Micromechanical modeling of the piezoelectric behavior of CNT cement-matrix composites

This study introduces a micromechanical model specifically formulated to capture the complex piezoelectric behavior of cement-based composites reinforced with multiwalled carbon nanotubes (MWCNTs). The proposed model simultaneously accounts for both dominant conduction mechanisms - conductive network formation and electron hopping - providing a more realistic and robust prediction of the overall electrical conductivity. The model explicitly integrates the effects of nanotube geometry, waviness (considering the intrinsic three-dimensional nature of the carbon nanotubes, CNTs), tunneling potential barrier height and the often-overlooked phenomena of nanotube agglomeration and segregation, which significantly influence the connectivity and performance of the conductive network. A key innovation of this work lies in the development of a novel quantum-mechanical approach to estimate the thickness of the inter-nanotube matrix region, by rigorously incorporating the physics of electrical tunneling. Furthermore, the model is extended to predict the piezoresistive response of the composite over a wide range of MWCNT concentrations, offering valuable insights for smart-sensing and structural health monitoring applications. The accuracy of the proposed model is validated through extensive comparison with experimental data from the literature, covering cement paste, mortar and concrete. Finally, a detailed sensitivity analysis highlights the most critical parameters controlling the electrical behavior of CNT-reinforced cementitious materials, providing practical guidelines for optimizing composite design.