*Article* **Effect of Multi-Walled Carbon Nanotubes on Strength and Electrical Properties of Cement Mortar**

**Elena Cerro-Prada \*,†, Rosalía Pacheco-Torres † and Fernando Varela †**

Construction, Infrastructure and Transportation Department, Universidad Politécnica de Madrid (UPM), Alfonso XII, 3, 28014 Madrid, Spain; rosalia.pacheco@upm.es (R.P.-T.); fernando.varela@upm.es (F.V.)

**\*** Correspondence: elena.cerro@upm.es; Tel.: +34-91-067-4612

† These authors contributed equally to this work.

**Abstract:** This work aims to investigate the effects of multi-walled carbon nanotubes (MWCNTs) on the strength and electrical properties of cement mortar. MWCNTs were added to cement mortar in four different concentrations: 0.00 wt.%, 0.01 wt.%, 0.015 wt.%, and 0.02 wt.% by the mass of cement. The consistency, density, setting time and compressive and flexural strength of mixes were tested and analyzed at 28 and 90 days curing time. Mechanical performance tests confirm an increase of 25% and 20% in the ultimate compressive and flexural strength respectively, which results from MWCNT 0.02 wt.% loading at 90 days curing time. The resistivity measurements in mortars with 0.01 and 0.015 wt.% MWCNT loading result up to 10% decrement at both 28 and 90 days curing. Activation energy calculations show fully accordance with these statements, resuming that 0.01 wt.% MWCNT appears to be the most effective loading scheme to produce certain conductivity enhancement in cement mortar.

**Keywords:** multi-walled carbon nanotubes; cement-based materials; flexural strength; compressive strength; electrical resistivity; activation energy
