Study On High Temperature Properties And Hydration-dehydration Dynamics Of Castables Bonded By Calcium Aluminate Cements Containing Spinel

With the development of metallurgy technology,refractories are suffering damage in worse environment during their service,the traditional corundum-spinel castables are difficult to adapt to the trend.In recent years,it was found that MgAl₂O₄ spinels can be formed during the preparation of calcium aluminate cement.The new kind of SP-CA cement would improve both the resistance to thermal shock and molten slags of refractory castables.The results of this research work will enrich the theory of microstructure design and performance optimization for refractory castables.To begin with,a literature review on SP-CA cements and optimizing design on phase composition and microstructure of calcium aluminate cement is made both at home and abroad.This serves as the historical background of the study.Base on the ternary phase diagrams,cement phase were designed to study the effect on castables.The dynamics of hydration-dehydration for castables bonded by the new cement will be studied.The influence mechanism of phase reaction among dehydration products of castables matrix and mosaic structure of microcrystal MgAl₂O₄ spinels on hot strength and thermal shock resistance of castables will be revealed by theoretic analysis and tests.At last,the crack propration of SP-CA cement bonded castables were studied,and extended finite element method?XFEM?calculations revealed influencing mechanism of submicro spinel on high temperature properties.The results of this research work will enrich the theory of microstructure design and performance optimization for refractory castables.In chapter two,using Industrial alumina,Magnesium hydroxide,dolomite as raw materials,submicron spinel–containing calcium aluminate?SP-CA?cements were prepared at differernt temperature.Control of spinel grain size can be achieved by controlling calcination temperature.For the cement specimens prepared at 1300°C,the spinel phases presented no definite borders or shapes with a diameter about 0.1-0.3?m.In chapter three,the working ability can be optimized by change the cement particle size,spinel content and phase ratio of cement.The spinel content in cements decreased from 72%to 50%,the water addition decreased and strength increased,but with a worse thermal shock resistance.Appropriately increase of CA₂ can accelerate sintering of cements.The work ability can be further optimized by decreasing the cement particle size appropriately.In chapter four,the hydration dynamics of castables bonded by the new cement will be studied,and the impacting mechanism of submicron spinel existence on dynamics have been put forward.As for the decrease of spinel grain size,the hydration rate is higer and K_NG,K_I and K_D are become larger.The Z potential test of SP-CA cements sullry calcined at 1450°C shows that cement particle are negative charge,while as for the spinel grain size decreased to submicron size,the surface charge changed to positive,which will promote the nucleation of CAH₁₀ and AH₃,and then promote the hydration rate of cements.In chapter five,the relativity among cement with mosaic structure,microstructure evolution of castables and their high temperature properties will be established.Both the elastic modulus and mechanical strengths of castable are significantly influenced by its microstructural characteristics,which are determined by heat treatment temperature.As the temperature changes,the evolution of the Young's modulus can be distinguished by seven domains:I:Mild dehydration stage,II:Rapid dehydration stage,III:Relative stable stage,IV:Formation of CA,V:Formation of CA₂,VI:Sintering stage,and VII:Formation of CA₆.In chapter six,different alumina spinel castables technical characteristics were contrasted.The formation of MA begin at 1100°C can increase elastic modulus and strength of castables,but the overexpansion at higer temperature will deteriorate material properties.AS for preformed spenel added in catables,the spinel can not contribute to the formation of CA₆.Small pore size and submicron spinel played relevant roles in corrosion behavior and thermal shock resistance.In chapter seven,the microstructural characteristics and high temperature properties of castables with SP-CA cement addition were investigated.For the alumina-spinel castables after heated,the submicron spinel particles bonded with corundum and hibonite?CA₆?grains could strengthen castable body.However,the large spinel size?<88?m?did not form linkage between spinel and alumina in the castable added preformed spinel?castable AS?.The submicron spinel can reduce pore sizes of castable from 2?m to 0.5?m.Extended finite element method?XFEM?calculations showed that the smaller pores not only prevent stresses concentration,but also resulting in more deformation at limited crack propagation.So,large amount of 0.5?m pores can act as the crack-stoppers to improve the thermal shock resistance of castables added SP-CA cements.