Experimental Research On Mechanical Properties And Durability Of Carbon Nanotube Ultra-high Performance Concrete

In recent years,the mass production and use of cement has led to the deterioration of living environment,acid rain,extreme cold and other extreme weather has seriously affected the safety of buildings.As a new material,UHPC not only has excellent mechanical properties and durability,but also has a green and environmentally friendly preparation technology,which can greatly reduce the use of cement and other materials.Since the discovery of carbon nanotubes at the end of the 20 th century,they have been widely used in cement-based composites due to their high bending strength,low density,high elastic modulus and other performance,which improve the microstructure of the matrix and improve the mechanical properties and durability of cement-based composites.The research and use of high-performance concrete is still in the initial stage,and the mix ratio,preparation process,curing mechanism,and changes in mechanical properties are still unclear.Therefore,it is of great significance to carry out related research on carbon nanotube ultra-high performance concrete for its popularization and practical engineering application.This paper takes carbon nanotube ultra-high performance concrete as the research object,focusing on the preparation of carbon nanotube dispersion,the test of the mechanical properties of carbon nanotube ultrahigh performance concrete,and the test of the sulfate corrosion resistance of carbon nanotube ultra-high performance concrete.Combined with SEM analysis,the structure of carbon nanotubes after dispersion,the changes of carbon nanotube ultra-high performance concrete structure and hydration products before and after sodium sulfate erosion were analyzed.The main research contents and results are as follows:(1)Using multi-walled carbon nanotubes as raw materials,combined with three methods of polyvinylpyrrolidone(PVP),mechanical damage and ultrasonic dispersion,the effects of mechanical damage temperature and ultrasonic dispersion time on the dispersion and stability of carbon nanotube dispersions were studied.The static method and the centrifugal method compare the best dispersion method.The results show that the stability and dispersibility of carbon nanotube dispersions prepared by mechanical stirring,ultrasonic dispersion and PVP are the best;the stability and dispersibility of carbon nanotube dispersions prepared by mechanical stirring and PVP are the worst.The optimal temperature of mechanical stirring is 30 ℃,and the optimal ultrasonic time is 40 min.After dispersing,the carbon nanotube dispersion liquid is black,and there is no obvious settlement in the tube,which can be stored for a long time.The carbon nanotube dispersion liquid can be used in the preparation of ultra-high performance concrete.(2)Ultra-high performance concrete was used as matrix and carbon nanotubes were used as fiber admixture.The effects of the content of carbon nanotubes,curing mechanism and curing age on the compressive strength and flexural strength of UHPC with carbon nanotubes were studied.The paper found that the compressive strength and flexural strength showed a trend of first increasing and then decreasing with the increase of the content of carbon nanotubes.The optimum content of 0.15%and 0.1% existed;from the point of view of strength,the best curing method is heat.Water curing and hot water curing can stimulate the "volcanic ash reaction" of the cementitious material for secondary hydration,increase the C-S-H gel content,and the maximum compressive strength and flexural strength values reach 125.3 MPa and 12.7 MPa.From the practical engineering application,the best maintenance method is straw curtain maintenance.Under hot water curing,the compressive strength and flexural strength in 7 days rise rapidly to more than 85% of that in 30 days.The strength of standard curing and straw curtain curing increased more than 73% within 7 d of 30 d.(3)The quality loss and mechanical property loss of ultra-high performance concrete with carbon nanotubes under different sulfate concentrations(0%,5%,10%)and different carbon nanotube content(0%,0.05%,0.1%,0.15%,0.2%)were studied.The research show that: the UHPC with carbon nanotubes has good resistance to sulfate erosion,and the incorporation of carbon nanotubes can improve the resistance to sulfate erosion of UHPC.When the content of carbon nanotubes is 0.05% and 0.1%,the flexural strength and the flexural strength and corrosion resistance coefficient are greater than 1.When the test piece is eroded by a 5% sodium sulfate solution,the compressive strength and flexural strength first decrease,then increase and then decrease,and the quality has increased,and the increase is greater than that of the control group;When the specimens were eroded by sodium sulfate solution with a concentration of10%,the compressive strength first decreased,then increased and then decreased,and the flexure strength first increased,then decreased and then increased.The mass of the specimens with carbon nanotubes of 0.05%,0.1%and 0.15% increased,while the mass of the specimens with carbon nanotubes of 0% and 0.2% decreased.(4)Scanning electron microscopy(SEM)was used to analyze the changes in the micro morphology and main elements of the concrete before and after carbon nanotubes and sulfate erosion.The results show that the dispersed carbon nanotubes are uniformly dispersed,mostly in short free state distribution,and the number of kink nodes is significantly reduced.A large number of C-S-H gels were formed by hydration in the specimens,and the carbon nanotubes were tightly wrapped by the gels without obvious pores and cracks.After erosion,the amount of C-S-H gel decreased,and a small amount of ettringite and gypsum were generated.The pores and cracks did not increase significantly.The content of Si and Ca elements decreased after erosion.The dispersion of carbon nanotubes under different dispersion modes,the basic mechanical properties and sulfate corrosion resistance of carbon nanotubes ultrahigh performance concrete were comprehensively analyzed.It is verified that it is feasible to use carbon nanotubes as fiber admixture to improve the mechanical properties and sulfate resistance of ultra-high performance concrete.