Relationship Between The Strength And Phase And Microstructure Of CAC Bonded Castables Under Low And Intermediate Temperatures

Calcium aluminate cement?CAC?has been widely used as a hydraulic binder to prepare the refractory castables due to the excellent rheological properties and higher green strength.During the heating process,the unreacted CAC particles will be further hydrated and the hydrates would gradually dehydrate,at the same time,the mechanical properties of CAC bonded castable will also change.When the castables fired at low and intermediate temperatures?110-1200??,the strength of CAC-bonded castables significantly decrease because of the dehydration of the hydrates.This decrease is generally attributed to the destruction of the hydrate network,coupled with an increase in the porosity of the castables.In recent years,there are numerous the research reports on thermal-mechanical and service property of CAC-bonded castables at high temperatures.However,few studies have investigated the causes of the change in the intermediate strength of castables.Our team research results demonstrate that there is a mismatch between the temperature of the intermediate strength drop of CAC-bonded castables and the temperature of hydrate decomposition.However,the relationship between the strength and phases and microstructure evolution of CAC-bonded castable after heating treatment at intermediate temperatures has not been illustrated.And the different amount of CAC contents and different curing temperature would generate varying amounts of hydrates after curing and drying.How the varied amounts of hydrates would influence the the castable strength at intermediate temperatures has not been fully evaluated.This work aims to investigate the strength changes of the CAC-bonded castables at intermediate temperatures,with emphasis on the structure evolution of the hydrates.On this basis,we changed the curing temperature and CAC contents to study the effect amount of hydrates on the strength of castables under the intermediate temperature.Furthermore,the relationship between the strength and the evolution of hydrate phase of castable at intermediate temperature is also discussed.We hope that this work would provide insight into enhancing the mechanical properties of the CAC bonded castable at the intermediate temperature,which would provide some reference for preventing structural spalling of CAC bonded castable used at 1ow and intermediate temperatures.The main conclusions of this paper are as follows:The relationship between the progression of the strength of the castable CAC-bonded castables heated tratement at different temperature?110-1200??and the phase and structural evolution of hydrates has been established.During the heating process,the hydrates C₃AH₆ decomposed into C₁₂A₇ and AH₃ into amorphous Al O?OH?and H₂O at 300?,which leads to an increased apparent porosity of castables.However,the dehydration of CAC hydrates below 300? only results in a slight reduction in the castable strength,because the dehydration products of C₁₂A₇ retain the morphology and cubic structure of C₃AH₆.The CMOR of castables decreases from 20.4 MPa to16.5 MPa,and the CCS of castable decreases from 135.1 MPa to 120.9 MPa when the temperature is raised from 110 to 300?.As the temperature increases,the significant reduction in the castable strength at approximately 600-800? does not only result from the decomposition reaction of Al O?OH?,but also from the drastic change in morphology of the original C₃AH₆ and AH₃ and their respective decomposition products.This leads to produce a loose structure of castables.The CMOR decreases from 20.4 MPa to 8.7 MPa and the CCS decreases from 135.1 MPa to 61.6 MPa as the temperature rises from 110? to 800?.After firing at 900-1000?,sintering between the decomposed C₁₂A₇ and decomposition products of AH₃?amorphous Al₂O₃?happens and agglomeration of the decomposition products of AH₃ occurs,which should be responsible for the increase in strength of castables after firing at between 900? and 1000?.When temperature is raised from 1000? to 1200?,CA reacts with Al₂O₃ to form CA₂,and the sintering between particles and the agglomeration of the amorphous Al₂O₃ increase,consequently enhancing the strength of the castables.The effect of curing temperature on the mechanical properties and the phase and structural evolution of CAC-bonded castables at low and intermediate temperatures has been studied.The increased curing temperature can accelerate the CAC hydration process,increase the hydration degree,and result in more hydrated products,which would lead to increase the demolding strength of castable.The higher curing temperature is beneficial to the homogenous distribution of decomposed C₁₂A₇.Therefore,the higher curing temperature is helpful to enhance the strength of the castables after heating at 300?.Similarly,when the curing temperature is increased,coalescence and sintering between the decomposed C₁₂A₇ and amorphous Al₂O₃ would occur to a higher degree in the castables fired at 800?,the higher curing temperature would result in more homogenous distribution of the hydrated products.Consequently,the strength of the castables has been increased after firing at 800?.The effect of CAC contents?2%,5%,8%,12%and 16%?on the macroscopic properties and the phase and structural evolution of CAC-bonded castables at low and intermediate temperatures has been studied.As discussed before,although the C₃AH₆are dehydrated after fired at 300?,the decomposed C₁₂A₇ retain the morphology and structure of hydration product C₃AH₆.Therefore,the higher CAC contents result in more C₁₂A₇ and endow the castables with higher strengths after heating at 300?.When the temperature increases to 800?,although the morphologies and structures of the C₁₂A₇ and amorphous hydration products derived from the dehydrated C₃AH₆and decomposed AH₃ change considerably at the temperature,the increase in CAC content is beneficial to the increase of decomposed C₁₂A₇ and amorphous products,and promotes the sintering between C₁₂A₇ and amorphous hydration products.Therefore,the castable with a higher CAC content has higher strength than that with a lower CAC content after heat treatment at 800?.At the temperature of 1000?,the castables with higher amounts of CAC have the C₁₂A₇ and amorphous Al₂O₃ derived from the dehydrated C₃AH₆ and decomposed AH₃.As a result,coalescence/sintering between the particles of the amorphous Al₂O₃ and the C₁₂A₇ happens to a higher degree,which in beneficial for improving the strength of the castables fired at 1000?.The major contributions of this work include two aspects.First elaborated is the relationship between the strength of the CAC-bonded castable and the phase and structural evolution of CAC hydrates at low and intermediate temperatures.Also illuminated are the influence mechanisms of CAC content?2-16%?and curing temperature on the strength of CAC bonded castable at low and intermediate temperatures from the perspectives of the phase and structural evolution of CAC hydrates.