On the aspect ratio effect of multi-walled carbon nanotube reinforcements on the mechanical properties of cementitious nanocomposites

For their novel mechanical, thermal, chemical, and electrical properties, carbon nanotubes (CNTs) are widely used in many fields of nanocomposite materials. In cementitious nanocomposites, CNTs can act as effective bridges to minimize and limit the propagation of micro-cracks through the matrix, under the conditions of well dispersion of the CNTs within the matrix and good bonding between the CNTs and the surrounding hydrated cement matrix. This study focuses on the effect of different concentrations of long multi-walled carbon nanotubes (MWCNTs) – high length/diameter aspect ratios of 1250–3750 – and short MWCNTs – aspect ratio of about 157 – in cement paste. Flexural bending tests were performed to evaluate four major mechanical properties of the cement/CNTs composites at ages of 7, 14, and 28 days. Results show that the flexural strength of short 0.2 wt.% MWCNT and long 0.1 wt.% MWCNT increased by 269% and 65%, respectively, compared to the plain cement sample at 28 days. The highest increase in ductility at 28 days for the short 0.1 wt.% MWCNT and the short 0.2 wt.% MWCNT was 86% and 81%, respectively. It is concluded that nanocomposites with low concentration of long MWCNTs give comparable mechanical performance to the nanocomposites with higher concentration of short MWCNTs. Clear evidence was obtained from scanning electron microscope images for micro-crack bridging; many of the MWCNTs were stretching across the micro-cracks showing CNTs breakage and pull-out.