Carbon nanofiber cementitious composites: Effect of debulking procedure on dispersion and reinforcing efficiency

The use of new reinforcing materials like carbon nanofibers (CNFs) makes it possible to produce cement based nanocomposites with revolutionary properties. However, in order to take advantage of the CNF’s excellent reinforcing efficiency it is necessary to achieve a uniform distribution in the matrix. In this work, nanofiber cementitious composites were produced containing CNFs at an amount of 0.048 wt.% of cement. To achieve good dispersion of the CNFs, a method utilizing a surfactant and ultrasonic processing, was employed. The method was optimized using two parameters: the effect of ultrasonic energy and the effect of surfactant to CNF (SFC/CNF) ratio. Initially, the SFC/CNF ratio on the dispersion of two types of CNFs, one subjected to a new special debulking method and one with minimal debulking process, was investigated. An ultrasonic energy of 2800 kJ/l and a SFC/CNF ratio close to 4.0 was found to be optimal for effective dispersion. Following these values, cement based nanocomposites reinforced with four types of CNFs, subjected to different debulking processes and having different morphology, were produced. Their nanostructure was studied using scanning electron microscopy (SEM). Their mechanical performance was evaluated using fracture mechanics tests. All four CNFs were found to control nanoscale cracking. As a result, both the flexural strength and the stiffness of the nanocomposites were significantly improved. Furthermore, the reinforcing efficiency of the CNFs in the cementitious matrix was shown to depend on the debulking procedure: at later ages, the use of the CNF subjected to the special debulking process was found to be more efficient in improving the mechanical performance of the nanocomposites.