Study On The Mechanism Of Alkali Activated Fly Ash/slag Autogenous Shrinkage

Alkali activated materials are the most potential green cementitious materials in recent years.They not only have high strength,high temperature resistance and other excellent properties,but also have great potential in environmental energy efficiency for their major raw material is industrial waste,such as fly ash and slag.However,alkali activated fly ash/slag cementitious material has large autogenous shrinkage deformation,and the cracks caused by shrinkage greatly limit the application of alkali activated fly ash/slag in engineering.Therefore,the correct understanding of the autogenous shrinkage deformation and autogenous shrinkage mechanism of alkali activated materials is the key to the large-scale production and application of alkali activated materials.In this paper,fly ash and slag were used as raw materials to prepare alkali activated fly ash/slag blended system cementitious materials.Firstly,three methods?capillary pressure,setting time,internal relative humidity?were used to study the starting time of autogenous shrinkage of alkali activated fly ash/slag blended system.On this basis,the effects of the modulus of activator,Na₂O content,water to powder ratio,fly ash content and mineral admixtures?silica fume,metakaolin?content on the autogenous shrinkage,capillary pressure and internal relative humidity of alkali activated fly ash/slag blended system were studied.The progress of reaction,pore structure and molecular structure of reaction products of alkali activated fly ash/slag blended system were investigated using isothermal calorimeter,nitrogen adsorption instrument and infrared spectrometer,respectively.The results showed that at the initial stage of the reaction,the capillary pressure inside the alkali activated fly ash/slag blended system would increase sharply,which corresponded to the accumulation of internal stress in the system,and the change of capillary pressure was consistent with the change of early shrinkage slope.Therefore,the time point corresponding to the rapid increase of capillary pressure could be determined as the starting time of autogenous shrinkage of the alkali activated fly ash/slag blended system.When the modulus of the activator increased,the pore structure type changed,the number of mesopores increased,and the autogenous shrinkage of the system increased.When the content of sodium oxide increased from 4%to 5%,the reaction degree increased,and the cumulative pore volume increased,resulting in the increase of autogenous shrinkage.When the water to powder ratio increased,the internal humidity of the system increased,the reaction rate and reaction degree decreased,the peak value of pore size distribution decreased,the number of mesopores with small pore size decreased,the capillary pressure decreased,resulting in the reduction of autogenous shrinkage.When the content of fly ash increased,the reaction rate and degree decreased,the pore size of mesopore increased,the capillary pressure decreased,and the autogenous shrinkage decreased.The addition of silica fume or metakaolin could reduce the autogenous shrinkage of alkali activated fly ash/slag,and metakaolin was better.The addition of silica fume slowed down the reaction rate of alkali activated fly ash/slag blended system,reduced the reaction degree,decreased the accumulated pore volume,and reduced the autogenous shrinkage of the system.The addition of metakaolin kept the alkali activated fly ash/slag blended system in high humidity,decreased the capillary pressure,reaction rate or reaction degree,changed the pore structure type greatly,rendered the pore size distribution wider,increasd the pore size,so the autogenous shrinkage of the system was greatly reduced.The internal humidity and reaction degree of alkali activated fly ash/slag blended system were important factors affecting the autogenous shrinkage.Generally speaking,the higher the internal humidity of the system was,the smaller the autogenous shrinkage would be.But they couldn't fully reflect to the regular pattern of autogenous shrinkage.Through the pore structure and molecular structure of products tests,it was found that the autogenous shrinkage of alkali activated materials was also closely related to the product pore structure type,mesopore pore size,number of mesopores,and molecular structure of product.It was found that the main factors that determine the size of autogenous shrinkage were pore structure type and mesoporous number.Compared with mesoporous number,pore structure type had more influence on autogenous shrinkage.The pore structure was"ink bottle"-type,the smaller the pore size and the more the number of mesopores,the larger the autogenous shrinkage,and the"ink bottle"-type pore was more likely to be generated after adding water glass.