Optimization Of Aluminate Cement–colloidal Silica Bonding System And Its Effect On The Properties Of Corundum-based Castables

The castables bonded with aluminate cement have excellent green strength and rheological properties,though their mid-temperature strength is low and curing process is more complex.Comparing with these,the castables bonded with silica sol could obtain good mid-temperature strength and simple curing process.But their green strength and flowability are low,which restrict their application in the refractory field.In order to fully utilize the advantages of aluminate cement and silica sol,complex bonding system of aluminate cement and silica sol was prepared to obtain excellent green strength,rheological properties and mid-temperature strength,as well as high high-temperature strength,slag resistance and thermal shock resistance.This thesis investigated the effect of silica sol on the rheological properties and hydration process.The rheological property results showed that there were a lot of Ca²⁺and Al?OH?₄^-in the pastes when aluminate cement contacted with water.And SiO₂ nanoparticles were homogeneously distributed in the pores and the surface of CAC particles as nucleus sites.Due to the flocculation effect of Ca²⁺,SiO₂nanoparticles were flocculated to network structure to strengthen the pastes,leading to the reduction of free water and the particles bonding together tightly.Many Ca²⁺ions were consumed in this process and prompted the early dissolution process of aluminate cement particles.As a result,the shear stress was increased with shear rate after the colloidal silica addition,the thixotropy and viscosity were improved.Besides that,the storage modulus and flow point were also increased,while the setting time was shortened.The hydration results showed that the addition of colloidal silica could promote the early hydration process of aluminate cement,on the contrary inhibit it at the late hydration process.There were less amount of hydrates in the pastes after the addition of colloidal silica,and the crystalline degree was also reduced,leading to the?dense?microstructure of hydrates,which was drastically different with the petal shaped microstructure of plain aluminate cement pastes.The SiO₂ nanoparticles promoted the hydration process into NG?Nucleation and crystal growth?process,increasing the electrical conductivity and intensity of the first heat releasing curves.Due to the coverage of hydrates on the surface of aluminate cement particels,the hydration process was transformed from I?Interactions at the phase boundaries?stage to D?Diffusion?stage earlier,making that the late hydration process was inhibited,the amount of heat releasing was decreased and both amount and crystalline of hydrates were reduced.The NMR?Nuclear Magnetic Resonance?spectra confirmed that there was a little SiO₂ nanoparticles was involved in the hydration process of aluminate cement,most of these nanoparticles existed in the form of Si-O tetrahedron in the network structure.Based on the above results,aluminate cement and silica sol were used as complex bonding system to fabricate corundum castables.The results revealed that the flow values were reduced and the strength at room temperature was improved,especially for the CMOR?cold modulus of rupture?.With the addition of silica sol,the CMOR of castables after curing at 110?C was increased from 7.5 MPa to 13.7MPa.After heating treatment at 1100?C and 1500?C,the CMOR reached the maximum values of 23.0 MPa and 46.7 MPa respectively,which was improved by more than 100%than those of castables bonded with pure aluminate cement.However,the HMOR?hot modulus of rupture?was decreased by as much as 79%,from 3.3MPa to 0.69 MPa,due to the formation of gehlenite and existence of Na in the silica sol.The residual thermal shock strength and retention rate of residual strength of castables with complex bonding system were 8.94 MPa and 19.1%respectively,which were higher than those of 3.97 MPa and 13.9%of castables with pure aluminate cement.What's more,the slag penetration indexes of castables were increased and decreased with the addition of silica sol.The penetration index was decreased to 58.1%when 3%CS was added,which was less than that of aluminate cement bonded castables samples.The ultra low aluminate cement and silica sol were used as the bonding agents to reduce the CaO content in the castables.The results showed that with the addition of silica sol,the CMOR were increased from 2.1 MPa to 3.3 MPa,from 2.7 MPa to 5.4MPa,from 2.8MPa to 30.5 MPa,and from 24.8 MPa to 39.1MPa,when castables were demolded,110?C curing,1100?C and 1500?C heating treatment.The CCS and displacement values were also improved,indicating that the addition of silica sol could strengthen and toughen the castables.But the HMOR values at 1000?C and1200?C were reduced from 31.5 and 26.9 MPa to 5.5 and 17.1 MPa,respectively.The HMOR value at 1400?C was reduced from 8.0 MPa to 1.8 MPa.Fortunately,the thermal shock resistance was improved.Therefore,SiO₂ micropowders were added into the castables to improve their HMOR,thermal shock resistance and slag resistance.Due to the addition of SiO₂micropowders,mullite was formed in the castables.XRD patterns revealed that the amorphous SiO₂ micopowders was transformed to quartz after heating treatment at1100?C,and mullite diffraction peaks could be found after the castables were heating treatment at 1500?C.These mullite crystals were cross distribution in the castables,resulting in low linear shrinkage and stable of apparent porosity.After the SiO₂micropowders were added,the cold strength values were stable.The HMOR values were increased from 2.3 MPa to 15.6 MPa,by as much as 6 times,and the CCS were increased from 193.9 Mpa to 243.0 Mpa,by as much as 25%.The increase of displacement in the force-displacement curves confirmed that the toughness of castables was improved.The improvement of physical properties can be attributed to the formation of in-situ mullite.The slag penetration indexes were reduced from45.3%to 19.4%,indicating that the slag resistance of castables was improved.