Research On In-situ Preparation And The High-temperature Mechanics Of Graphene-reinforced Cement-based Materials

At present,using nano materials as additives to modify cement-based composites has many advantages.These nano materials,such as carbon nanofibers,nano silica,carbon nanotubes and graphene-based materials,can not only serve as the seeding sites of hydration reaction and provide a place for early hydration reaction,but also effectively fill the pores between C-S-H(hydrated calcium silicate)matrix.However,the effective use of these nano materials to reinforce the cement-based composites needs to meet two basic prerequisites: Firstly,these nano materials need to be evenly dispersed in the cement matrix,so as to give full play to their excellent physical and chemical properties;Secondly,these nano materials need to be closely combined with cement-based materials.Therefore,the uniform dispersion of nano materials in cement matrix is of great significance for the engineering application of nano materials.On the premise of ensuring the basic properties of cement-based materials,graphene cement-based composites were prepared by a new and low-cost graphene preparation method.In brief,graphene was prepared and dispersed on the surface of cement particles by in-situ growth strategy.Further microscopic morphology and characterization results showed that graphene attached to the surface of cement particles was uniformly dispersed in the cement matrix after simple physical dispersion.Compared with the control group,the compressive strength and flexural strength of the test group containing 3% wt(mass ratio of cement)carbon source increased by 38% and 48.9% respectively.Through the experimental characterization of environmental scanning electron microscope(SEM)and thermogravimetric analysis(TGA),the authors found that the uniformly dispersed graphene significantly improved the hydration degree of cement,and the cement matrix also formed a denser microstructure compared with the control group.Mercury intrusion test(MIP)shows that graphene grown in situ can significantly regulate the pore structure.In addition,due to the use of cheap glucose as carbon source material,the cost of graphene grown in situ is about 14% of that of commercial graphene.The graphene in-situ growth strategy proposed in this thesis will help to reduce the engineering application cost of graphene-based cement materials and further promote the multifunctional application of cement-based materials.In the second half of this thesis the author focuses on the high temperature properties of graphene reinforced cement-based composites.By summarizing the research on the mechanical properties of graphene-based cement materials at high temperature,the author found that graphene-based materials not only have a good toughening effect on cement at room temperature,but also can effectively inhibit the internal damage and deterioration of cement materials at high temperature.At present,graphene reinforced cement-based composites have attracted extensive attention because of their excellent high temperature resistance.However,the mechanism of graphene in thermal damage is not clear.Therefore,the second half of this paper focuses on the mechanical properties,pore structure changes and interface evolution of graphene toughened cement-based materials at high temperature.SEM test results show that graphene dispersed in the cement matrix deforms out of plane at high temperature,and the interface between graphene and cement matrix changes with the increase of temperature,which finally drives the evolution of pore structure.Accordingly,the toughening effect of graphene on cement-based materials first decreases and then increases.When the temperature exceeds 400°C,the strengthening domain of graphene changes from mesopores less than 50 nm to capillary pores,which helps to improve the strengthening efficiency of graphene on cement mortar.In general,the second half of the article focuses on the interface evolution process between graphene and cement-based materials at high temperature,and deeply explores the influence of interface evolution on the pore structure of cement matrix through analysis tools such as MIP,SEM and XRD,which will help to optimize the design of graphene composites at high temperature.