Calcium-alumino-silicate-hydrate (C-A-S-H) and calcium hydroxide (CH) are the two primary hydration products that play crucial roles in the mechanical properties of cementitious materials, particularly in the viscoelastic behavior known as creep. This study used hydrated C3S and a mixture of the synthesized C-A-S-H and CH to identify the interactions between CH and C-A-S-H. The interactions that dominate the mechanical properties mainly operate from physical and chemical perspectives. CH exhibited a filling effect on C-A-S-H in the physical interactions, demonstrating multi-scale particle packing efficiency. The particle size of CH per unit volume of the mixture was proposed as a critical parameter for the filling effect. Regarding chemical interactions, the synthetic system exhibits interactions primarily through van der Waals forces through compaction. In contrast, the interaction in the hydration system primarily originates from the strong atomic bonding between C-A–S–H and CH at the nanoscale.