The Use of Microfine Cement to Enhance the Efficacy of Carbon Nanofibers and Microfibers in Portland Cement Mortars

Abstract

The research herein discusses the mixing techniques for multi-scale fiber-reinforced Portland cement-based materials and their results on the mechanical properties of the overall composite with emphasis on drying shrinkage cracking resistance. The primary fibers used in this dissertation were carbon nanofibers (CNFs) and milled carbon microfibers (MCMFs). Ordinary Type I/II Portland cement (OPC) was found to limit the concentration of CNFs that can be successfully incorporated into the material due to geometric clustering. Using a microfine Portland cement eliminated geometric clustering of CNFs and stabilized the mixture, allowing for high concentrations of CNFs up to 5% by weight of cement. CNFs were dispersed in the cement by sonicating the CNFs with the cement in pure ethyl alcohol in fixed proportions, and then the alcohol was evaporated and recaptured with a distillation column. The hybrid CNF/cement powder produced was used to make high-concentration fiber-reinforced mortars. MCMFs were added during sonication for multi-scale fiber reinforcement in concentrations up to an additional 5% by weight of cement, and larger microfibers were added during mixing for up to 13% total fibers by weight of cement.

Dispersion of CNFs in cement was analyzed using a theoretical model and a finite element algorithm that utilized SEM images of the hybrid powders. In both scenarios, CNF dispersion was hindered in OPC but not in microfine cement. Restrained ring drying shrinkage tests revealed that high concentrations of CNFs did not significantly benefit hybrid OPC mortar, but the CNFs did bridge microcracks in the hybrid microfine cement mortar and extended time until failure by up to 640%. The addition of MCMFs to the hybrid microfine cement mortar extended time until failure and peak microstrain in the restraining ring by at least 5,200% and 390%, respectively. Further testing revealed that high concentrations of CNFs can prove detrimental to OPC while high concentrations of MCMFs could prove highly beneficial. Multiple experiments also revealed that while high concentrations of CNFs can prove beneficial, the addition of high concentrations of MCMFs with CNFs can provide compounded benefits that are greater than the sum of the benefits from CNFs or MCMFs alone.