Effect of Carbon Nanofiber Clustering on the Chemo-mechanical Behavior of Cement Pastes

Abstract

The degradation of cementitious materials typically begins on the nanoscale and progressively worsens to the macroscale, therefore, carbon nanofibers (CNFs) are excellent candidates for reinforcement of cementitious materials at the nanoscale due to their distinctive properties (high aspect ratio, high strength, low density, and corrosion resistance). The goal of this research was to develop a fundamental understanding of the relationships between the CNF dispersion state, microstructure, and chemo-mechanical behavior of cement pastes from the micro- to the macroscale during exposure to aggressive environments (i.e., decalcifying environment and sulfate attack). Destabilization of the CNF dispersion in the hydrating cement environment resulted in nano- and microscale CNF clustering in cement pastes. The CNF clusters influenced the chemo-mechanical behavior of the cement pastes with a chemical dissolution-filling effect seen in the clusters during decalcification that slowed the loss of flexural strength and the development of a reinforcement network that delayed cracking and spalling while allowing expansive pressures to grow during sulfate attack. Correlation between grid nanoindentation data and chemical phase composition revealed that the CNF clusters promoted the formation of high density C-S-H and an increase in the number of indents associated with high calcium phases. A decrease in indentation modulus and hardness that was independent of the silicon to calcium ratio was seen for the cement pastes with and without CNFs during decalcification. A micro-macro homogenization approach based on the Mori-Tanaka scheme and an area-averaging method was developed to upscale the local micromechanical properties derived from grid nanoindentation to the overall macroscale elastic modulus of the decalcified material. Overall, the micro-macroscale homogenization approach was able to capture the microscale mechanical changes due to decalcification as well as the influence of the CNF clusters, and a good correlation was achieved between the homogenized modulus and macroscopic experimental measurements.