Research On Carbonation Behavior Of Alkali-activated Slag-metakaolin Composite Cementitious Material

Alkali-activated material is a new type of green construction material,which has good strength and can also realize the resource utilization of industrial solid wastes.It has the potential to become a substitute for cement in the future.Due to the wide variety of raw materials,two or more composite systems can achieve complementary advantages and further improve their own performance.At present,some achievements have been made in the research on alkali-activated materials,in order to put alkali-activated materials into production and application,it is necessary not only to study their mechanical properties,but also to ensure their durability.As one of the key factors affecting the durability of reinforced concrete structures,the carbonation problem needs to be solved urgently.The research on the carbonation of alkaliactivated materials is not sufficient,the research on the carbonization of composite systems is even less,and there is almost no established carbonation model of alkali-activated materials.The above problems make alkali-activated materials suffer great resistance in application.In this project,slag and metakaolin were used as raw materials to prepare alkali-activated materials.The changes of material properties under the influence of slag content and alkali content were studied.At the same time,accelerated carbonization test was adopted,the influence of carbonization on alkali-activated materials was analyzed by carbonization depth test,compressive strength test,material alkalinity test,microstructure test and phase test.The main results of the Institute are as follows:(1)The samples prepared by slag-metakaolin composite system have higher compressive strength than those prepared by single metakaolin.The compressive strength increases first and then decreases with the increase of alkali content of activator,there is the most suitable alkali content range to make the compressive strength reach the peak value.The higher the slag content is,the lower the alkali dosage is,under the same alkali content,the alkalinity of the material after hydration increases with the increase of slag content.(2)The carbonization depth of alkali activated materials decreases with the increase of alkali content and slag content at the same carbonization time.Carbonization makes the alkalinity of the material decrease rapidly,but it will eventually tend to be stable,adding a large amount of slag as a calcium source will lead to a greater decrease in the alkalinity of the material.The pore structure of the paste after carbonization has adecreasing trend but it is not obvious,the gel after carbonization is more prone to cracks in the high slag content group,when the slag content exceeds 20%,the compressive strength of the sample decreases sharply after complete carbonization.After carbonation,the carbonation product of the single metakaolin group is Na2CO3,and the carbonation product of the slag-doped metakaolin group is mainly CaCO3.The group with low slag content mainly generates calcite with stable structure after carbonization,compared with the group with low slag content,the group with high slag content will generate a large number of aragonite with worse stability.The alkali-activated slagmetakaolin has weak ability to mineralize CO2,and the formed carbonate is more easily decomposed at high temperature.(3)Based on the real pore structure correlation parameters,the model of carbonization depth and material alkalinity was constructed.The denser the pore structure,the higher the theoretical pore water saturation,and the stronger the ability to resist the intrusion of CO2.The carbonization depth model based on pore structure and Fick’s first law is feasible.The alkalinity model based on Fick’s second law and solution ion balance can reflect the change trend of alkalinity with carbonization time,however,due to the influence of atmospheric CO2 concentration,the theoretical calculated value will be lower than the experimental value.From the perspective of material alkalinity,the accelerated carbonization test underestimates the carbonization resistance of alkali-excited materials to a certain extent.