Study On The Hardening Reaction And Shrinkage Property Of Alkali-activated Materials

Alkali-activated slag cementitious materials(AAS)as a new type of green building materials have not been widely promoted,mainly because their quality are not easy to control in the actual application process.Among them,the shrinkage and microcrack of AAS are the key and difficult problems to be solved in AAS quality control.The hardening process of AAS is more complex than Portland cement.The type and structure of hardening products change with the many factors,which will lead to different shrinkage behavior(size and variation rule)of AAS.At present,the research on Shrinkage Properties of AAS is not yet mature,mainly in the following two aspects: First,the hardening mechanism of AAS is still lack of systematic and comprehensive basic research,so the theoretical reference knowledge of AAS shrinkage performance research is insufficient.Secondly,chemical shrinkage has a great influence on the early autogenous shrinkage of materials,and AAS shrinkage is closely related to its hardening reaction process and products.However,there are many studies on the single shrinkage type and shrinkage reduction methods of AAS,while there are few studies on the correlation between chemical shrinkage and autogenous shrinkage based on hardening reaction mechanism of AAS is less studied.Based on the above problems,this paper systematically and comprehensively studied the effects of water-binder ratios,alkali content and mineral admixtures on the hardening reaction process of AAS,and further analyzed the causes and changing rules of chemical shrinkage and autogenous shrinkage of AAS.In this paper,the effects of three factors on the hardening reaction process and products of AAS were studied by means of electrical resistivity method,thermogravimetric infrared combined test and hydration heat test,so as to further understand the causes of volume deformation of AAS.The change of porosity during hardening reaction of AAS was measured by mercury intrusion test,and the conclusions and hypotheses were further analyzed and verified.Finally,by summarizing the experimental data,the changing law of chemical shrinkage and autogenous shrinkage of AAS under different influencing factors were explored,and the chemical shrinkage and autogenous shrinkage of AAS were predicted based on the data.Based on the above research purposes,this paper draws the following conclusions:(1)The changes and causes of chemical shrinkage and autogenous shrinkage of AAS with different water-binder ratios(0.3,0.35,0.4,0.45,0.5)were systematically studied.It was found that the chemical shrinkage and autogenous shrinkage of AAS with 0.4 water-binder ratio were the largest during 30 days of hardening reaction.If the water-binder ratio was too small,less the water content in AAS pore would lead to the decrease of reaction rate at the later stage of hardening,which would make the shrinkage of AAS smaller.When the water-binder ratio is large,the amount of water added into AAS,which reduces the alkali concentration of AAS and reduces the hardening reaction rate and shrinkage.(2)The changes of chemical shrinkage and autogenous shrinkage of AAS with different alkali content(4%,5%,6%,7%,8%)were systematically studied.It was found that the chemical shrinkage of AAS with 6% alkali content was the largest in 30 days of hardening reaction.Which due to the moderate water content in the hardening reaction,and the reaction rate is faster.while the autogenous drying effect of AAS with 8% alkali content is greater because of excessive alkali and less water content,which makes the autogenous shrinkage of AAS maximum in 30 days of hardening reaction.(3)The effects of metakaolin and its content on the hardening process and shrinkage properties of AAS were studied.It was found that the chemical shrinkage and autogenous shrinkage of AAS were improved best when the content of metakaolin was 30%.The addition of metakaolin can slow down the hardening reaction rate of AAS and the hardening products can improve the shrinkage properties of AAS.However,because metakaolin can absorb water in the pore of slurry,excessive addition of metakaolin can lead to greater autogenous shrinkage of AAS.(4)During the hardening reaction of AAS,the development of chemical shrinkage and autogenous shrinkage has a good linear relationship with the development of resistivity.The slope of the linear relationship decreases with the increase of water-binder ratio,alkali content and metakaolin content.Therefore,the changes of autogenous shrinkage and chemical shrinkage within 24 hours of hardening reaction can be predicted and analyzed by measuring the resistivity of samples.