Multiscale Performance of Cement-Based Composites with Carbon Nanofibers

Abstract

The use of short, randomly distributed fibers in cement-based composites has improved properties and allowed for multifunctional capabilities such as the ability to sense damage. Because flaws exist in cement-based composites from the nano- to the macroscale, the addition of nano- to macroscale fibers has the potential for further improving cement-based composites by supplying a hybrid effect. Carbon nanofibers (CNFs) have properties such as high aspect ratios and strength, which make them excellent candidates for nanoreinforcement.

The objective of this research was to investigate the inclusion of CNFs in cement pastes for use as nanoreinforcement. The specific objectives were to: (1) determine the effect of dispersion methods and CNF loading on CNF dispersion in solutions and cement pastes; (2) investigate the micromechanical properties of hydrated cement pastes containing CNFs, including representative major cement phases and phases in and around CNF agglomerates; (3) determine the effect of CNFs on the macromechanical properties (compression, splitting tension, and flexure) of cement pastes; and (4) evaluate the hybrid effect of CNFs and carbon microfibers on the microstructure and multiscale mechanical properties of cement pastes.

An integrated multiscale experimental approach including state of the art experimental characterizations with scanning electron microscopy, x-ray microanalysis, nanoindentation, optical microscopy, and traditional mechanical testing was used to better understand the processing-microstructure-dispersion and dispersion-property relationships for cement-based composites containing CNFs. Several CNF dispersing methods were examined, and the dispersion state in solution was found to not be indicative of the dispersion state in cement pastes. The best CNF dispersion was found when polycarboxylate-based high-range water reducer (P-HRWR) was used, and only this dispersion showed improvements in the macroscale flexural properties. Increases in flexural strength up to 65% were found when P-HRWR was used with the addition of 1% CNFs per weight of cement even with the presence of agglomerated CNFs. On the microscale, the CNFs were found to increase the percentage of high stiffness calcium silicate hydrates (C-S-H) at the expense of low stiffness C-S-H. However, around the CNF agglomerates, the micromechanical properties were significantly decreased.