Embodiments relate to use of graphite nanoplatelets (GnP) to enhance the mechanical and durability characteristics of cement that may be used as cement sheaths in wellbores of oil and gas wells. Generally, undesired permeability of cement is caused by diffusion of trapped oil and/or natural gas through the cementitious matrix of the cement, leading to material degradation of the cement. Methods disclosed involve using modified GnPs (having physically modified surfaces or chemically modified surfaces energies) to generate a cementitious nanocomposite with uniformly dispersed GnPs, which can effectively arrest the undesired diffusion mechanism. Modified GnPs can also increase the strength of interfacial adhesion (e.g., interfacial bonds and interfacial energies) between the GnP and the cement matrix (e.g., hydrations of the cement). Physical modification of GnP can involve non-covalent treatment techniques. Chemical modification of GnP can involve covalent treatment techniques.