| Alkali activated materials not only have the characteristics of early strength and high strength,excellent corrosion resistance and low permeability,but more importantly,alkali activated materials usually take industrial solid waste as the main raw material,which can be cured and formed under natural conditions.It is a low-carbon,green,energy-saving and environmental protection building material,which has a great trend to replace Portland cement as the main building material.However,alkali activated materials also have the characteristics of large shrinkage and easy cracking,which seriously hinders the wide application of alkali activated materials.Aiming at the defects of large shrinkage and easy cracking of alkali activated materials,taking the compressive strength,flexural strength and linear shrinkage as the evaluation indexes,this paper compares and studies the effects of the type and proportion of raw materials,the type and content of activator,the content of mineral additives and the type and content of external fibers of different alkali activated materials on the mechanical properties and dry shrinkage characteristics of alkali activated specimens.The results show that:(1)The compressive and flexural strength tests show that the mechanical properties of alkali excited specimens are obviously better than those of cement mortar specimens.Compared with the four raw materials of slag,red mud,garbage ash and fly ash,slag has the highest polymerization activity and the best mechanical properties,while the reaction activity of red mud,garbage ash and fly ash is lower.(2)The dry shrinkage test of raw material type and ratio shows that the dry shrinkage rate of slag alkali excited specimen is significantly higher than that of cement mortar specimen.With the increase of curing age,the dry shrinkage rate increases gradually,and the increase range decreases gradually.The addition of red mud will further increase the dry shrinkage of alkali excited specimens,the addition of waste ash will lead to the violent expansion of alkali excited specimens,and the addition of fly ash can significantly alleviate the dry shrinkage of alkali excited specimens.The low consumption of free water in the polymerization reaction of alkali excited specimens leads to the volatilization of a large amount of free water,which is the main reason why the dry shrinkage of alkali excited materials is greater than that of cement mortar specimens.The addition of fly ash can promote the formation of a variety of mineral crystals in the alkali excited specimen,give play to the "polycrystalline effect" and form a more reasonable "crystal binder ratio",which is the main reason why the addition of fly ash can alleviate the dry shrinkage.(3)The dry shrinkage experiment of activator and mineral admixture shows that with the increase of sodium silicate content,the dry shrinkage rate of alkali excited specimens first decreases and then increases significantly.The control effect of liquid sodium silicate on the dry shrinkage of alkali excited specimens is better than that of solid state.When its content reaches 120 g,the control effect is the best.The addition of an appropriate amount of calcium hydroxide(11.3g)can further reduce the dry shrinkage of alkali excited specimens,but with the increase of calcium hydroxide content,the dry shrinkage of specimens first decreases slowly and then increases significantly.SEM analysis shows that the addition of sodium silicate can significantly improve the polymerization efficiency of alkali activated materials,promote the formation of high-strength polymerization products,reduce the number of pores and cracks,and further reduce the dry shrinkage.However,the high content of sodium silicate will lead to a large amount of deposition,which will lead to the increase of dry shrinkage.The addition of an appropriate amount of calcium hydroxide can make up for the deficiency of active calcium content,further promote the polymerization reaction and reduce the dry shrinkage.However,the high content of calcium hydroxide also leads to a large amount of deposition,which increases the number of microcracks and improves the dry shrinkage rate.(4)The fiber dry shrinkage experiment shows that the addition of carbon fiber,glass fiber and polypropylene fiber can reduce the dry shrinkage rate.With the increase of fiber content,the dry shrinkage first decreases slowly and then increases significantly.Due to the low density and high tensile strength of carbon fiber,the improvement effect of carbon fiber is obviously better than that of glass fiber,and the improvement effect of polypropylene fiber is the worst.The improvement effect is the best when the content of carbon fiber is 0.3%.SEM analysis shows that excessive fiber content will lead to serious agglomeration and agglomeration in the microstructure of alkali stimulated specimens,increase cracks and pores,and lead to the increase of dry shrinkage.Fig [39] Table [12] Reference [83]... |
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