks (refer to Table 4-1 for the initial value of MOE). 63 -10 J Hydration Time, hrs Fig. 5-2-4. MOE variation versus time for hydration in 98% relative humidity at 80°C for magnesia-spinel and magnesia bricks (refer to Table 4-1 for the initial value of MOE). 25 n Hydration Time, hrs Fig. 5-2-5. MOE variation versus time for hydration in 98% relative humidity at 60°C for magnesia-spinel and magnesia bricks (refer to Table 4-1 for the initial value of MOE). 64 0 14 28 42 56 70 84 Hydration Time, hrs b) Fig. 5-2-6. MOE variation versus time for hydration in steam at 109°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI, MC4 and MC6 (refer to Table 4-2 for the initial value of MOE). 65 a) b) Fig. 5-2-7. MOE variation versus time for hydration in steam at 121°C. (a) for brick MC2, MC3 and MC5 and (b) for MCI, MC4 and MC6 (refer to Table 4-2 for the initial value of MOE). 66 -20 J Hydration Time, hrs Fig. 5-2-8. M O E variation versus time for hydration in steam at 130°C for the magnesia-chrome bricks MCI , MC2, MC3, MC5 and MC6 (refer to Table 4-1 for the initial value of MOE). For all the magnesia-chrome brick samples, the profiles of the M O E variation were similar. The M O E values first quickly reached their maximum, followed by slow decreases below the initial levels, until the samples disintegrated (Figs.5-2-6 to 5-2-8). The change in the M O E with hydration time for all brick samples obviously had three stages: i) an increase from the initial value to reach a maximum value, ii) a decrease from the maximum to the initial value, and iii) below the initial value until sample disintegration. This was a typical pattern for the M O E variation. The stages for the M O E were not exactly the same as the stages for the weight increase. Obviously, the weight 67 increase has a direct influence on the MOE variation, but the mechanisms affecting the MOE value will be discussed in Chapter 5.3. Under different temperatures, the changes in M O E versus hydration time were of the same profile. With increasing temperature, both the M O E maximum values and the time to reach the maximum tended to decrease. For example, for MSI sample, in steam at 100, 109 and 121°C, the maximum M O E variations were 37.6%, 34.3% and 33.4%, and the time to reach the maximum was 10, 6 and 4 hours, respectively (Figs. 5-2-1 to 5-2-5). 5. 3. Influence of Brucite on MOE Variation In early stages of hydration, the modulus of elasticity generally increases with the degree of hydration for the magnesia-based refractory materials, owing to the development of brucite. The presence of brucite may affects M O E in the following ways: the produced brucite grows on the walls of the open pores and forms a film, which reinforces the brick and increase the MOE; due to the larger volume of brucite the formation of microcracks along the grain boundaries weakens them and have a negative effect on the MOE increase; the brucite fills in the small pores of the hydrated material, and reducing the porosity increase MOE. The hydration of MgO-based bricks produces brucite, which grows on the walls of the open pores and on the surfaces and gradually forms a film. This film serves to reinforce the brick structure and it will maintain its elasticity as long as it remains continuous. The hydration process of brick MC3 in steam at 109°C was chosen as an example to present this mechanism. The weight increase and M O E variation are shown in Fig. 5-3-1. 68 The samples hydrated for 0, 8, 24, and 38 hours were chosen to observe the microstructures. Hydration Time, hrs Fig. 5-3-1. Weight increase and MOE variation versus the time for hydration in steam at 109°C for brick MC3. When the SEM specimens were polished, the soft brucite was easily washed away. In order to be able to verify that the film of brucite formed inside the open pores, SEM/EDS analyses was performed on fresh fractures of sample. The SEM image of the fractured non-hydrated MC3 sample is shown in Fig. 5-3-2. The compositions of the grains in sample MC3 are shown in Table 5-3-1. 69 Fig. 5-3-2. SEM image for the fracture section of the non-hydrated MC3 sample. Table 5-3-1. The compositions of different grains in MC3 shown in Fig. 5-3-2, wt% Oxide Point A Point B Point C Point D MgO 89.1 14.9 58.0 22.7 A1 2 0 3 - 13.9 7.63 _ Si0 2 0.84 - 1.00 37.2 CaO - - 0.35 38.4 C r 2 0 3 3.90 50.3 20.9 0.82 Fe 2 0 3 6.14 19.6 12.1 0.90 T i 0 2 - 1.36 - -Fig.5-3-2 showed that the MC3 sample consisted of fused magnesia-chromite spinel grains, chrome ore grains, isolated impurities, and pores. The bond between the fused grains appeared strong, as the sample fractured intragranularly. Brucite deposited on the nore walls WD15.2mm 20.OkV"x7.Ok " *5um Fig. 5-3-3. The film of brucite on the wall of a pore after hydration for 8 hours. As seen in Fig.5-3-1, after hydration for 8 hours at 109°C, the M O E value reached the maximum value. The SEM image of the MC3 sample displayed a film of brucite on the walls of pores (Fig. 5-3-3). The film consisted of brucite crystals of about 1 u.m, with a platelet-like shape. This crystal morphology indicated that the nucleation of the brucite was slow and the crystals developed very well [13]. This film can strengthen the structure of the brick and enhance modulus of elasticity and the strength. This result is in accordance with another work [8] on magnesia-chrome bricks, in which the authors found that the hydration of a small amount of magnesia in magnesia-chrome bricks could increase brick strength (modulus of rapture). In addition, the formation of brucite, during which the volume increases, induces a compressive stress on the neighborhood of a pore. 71 The compressive stress could affect the bonding of periclase grains, but for a low degree of hydration, when the stress was much less than the bonding strength, the modulus of elasticity would increase with the thickness of brucite film. At the beginning of hydration, a large weight increase and a high increase in MOE takes place. A film of brucite forms on pore walls, which reduces the ulterior hydration rate. During the early stage of hydration, an increasing thickness of the film is accompanied by the rising MOE. The maximum values of MOE would correspond to a maximum thickness of the film, called "critical thickness". When the brucite film exceeds the "critical thickness", the stress accompanying its shrinkage during drying between hydration cycles, will exceed the maximum strength and cracking occurs, and then the MOE begins to decrease. An empirical model [49] of the hydration process was used to calculate the thickness, h [um], of the brucite film. For a hydrated MgO brick of perfectly cylindrical open pores of median diameter do [p.rn], the thickness of the film of brucite, h was calculated with the following equation: h=d0[l -(1 -AWk/Pf ] (5-1) with k = Pbrick MMgo/pbrucite MH20 (5"2) where AW = the weight increase, %, P = apparent porosity of the brick, %, MMgo = molar mass of MgO, g/mol, MH2o - molar mass of H 2 O , g/mol, 72 Pbrick = density for brick, g/cm3, Pbrucite = density for brucite, g/cm3, do = median pore diameter, jum. For this hydration condition, Fig. 5-3-1 shows that when MC3 sample reaches the maximum MOE, its weight increase, AW is 0.33% and the porosity, P is 13.0% (Table 5-3-2), and the pore size, do is 15 -20 um (Fig. 5-3-3). From the equations (5-1) and (5-2), the maximum thickness of the film of brucite is calculated at about 0.6-0.8 um, which was in agreement with the SEM observation (Fig. 5-3-3). After the reaction takes place at grain boundaries with the hydration process, the brucite at periclase boundaries tends to separate periclase grains due to the larger volume of brucite than mangesia and gradually forms microcracks along the grain boundaries, weakening the grain boundaries. The weakening of the bonding of periclase grains further reduces the MOE. After hydration for 24 hours, the modulus of elasticity was only about 70% of the maximum value. The fractograph of the MC3 sample is shown in Fig. 5-3-4. The fact that the weak grain boundaries make the sample most transgranularly fracture and microcracks take place around magnesia particles indicates that the hydration proceeds along grain boundaries and the stress induced by the higher volume of brucite weakens the grain boundaries. 73 Fig. 5-3-4. The fractograph of the MC3 sample after hydration for 24 hours. During further hydration to 38 hours, the MOE decreased to almost the initial value and the grain boundaries of magnesia particles were weaker. "Dusting" was observed on magnesia particles. Fig. 5-3-5a showed the fracture surface of the MC3 sample after hydration for 38 hours. In order to more clearly observe the "dusting", the SEM image of a polished sample is shown in Fig. 5-3-5b. After this point, "dusting" would occur through the whole brick, MOE continued to decrease with hydration, and the sample would lose its physical integrity. It is not clear why "dusting" takes place when the MOE reaches around its initial values yet. 74 Fig. 5-3-5. SEM images of the sample MC3 after hydration for 38 hours in steam at 109°C, (a) the fracture surface and (b) the polished sample. 75 The apparent porosity was tested at the different hydration time points mentioned above. The results were presented in Table 5-3-2. The results indicated that when the apparent porosity decreased to the lowest value of 13.0%, the MOE reached the maximum value. Table. 5-3-2. Apparent porosity of MC3 after hydration in steam at 109°C Hydration Time, hrs 0 8 24 38 Apparent Porosity, % 13.6 13.0 13.9 14.9 MOE, GPa 35.3 40.6 38.0 35.5 MOE Variation, % 0.00 15.1 7.70 0.48 Although the film of brucite, cracking, and porosity affect the MOE, it is difficult to distinguish the effect of each of them on the MOE separately. The MOE variations are caused by their convolution results. The single mechanism of the influences will be studied in the future. 5.4. Influence of Hydration on Pore Size Distribution The pore size distributions (PSD) versus hydration time presented in Figure 5-4-1 are curves of bi-modal type (at 0.42|im and at 2.5u.m for as-received sample), which maintain their shape during the hydration process, with changes only in the median pore diameter. Brick MC3, used here as an example, had 10% of 0.3um diameter pores before hydration, with the smaller pores mainly distributed at 0.42um and larger pores at 2.5um. After 8 hours of hydration in steam at 109°C, the 0.3 urn pores disappeared and the number of pores of 0.35um, 0.4um, and 0.5um increased. After 16 hours of hydration there were no pores smaller than 0.35um and the number of those of 0.4um, 0.42um, and 2.5um slightly increased. The disappearance of the small pores and the relative decrease 76 of the large pores were considered the result of brucite deposition in the open pores. The increase in MOE at the beginning of hydration was definitely related to the disappearance of these small pores. Going back to the MOE variation with the hydration time (Fig. 5-2-6a), brick MC3 started to show a decrease in MOE after 8 hours and reached the initial value of the unused brick after 38 hours. A continuous increase in the number of pores of 0.4-0.5 urn after 24 hours and a jump to larger pores (0.7|j.m) after 38 hours were observed in Figure 5-4-la. This behavior could be attributed to the occurrence of the massive cracking at the grain boundaries. For the coarse pores (larger than lum), there was a continuous increase in the number of pores of ~ 2.5 (im, accompanied by the appearance and the increase of the number of pores larger than 10 ju.m (Figure 5-4-1), which did not exist in the brick before hydration. 77 8 Hydration Time, hrs _ - o - _ o A-24 8 16 2.5 5 7.5 10 25 100 Pore Size, um b) Fig. 5-4-1. Variation of PSD for different hydration periods in steam at 109°C for brick 5.5. Influence of Hydration on Permeability Permeability was measured after each hydration cycle and the changes during hydration are presented in Fig.5-5-1 for magnesia-spinel and magnesite bricks. It can be seen that for all the bricks, the permeability decreased at first to a minimum value, and then showed a rapid increase. Comparing Fig. 5-5-1 and the changes in MOE in Fig. 5-2-6, permeability reached a minimum when the MOE was at its maximum. The increase in MOE corresponded to the decrease in the permeability, and vice versa. The decrease in permeability was related to filling with brucite inside the open pores, MC3. (a) smaller than lum pores and (b) larger than lum pores. 78 resulting in reduction in porosity. After the maximum thickness of the brucite film is reached and cracking.occurs along the grain boundaries, permeability gradually increases until "dusting" occurs. Then macrocracking through the grain boundaries appears, causing the sharp increase in permeability. The change in the permeability for magnesite-chrome bricks at 109°C was presented in Fig. 5-5-2. For magnesia-chrome bricks, the changes in permeability were also well correlated with the opposite changes in MOE (Fig. 5-5-2 and Fig. 5-2-6). These results confirm the mechanism described above. Hydration Time, hrs Fig. 5-5-1. Permeability versus hydration time for brick MSI, MS2, M l , and M2 in 98% relative humidity at 80°C. 79 1000 800 600 400 a> CH 200 0 • MCI * MC4 - MC6 -+-MC3 i * T T , J T T 1 12 24 36 48 60 72 Hydration Time, hrs 84 Fig. 5-5-2. Permeability versus hydration time for brick MCI, MC3, MC4 and MC6 in steam at 109°C. 5.6. A Simplified Kinetic Analysis During the kinetic analysis, the results are considered on the basis that the reaction takes place in the open pores, pores are prefect cylinders and distributed uniformly, and the reaction proceeds by propagation of a reaction front. The hydration model, which I have named "cylindrical pore model", is shown in Fig. 5-6-1, and the original radius and length are ro [cm] and Lo [cm], respectively. Other models have been considered, but so far have yielded results in less satisfactory agreement with the experimental data. If the reaction is limited by. the rate of chemical reaction at an interface, the basic kinetic equation could be of the following form: 80 dmldt = u-S (5-4) where m = reacted MgO in mass around a pore at time t [hr], g, u = reaction rate by weight per unit area, g/hr/cm"2, S = reaction area, cm2, For a cylindrical pore of radius r at time t and the density of magnesia particle pMgo, Eq. (5-4) becomes 2xpMgOL0r(dr I dt) = InL^ru (5-5) Integration between the original ro and r, t from zero and t gives r-r0 =(u/pMg0)t (5-6) 81 If a is the fraction of hydrated MgO, and w [g] is the average MgO mass around each pore, then, a = n(r2 - r2)L0pMgO I w (5-7) According to the definition of apparent porosity, we obtain the following equations P = NV0/Vsamp/eor N = P- Vsample/V0 (5-8) where P = apparent porosity of a sample, %, N = number of open pores in the sample, Vmmp\e= volume of the sample, cm3, Vo = nro Lo, the original volume of each open pore, cm . Because only the open pores affect hydration, the average MgO mass around each pore w is obtained from Equation (5-8): W = Vsample • Pbrick ' ^ = ™tUP brick ' P (5"9) Substituting Eq.(5-9) into Eq.(5-7) gives a = {(rlr0)2-\)(pMgO-Plpbrick) If let k,= P h r i c k ,then PMSQP 82 r/r0 = (1+kia) 1/2 (5-10) Combining Eq.(5-10) and Eq.(5-6) gives R,(a) = (l+k,a),/2 -1 = (ulp r0)*t = KH (5-11) where K = u/pMOr0 is the rate constant, which is dependent on the hydration temperature. Eq.(5-ll) is the rate equation controlled by the chemical reaction for the hydration with the "cylindrical pore model". If the diffusion controls the hydration, under isothermal conditions, the growth rate of the interface layer is given with a good approximation by the parabolic relationship for the initial stage of the reaction, for which the equation can be written as follows [50]: d = (DTt)'/l (5-12) where 5 [cm] is the thickness of the reaction product layer (brucite), and DT [cm2/sec] is the diffusion coefficient at temperature T, and t [hr] is time. From this hydration model (Fig.5-6-1), the thickness of the reaction product, S, can be expressed by S = r0- [(l+k,a)l/2-((l+kia)- (l-k2) + k2)'/2J (5-13) • with _ P brick PMSQP and k2 = 83 where a = fraction of hydrated MgO, r0 = the original radius of pores, cm, Pbrick - the density of the brick, g/cm3, PMgo = the density of magnesia, g/cm3, pbrucite = the density of brucite, g/cm3, P - the apparent porosity of the brick, %, MMgo = the molar mass of MgO, g/mol, MMg(OH)2 - the molar mass of Mg(OH)2, g/mol. Substituting Eq.(5-13) into Eq(5-12) gives R2(a) = [(l+k,a)1/2 - ((l+k,a)- (l-k2) + k2)l/2J2 = (DT/r0) t = K- t (5-14) where K - Dj/ro is the rate constant, which is also dependent on the temperature. Eq.(5-14) is the rate equation for the hydration controlled by the diffusion through the product layer within the "cylindrical pore model". The activation energy and rate of a reaction are related by Arrhenius equation: K = A*exp(-E/RT) (5-15) where K = the rate constant, 1/hr, A = frequency factor, E = the activation energy, J/mol, R = gas constant, 8.314 J/K»mol, 7= absolute temperature, K. 84 Activation energies are determined by plotting the logarithm of K against l/T on a graph, and determining the slope of the straight line that best fits the points. In the kinetic analysis, brick M l was chosen as an example, because it had the high content of MgO and low CaO/Si02 ratio, meaning that calcium oxide has little influence on the hydration of magnesia, and assume that the weight increase is obtained only by the magnesia hydration. The discussion in Chapter 5.1 indicated that there are three stages of hydration in terms of the weight increase. The first stage of relatively fast hydration is followed by the second stage with the slower hydration rate. The third stage is a fast hydration. The hydration duration for each stage depends on the hydration rate of a specific brick. The three stages are controlled by different hydration mechanisms. Studying the first two stages is more important to understand the hydration process, as the third stage corresponds to the "dusting" process. For M l there was no third stage because of a low hydration rate. Therefore, the first two stages will be analyzed in this section. In the mathematical analysis of the results of this work, Eq. (5-11) and Eq. (5-14) were the best adapted to the data of the first and the second stage, respectively, for brick M l . The curves of the M l experimental data are shown in Fig.5-6-2. The slopes of the straight lines of R/(a) and R2(a) versus time t are the rate constants of the M l brick hydration under different conditions for the first two stages. The reaction rates and R-square are presented in Table 5-6-1. 85 Fig. 5-6-2. Hydration rate equations, Rl(a) and R2(a), versus time, t, for Ml samples. (a) the first stage of hydration in steam at 121, 109, and 100°C, (b) the first stage of hydration in humidity at 100, 80, and 60°C, (c) the second stage of hydration in steam at 121, 109, and 100°C, (d) the second stage of hydration in humidity at 100, 80, and 60°C. 86 Table 5-6-1. The reaction rate constants, K [1/hr], for M l samples. Conditions Stage I Stage II y^-lO 5 R-Square ^ • l O 5 R-Square Steam 121°C 180 0.990 4.47 0.984 109°C 113 0.972 2.35 0.996 100°C 96.2 0.975 1.55 0.993 Humidity 100°C 96.2 0.975 1.55 0.993 80°C 48.4 0.999 1.32 0.993 60°C 8.40 0.994 0.19 0.998 The calculation showed that the rate constant in the first stage was 30 ~ 60 times higher than those in the second stage. During the first stage of hydration, the chemical reaction is the governing process, since at the beginning of hydration the surfaces of the magnesia particles are relatively "clean". The water molecules can directly contact the magnesia surface and hydration occurs easily. With the film of brucite formed on the pore wall, for M l there was about 0.2% of MgO hydrated when the second stage started. During the second stage, the water molecules had to diffuse through the film of brucite to continue the reaction, making the hydration much slower. This stage is controlled by the diffusion process. The MSI showed clear three stage curves of the weight increase versus hydration time. After examining all equations it was found that Equations (5-11) and (5-14) were the best suited to analyze the first and second stage, respectively. The rate constants are calculated and are presented in Table 5-6-2. The experimental data for the samples MS2 and M2 are best conformed to Equation (5-11). It is indicated that the hydration of MS2 and M2 are processes controlled by chemical reaction. The calculated rate constants for MS2 and M2 are shown in Table 5-6-2. 87 Table 5-6-2. The reaction rate constants, K*103 [1/hr], of MSI, MS2 and M2 samples in steam and humidity at different temperatures. Conditions MSI MS2 M2 Stage I Stage II Steam 121°C 17.1 2.12 36.3 27.3 109°C 14.1 1.46 17.2 14.6 100°C 9.41 0.61 14.6 9.33 Humidity 100°C 9.41 0.61 14.6 9.33 80°C 2.54 0.40 4.51 5.95 60°C 0.28 0.01 0.34 0.44 Comparing the rate constants at 121°C, the following order of hydration rate is obtained: MS2>M2>MS1 >M1 Equation (5-14) is the best equation to fit the experimental data for all the magnesia-chrome bricks after the second cycle of hydration. This means that after the second cycle, hydration was controlled by diffusion, which is proved by the results of the weight increases and the M O E variations. During the first cycle, the largest weight increase and highest values for M O E were achieved. The calculated rate constants are shown in Table 5-6-3. Table 5-6-3. The rate constants, K-104 [1/hr], for the magnesia-chrome bricks in steam. Temperature MCI MC2 MC3 MC4 MC5 MC6 130°C 2.14 2.70 28.4 86.2 (146°C) 4.57 15.9 121°C 2.02 2.55 20.2 12.2 3.67 15.0 109°C 0.13 0.74 9.20 2.60 1.01 1.80 The hydration of magnesia-chrome brick was much slower than that of magnesia-spinel and magnesite bricks, and the reason is probably that the periclase (MgO) content in 88 magnesia-chrome brick is less than that in the other types of bricks. The effects of temperature on the hydration are different. For example, at 121°C, for M C I the hydration rate constant was 14 times higher than at 109°C, but for M C 2 and M C 3 , the hydration rates increased only 2-3 times. The order in terms of the rate constants from high to low at 109°C is: M C 3 > M C 4 > M C 6 > M C 5 > M C 2 > M C I . This order is consistent with that of the CaG7SiC>2 ratio from high to low for the same M g O content; also at low M g O contents there are low rate constants. -4 -• -14 T — 1 1 1 1 1 — — I 2.5 2.6 2.7 2.8 2.9 3 3.1 in x io3, K 1 Fig. 5-6-3. The plots of ln AT versus 1/Tfor sample M l . The activation energies can be calculated using the logarithm form of Arrhenius equation (5-15) from the plots shown in F ig. 5-6-3 and are listed in Table 5-6-4. The difference is '89 probably caused by the chemical compositions, microstructures, and the physical properties, such as porosity and permeability. Informatively, other values for activation energy for the hydration reaction of magnesia from other studies performed by other researchers are shown in Table 5-6-5. Table 5-6-4. Activation energies, E (kJ/mol), for the hydration of all the experimental bricks in steam and humidity. MgO and MgO-Spinel Bricks MgO-Cr 2 0 3 Bricks M l Stage I 52.5 MCI 178 Stage II 50.5 MC2 82.7 M2 68.5 MC3 69.5 MSI Stage I 73.8 MC4 123 Stage II 88.9 MC5 94.6 MS2 80.6 MC6 140 Table 5-6-5. Activation energies from Literature for the hydration of magnesia Reference: Activation Energy, kJ/mol Type of MgO Layden & Brindley (1963)1" 151 72.7 Sintered magnesia (at 1800°C) Bratton & Brindley (1965)r231 67.3 Sintered magnesia (at 1800°C) Smithson & Bakhshi (1969)[12] 58.9 Caustic magnesia Fruwirth et al (1985)r71 70.6 MgO single crystals (100-200um) Kitamura et al (1996)[91 65.5 Single crystalline magnesia 5.7. The Mechanism of Hydration of Magnesia-Based Refractory Bricks From the discussion above, the mechanism of hydration for magnesia-based refractory bricks is suggested. The hydration occurs in three stages, and each stage is governed by different process. 90 Initially water or water vapor goes into open pores of bricks and a layer of water forms on the walls of pores. The water reacts with magnesia and the brucite nucleates and grows. A film of brucite consisting of intercrossed crystals forms on the walls of pores and works as a reinforcement for the brick structure, resulting in an increase in modulus of elasticity, which corresponds to an increase in the thickness of the film. During the first stage, the chemical reaction is the governing process, since at the beginning of hydration the surfaces of the magnesia particles are relatively "clean". The water molecules can directly contact the magnesia surface and hydration occurs easily. With the hydration progress the MOE increases accompanying the thickening of the brucite film, which slows down the hydration rate. In the second stage, the hydration reaction is controlled by the diffusion of water through the brucite film. When the film exceeds the "critical thickness", cracks form in the film because the stress caused by its shrinkage during drying between hydration cycles exceeds the maximum strength, and M O E begins to decrease. When hydration takes place at periclase grain boundaries, brucite weakens the bonding of periclase grains, and the M O E further decreases. When the M O E decreases to the initial value, the third stage starts, and cracks gradually accumulate and link, leading eventually to the total destruction of the bricks' integrity. The phenomenon is known as "dusting". 91 6. CONCLUSIONS In this research, the hydration of magnesite, magnesia-spinel, and magnesia-chrome refractory bricks in 98% relative humidity, water, and steam environment at temperatures between 60 and 130°C has been studied. The main conclusions from this research are the following: 1. The chemical composition of a magnesia-based brick, especially CaO/Si02 ratio, has a strong effect on its hydration, and increasing this ratio will cause an increase in the hydration rates. For example, 0.26% of MgO hydrated in the magnesite brick of CaO /S i0 2 = 1.8 in water at 60°C for 12 hours, but 0.85% of MgO hydrated when CaO/Si02 = 7.5. It was also confirmed that a uniform distribution of high-in-silica glass phases would protect against hydration. The hydration rate increases with periclase (MgO) content. 2. The hydration rate increases with temperature. For example, only 1.18% of MgO hydrated in the M2 brick (magnesite brick of 96.5%MgO) in steam at 100°C for 4 hours, but 4.15% of MgO hydrated at 121°C during the same time. Kinetically, for the M2 brick, the hydration rate constant at 100°C is only one-third of that at 121°C, and they are 0.009 [1/hr] at 100°C and 0.027 [1/hr] at 121°C, respectively. 3. During the course of hydration, a film of brucite forms and modulus of elasticity (MOE) of a magnesia-based refractory brick increases from the initial value MOEo, to a maximum M O E m a x , corresponding to the increase in the film thickness, followed by a relatively slow decrease. When modulus of elasticity decreases below the initial value 92 MOEo, the sample progressively disintegrates. The maximum value of modulus of elasticity corresponds to a "critical thickness" of the brucite (MgCOFfh) film. When the brucite film exceeds the "critical thickness" it cracks, the grain boundaries weaken, and consequently the M O E begins to decrease. The cracks gradually accumulate and link, leading eventually to the total destruction of the bricks' integrity. This phenomenon is known as "dusting". For example, modulus of elasticity for brick MC3 (magnesia-chrome brick of 62.0 % MgO and 18.0 % CV2O3) increases from 35.3 GPa (i.e. for initial, un-hydrated state; hydration film thickness is zero), to the maximum of 40.6 GPa when the "critical thickness" of the brucite film reaches about 1 um. 4. The study of the influence of hydration on pore size and pore size distribution indicates that the hydration starts with filling the small pores, below 0.42um with the product of hydration (i.e. brucite), consequently, relatively increasing the number of the larger pores, for example, pores of 0.5um increases from 7.5% (as-received sample) to 11.2% (hydration for 8 hours). After hydration takes place at periclase grain boundaries, microcracking occurs at grain boundaries and the microcracks density increases with hydration progress. Therefore the pore size distribution shifts to larger sizes. For example, the peak value of the pore size distribution for brick MC3 shifted from 0.42um (hydrated for 8 hours) to 0.50um (24 hours), further to 0.75 um (38 hours). 5. The brick permeability to gas decreases with hydration in the early stage. However, after "dusting" occurs, the permeability increases rapidly. During hydration, the permeability of brick M2 increases by factor of 14, i.e. from 72 to 1032 milliDarcys. 93 6. The hydration mechanism of magnesia-based refractory bricks is proposed. The hydration occurs in three stages, and each stage is governed by different process: I. During the first stage the brucite film forms and the hydration progress is controlled by chemical reaction between M g O and water. For example, for brick M l (magnesite brick of 93.0% MgO) , the rate constant is 1.80xl0" 3 [1/hr] in steam at 121°C in Stage I and activation energy is 52.5 kJ/mol. II. In the second stage, hydration slows down. The hydration progress is now controlled by diffusion of water through the brucite film. For example, for the M l brick, the second stage starts when there is about 0.2% M g O hydrated and the rate constant is 4.47x 10"5 [1/hr] and activation energy is 50.5 kJ/mol in steam at 121°C in Stage II. III. The third stage starts when "dusting" takes place. Due to the formation of new surfaces accessible to water through cracks at grain boundaries, hydration accelerates again, and eventually leads to total disintegration of the refractory brick. 7. The modulus of elasticity measurement by the impulse excitation technique can be used as a relatively simple nondestructive method to assess the hydration degree of magnesia-based refractory bricks. 94 7. FUTURE WORK This work is an initial stage of the larger study of the hydration of magnesia-based refractory bricks. For further studies are recommended as follows: 1. A qualitative relationship between the hydration degree and modulus of elasticity determined by the impulse excitation technique has been established. However, for more accurate evaluation of the degree of hydration by the non-destructive method of impulse excitation technique, the quantitative correlation between the degree of hydration and the modulus of elasticity must be developed. 2. The brucite film affects modulus of elasticity, so do porosity and cracking. It is important to understand the relationship between the characteristics of the brucite film, porosity, cracking and modulus of elasticity. 3. Hydration of basic bricks depends on their products. Studying the effect of microstructure, such as pores size and pore distribution, on hydration is necessary to improve our understanding of this process, and thus to enhance the hydration resistance of basic bricks. 4. The ultimate goal of this research is to develop solutions to suppress the hydration of magnesia-based refractories. Future work must therefore include careful evaluation of all possible factors, including those related to installation and in-service process parameters. 95 8. REFERENCES 1. H. He and G. Oprea, "Test Results on Brick Smaple for Impala Platinum Furnace," Technical Report, UBCeram, unpublished, (2001). 2. E. G. 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