| The castables bonded with hydratable alumina（HA）has excellent thermal shock resistance and slag erosion resistance,which has great application potential in ladle lining and other refractory structures.However,the mid and low-temperature strength of HA-bonded castables is poor,especially after calcination at 1000℃,the bonding strength is almost completely lost,which will lead to the collapse of castables during the mid-temperature heating process.Therefore,it is very important to improve the mid and low-temperature strength of HA-bonded castables for its application in ladle lining.Comparing with these,the castables bonded with silica sol（CS）could obtain good mid-temperature strength,adding appropriate amount of CS into HA-bonded castables,in coordination with hydration of HA and condensation of CS,is expected to improve the strength of castables at mid and low-temperature.Some studies have explored the strength changes of HA-CS bonded castables at mid and low-temperature,but there are no reports have investigated the causes of the change,the relationship between the strength and phases and microstructure evolution of HA-CS bonded castables after heating treatment at mid and low-temperature has not been reported.The curing temperature significantly affects the hydration of HA and the gelation of CS,which would affect the early strength of HA-CS bonded castables.In addition,the difference in the degree of CS coagulation at different pH values will also lead to different bonding strengths of HA-CS castables.In view of the above problems,this work first studied the influence of CS with three different pH values on the hydration behavior of HA-CS system at different curing temperatures（20℃,30℃,40℃ and 50℃）,including the hydration exothermic curve,pH value and conductivity changes.The effects of adding CS with three different pH values at different curing temperatures on the early properties of HA bonded castables were analyzed,and the essential relationship between the phase composition and microstructure and the demoulding strength and drying strength of castables was established.The hydration results showed that the addition of CS reduced the heat release and delayed the hydration rate of HA.For example,at 40℃,HA reached the peak of hydration heat release at 61 min,and the peak time of hydration heat release was prolonged to 75 min,75 min and 80 min with the addition of acidic,neutral and basic CS.This is because SiO2 spherical particles cover the surface of HA particles,blocking the contact between HA and water and delaying the hydration rate of HA.Moreover,the slurry mixed with basic CS has the highest concentration of OH-,which is conducive to the extraction of H+from the Si-OH group on the surface of SiO2 particles and thus promoting chain formation,and a more dense gel layer greatly delayed the hydration of HA.In addition,there were less amount of HA hydrates in the pastes after the addition of CS,and the crystalline degree was also reduced,which led to the decrease of demolding strength of castables.With the increase of curing temperature,the degree of CS condensation increases,leading to the decrease of demolding strength loss caused by HA hydration inhibition.Among them,basic CS has the highest degree of condensation and the most compact microstructure,the CMOR of the castable mixed with basic CS was 1.69 MPa after curing at 50℃ for 24 h,which was very close to that of the castable without CS（1.79 MPa）.However,curing at 50℃will make the slurry harden quickly and lose fluidity after holding for 1 h,thus adversely affecting the construction performance of castables.Curing at 30-40℃ allows for good workability and early strength of HA-CS bonded castables.The effects of three different pH values of CS on the physical properties（especially the CMOR and CCS）of HA-bonded corundum castables after heat treatment at 1000℃were compared and analyzed in different dosages（1%,2%and3%）.After heat treatment at 1000℃,the CMOR of HA bonded castables mixed with 2%acidic,neutral and basic CS were 7.68 MPa,8.66 MPa and 8.67 MPa,respectively,and the CCS were 58.64 MPa,64.93 MPa and 57.51 MPa,respectively,which were improved by 10 times and 5 times than those castables bonded with HA.The continuous increase of CS content will lead to the increase of water demand in the molding process and adversely affect the strength,and 2%is a more appropriate content of CS.At the same time,the effects of CS with three different pH values on the mechanical properties of HA bonded castables after heat treatment at 110-1250℃were discussed.Among the three different pH values of CS,the basic CS had the greatest improvement in the mid and low-temperature strength of HA bonded castables and significantly increased the apparent porosity of the castable.By analyzing the changes of the phase composition and chemical bond of each component sample during the heating process,as well as the micromorphological evolution and the TG-DSC curve before and after adding basic CS,it was observed that the thickness and density of SiO2 spherical particle aggregates in the HA-CS bonding system continued to rise,and the bonding force generated by silica sol condensation continued to increase.Meanwhile,CS inhibited HA hydration,leading to a significant reduction in strength loss caused by dehydration and decomposition of HA hydrates in the heating process.In addition,the addition of CS delayed the crystal transformation of transition phase alumina and the disappearance of morphology of HA hydrates,and improved the strength of HA bonded castables after heat treatment at 400-1000℃. |
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