| In recent years,in order to avoid the damage of calcium aluminate cement(CAC)bonded castables during heating up,admixtures are usually added to improve the burst resistance of castables,such as fibers,metal aluminum powder or aluminum lactate.However,these admixtures can also cause disadvantages such as uneven distribution of composition and structure of the castables while improving the explosion resistance of the castables.Hydration of CAC can produce hexagonal columnar hydration product CAH10 in an environment below 21?.Since the curing time has a great influence on the crystal size and crystallinity of the hydrated product CAH10,prolonging the curing time can increase the crystal size and crystallinity of CAH10.In addition,alumina micro-powder is a commonly used micro-powder in castables.The addition of alumina micro-powder can provide more nucleation sites for the precipitation of CAC hydration products and affect the hydration of CAC,which in turn affects the crystal size of hydration product CAH10 and increases its crystallization degree.But so far,when CAH10 continues to convert at higher temperatures,there is no relevant study on the effect of curing time and the addition of alumina micro-powder on the conversion of metastable hydration products to stable hydration products C3AH6 and AH3.Based on the confirmation that the conversion of CAC hydration products takes place through the "dissolution-precipitation",this work investigated the effects of curing time and the addition of different alumina micro-powders on the conversion of CAC hydration products CAH10 to C3AH6 and AH3.The effect of specific surface area of alumina micro-powder on the conversion of CAC hydration products was also studied.First of all,this work examined the effect of curing time on the conversion of CAC hydration products.CAH10 with different crystal size and crystallinity was produced from the pastes of CAC cured at 10? or 20? for 1d and 3d respectively,and the cured pastes were continually cured at 50?.The results indicate that the crystallinity of CAH10 is increases with extending curing time at 10? or 20?,and the hydrate CAH10 with a larger crystallinity and higher crystallinity transforms to C3AH6 and AH3 at a slower rate during further curing at 50?.Secondly,this work also studied the effect of curing time on the conversion of hydrated products in mixture paste of CAC and micro-alumina powder.CAH10 with different crystal size was prepared from the mixture pastes cured at 10? or 20? for Id and 3d respectively,and the cured pastes were continually cured at 50?.The results showed that the crystallinity of CAH10 increases with increasing curing time at 10? or 20?,and the hydrate CAH10 with a larger crystallinity and higher crystallinity converts to C3AH6 and AH3 at a slower rate during further curing at 50?.Thirdly,the effect of adding alumina micro-powder on the conversion of CAC hydration products was investigated.The pastes of pure CAC and mixture of CAC and alumina micro-powder were cured at 10? or 20? for 3d to produce CAH10 with different crystal sizes,and further cured at 40? and 50? for different times to examine the conversion of CAH10 to C3AH6.The results indicate that the crystallinity and crystal size of CAH10 of CAC is promoted by addition of alumina micro-powder,and CAH10 with higher crystallinity and larger crystal sizes dissolves at a slower rate during further curing at high temperature for different times and consequently the precipitation rate of C3AH6 is decreased.Finally,this work also investigated the effect of alumina micro-powders with different specific surface areas on the conversion of CAC hydration products.It was found that the larger the specific surface area of the added alumina micro-powders,the larger the crystal size of CAH10 produced by curing at 10? for 3d.CAH10 with higher crystallinity and larger crystal sizes dissolves at a slower rate during further curing at 50? and consequently the precipitation rate of C3AH6 is retarded. |
PDF Full Text Request