Investigation On Mechanism Of HMOR Decline And Fracture Behavior Of Alumina Based Castables With Different Binding Systems

As an important part of refractories,binding systems have significantly influence on the properties of refractories.At present,the most common binding mode is hydraulic binding using calcium aluminate cement（CAC）as binding agent.Unfortunately,in Al₂O₃-SiO₂ system,a sharp decline of hot modulus of rupture（HMOR）occurs after 1200℃ in the case of CAC addition in binding system.Therefore,it’s critical important to investigate the influence of CAC addition on relation of HMOR-temperature of Al₂O₃-SiO₂ system castables and reveal the mechanism of HMOR Decline.In order to clarify the effect of binding-system on the relation of HMORtemperture and fracture behavior of alumina based castables,LC（low cement）,ULC（ultra low cement）and NC（no cement）alumina based castables were prepared by changing the addition of CAC.The fracture behavior of alumina based castables with different bonding phase was investigated by methods of wedge splitting test.The mechanism of HMOR decline is revealed and analyzed according to results such as hot stress-strain behavior,content of glass phase,phase composition,microstructure and so on.In terms of non-oxides bonded Al₂O₃ based castables,the introduction of cement leads to a remarkable drop of HMOR at 1400℃,which changes from 21.6MPa in samples without cement to 0.64 MPa in samples with 5% cement.Plastic deformation occurs at 1200℃ in the CAC containing system,and viscous flow appears at 1400℃in the specimens with 3-5% CAC addition.In cement free system,however,little plastic deformation takes place even at 1400℃.With the increase of CAC addition,content of glassy phase increases linearly.Besides the in situ formed nonoxide bonding phases,such as β-Sialon,O’-Sialon and so on,glassy phase and anorthite（CAS₂）are formed in the cement containing castables.The morphology of Sialon is granular and the distribution of SiAl ON is discontinuous;while the content of glassy phase is much and the distribution of glassy phase is continuous.However,in the cement free system,the Sialon crystals interlocked in matrix are needle-or prism-like,the content of glassy phase is less and the distribution of glassy phase is discontinuous,favorable to the hot strength maintenance.HMOR decline at 1400℃ is due to the viscous deformation in the cement addition specimen,and the present of glassy phaseand its softening effect accounts for the viscous deformation.As to oxide bonded Al₂O₃ based castables,the introduction of cement also leads to a remarkable drop of HMOR at 1400℃,which changes from 13.7MPa in samples without cement to 1.68 MPa in samples with 5% cement.Glassy phase and anorthite（CAS₂）are formed as the bond phase in the cement containing bonding system,the content of glassy phase is much and the distribution of glassy phase is continuous.While in the cement free system,needle-like or prism-like mullite is formed as the bond phase,discontinous glassy phase present in the network of mullite,which is favorable to HMOR at 1400℃.Binding-system exerts a significant influence on fracture behavior of oxide bonded alumina based castables.The load-displacement curves of LC and ULC alumina based castables are linear,which indicates that the fracture of the castables is a typical brittle fracture and the toughness of the castables is poor.The loaddisplacement curves of NC alumina based castables are nonlinear,which means that the fracture of the castables is a quasi-static fracture and the toughness of the castables is better.Compared with oxide bonded Al₂O₃ based castables,the toughness of nonoxide bonded Al₂O₃ based castables is better,and the effect of binding system on fracture behavior of non-oxide bonded Al₂O₃ based castables is not obviously.The effect of binding-system on the curve of HMOR-temperture and fracture behavior of alumina based castables was investigated,the mechanism of HMOR decline was revealed.The achieved by this work can provide technical references to the selection,optimization of binding system and achieving high performance of such castables.