Effect of multiwalled carbon nanotube size on electrical properties of cement mortar composites

Cement mortars (CM) containing different contents and sizes of multiwalled carbon nanotubes (MWCNTs) were prepared using surfactants in combination with ultrasonic dispersion. The effects of the content, diameter and length of MWCNTs on the electrical conductivity of cement mortar and the self-inductive piezoresistive rate under single and cyclic loading were studied using the four-electrode method. The results show that the conductive percolation thresholds of CMs containing MWCNTs with different length–diameter ratios occur at an approximate MWCNT content of 0.1 wt%. The electrical conductivity of cement mortars with MWCNT diameters of 10–20 nm was the largest for a content of 0.1 wt%, and the values were 112% and 128.3% of the conductivity of the MWCNTs/CM with diameters of 20–40 nm and 40–60 nm, respectively. In addition, the fractional change in resistivity of the cement mortar specimens varied most significantly at a content of 0.75 wt% of MWCNTs; the fractional change in resistivity reached 14.27%. Adding small-diameter MWCNTs to cement mortar can maximise the sensitivity of electrical resistance of cement mortar to compressive stress.