Effect Of Free Water Loss On Phase Transformation Of CAC Hydrates And Physical Properties Of CAC Bonded Castables

In the curing process of calcium aluminate cement(CAC)-bonded alumina castables,some people believe that the closed environment curing(with film)can avoid the loss of free water in the hydration process,which is beneficial to the hydration of cement,and is beneficial to the improvement of the strength of castables during curing and drying process.However,some people believe that although there is more free water loss during open environment curing(without film),the strength of the castables is higher after curing and drying.So far,there has been no systematic study on the effect of the difference in free water loss between closed environment curing(with film)and open environment curing(without film)on the curing and drying strength of CAC-bonded castables and its causes.In order to reveal the effect of different free water loss during the curing process on CAC hydration and the conversion of hydration products and the properties of CAC-bonded castables,this paper takes CAC matrix and CAC-bonded castables with and without film as the research objects.Curing with film and without film at 50℃ and the humidity of 30%,the effect of the difference of free water loss on CAC hydration and hydration products and the castable properties during curing and drying at 110℃was investigated.By changing the loss of free water and the amount of cement added in the curing process,the types and quantities of hydration products were regulated,and the formation and transformation of CAC hydrate phases and the mechanical properties of castables during curing and drying processes were further explored.The main research results of this paper are as follows:The law of the effect of the difference of free water loss during curing at 50℃ on CAC hydration and the conversion of hydration products in the curing and drying process was proved.The results show that the intermediate phases C2AH8(2CaO·Al2O3·8H2O)and AH3(Al2O3·3H2O)are first formed at the initial stage of CAC hydration at 50℃.With film,the intermediate phase C2AH8 generated in the early stage of hydration is converted into C3AH6(3CaO·Al2O3·6H2O)and AH3 through dissolution-precipitation,and the hydration products are no longer converted during drying at 110℃.Due to the large amount of free water loss of samples without film,there is not enough free water to dissolve C2AH8 during the hydration process,and C3AH6 and AH3 cannot be precipitated.Therefore,after drying at 110℃ for 24 h,the undissolved C2AH8 in the sample absorbs the CO32-generated by dissolving CO2 in the air,and keeps CO32-firmly in its interlayer,and finally dehydrates to form the C4ACH11(3CaO·Al2O3·CaCO3·11H2O).The effects of the difference in the loss of free water during curing at 50℃ on the microstructure and properties of CAC combined castables after curing and drying were investigated.The results show that when the curing temperature is 50℃,the conversion of C2AH8 to C3AH6 brings a huge change in the morphology of the hydration products due to the less loss of free water in the castable with film.This means that the evenly distributed plate-like C2AH8 in the castables is transformed into a relatively compact and regular hexagonal pomegranate-like C3AH6 in physical space,thereby reducing the mechanical properties of the castable after curing and increasing its apparent porosity.After the castable samples without film are dried at 110℃ for 24 h,the undissolved hydration product C2AH8 absorbs CO2,dehydrates and transform into lamellar C4ACH11,which interlaces each other and formes an interlocking structure,thereby improving the strengths of the castables and reducing apparent porosity of castable samples after drying.The effects of free water loss during curing at 50℃ on the phase transformation and properties of CAC-bonded castables with different cement contents(2 wt.%～8 wt.%)during curing and drying were studied.The results show that the free water consumption of CAC hydration increases with the increase of cement content,which inhibits the dissolution-precipitation conversion of C2AH8 to C3AH6,thereby improving the mechanical properties of the castable without film after curing and reducing its apparent porosity.With the increase of cement content,the wellcrystallized and uniformly distributed plate-like C4ACH11 is obtained in the castable without film by drying at 110℃ for 24 h,thereby improving the mechanical properties of the castable samples and reducing its apparent porosity.The main contribution of this work is to discover that the intermediate phases C2AH8 and AH3 are first generated in the initial stage of CAC hydration curing at 50℃,and then the C2AH8 is dissolution-precipitation converted into C3AH6 and AH3,which is limited by the amount of free water loss during the curing process.At the same time,the relationship between the free water loss during curing,the transformation of CAC hydration products and the evolution of physical properties of CAC-bonded castables was elucidated.The results of this work will further enrich the hydration theory of calcium aluminate cement,and provide new ideas for the curing regime of CACbonded castables cured at higher temperatures.