Hydration And Dehydration Mechanism Of Hydratable Alumina And Its Application In Al₂O₃-MgO Castables

The molding methods of Al₂O₃-MgO unshaped materials mainly include automatic-flowing,vibrating,jetting which provides relative lower pressure.Hence,the densification,structure and properties of the refractories are largely determined by the binding system.It is well known that hydratable alumina could react with water quickly to form gibbsite and boehmite gel at 25 °C,which results in the high ceramic-bonding strength.Additionally,it is efficient to sustain better performance of refractories in high temperature,by avoiding the formation of low-melting phases such as feldspar and plagioclase due to the introduction of Ca element from calcium aluminate cement.However,there are few researches on hydration mechanism,dehydration behavior of hydratable alumina and the effects of hydration on the properties of castables.Given the above,it is of vital significance to study the hydration behavior and the dehydration mechanism of hydratable alumina and its application in the Al₂O₃-MgO castables.Based on the design ideas “Regulate hydration rate,improve hydration efficiency,control the dehydration structure and optimize combination performance”,the hydration behavior,the dehydration mechanism and its application in the Al₂O₃-MgO castables of hydratable alumina are investigated.The researches focus on the hydration behavior and the dehydration mechanism of hydratable alumina,including: the effects of particle size distribution and morphology characteristics,the synergistic effects of water reducer,the surface adsorption of micro powders and the pretreatment of acid on the hydration rate and desorption structure as well as combination performance.Moreover,micromorphology after dehydration of hydratable alumina and the structural evolution in the castables are analyzed by nitrogen adsorption-desorption and mercury porosimeter.By analyzing the microstructures and properties of the prepared materials,the main conclusions are as follows.（1）Particle size distribution and morphology characteristics can affect the hydration behavior of hydratable alumina obviously.Smaller particle size and the higher sphericity enhance the dispersion stability and solidification of hydratable alumina.After ball-milling for 3h,N2 adsorption-desorption isotherms of hydratable alumina(d₅₀=8.7μm,0.93 sphericity)heated at 300 oC belong to Ⅳ type and the hysteresis belongs to H3 type,indicating the structure of hydratable alumina is disordered which forms by layered aggregates.Hence,decreasing the particle size and increasing the sphericity could improve the flow ability and densification of the Al₂O₃-MgO castables.（2）The dispersant synergistically contributes to the diffusion of the boehmite gel into the solution,increasing the concentration of the boehmite gel in the slurry,thereby increasing the hydration efficiency of the hydratable alumina.After 2h hydration process,the hydrated alumina with sodium metasilicate added had a lower crystallinity,reflecting its lower crystallinity.N₂ adsorption-desorption isotherms after treatment at 300 °C belong to type IV and the hysteresis belongs to H2 type,indicating the structure of hydratable alumina is ink bottle-like which forms by layered aggregates.By synergistic hydration of water reducer,both the efficiency and degree of hydration improve,and then attribute to the flow ability of the Al₂O₃-MgO castables.Moreover,the uniform gel network and pore structure are formed in the castables,leading to the higher mechanical strength and the better thermal shock resistance.(3Al₂O₃/SiO₂ micropowders can adsorb the hydroxyl group（-OH）generated by the hydrolysis of boehmite at the initial stage of the hydration reaction and promotes the hydration reaction to the positive reaction direction,thereby increasing the hydration degree of the hydratable alumina.Additionally,the van der Waals repulsion between the negatively charged Al₂O₃/SiO₂ micropowder and the negatively charged boehmite helps to increase the molecular spacing and prolong the time of solidification,thereby optimizing the constructability of Al₂O₃-MgO castable.（4）The acidic hydration environment can significantly inhibit the hydration reaction of hydratable alumina and control its gel release rate.The pretreatment of hydratable alumina with a citric acid solution（PH=3）would form the organic complexes and adsorb on the surface of the gel layer to achieve the function of inhibiting the hydration reaction.During the dehydration process,the compact layered hydration structures contribute to the slower release of water molecules,and then buffer the thermal stress.Further,the hydratable alumina pretreated by citric acid solution could improve the flow ability and constructability of the Al₂O₃-MgO castables.（5）Effects of the hydrated alumina dehydration pore structure and the dispersed matrix pore structure on the slag penetration resistance of the castable are analyzed by COMSOL Mutiphysics software.The pore structure formed by desorption of hydratable alumina and microstructures of castables matrix are optimized by controlling the particle size and microstructure,dispersant modification,micro-powder surface adsorption and acid pretreatment,and then the pore structures influence the slag penetration resistance of the castables.Wherein the dehydration structure of hydrated alumina,the layered discrete composite structure is more favorable for the slag penetration resistance of Al₂O₃-MgO castable.