Properties And Mechanisms Of Ultra-High Performance Concrete With Ni Coated Carbon Nanotubes

Ni Coated Multi-walled Carbon Nanotubes(NiMCNTs)are excellent reinforcing/modifying nanofillers for developing(Ultra)high-performance,multifunctional/smart concrete due to their excellent mechanical properties,electrical conductivity,thermal properties and dispersibility(good surface wettability).This paper mainly studies the influence of the length,diameter and content of NiMCNTs on the mechanical and electrical properties of Ultra-High Performance Concrete(UHPC),and establishes the uniaxial compressive constitutive relationship of composites.The effects of NiMCNTs on the mechanical properties of UHPC under different water to cement ratios and different cement varieties are also investigated.In addtion,the reinforcing/modifying mechanisms of the mechanical/electrical properties of UHPC filled with NiMCNTs are discussed through the analysis of microstructure and hydration product characteristics as well as theoretical calculations.The specific research contents and results are as follows:(1)NiMCNTs filled UHPC with different aspect ratios and different contents were prepared,and their mechanical and electrical conductivity properties were tested to study the effects of NiMCNTs aspect ratio and content on the performance of composites.The results show that low content of NiMCNTs with a large aspect ratio(carbon nanotube content is only 0.1wt.%)can significantly improve the mechanical properties of UHPC,while NiMCNTs with a small aspect ratio play a better strengthening effect at high content.The flexural strength/compressive strength of UHPC filled with 0.5wt.%NiMCNTs with an aspect ratio of 125 can be increased by 33.98%/25.24%.NiMCNTs with an aspect ratio of 1500 at 0.5wt.%content can increase the compressive toughness and bending toughness of the composite by 46.77%and 41.25%,respectively.The elastic modulus and Poisson's ratio of UHPC containing 0.25wt.%NiMCNTs(1000 aspect ratio)can be increased by 17.80%and 41.68%,respectively.The uniaxial compressive constitutive model established by continuous damage mechanics theory can well describe the stress-strain relationship of the specimen.Besides,adding NiMCNTs can reduce the early resistivity of UHPC,0.25wt.%NiMCNTs with an aspect ratio of 200 cause a decrease in resistivity of composites cured for 3d/7d of 35.76%/52.32%.(2)NiMCNTs filled UHPC were prepared by using two types of cement and two water to cement ratios to study their mechanical reinforcement behavior.The research results show that Portland cement is more effective than Ordinary Portland cement in enhancing the flexural strength of NiMCNTs filled UHPC,and the flexural strength enhancement effect of composite with a water to cement ratio of 0.3 is better than that of NiMCNTs filled UHPC with a water-cement ratio of 0.24.The flexural strength of NiMCNTs filled UHPC using Portland cement and 0.3 water to cement ratio can be increased by 20-30%.The compressive strength of composites fabricated with Portland cement as cementing material and 0.3 water to cement ratio and composites fabricated with Ordinary Portland cement with 0.24 water to cement ratio can both be improved,and the improvement range are 20-25%and 15-20%,respectively.Overall,the condition of using Portland cement and 0.3 water to cement ratio can maximize the enhancement effect of NiMCNTs on the mechanical properties of composites.(3)The microstructure and hydration product characteristics of NiMCNTs filled UHPC were analyzed,and the mechanical calculation theory of fiber-reinforced cementitious composites was carried out to explore the reinforcing and modifying mechanisms of NiMCNTs on the mechanical and electrical conductivity properties of composites.The analysis shows that UHPC filled with an aspect ratio of 1000 and 125 of NiMCNTs has high interface bonding strength,small porosity and most probable pore-size,uniform distribution of pore size,and high degree of polymerization of hydrated calcium silicate(C-S-H)gels,thus their mechanical enhancement are more obvious.UHPC filled with an aspect ratio of 1500 of NiMCNTs has a large number of NiMCNTs with pull-out effect and high composite efficiency.This can effectively inhibit the formation and development of microcracks,so the toughening effect of NiMCNTs is effective.In summary,the incorporation of NiMCNTs is beneficial to prevent the generation and development of microcracks in composites,improve the pore structure,decrease the crystal orientation of calcium hydroxide,promote the transition of the silicate tetrahedral structure in C-S-H from dimer to dimer short chain and increase the polymerization degree of C-S-H gels,thus effectively improving the mechanical properties of UHPC.The calculation results show that NiMCNTs are pulled out when composite is damaged.This can consume a lot of energy,thereby enhancing the mechanical strength and toughness of composites.The excellent electrical conductivity of NiMCNTs and the ability to contact each other in concrete are conducive to the formation of the conductive network structure,which has a positive effect on improving the electrical conductivity of UHPC.However,the contact points of NiMCNTs are reduced during the hydration process because the hydration products attached to the surface of NiMCNTs.This results in the weakening modification effect of NiMCNTs on the electrical conductivity of UHPC along with curing age.