Microsilica Addition as an Antihydration Technique for Magnesia-Containing Refractory Castables

Microsilica Addition as an Antihydration Technique for Magnesia-Containing Refractory Castables Microsilica addition, which generates a protective magnesium silicate coating, is used as a magnesia antihydration technique to improve castable properties by decreasing volumetric expansion and, thus, microcracking. Rafael Salomão and Victor C. Pandolfelli Magnesia additions in refractory castable compositions are usually limited because of the expansion that follows their hydration. 1,2 This expansion is related to the difference of density between magnesium oxide (3.5 g/cm 3 ) and its hydroxide (2.4 g/cm 3 ), which leads to the rupture of the shaped material. 2 4 Because of the technological importance of magnesia-containing castables, various works in the literature describe magnesiaantihydration techniques (MAHT) as a possible solution to this problem. 5 12 MAHTs have been developed to avoid the damage of magnesia hydration by halting its reaction. Recent work has shown that, if magnesia hydration is minimized during the mixing, curing and drying steps, castable mechanical properties can be highly improved. 4 Some of the most important MAHT mechanisms are coating of the magnesia particle surface by hydrophobic barriers (such as organosilicon-based polymers 5,8 or a thin alumina film applied by plasma sputtering), 10 adding impurities (such as CaO, Ba 2 or Fe 2 ) during calcination, 11 controlling the drying step 4 and adding microsilica (SiO 2 ) to the compositions. 7,9,12 Good results have been attained using the first technique when in the magnesia is a powder. However, there are significant drawbacks when applied to refractory castables, such as higher costs and large-scale application difficulties. Adding impurities, on the other hand, usually decreases the refractoriness of the product. The drying-step control has been explored recently and has presented good results for sintered-magnesia-containing castables. 4 However, drying-step control did not offer significant protection during the curing step. Microsilica addition, on the other hand, is one of the most explored MAHTs in refractory castables because of its low cost and effectiveness. Magnesia- and Microsilica-Containing Castables Various works have reported application of the MgO SiO 2 and Al 2 MgO SiO 2 systems in refractory castables. 7,9,13 19 The best documented effects of microsilica addition are increase in flowability of castables 18 and softening effect that compensates the disruptive expansion of spinel formation. 16,17 In the first case, because of its spherical shape, microsilica particles promote a ball-bearing effect, which decreases castable particle friction and eases flowability. 18,20 In the second case, the presence of a small content of low-melting-temperature phases (such as cordierite, 2MgO 2Al 2 5SiO 2 ) allows for a better accommodation of expansive tensions generated by in-situ spinel formation in the castable structures. 16 Another important aspect is the binding effect that the association of microsilica and magnesia presents. 19,21 This mechanism usually is described as the building up of a gellike hydrated magnesium silicate compound (MgHSiO 4 nh 2 O) between magnesium oxide or hydroxide and microsilica or colloidal silica parti- American Ceramic Society Bulletin, Vol. 86, No. 6 9301

Fig. 2 Aspect of the magnesia-containing castables with various amounts of microsilica (after 7 d at 50 C in humid environment). cles. 7,18,19,22 This reaction decreases the magnesia hydration rate 12 and, in some cases, can be used to improve the consolidation of the castable without the use of other binders. 19 Nevertheless, the peculiarities of this mechanism remain properly unexplored because of experimental difficulties and technological interests involved. The present work is based on recent progress attained in magnesia hydration behavior. 3,4 It uses a systemic approach to evaluate the impact of microsilica addition on magnesia hydration. Using flowability and hydration dehydration tests, 4 mechanical strength and apparent volumetric expansion (AVE) measurements, 3 drying-rate profiles 23 and X-ray diffractometry (XRD), the authors propose a protection mechanism based on the solubility behavior of silica. 24 The benefits and limitations of this antihydration technique also are presented. Castables Preparation A vibratable high-alumina refractory castable composition that contained 6 wt% of sintered-magnesia (D 50 = 15 µm, Magnesita S.A., Brazil), 6 wt% of calcium aluminate cement (CAC; CA14M, Almatis, United States), 5.5 wt% of water and 0.25 wt% of poly(ethylene glycol)-based dispersant (Bayer, Germany) was used in the tests (Table I). Microsilica (971-U, Elkem, Norway) was added to the castable formulation in various amounts (0.25 2 wt%). Magnesia-free and magnesia-free/microsilica-free (replaced by calcined alumina) reference compositions also were tested. Castable mixing and water addition were conducted in a paddle mixer for 10 min. At the end of this time, the flowability of the castables was measured after 60 s of vibration. Compositions were cast under vibration into 40 40 mm cylindrical molds for drying and hydration tests and mechanical strength measurements and into 70 70 mm cylindrical molds for AVE evaluation. 3 The initial curing time was done in an acclimatized chamber (Model 2020, Vöetch) at 8 C for 24 h. These conditions were used to ensure there was a minimal mechanical strength of the samples for cement hydration and demolding 25 with no significant magnesia reaction. 3 The hydration tests were conducted in samples exposed at 50 C in a humid environment for 7 d. During this period, the mechanical strength, dehydration profile and AVE of the samples were evaluated every 24 h. The splitting tensile strength was obtained (TestStar II, MTS) according to Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens, ASTM Designation C496-96. ASTM Book of Standards, ASTM International, West Conshohocken, Pa. A constant loading rate of 40 N/s (1000 kpa/min) was used in samples previously fired at 350 C for 5 h at a heating rate of 10 C/min. The drying tests were conducted using green humid samples up to 600 C at a 10 C/min heating rate in a thermogravimetric apparatus developed by the authors research group. 21,25 Mass variations and temperature profile inside the furnace and at the sample surface (a thin K-type thermocouple was placed in the halfheight of the samples, 1 mm beneath their surface) were recorded simultaneously. The amount of water loss (W, in percent) up to a certain temperature or during a specific drying stage represented the cumulative fraction of water released up to this point divided by the total amount of water initially present in the body. The W parameter and its time derivative (dw/dt, in %/min), which represents the dehydration rate, were calculated using the following equations: W(%) = 100[(M 0 M)/(M 0 M F )] (1) 9302 American Ceramic Society Bulletin, Vol. 86, No. 6

(dw/dt) i = (W i+10 W i 10 )/(t i+10 t i 10 ) (2) where M is the instantaneous mass recorded at time t i during the heating stages, M 0 the initial mass and M F the final mass of the tested sample. The AVE test consisted of measuring the dimensions of a 70 70 mm cylindrical sample during humidity exposure condition for 7 d. 3 The AVE was calculated considering the initial volume of the samples as a reference and using the following equations: V i = [H i π (Di 2T) 2 ]/4 (3) Drying rate dw/dt (at 10 C/min heating rate) (%/min) AVE = 100[(V E V 0 )/V 0 ] (4) where V i is the volume, H the height, D the diameter, T the mold-wall thickness, V 0 the initial volume of the sample and V E the corresponding volume after hydration Time (min) and expansion. Equation (4) is used to calculate the volume of cylindrical samples at a particular time. A detailed at 8 C) and (b) after (7 d at 50 C) magnesia hydration. Fig. 3 Drying-rate profiles for microsilica-containing castables (a) before (24 h description of this technique and its use can be found in the literature. 3 To identify the chemical compounds formed during the magnesia hydration in the presence of microsilica, sintered-magnesia aqueous suspensions containing various amounts of microsilica ( 10 wt%), 25 wt% of water and 1.5 wt% of poly(ethylene glycol)-based dispersant were prepared. The mixing, casting and curing steps of the suspension samples were conducted under the same conditions used for the castables. The samples then were dried overnight in silica gel, ground-milled (D Part < 45 µm) and characterized using XRD (CuKα, nickel filter, Model D 5000 Kristalloflex 710, Siemens). These results also were presented as the relative intensity of the main diffraction peak of each compound identified. Effect of Microsilica Addition Microsilica addition decreases the impact of magnesia hydration, as evidenced by AVE (Fig. 1(a)) and mechanical strength (Fig. 1(b)) results. For the microsilica-free sample, the green mechanical strength begins to decrease after 3 d of exposure, when the first evidence of AVE is observed. The mechanical strength of the magnesia-free reference samples (Fig. 1(c)) increases continuously with curing time (similar results can be found in the literature 25 ). This behavior is understood to be indicative that tensioning caused by magnesia hydration attains higher levels than the castable mechanical strength. 3 On the other hand, the higher the microsilica content, the lower the AVE values (AVE is not observed for the 1 and 2 wt% microsilica content samples) and the greater the mechanical strength attained after 7 d (Fig. 2). The results of AVE and mechanical strength measurements can be associated with the drying-rate profile of the samples before (Fig. 3(a)) and after (Fig. 3(b)) they are exposed to hydration. Before magnesia hydration (after 24 h of curing at 8 C), the main difference between the microsilica-free and microsilicacontaining sample drying-rate profiles is a small decrease in the intensity of the peak related to water ebullition (25 40 min or 100 300 C). This effect is a result of permeability decease caused by better particlepacking efficiency promoted by microsilica addition. 13,20,21 The peaks that correspond to the decomposition of the cement and magnesia-hydrated products present low and similar intensities for both samples. This aspect is compatible with the literature, which shows that Sample temperature ( C) American Ceramic Society Bulletin, Vol. 86, No. 6 9303

this curing condition (temperature <10 C) can decrease the cement 25 and magnesia hydration rate. 3 After hydration (7 d at 50 C, Fig. 3(b)), the addition of microsilica changes the drying-rate profiles considerably. The three peaks observed can be described as corresponding to the ebullition of water (110 300 C), decomposition of cement hydrates (300 400 C) and decomposition of Mg(OH) 2 (400 600 C). 3,4 The increase in the microsilica content does not affect the cement hydrate decomposition peak. However, it decreases the Mg(OH) 2 decomposition peak considerably, which indicates that the magnesia hydration rate is decreased for the sample that contains 1 wt% of microsilica and totally halts hydration for samples that contain 2 wt%. Another important consequence of microsilica addition is associated with the increase of the ebullition peak and its shift to higher temperatures. Reports in the literature that concern castable drying behavior describe this effect as typical for low-porosity and less-permeable structures 23,25 and, therefore, without cracks caused by magnesia hydration. The results attained associated with the reference samples (Fig. 1(c)) indicate that the microsilica antihydration mechanism is not based on castable strengthening. Rather, it most likely is based on a chemical interruption of the magnesia reaction with water and the consequent decrease of Mg(OH) 2 generation. To evaluate this hypothesis, these results have been related with silica dissolution behavior 24 (Fig. 4), XRD of the magnesia microsilica suspension after the hydration period (Fig. 5) and castable flowability under vibration (Fig. 6). The silica dissolution behavior as a function of the environment ph is based on the silicon Pourbaix diagrams. 24,26,27 These have been used recently in the study of aluminum and silicon powder as antioxidizing agents in high-carbon-containing castables. An aqueous suspension of SiO 2 (or silicon powder partially oxidized) is stable within a Table I Castable and Sintered Magnesia Suspension Composition Composition Raw material (wt%) Sintered-magnesia-containing refractory castable White electrofused and calcined alumina 86 88 Calcium aluminate cement 6 Sintered magnesia (98 wt% of MgO) 6 Microsilica 0 2 Water 5.7 Dispersant (poly(ethylene glycol)-based polymer) 0.25 Sintered-magnesia aqueous suspension Sintered magnesia (98 wt% of MgO) 90 100 Microsilica 0 10 Water 30 Dispersant (poly(ethylene glycol)-based polymer) 1.5 Elfusa, Brazil. Almatis, United States. Magnesita S.A., Brazil. Elken, Norway. wide range of ph (0 10, i.e., acidic to alkaline) (Fig. 4). 21,26,27 However, when an amorphous and high-surface-area SiO 2 source, such as microsilica, is exposed to a highly alkaline environment (ph of 10 12), such as magnesia- or CAC-containing refractory castables, it partially dissolves and generates silicic acid (HSi ). 21,22,26 If this dissolution occurs in the presence of magnesia, the silicic acid can be strongly attracted to the alkaline Mg(OH) 2 layer that covers the surface of magnesia particles. This generates a poorly crystallized, layered magnesium silicate hydrate, 2 HSi + Mg(OH) 2 + H 2 O MgHSiO 4 2H 2 O (5) The magnesium silicate coating formed presents low solubility in water when in alkaline ph. 21,22 Therefore, it behaves as a hydrophobic barrier that inhibits further magnesia hydration reaction (Fig. 7). This effect is observed in the magnesia microsilica suspension XRD results (Fig. 5). After the hydration period (7 d at 50 C), a mix of anhydrous (periclase, 2θ = 43 ) and partially hydrated (brucite, 2θ = 43 ) sintered magnesia can be observed for the microsilica-free sample. When microsilica is added to the composition, the brucite intensity decreases and the periclase intensity increases to the level observed for the nonhydrated sintered magnesia. Simultaneously, an increase of the peaks related to magnesium silicate (MgHSiO 4 2H 2 O, 2θ = 60 ) protective coat generation also is observed. The low intensity of these peaks is related to the intrinsically poor crystallinity of the magnesium silicate and to the small content formed (a thin coating on the magnesia particle surface). The efficiency of this technique depends on the silicic acid content and, consequently, microsilica available to react with the magnesia to follow the stoichiometry of the reaction (Figs. 1(a), 3(b) and 5). This 9304 American Ceramic Society Bulletin, Vol. 86, No. 6

aspect also presents a close relationship with the impact of microsilica on the flowability of the castables (Fig. 6). Because of the ball-bearing effect, microsilica spherical particles decrease the friction among castable particles, which increases their flowability. 18,20 The optimum microsilica content for a certain castable formulation is attained when its particles are fully covered by the microsilica particles, similar to the coverage of dispersants. Above this content, if the water amount in the formulation is kept constant, such as in this work, the microsilica high surface area decreases the water available to maintain particle separation, which causes a decrease in flowability. For the magnesia-containing or magnesia-free systems tested, the highest flowability values are attained with a microsilica content of 0.5 wt%. Content above this value (at least 1 wt%) is the minimum amount required to generate a significant decrease in AVE values (Fig. 1(a)) and in the magnesia hydration (Fig. 2(b)). Therefore, the microsilica addition behaves properly as an antihydration technique when the amount added to the formulation is enough to cover the surface of all the magnesia particles in the castable matrix. Therefore, it is reasonable to assume that castable compositions with a higher surface area (generated by a larger content of fine particles or by different magnesia sources) require a greater amount of microsilica to fully inhibit magnesia hydration and vice-versa. The minimum content of microsilica needed is defined mainly by the total surface area of the castable and by the amount of magnesia present in the system. Important aspects of this antihydration technique are that It is inexpensive and can be applied easily to the actual magnesia castable formulations after minor modifications; For magnesia amounts <10 wt%, the microsilica content required (1 wt%) might not lead to a major loss of refractoriness; and In alumina/magnesia castables, the microsilica addition can lead to extra benefits that increase the flowability of castables and accommodate the expansive tensions generated by in-situ spinel formation, as described in the literature. 14,16,17 Acknowledgments The authors are grateful to the Brazilian Research Founding FAPESP, Alcoa Alumínio (Brazil) and Magnesita S.A. (Brazil) for supporting this work. About the Authors Rafael Salomão and Victor C. Pandolfelli are faculty members in the Materials Engineering Dept., Federal University of São Carlos, São Carlos, S.P., Brazil. Correspondence regarding this article should be addressed to Victor C. Pandolfelli via e-mail at pers@iris.ufscar.br or vicpando@power.ufscar.br References 1 A. Nishikawa, Technology of Monolithic Refractories, Tech. Rept. No. 33-7, PLIBRICO, Tokyo, 1984; pp. 98 101. 2 A. Kitamura et al., Hydration Characteristics of Magnesia, Taikabutsu Overseas, 16 [3] 3 11 (1995). 3 R. Salomão, L.R. Bittencourt and V.C. Pandolfelli, A Novel Approach for Magnesia Hydration Assessment in Refractory Castables, Ceram. Int. (2007); in press. 4 R. Salomão and V.C. Pandolfelli, Magnesia Sinter Hydration Dehydration Behavior in Refractory Castables, Ceram. Int. (2006); submitted for publication. 5 A. Kaneyasu et al., Magnesia Raw Materials with Improved Hydration Resistance, Taikabutsu Overseas, 17 [2] 21 26 (1996). 6 Y. Koga et al., Effects of Alumina Cement Grade and Additives on Alumina Magnesia Castable Containing Aluminum Lactate, Taikabutsu Overseas, 18 [1] 43 47 (1997). 7 M.M. Ali and A.K. Mullick, Volume Stabilization of High-MgO Cement: Effect of Curing Conditions and Fly Ash American Ceramic Society Bulletin, Vol. 86, No. 6 9305

Addition, Cem. Concr. Res., 28 [11] 1585 94 (1998). 8 S. Chen et al., Effect of Additives on the Hydration Resistance of Materials Synthesized from Magnesia Calcia System, J. Am. Ceram. Soc., 83 [7] 1810 12 (2000). 9 C. Ödegård et al., Magnesia Silica Hydrate Bonded MgO Castables ; pp. 4 7 in UNITECR 01 Proceedings (Cancun, Mexico, 2001). 10 J.H. Eun et al., The Protection of MgO Film Against Hydration by Using Al 2 Capping Layer Deposited by Magnetron Sputtering Method, Thin Solids Films, 435, 199 204 (2003). 11 A. Yoschida et al., Evaluation Method for Hydration Resistance of Magnesia Fine Powder and Effect of B 2 Content in Magnesia Raw Materials ; in UNITECR 03 Proceedings (Osaka, Japan, 2003). 12 K.G. Ahari et al., Hydration of Refractory Oxides in Castable Bond Systems Part II: Alumina Silica and Magnesia Silica Mixtures, J. Eur. Ceram. Soc., 23, 3071 77 (2003). 13 B. Sandberg and T. Mosberg, Use of Microsilica in Binder System for Ultra-Low-Cement Castable and Basic Cement- Free Castables ; pp. 245 58 in Advances in Refractory Technology, Ceramic Transactions, Vol. 4. Edited by R.E. Fisher. American Ceramic Society, Westerville, Ohio, 1989. 14 B. Myhre, Cement-Free Castables in the System MgO SiO 2 : The Effect of Bond Phase Modifiers on Strength ; Presented at the 93rd Annual Meeting of the American Ceramic Society, Cincinnati, Ohio, 1991. 15 B. Myhre and B. Sandberg, Mullite Formation in Tabular Alumina-Based Refractory Castables with Hydraulic Alumina as Binder ; Presented at the 93rd Annual Meeting of the American Ceramic Society, Cincinnati, Ohio, 1991. 16 B. Sandberg and B. Myhre, Castables in the System MgO Al 2 SiO 2 ; pp. 19 22 in UNITECR 95 Proceedings (Kyoto, Japan, 1995). Technical Association of Refractories, Kyoto, Japan, 1995. 17 B. Myhre et al., Castables with MgO SiO 2 Al 2 as Bond Phase ; Presented at the XXVI ALAFAR Congress, San Juan, Puerto Rico, 1997. 18 B. Myhre et al., Flow and Flow Decay of Refractory Castables ; pp. 11 13 in the Proceeding of the 3rd India International Refractories Congress (Calcutta, India, 1998). 19 A. Hundere et al., Magnesium Silicate Hydrate-Bonded MgO Al 2 Castables ; Presented at the 38th Annual Conference of Metallurgists, Quebec, Canada, 1999. 20 I.R. de Oliveira, A.R. Studart, R.G. Pileggi and V.C. Pandolfelli, Particles Packing: Principles and Applications in Ceramic Processing (in Portuguese); pp. 112 16. Fazendo Arte Editora, São Paulo, Brazil, 2000. 21 R.K. Iler, The Chemistry of Silica; pp. 3 104. Wiley, New York, 1979. 22 J. Temuujin et al., Role of Water in the Mechanochemical Reactions of MgO SiO 2 Systems, J. Solid State Chem., 138, 169 77 (1998). 23 M.D.M. Innocentini, F.A. Cardoso, M.M. Akiyoshi and V.C. Pandolfelli, Drying Stages During the Heat-Up of High- Alumina, Ultra-Low-Cement Refractory Castables, J. Am. Ceram. Soc., 86 [7] 1146 48 (2003). 24 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution; pp. 458 63. Pergamon Press, New York, 1966. 25 F.A. Cardoso, M.D.M. Innocentini, M.M. Akiyoshi and V.C. Pandolfelli, Effect of Curing Time on the Properties of CAC-Bonded Refractory Castables, J. Eur. Ceram. Soc., 24 [7] 2073 78 (2004). 26 V.G. Domiciano, J.R Garcia and V.C. Pandolfelli, Hydration Resistance of Silicon Powder in High-Carbon- Containing Refractory Castables, Am. Ceram. Soc. Bull., 84 [10] 31 36 (2005). 27 V.G. Domiciano, I.R. Oliveira, R. Salomão and V.C. Pandolfelli, Hydration of Aluminum Powder in High-Carbon- Containing Refractory Castables, Am. Ceram. Soc. Bull., 84 [5] 9101 9105 (2005). 9306 American Ceramic Society Bulletin, Vol. 86, No. 6

Mechanical strength (diametrical compression) (MPa) Mechanical strength (diametrical compression) (MPa) Apparent volumetric expansion, AVE (50 C, humid environment) (%) Hydration time (d, at 50 C, in humid environment) Fig. 1 Hydration tests (up to 7 d, at 50 C, in humid environment): (a) AVE; (b) evolution of mechanical strength; and (c) reference samples (magnesia-free and magnesia-free/ microsilica-free).

log [(H 2 Si ) + (HSi ) + (Si 2 )] ph Fig. 4 Silica dissolution behavior. 21,24 Arbitrary intensity of the main XRD peak (counts) Microsilica content (wt%) Fig. 5 Phase evolution for magnesia microsilica aqueous suspensions (after 7 d at 50 C).

Flowability under vibration (%) Apparent volumetric expansion, AVE (after 7 d at 50 C) (%) Microsilica content (wt%) Fig. 6 Impact of microsilica content on castable flowability under vibration and AVE. Fig. 7 Schematic view of microsilica antihydration mechanism for magnesia.