Dispersion Of Carbon Nanotubes And The Mechanical Properties Of Carbon Nanotubes Reinforced Cement-based Composites

Carbon nanotubes are nanoscale fibers, they can be divided into single-walled Carbon nanotubes (SWCNTs) and muiti-walled carbon nanotubes (MWCNTs) according to the layer number of carbon atoms. Single-walled Carbon nanotubes were observed for the first time by Iijima S in1991, and muiti-walled carbon nanotubes were prepared two years later, and this tremendous study have raised a research interest worldwide due to the specific structure and excellent physical and chemical properties of carbon nanotubes. Among them, the studies on the composites of MWCNTs are one of the most important fields. Their marvelous mechanical properties can enhance the strength and toughness of cement-based materials greatly, their unique electrical and photoelectric properties make them good candidates for improving the conductivity of polymer materials and preparing new photoelectric polymer composites, their characteristic structure makes it possible to be used for synthesizing one-dimensional nanometer composites filled with metal or metallic oxide.Portland cement concrete is the most widely used construction material because of its low cost, room temperature setting, wide availability and well documented properties for design and construction. While, its self-weight is high and the flexural strengths are relative low. Various methods have been used to improve the toughness of cements and concrete, and the most effective is fiber reinforcement. Recently, MWCNTs have been used as fillers in cement matrix composites to improve both the mechanical and electrical properties, as well as the electromechanical and electromagnetic behavior.However, the enormous aspect ratio and strong van der Waals attraction between MWCNTs is a major obstacles to the production and application of these composite materials. Therefore, it is necessary to precondition MWCNTs to improve the dispersing properties and eliminate aggregation. Aiming at the dipersion of carbon nanotubes, the main work and achievements are as following:(1) Four surfactants, gum arabic (GA), triton x100(Tx100), cetyl trimethyl ammonium bromide (TB) and sodium dodecyl sulfate (SDS) were individually or combinatorially employed as the dispersants to evaluate the dispersion of MWCNTs. Combined with ultrasonic treatment and acid treatment, eleven suspended solutions were fabricated with surfactant. Observation the dispersion with stewing-centrifugation method, the experimental results show that GA has the best dispersion impact on MWCNTs as the solution retained stabilization till after170minutes. The UV-vis was first used to evaluate the absorbency variation of MWCNTs over time, the test results show that Tx100blends with GA has the best effect on MWCNTs; when the concentration of GA is0.45g/L. and the variation is little; comparison results show that suspension of GA with concentration of0.45g/L has the best dispersibility. FTIR was used to measure the surface group of MWCNTs, the results show that there are hydroxide radical and carboxyl group on the surface of MWCNTs when treated by mixed acid.(2) Stable homogeneous suspensions were prepared using gum arabic(GA) as dispersant with ultrasonic treatment, in order to probe the dispersity and mechanism of MWCNTs in aqueous solution. Observation the dispersion with UV-vis and TEM. The concentration measurement shows that the optimum concentration of GA is0.45g/L; furthermore, the suspension is rather stable, the concentration of MWCNTs decrease only by11.45%when standing for80h. The stabilization mechanism of the MWCNTs in aqueous solution with surfactant GA was discussed by measuring isotherm adsorption, and the isotherms of GA adsorption on MWCNTs indicates a "SL" two-step mechanism of adsorption, and the isotherms reach the saturation plateau at GA concentration of0.45g/L. GA can improve the hydrophily and dispersibility of MWCNTs by means of the cladding of long molecule.Stable homogeneous suspensions of MWCNTs were prepared and incorporated into Portland cement pastes. The MWCNTs were surface-modified prior to being dispersed in the cement paste matrix. Both the mechanical behavior and microstructure of the newly formed composites were studied. The main work and achievements are as following:(1) The mechanical strength of the cement paste presented increasing and then decreasing trend with the increasing of the M WCNTs content, and it was developed with the increase of the curing age; varied with the water-cement ratio, the mechanical strength presented increasing and then decreasing trend, and the strength of samples doped with MWCNTs was higher than the referee samples; at28days curing, with the water-cement ratio of0.35. a0.08wt.%MWCNTs addition increased flexural strength by43.6%to a maximum of15.8MPa; the highest compressive strength is close to84.5MPa for a0.10wt.%MWCNTs addition, it was9.2%higher than the referee sample.(2) Addition of MWCNTs increases both the compressive and flexural strengths of cement mortar. The compressive strength of mortars reinforced with0.10wt.%addition of MWCNTs resulted in an increase of21.6%. The addition of MWCNTs also improved the flexural strength of the composites. A0.08wt.%addition of MWCNTs increased flexural strength by38.5%. I lowever. the maximum addition of0.15wt.%resulted in a slight decrease. The mechanical strength increased with the increase of the curing age. (3) After curing, the flexural toughness tests were performed by three-point bending tests in a computer controlled electro-hydraulic servo universal tester (MTS318). Addition of MWCNTs increases both the fracture energy and flexural toughness index of cement composites. MWCNTs addition also improved flexural toughness index of the samples. A0.08wt.%MWCNTs addition with the water-cement ratio of0.35increased flexural toughness index of the samples by52.8%and the fracture energy165.1%.The mechanism of MWCNTs on cement-based composites has also been analysed using micro measuring and testing technique integrated with the theory of composites. It indicated the mechanism was that the MWCNTs were highly dispersed in the suspensions and the matrix, MWCNTs could promote the hydration of cement and improve the microstructure of the cement-based composites, and there was strong binding strength between MWCNTs and matrix.