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Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are quickly becoming two of the most promising nanomaterials because of their unique mechanical properties. The size and aspect ratio of CNFs and CNTs mean that they can be distributed on a much finer scale than commonly used microreinforcing fibers. As a result, microcracks are interrupted much more quickly during propagation in a nanoreinforced matrix, producing much smaller crack widths at the point of first contact between the moving crack front and the reinforcement. In this study, untreated CNTs and CNFs are added to cement matrix composites in concentrations of 0.1 and 0.2% by weight of cement. The nanofilaments are dispersed by using an ultrasonic mixer and then cast into molds. Each specimen is tested in a custom-made three-point flexural test fixture to record its mechanical properties; namely, the Young's modulus, flexural strength, ultimate strain capacity, and fracture toughness, at 7, 14, and 28 days. A scanning electron microscope (SEM) is used to discern the difference between crack bridging and fiber pullout. Test results show that the strength, ductility, and fracture toughness can be improved with the addition of low concentrations of either CNTs or CNFs.

Carbon nanofibers (CNFs) and nanotubes (CNTs) are known to be extremely strong and stiff, and their potential as reinforcement has been of interest to many investigators in the past decade. One of the most important keys for fully harnessing the properties of any type of fiber is to control the distribution in the material matrix. As far as CNFs–CNTs are concerned, the strong attraction among nanoscale fibers due to van der Waals forces makes this task difficult. This study focuses on some of the problems that prevent a uniform distribution of CNFs–CNTs in cement paste and the methods used in the past to enhance dispersion. The first phase of the experimental program investigates the effect of using superplasticizers accompanied by sonication on the dispersion of CNFs in water and paste. The second phase focuses on the problem of cement grain size and limitations that the use of fine grain cement causes. Finally, on the basis of results and past studies, suggestions are made for achieving enhanced dispersion of CNFs–CNTs in cement paste.

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To study the effects of functionalized carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the mechanical properties of cement composites, both untreated and treated CNFs and CNTs were added to cement paste in concentrations of 0.1% and 0.2% by weight of cement. The surface-treated nanofilaments were functionalized in a solution of sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ). The nanofilaments were dispersed by using an ultrasonic mixer and were then cast into molds. Each specimen was tested in a custom-made three-point flexural test fixture to record the mechanical properties (i.e., the Young's modulus, flexural strength, ductility, and modulus of toughness) at the age of 7, 14, and 28 days. The microstructure was analyzed by using a scanning electron microscope. Untreated CNTs and CNFs were found to enhance the mechanical properties of cementitious materials, whereas the acid-treated CNTs and CNFs degraded the mechanical properties.