| Hydratable alumina(HA)is a binder applied for refractory castables,which can supply the bonding strength for castables due to its hydration to form the hydrates of boehmite(β-AlOOH)and bayerite(β-Al(OH)3)at room temperature.These hydrates can also transform into α-Al2O3 at high temperatures.Besides,formation of low melting point phases,such as anorthite and anorthite,via reaction with Al2O3 and SiO2 in castables or slag at high temperatures can be avoided,because there is almost no CaO in this binder.However,potential wide applications of HA as binder are negatively restricted by its slow development of curing strength and probable collapse issue of HA-bonded castables at the medium temperature heating stage.Previous researches reported that strength deduction of the castables bonded by calcium aluminate cement(CAC)at the medium temperatures is not oly related to the decomposition of CAC hydrates,but also is closely associated to their structure collapse.However,there is no researches to clarify the strength change of HA-bonded castables at medium temperatures is also not only related to the decomposition of HA hydrates,but also to their corresponded structural evolution.It is also lack of systematically comparative researches to demonstrate whether CAC-and HA-bonded castables can show different slag resistance properties when they were used in the ladle lining for stainless steel refining,because there may be differences on pore size distribution in the castable matrix,as well as differences of the content and viscosity of the liquid formed at the interface between castables and slag at high temperatures.In addition,it is normally expected short curing and drying time for the production of HA-bonded pre-cast shapes.But,there is no in-depth research to reveal the difference of hydration rate of HA during different curing and drying stages.To clarify the effective curing and drying time which can significantly improve the strength of castables,relationship between hydration characteristics of HA in the curing and drying stages and strength changes of the HA-bonded castables was systematically investigated.Then,effects of phase decomposition and microstructural evolution of HA hydrates on the strength change of HA-bonded castables at medium temperatures were researched.Accordingly,the correlation between properties of the castables and phase and microstructure evolution of the HA hydrates was established.Lastly,corrosion mechanism of HA-bonded castables by the stainless steel refining slag was analyzed by investigating the relationship between slag resistance of HA-bonded castables and the pore size distribution in its matrix,as well as amount and viscosity of liquid formed at the interface between castables and slag.1.Hydration Rates of HA in Different Curing and Drying Stages of Castables.The characteristics of HA hydration behavior at 20℃ during the 72 h curing process,especially the difference of hydration rate between the early and the late stage,and their influence on the strength of castabels are researched.The results show that the X-ray diffraction intensities corresponed to the hydrates of β-AlOOH andβ-Al(OH)3 increased rapidly in the early sage within 18 h.Besides,velocity of the ultrasonic wave through the castables significantly increased from 350 to 5000 m/s.It means that amount of β-AlOOH and β-Al(OH)3 increased rapidly in the early stage.In the late stage of curing(18～72 h),intensities of X-ray diffraction peaks ofβ-AlOOH and β-Al(OH)3 did not increase remarkably with the prolongation of curing time,and velocity of ultrasonic wave through castables only increased from 5000 to 5150 m/s.These results demonstrate that there was an insignificant hydration in the late stage.The reason for such behaviour is that there is a thick coating layer on the surface of HA particles was gradually formed after the early curing stage,which prevented the contact between HA and water.Accordingly,the strength of castables was rapidly increased to 3.7 MPa during the early stage within 18 h,but the strength was only improved to 4.0 MPa in the late stage.Therefore,the effective curing time to significantly improve strength of the HA-bonded castable is mainly the early curing period.The characteristics of rehydration of HA during the 110℃ drying process and its influence on the strength of the castabels were investigated.During the early drying stage before free water was completely evaporated(0～12h),intensities of X-ray diffraction peaks corresponed to the hydrates of β-AlOOH and β-Al(OH)3 increased rapidly,indicating hydration degree of the residual HA was further drastically increased,and strength of the castables was significantly improved from 3.8 to 9.7 MPa.However,during the late drying stage(12～48 h),intensities of the X-ray diffraction peaks of β-AlOOH and β-Al(OH)3 did not increase significantly with the increasing of drying time,and strength of the castables only increased from 9.7 to 9.8 MPa after drying for 48 h.This is caused by that there is no free water and little amount of residual HA,resulting in no clear increasing of hydrates.Therefore,the effective drying time to remarkably improve strength of the HA-bonded castable is mainly the early drying period.2.Microstructure and Phase and Evolution of HA Hydrates and its Effect on the Strength of the HA-bonded Castables at Medium TemperaturesThe relationship between phase and micro structure evolution of HA hydrates and strength of the castable at medium temperatures was revealed.The dehydration of the HA hydrates of β-AlOOH and β-Al(OH)3 takes place at approximately 260℃,but strength of the castable after drying at 110℃ and 400℃ are 13.6 MPa and 13.2 MPa,respectively.The results show that strength of castables did not decrease obviously with the temperature increasing from 110 to 400℃,indicating that decomposition of the hydrates has no clear effect on the strength drop of the castables.Structure of the dehydrated hydrates starts to collapse after firing at 600℃,therefore,strength of the castable significantly decreased to 5.1 MPa accordingly.Structure of the dehydrated HA hydrates collapses completely after firing at 1000℃,resulting in strength of the castables remarkably decreased to 1.1 MPa.Moreover,the strength of the castables after firing at 1000℃ can not be improved by increasing HA content.Therefore,strength reduction of the HA-bonded castable at medium temperatures is not only related to the phase decomposition of HA hydrates,but also is closely associated with their structural collapse of the dehydrated HA hydrates.It is found that the dehydrated CAC hydrates of 3 CaO·Al2O3·6H2O(C3AH6)and Al(OH)3(AH3)still can exhibited a certain bonding strength after their structure collapse at 800℃.However,structure of the dehyrated HA hydrates show completely catastrophic collapse at 1000℃,resulting in the lowest strength of the HA-bonded castables(1.7 MPa)is significantly lower than that of CAC-bonded castables(7.7 MPa).Therefore,due to the differences of the temperature point and collapse degree of the dehydrated HA hydrates,then the lowest strength values and corresponed temperature points are different between HA-bonded and CAC-bonded castables.3.Relationship between Phase/Microstructural evolution and properties of the HA-bonded castable at high temperaturesRelationship between slag resistance and phase composition and microstructure of HA-and CAC-bonded Al2O3-MgO castables at high temperature was investigated.After the characterization of the microstructure and thermodynamic calculation of the HA-bonded and CAC-bonded castables at high temperatures,it is achieved that the median diameter(D50)of the pore size of the matrixes corresponded to CAC-bonded and HA-bonded castables after firing at 1600℃ are 11.52 μm and 9.48 μm,respectively.It reveled that pore size distribution of HA-bonded castables is smaller than that of CAC-bonded castables,which is beneficial to prevent the diffusion of the refining slag into the castables matrix.In addition,comared to CAC,there is almost no CaO in the HA binder,this not only can reduce the liquid amount at the reaction interface between castables and the stainless steel slag,but also can further improve the liquid viscosity at 1680℃.Therefore,the HA-bonded castables show better slag resisitance than that of CAC-bonded castables. |
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