Effects Of Na2o And Si O2 In Reactive Alumina Powders On The Properties Of Calcium Aluminate Cement Bonded Castables

In recent years,the grade of minerals available for refractory enterprises continuously declines,which results in the rising cost of raw materials with high purity.In order to maintain balance between the cost and performance of refractories and reasonable utilization of resource,raw materials with high performance-price ratio attract more attention in the design of refractories.Although it was demonstrated that the impurities in raw materials could have a significantly influence on the performance of refractories,the action mechanism of impurities in raw materials is not well clarified,which seriously restricts the technological progress and enhancement of competitiveness for castables.In view of this,the work focused on reactive alumina powders.The influence of various characteristics of powders on the properties of castables was evaluated.Besides,the action mechanism of impurities Na₂O and SiO₂ on the phase evolution,microstructure and properties of castable was investigated.This study tried to optimize the properties of materials by impurities and make the best use of everything.Based the above investigations,the following conclusions are drawn:（1）Reactive alumina powders with smaller particle size were easier to fill into the gap between aggregates and fine powders,which contributed to release the free water in the gap and dense structure of castables and furtherly resulted in better flowability and mechanical property of castables.The larger specific surface area of powders can provide more nucleation sites for hydration products of calcium aluminate cement,so castables added with this kind of powders exhibited faster setting and hardening time.The content of Na₂O and SiO₂ in reactive alumina powders influenced the liquid phase sintering of powders.The results indicated that the addition of powders with better sinterability optimized the mechanical properties of castables.（2）Adjusting the content of Na₂O and SiO₂ in reactive alumina powders can enhance the sinterability of powders itself,and then optimize the strength of castables.Nevertheless,the correlation between the mechanical properties and the content of Na₂O and SiO₂ in reactive alumina powders was not simple linear.When Na₂O/SiO₂ in reactive alumina powders was greater than 0.5,higher SiO₂ content had a positive effect on the sintering of reactive alumina powders.When 0.25<Na₂O/SiO₂<0.5,powders with higher Na₂O and SiO₂ content presented better sinterability.As to castables,specimens added powders with higher sinterability finally exhibited better mechanical properties.（3）The solubility of Na₂O in reactive alumina powders was higher than that of SiO₂.With the increasing of soluble Na₂O in reactive alumina powders,the hydration of cement was accelerated,which finally led to a rapid flowability decay of castable.In addition,the occurrence status of Na₂O derived from reactive alumina powders affected the in-situ reaction of CA₆ in alumina based castables.When Na₂O in reactive alumina powders participated in the formation of liquid,the increasing of sodium content in the low molten phase（C₂AS）promoted the generation of liquid and reduced its viscosity,which was beneficial for the dispersion of liquid phase,and promoted the CA₆ formation by liquid reaction.When the impurity Na₂O in reactive alumina powders existed as the formβ-Al₂O₃,it blocked the diffusion pathway of Ca²⁺ions and finally hindered the CA₆ formation by solid-state reaction.（4）The impurities especially Na₂O in reactive alumina powders significantly enhanced the expansive reactions of CA₆ and spinel formation in alumina-magnesia castables.When Na₂O content in reactive alumina powders was increased from 260 ppm to 1810 ppm,the permanent linear change of alumina-magnesia castables after calcined at1500°C increased from 2.7%to 4.2%.Excessive volume expansion resulted in the reduction of densification and strength of castables after calcination,and also weakened the thermal shock resistance of alumina-magnesia castables in a certain degree.（5）The introduction of titanium in low melt phase system（CaO-Al₂O₃-SiO₂）promoted the formation of liquid phase,thus,the addition of Ti-bearing compounds,especially Ca TiO₃,accelerated the densification of alumina-magnesia refractory castables compared with mineralize MnO.The impurity Na₂O in reactive alumina powders had similar effects on the formation of liquid phase.Under the cooperative effects,more liquid generated at TiO₂ containing alumina-magnesia castables added reactive alumina powders with higher Na₂O content.Therefore,in the composition design of TiO₂ containing alumina-magnesia castables,choosing reactive alumina powders with higher Na₂O content was beneficial for controlling the expansion behavior of castables.