Fabrication And Functional Prorperties Of Multi-Walled Carbon Nanotube/Cement Composites

Most construction cements today are hydraulic, and generally based on Portland cement, composed primarily of limestone, certain clay minerals and gypsum. Effort to mitigate structural failures in cement is a constant endeavor that has employed a range of materials. Owing to the high aspect ratio and elastic modulus, microfiber reinforcement has led to significant improvement of cement mechanical properties. Recently, the cement-based composite filled with some conductive microsize carbon fiber (CF) has attracted considerable interest. This type of composite not only presents superiority in strengthening, anti-cracking, and toughness, but also smart self-sensing functions. It could be embedded into key components of civil infrastructure as an intrisic sensor, real-time diagnosing the stress and possible damages in long-term service. Yet, the dimension of the domain hydration product of cement, calcium silicate hydrates, is generally hundreds of nanometers, flaws at the nanoscale remain with microfiber reinforcement. Since carbon nanotube (CNT) was documented in 1991, CNT has increasingly drawn great attention as the ultimate fiber with ultrahigh aspect ratio. CNT exhibits excellent mechanical property, good chemical and thermal stability, predominant electrical and microwave absorption property, and has the unique nano-effect of one dimensional nano-structure. CNT has developed to be the perfect reinforce for various matrix material, and gives the composite many novel functions. Hence, CNT could be a superior candidate substituted CF as the reinforce to cement matrix to develop a type of intrinsic nanocomposite with single or multiple functions.However, during processing such nano-material, CNT tend to tangle and form aggregates due to strong intermolecular van der Waals interactions, which leads to the poor dispersion of CNT in cement matrix. How to disperse CNT uniformly in cement matrix and increase the interfacial interaction between CNT and matrix are the primary problems urgent to be solved. Several processes of surfactant (bath or tip) ultrasonic dispersion, chemical functionalization, or electrical field-induced were experimentally utilized to disperse multi-walled CNT (MWNT) in aqueous solution, associated with beating or high-speed homogenizing mixture methods, to provide MWNT dispersion in the cement matrix. Results reveals, after suitable surfactant selection and sufficient tip ultrasonic dispersion, and high-speed homogenized mixing, isolated MWNT fibers with a high degree of dispersion in the matrix have been achieved. The uniform MWNT distribution at the microscale contributes to the observed increase in mechanical property (flexural, compressive strength), electrical conductivity of the cured MWNT/cement composite (MWNT/CC), with respect to the reference cement paste (Plain/C).The loading-crack mouth opening displacement (P-V) curve of MWNT/CC was acquired with three-point bending method under ASTM procedure. The test results indicate that modest MWNT addition inhibits cracking at the nanoscale level, and improve the fracture property of cement matrix. The resultant fracture toughness (KIC) and critical opening displacement (δc) of MWNT/CC can be enhanced by 175.21%, 54.77%, relative to the Plain/C, respectively.The vibration-reduction behavior of MWNT/CC beam suspended with elastic backups was tested with free attenuation method. Time domain, model frequency domain technique was utilized to acquire its critical damping ratio (ξ) and basic frequency (f1), respectively. The flexural strength (σt) of MWNT/CC was subsequently studied. The in-filling and bridging and the elastic-plastic fluctuation effect of MWNT, the inner surface frictions between the network MWNTs and the interface between well-dispersed MWNT and cement hydration are attributed to the observed good damping capacity and mechanical reinforcement afforded by the MWNT/CC. Relative to the Plain/C, theξandσt of MWNT/CC can increase by 58.29%, 31.63%, respectively.Effect of environment moisture content and temperature on initial resistivity (ρ0) of MWNT/CC were tested and analyzed. The DC I-V and AC impedance characteristic of MWNT/CC was measured. The polarized effect is dramatically reduced with high MWNT loading and low moisture content, hereby MWNT/CC was encapsulated with epoxy after oven-dried to isolate it from moisture. Theρ0 of MWNT/CC with 2.0% MWNT loading almost linearly changes with temperature in the range of -10~50℃. The DC I-V characteristic of MWNT/CC exhibits nonlinear feature within applied±2.5 V. The feature of AC impedance module or complex impedance (Z) versus frequency (f) of MWNT/CC show similar"U"shape, and the Z steadily decreases with f higher than 105 Hz.The resistivity (ρ) of MWNT/CC tested with four-electrode method, is dramatically affected by the addition of MWNT, its percolation threshold is around 1.526 vol.%. Theρof MWNT/CC was real-time sampled by voltage-dividing method, associated with the applied pressure of a transducer, and the longitudinal stains of the strain guages. The piezoresistivity property of MWNT/CC under monotonic and cyclic loading with two different loading rates and stress amplitudes was repectively studied. Results reveal, there exists good stress-strain relationship of MWNT/CC with 0.5% MWNT loading, and the sensitivity and repetitive stability of piezoresistivity feature of the MWNT/CC under either monotonic loading phase, or cyclic upload phase, or cyclic download phase, is the highest and best of all compositions nanocomposites. The tunneling current density and possible contact resistance of MWNT distribtuted in the corresponding composite can be effectively altered with MWNT fiber spacing (the barrier width).Acid oxidation treatment can greatly improve the dispersing efficiency of MWNT, and the bonding between it and cement matrix, which is helpful to further improvement of the mechanical property and fracture toughness of MWNT/CC. Hybrid nanoscale carbon black (CB) is helpful to further enhance the fracture toughness and piezoresistivity property of MWNT/CC, rather than hybrid CF.The self-reinforced (mechanical, fracture toughness, damping) properties of MWNT/CC faciliate to develop it into a promising multifunctional pavement reinforcement material. Its intrinsic stress/strain-sensing property faciliate to develop this type of nanocomposite into a kind of new smart sensor, which could also be embedded into concrete structure for"real-time, on-site"monitoring.