Influence of carbon nanotubes structure on the mechanical behavior of cement composites

Cement matrix composites have been prepared by adding 0.5% in weight of multi wall carbon nanotubes (MWCNTs) to plain cement paste. In order to study how the chemical–physical properties of the nanotubes can affect the mechanical behavior of the composite, we compared the specimen obtained by mixing the same cement paste with three different kinds of MWCNTs. In particular, as-grown, annealed and carboxyl functionalized MWCNTs have been used. In fact, while high temperature annealing treatments remove lattice defects from the walls of CNTs, hence improving their mechanical strength, acid oxidative treatments increase chemical reactivity of pristine material, consequently chemical bonds between the reinforcement and the cement matrix are supposed to enhance the mechanical strength. Flexural and compressive tests showed a worsening in mechanical properties with functionalized MWCNTs, while a significant improvement is obtained with both as-grown and annealed MWCNTs. The phase composition of the composites was characterized by means of thermo gravimetric analysis coupled with mass spectroscopy, while the mineralogy and microstructure were analyzed by means of an X-ray diffractometer and scanning electron microscope. The results are interpreted and discussed taking into account the chemical and physical properties of the MWCNTs by means of EDX, TGA, SEM and Raman analysis.