Effect Of Graphene Oxide On Properties And Mechanism Of Cement Fly Ash System

In modern concrete,fly ash has become an indispensable part of cementitious materials.It can not only enhance the strength and durability of concrete at the later stage,but also save energy and reduce emissions and environmental load.However,the impact of fly ash on cement and materials is also very prominent.On the one hand,it will reduce the early performance of cement-based materials.On the other hand,adding fly ash will increase the mobility of paste and float itself.Graphene oxide(GO for short)is a new type of two-dimensional nanomaterials.It can not only improve the strength of cement based materials at all ages,but also increase the viscosity of paste and improve the durability of test samples.However,adding graphene oxide into cement paste may cause paste sticky and affect its performance.Meanwhile,GO still has many shortcomings such as large shrinkage and so on.The addition of GO into the cement fly ash(C-FA for short)system can make up the deficiency of graphene oxide and fly ash.At the same time,it can also strengthen its advantages.At present the study on the influence of GO on the C-FA system is less,the influence cannot form a unified conclusion.Therefore,based on the above research,a series of experiments were carried out to systematically explore the effect of GO on the rheological,hydration and performance of C-FA cementitious materials.The fluidity,rheological parameters and the correlation between fluidity and parameters of GO on C-FA system was studied by a fine rheometer.The results showed that due to the large surface area of GO,when adding into the cement fly ash system,the formation of flocculation structure in the paste will increase with the increasing of GO.When the amount of fly ash is 20%,the yield stress and plastic viscosity of GO doped 0.03wt% are increased by24.9% and 34% respectively than that of 0.01wt%,while the fluidity of the paste is reduced by 30.6%.The flocculation structure formed by the presence of GO can be destroyed to some extent by the fly ash in the paste.Therefore,on the one hand,the synergistic effect of both of them prevents the paste from dilute and floatation caused by the incorporation of fly ash.On the other hand,it also prevents the increase of viscosity and the workability of paste caused by the addition of graphene oxide.Experiments were carried out to study the combined water content of fly ash system,reaction degree of fly ash,thermogravimetric analysis,hydration temperature rise,calcium hydroxide crystal form and hydrated product morphology.The results show that the GO has no obvious effect on the early hydration of cement fly ash cementitious material.But GO can refine the crystal size of the total calcium hydroxide in the cement stone.When the amount of fly ash is 20%,the size of GO 0.03% is reduced by 9.2% than that without GO,which effectively promotes the bonding force with the gel and strengthens the early performance of the system.GO can promote the secondary hydration of fly ash in the composite cementitious material system at the later stage,thus improving the performance of the system in the later period.The strength,pore structure,shrinkage performance and resistance to chloride ionpermeability of the cement fly ash system were studied.The results show that the compression and flexural strength of the whole age of the test block can be significantly enhanced after the incorporation of the GO into the C-FA system.Especially during the 3 day age,the group GO-3 FA-10 increased by 12.1% compared with that of group GO-0 FA-10,and the flexural strength increased by 21.4%,which significantly increased the early strength of hardened paste and improved the low flexural strength of C-FA system at the early stage.GO can improve the pore structure of cement paste at later stage,and reduce porosity of 60 day cement paste.And the total pore volume of group GO-3 FA-20 is lower than that of group GO-0 FA-20 by about 13.7%.The addition of graphene oxide increased the pore volume of<100nm,and increased the volume of transition pore and gel pore.Meanwhile,with the increase of GO content,the shrinkage of cement paste increased,and the incorporation of fly ash could reduce shrinkage.GO reduces the number of connected liquid phase in the test block,thus enhancing the resistance to chloride ion permeability of cement fly ash mortar.