Early-age hydration of ordinary Portland cement is now acknowledged to originate from nucleation and crystal growth of calcium–silicate–hydrates. The acceleration mechanism of water-soluble inorganic substances, such as calcium chloride, is still unclear. In the present study, the acceleration mechanism was approached in two independent ways. First, the acceleration effect of filler materials (limestone and quartz powder) and calcium chloride (CaCl₂) were studied experimentally with a conduction calorimeter. Second, the calcium chloride effect was studied with thermodynamic modelling. The experimental test results showed that the filler materials accelerated the acceleration period of ordinary Portland cement hydration, whereas calcium chloride accelerated hydration throughout the 24 h measuring period. A synergistic effect was observed with fillers and calcium chloride. Thermodynamic modelling indicated that calcium chloride provides a supersaturation with respect to pure calcium–silicate–hydrate. It was concluded that the supersaturation provides a qualitative explanation for both calcium chloride-induced acceleration and the synergistic effect observed with fillers