On the mechanisms for improved strengths of carbon nanofiber-enriched mortars

Abstract

As nanomaterials, carbon nano-fillers have manyfold benefits in concrete applications, and while of considerable research interest, their impacts on cement phases are still not fully understood. We applied multiple techniques, including mercury intrusion porosimetry, X-ray diffraction, thermal gravimetry, infrared spectroscopy and electron microscopy to investigate in detail the effects of a pre-mixed carbon nanofiber admixture in mortars. The addition of carbon nanofibers was observed to: (i) alter pore structure to a lower total porosity with greater proportion of micropores, (ii) delay the hydration reaction of alite, (iii) shift hydration of alite towards the formation of portlandite and (iv) foster formation of calcium carbonate phases. The ensemble of results indicate that improved performance is unlikely to be due to formation of additional calcium silicate hydrates, as has been suggested in the literature, but instead is due to the combined effect of nano-fiber bridging and pore-filling by portlandite and calcium carbonates.