Acoustic emission investigation of the mechanical performance of carbon nanotube-modified cement-based mortars

•The first AE investigation of CNT-modified cement mortars.•Air entrapment in matrix overcame by vacuum processing before mixing with the cement.•Maximum improvements in flexural strength at tube loading of 0.4 wt% of cement.•Simultaneous maximization of two key AE descriptors at the critical nanotube loading.•Mortar’s fracture mode changes at the critical tube loading, from shear to tensile. Concrete’s inherent brittleness, low tensile strength and premature micro-cracking phenomena can be improved by embedment, in the bulk material, of carbon nanotubes (CNT), the 1-D allotrope of carbon exhibiting a remarkable combination of mechanical and transport properties. The present study reports on the flexural and compressive mechanical performance of mortars reinforced with variable loadings of multi-walled carbon nanotubes. Four point bending tests were performed with simultaneous in situ monitoring of the acoustic emission (AE) activity of the mortars, a technique herein applied for the first time in nanotube-reinforced cement, and the data were used to evaluate the efficiency of the non-destructive method in assessing the sensitivity of the mortars’ properties on tube loading. A CNT concentration of 0.4% relative to cement mass, was found related to optimal improvements in flexural strength. Impressively, two key AE descriptors, namely average frequency and the rise time over amplitude ratio were independently found to simultaneously maximize at the same tube loading while their temporal variation revealed a transition in the material’s fracture mode with tube loading. Complications due air entrapment in the materials facilitated by nanotube presence are discussed in the text and a viable resolution is suggested.