A Study Of The Phase Transition And Mechanical Properties Evolution Of CAC Combined Castables During Drying

Calcium aluminate cement(CAC),as a widespread used binder in castables,tends to form metastable hydration products CAH₁₀(Ca O·Al₂O₃·10H₂O)and C₂AH₈(2Ca O·Al₂O₃·8H₂O)at lower temperature.When the temperature rises,the metastable hydration products gradually convert into stable hydration products C₃AH₆(3Ca O·Al₂O₃·6H₂O)and AH₃(Al₂O₃·3H₂O).There are still some residual CA(Ca O·Al₂O₃)and CA₂(Ca O·2Al₂O₃)because of the incomplete hydration of CAC during the curing process.Therefore,the subsequent drying process is considered to affect the formation and conversion of CAC hydration products,which has a great impact on the mechanical properties of CAC-bonded castables.However,there are limited researches on the conversion rate of hydration products,the hydration rate of residual CAC and the strength growth rate of castables during the drying process.The purpose of this work is to explore the mechanical property evolution of CAC-bonded alumina castables during the drying process,with emphasis on the hydration of residual CAC and the transformation of hydration products.The types of hydration products and the hydration degree of CAC are adjusted by changing the curing conditions and the contents of CAC,which aims to further study the formation and conversion of hydration products in the drying process,and their effect on the mechanical properties of castables.The main conclusions are drawn as follows:The effect of drying time on phase transformation and mechanical properties of CAC-bonded alumina castables was investigated.The results show that the rapid hydration of residual CA₂ in the castables promotes the formation of hydration products during drying at 110°C,which improves bonding strength of aggregate and matrix in castables.Therefore,the strength of the castables exhibits a rapid growth trend in the early drying stage.The effects of curing conditions,in terms of curing temperature and curing time,on the phase transformation and mechanical properties of CAC-bonded alumina castables during the drying process were studied.The results indicate that the hydration products CAH₁₀ or C₂AH₈ obtained by curing at relatively low temperatures(10°C,30°C)can quickly transform into C₃AH₆ and AH₃ during the drying process.In addition,when the curing temperature is low(10°C)or the curing time is short(0.5 d),there are more residual CA and CA₂in the castables after curing.Due to the rapid hydration of residual CA and CA₂ during the drying process,the strength of the castables increases significantly in the early drying stage drying(within 4 h).That is to say,the strength of the castables cured at relatively low temperature or cured for relatively short time increases noticeably in the early drying stage at 110°C.The influence of CAC contents(2 wt%?8 wt%)on the phase transformation and mechanical properties of corundum-based castables during the drying process was studied.The results show that the hydrates C₂AH₈ and AH₃generated after curing at30°C for 1 d increases as the CAC content increases from 2 wt%to 8 wt%.Meanwhile,unhydrated phases(CA and CA₂)in castables with high CAC content(8 wt%)are more than those of castables with low CAC content(2 wt%and 5 wt%).Based on the above work,the relatively high temperature of the drying procedure facilitates the further hydration of residual CAC and the conversion of hydration products.There are a large amount of hydration products C₃AH₆ and AH₃ in the castables with high cement content(8 wt%),which improves the ability of bonding tightly each component in the castables.Consequently,the mechanical properties of castables with relatively high CAC content significantly increases in the initial drying stage(within 4 h).The main contribution of this work is to find that the hydration rate of residual CA and CA₂ is very fast in the early drying stage(within 4 h),the hydrates CAH₁₀ or C₂AH₈can rapidly transform into C₃AH₆ and AH₃ in the drying process.The reason why the strength of castables increases rapidly in the early drying stage at 110°C(within 4 h)was explained,and consequently revealed why the strength of castables with lower curing temperature,shorter curing time and higher CAC content is enhanced more obviously in the early drying stage at 110°C(within 4 h).The results of this work further enrich the theoretical basis of calcium aluminate cement research,and provide certain reference for guiding the drying system of CAC-bonded castables.