Optimum carbon nanotubes' content for improving flexural and compressive strength of cement paste

This study investigated the effect of multi-walled carbon nanotubes' (MWCNTs) weight fraction on the setting time and mechanical properties of cementitious composites. Different cement mixes containing CNT-to-cement weight fractions of 0.03, 0.08, 0.15, 0.25, 0.35 and 0.5 wt% were prepared in addition to the control mix. The initial and final setting times of the fresh pastes were measured on the cast day and the flexural and compressive strengths of the hardened samples were determined after 28 days of moist curing. The fractured surfaces of the samples were then examined using a scanning electron microscope (SEM). The results showed that the 0.25 wt% CNTs is the optimum weight fraction in terms of achieving maximum strength at a reasonable cost. Batches with lower CNTs' contents than 0.25 wt% demonstrated lower flexural and compressive strengths, whereas batches with higher CNTs' contents than 0.25 wt% produced similar or slightly higher strengths. Analysis of variance (ANOVA) confirmed that increasing CNTs' concentration above 0.25 wt% will not have a significant effect on the compressive and flexural strengths. Investigations of the microstructure, which was carried out using SEM, showed good dispersions of the nanofilaments within the cement matrix. Spots of agglomerations were noticed in batches containing 0.25, 0.35 and 0.5 wt%. SEM images have also indicated that CNTs were embedded within the cement hydration products. The study findings were useful for determining the CNTs' content required to achieve both optimum dispersion and maximum strength enhancement of cementitious composites.