Properties And Mechanisms Of Multifunctional Cementitious Composites With Carbon Nanotubes

Because carbon nanotubes（CNTs）have outstanding mechanical property,electrical property and chemical stability,they are promising fillers for developing high-performance,multi-functional and smart cementitious composites.In this thesis,the effects of the types and contents of CNTs on the mechanical,electrical,electromagnetic wave shielding and absorption properties of cementitious composites are investigated.Moreover,the properties of CNTs and the hydrated products（morphology,composition,structure and content）,AC impedance spectroscopy and the electromagnetic parameters of composites,and mechanical calculation of fibers reinforced cementitious composites are studied to analyze the reinforcing mechanisms of CNTs to cementitious composites.The main research contents and conclusions are as follows:（1）Cementitious composites with different types（including different diameters and lengths,functionalization,graphitization,nickel plating,helical and large-inner thin-walled CNTs）and the contents of CNTs were prepared,and their compressive and flexural strength were tested.The modified mechanisms of CNTs to mechanical properties of cementitious composites was analyzed by XRD,²⁹Si NMR,TG,SEM tests and mechanical calculation of fibers reinforced cementitious composites.The results show that CNTs with large diameter are very impactful for improving the mechanical properties of cementitious composites.Long CNTs are beneficial to improve the flexural strength of cementitious composites.The effect of functionalization,nickel plating and helical CNTs on the strength of cementitious composites is very significant.The relative and absolute compressive strength of cementitious composites with 0.5%of nickel coated CNTs increases by 78.8%/74MPa,and the relative and absolute flexural strength of cementitious composites with 0.5%of short hydroxylated CNTs is improved about 76%/6.6MPa compared to that of cementitious composites without CNTs.CNTs have no obvious effect on the structure of C-S-H gels,and inhibit cement hydration.However,widely distributed CNTs can reduce the crystal orientation of calcium hydroxide and improve microstructure of composites.Futhermore,bridging and pulling effect of CNTs are also important factors for enhancing mechanical property of composites.（2）The AC resistance and AC impedance of cementitious composites with CNTs were tested.The conductivity of cementitious composites with CNTs was studied,and the equivalent circuit model was established.The results show that 0.8%of long CNTs with large diameter can reduce the resistivity（193Ω·m）of cementitious composites by about 56%.The functionalization of CNTs is beneficial to improve the conductivity of the composites.Cementitious composites with 0.8%of graphitized CNTs present the highest conductivity,and their resistivity（71Ω·m）is about 63%lower than that of cementitious composites without CNTs.AC impedance spectra of cementitious composites with different types of CNTs are consistent with“Quasi Randles”type,and the equivalent circuit of cementitious composites with CNTs is Rs（Q₁（R₁W₁））（Q₂（R₂W₂））（Q₃R₃）.（3）The electromagnetic parameters of cementitious composites with CNTs were measured by indirect method,and shielding effectiveness and reflectivity of the composites were calculated by the theoretical formula.It is shown that the effect of CNTs on the electromagnetic wave shielding performances of cementitious composites is not obvious.The electromagnetic wave absorption properties of cementitious composites increase with the increasing contents of the CNTs.The electromagnetic wave absorption performance of cementitious composites containing carboxylated CNTs with short length is the best,and their minimum reflectivity（-36dB）is 4.3 times that of cementitious composites without CNTs.The absorption loss to electromagnetic wave of cementitious composites with CNTs can mainly be atributed to dielectric loss.The contribution of magnetic loss is near zero.