Due to their remarkable mechanical properties, graphene nanoplatelets (GNPs) are widely employed to reinforce matrix components such as cement materials. Recent findings have indicated that GNPs can be used as additives to improve the mechanical properties of cement. However, the enhancement mechanism is still indefinite. In order to investigate the reinforcement mechanism, this work studied the effect of GNPs on the hydration behaviour of Portland cement through an experimental investigation with isothermal calorimetry and thermogravimetric analysis. The results indicate that the addition of GNPs has little effect on the cement hydration process and the degree of hydration. As revealed by scanning electron microscopy and mini-slump tests, the GNPs were mechanically dispersed in the cement matrix. Special attention was paid to the physical functions between GNPs and cement and a possible mechanism is proposed to explain the formation pathway for GNP–cement composites. This research provides deeper understanding regarding the use of GNPs for reinforcing and toughening cement-based composites.