The use of microfine cement to enhance the efficacy of carbon nanofibers with respect to drying shrinkage crack resistance of portland cement mortars

Significant research has recently been aimed at quantifying the effects of carbon nanofibers and carbon nanotubes in portland cement pastes and mortars. Such efforts have shown that mechanical properties can increase with low concentrations of carbon nanofibers but have marginal improvement or are negatively affected with high concentrations. The objective of this paper is to evaluate the use of a microfine cement to enhance the efficacy of carbon nanofibers in portland cement mortar with respect to cracking resistance via enabling higher nanofiber concentrations. Experiments are performed with concentrations of carbon nanofibers up to 3% by weight of cement using either Type I/II or microfine cement. The primary test implemented was a restrained ring drying shrinkage test; unrestrained drying shrinkage tests, elastic modulus tests, and scanning electron microscopy imaging were performed to provide supplemental data to explain the observations from the restrained ring drying shrinkage tests. It was found that Type I/II cement mortars either lost performance or had insignificant gains with respect to cracking resistance, and all Type I/II mortar mixtures had losses in stiffness with the addition of high concentrations of carbon nanofibers. In contrast, microfine cement mortars had increased shrinkage cracking resistance and no loss in stiffness with increasing amounts of carbon nanofibers (up to the 3% by weight of cement tested in this research). The microfine cement mortar with 3% carbon nanofibers by weight of cement delayed the experimentally measured time of cracking in the ring test by a factor of up to 3.89. The delay in visible cracking time was attributed to microcrack bridging by the carbon nanofibers as imaged by scanning electron microscopy.