Functionalized Graphitic Nanoreinforcement for Cement Composites

Physical and mechanical properties of graphitic nanomaterials, in particular multiwalled carbon nanotubes (MWCNTs) and graphene nano-platelets (GNPs) make them promising candidates for nanoreinforcement of cement composites. The two key challenges associated with the incorporation of MWCNTs and GNPs are to attain uniform dispersion and interfacial bonding within the composite matrix. The effects of three main-stream dispersion techniques (namely, ultrasonication, acid-etching, and surfactant-coating) on the mechanical properties and microstructure of MWCNT- and GNP-cement composites were experimentally studied. Compressive strength tests and different characterization techniques including dynamic light scattering, Raman spectroscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron spectroscopy were employed to evaluate the dispersion and embedment of nanoreinforcement in cement mortar and paste. As a result of significant and consistent compressive strength enhancements, further supported with material characterization results, acid-etching and surfactantcoating were selected as suitable functionalization techniques to manufacture MWCNTand GNP-cement composites, respectively. The validity of the selected functionalization techniques was further investigated through bending tests on single-edge notched cement paste beams. The results were studied with respect to flexural strength and stiffness. In addition, the effects of incorporating well-dispersed acid-etched MWCNTs on the fracture behavior of cement paste were studied through bending tests on notched beam samples. This research contributes to filling the gap in understanding whether dispersibility of MWCNTs and GNPs in aqueous solutions by means of well-known dispersion and functionalization techniques results in good dispersion and embedment (i.e., resulting in consistent and repeatable enhancement in relevant mechanical properties) in cement matrices. This gap is addressed by presenting new experimental evidence on improved mechanical properties as well as supporting evidence from material characterization tests, in particular for the case of GNP-reinforced mortar and cement paste. A novel contribution of this work is offered by the results of digital image correlation measurements aimed at visualizing full-field strain maps from the area surrounding the notch in cement paste beams. These results provide insight into the morphology and evolution of the fracture process zone in nanoreinforced cement paste vis-à-vis unreinforced counterparts, and constitute new evidence on the potential fracture toughening effect of MWCNTs