Effect Of Al-O₅ Unit Adjustment On The Hydration Behavior Of Hydratable Alumina

Hydratable alumina has been increasingly employed as the hydraulic binder in preparing Ca O-less and/or Ca O-free refractory castables.This is because the absence of Ca O increases the generation temperature of liquid phase in refractory castables during heating and improves the thermo-mechanical properties and slag corrosion resistance of castables.However,hydratable alumina with high specific surface area reacts with water extensively,leading to the generation of a large number of hydration products in a short time.These hydration products fill in the gaps among the components of the castable.As a result,the flow value of the castables is rapidly decreased,resulting in a quite short setting time/working time of hydratable alumina-bonded castables during industrial practice.Up to now,it is not clear how to adjust the hydration rate of hydratable alumina,especially the hydration rate in the initial stage of hydration,to prolong the working time of the castables.In previous work,the hydration behavior,the phase composition and morphology changes of hydratable alumina during hydration have been characterized.The underlying mechanism resulting in the fast hydration rate of hydratable alumina is,however,unclear.It has been reported that there are three kinds of Al-O_nunits,i.e.Al-O₄,Al-O₅and Al-O₆units,in alumina with different crystal structure,and the proportions of the three Al-O_nunits in different types of alumina are not the same.By comparingγ-Al₂O₃with hydratable alumina,it is found that these two kinds of alumina have similar specific surface areas,but the hydration rate is quite different.Further research found that the proportions of the three Al-O_nunits inγ-Al₂O₃and hydratable alumina are different.Compared withγ-Al₂O₃,hydratable alumina contains a higher proportion of Al-O₅,which may be the reason leading to the fast hydration rate of hydratable alumina.In addition,it has been reported that the hydration rate ofγ-Al₂O₃can be lowered by decreasing the ratio of Al-O₅units via dopingγ-Al₂O₃with alkaline earth metal oxides.Therefore,it is expected that the hydration rate,especially the initial hydration rate,of hydratable alumina can be also decreased by decreasing the Al-O₅units through doping hydratable alumina particles with alkaline earth metal oxides.As a consequence,fluidity decay of hydratable alumina-bonded castables will be lowered and the working time of the castables will be extended.The present work aims to exploring the underlying mechanism that affects the hydration rate of hydratable alumina,especially the hydration rate in the initial stage of hydratable alumina.In this work,the hydration behavior of hydratable alumina was systematically studied.Approaches were studied to adjust the hydration rate of hydratable alumina to decrease the hydration rate.Therefore,the fluidity decay of hydratable alumina-bonded castables is adjusted,and the working time of the castables is prolonged.The main work and results of this work are as follows:The hydration behavior of hydratable alumina at 40℃ was studied,and the phase compositions,micromorphology,particle size,viscosity,specific surface area changes and the proportion changes of three Al-O_nunits in the particles of hydratable alumina after hydration for different times were systematically characterized.The results show that hydratable alumina hydrated rapidly within 20 minutes after contacting with water at 40℃,resulting in the generation of a large number of hydration products.The particle size and viscosity of the slurry increased sharply in the first 20 minutes.The Al-O₅unit decreased by as much as 81.9%within the first 10min during hydration,and completely disappeared after 20 min.Because the hydration of hydratable alumina occurs through the surface to the interior of the particles,and Al-O₅unit mainly presents on the surface of hydratable alumina particles,the changes of the three Al-O_nunits in hydratable alumina,especially the changes in Al-O₅unit proportion is the predominant reason leading to the fast hydration rate of hydratable alumina,especially in the initial stage of hydration.The effect of magnesium lactate on the hydration behavior of hydratable alumina and the properties of hydratable alumina-bonded castables was studied by doping hydratable alumina with magnesium lactate.The results show that magnesium lactate absorbed on the surface of hydratable alumina particles,forming a weak chemical bonding with Al-O₅units on the surface of hydratable alumina,inhibiting the reaction between the Al-O₅unit and water.The hydration rate of hydratable alumina,especially the hydration rate in the initial stage of hydration was,therefore,decreased,thereby suppressing the formation of hydration products.As a result,the flow value of hydratable alumina-bonded castables was improved and the working time of castables was prolonged.In addition,the addition of magnesium lactate promoted the sintering of hydratable alumina bonded castables during heating,improving the thermo-mechanical properties of the castables.The effect of nano-scale magnesium-aluminum hydrotalcite on the hydration behavior of hydratable alumina and the properties of hydratable alumina-bonded castables was studied by mechanically grinding magnesium-aluminum hydrotalcite with hydratable alumina.Magnesium-aluminum hydrotalcite with thin plate morphology was pressed into the cracks and pores on the surface of hydratable alumina.Since Al-O₅unit mainly presents on the surface of hydratable alumina particles,the direct contact of Al-O₅and water,therefore,physically selected.As a result,the hydration rate,especially the initial hydration rate,of hydratable alumina was lowered.Therefore,the flow value of castables bonded with MAT-bearing hydratable alumina was improved and the working time of castables was prolonged.However,with the CO₂and H₂O were released from the decomposition of MAT during heating,leading to increased apparent porosity of castables and thus decreased mechanical strength of castables.The major contributions of this work include two aspects.First,the hydration behavior of hydratable alumina was systematically studied and the underlying mechanism of fast hydration rate was explored.The change in Al-O₅unit proportion is the predominant reason leading to the fast hydration rate of hydratable alumina,especially the initial hydration rate.Second,two approaches were developed to adjust the hydration rate of hydratable alumina.The hydration rate of hydratable alumina was lowered by adding magnesium lactate and magnesium-aluminum hydrotalcite,because of the inhibited interaction between Al-O₅and water.The lowered hydration rate consequently increased the fluidity and thus prolonged the working time of castables bonded with hydratable alumina.The results attained from this work are believed to have both scientific and practical values for the large scale industrial application of hydratable alumina as hydraulic binder.