Graphene Nanoplatelet Reinforcement of Interfacial Transition Zones in Expansive Agent-Cement Composites

Expansive agents (EAs) are widely used in concrete projects but often aggravate interfacial transition zone (ITZ) weakness due to localized expansive crystal accumulation. This study proposes the incorporation of graphene nanoplatelets (GNPs) to reinforce the micromechanical and microstructural properties of the ITZ in EA-cement composites, with particular emphasis on changes in C−S−H phase assemblage, microstructural characteristics, and elemental distributions. Results showed that 0.10% GNP maximized mechanical performance, increasing total average indentation modulus and hardness by 65% and 90%, respectively, compared with the GNP-free system, while reducing ITZ thickness to 25−30 μm. ITZ porosity was reduced by 5−35%, accompanied by enhanced formation of C−S−H, portlandite, and ettringite, and an 87−90% increase in hydrated phase fraction. GNP incorporation increased the indentation modulus and hardness of loose-packed (LP), low-density (LD), and high-density (HD) C−S−H by 8−45% and redistributed phase fractions from loose-packed toward low/high C−S−H. The calculated bulk density of LP, LD, and HD C−S−H increased by 2.1−5.1%, 2.8−7.6%, and 3.6−7.8%, respectively. The local ITZ Ca/Si ratio decreased from 2.0 to 0.8−1.0, showing more polymerized, silica-rich C−S−H associated with expansive crystal regulation. This work provides a viable strategy for designing high-performance, nano-engineered, and expansive cement composites.