This study analyzed the behavior of two laboratory-synthesized calcium silicates, C3S and C2S, after hydration in 8-M NaOH and in water as a control. Two- and 28-d mechanical strength values were determined and the products were characterized with XRD, TEM, and Si-29 and Na-23 MAS NMR. The results showed that hydrating C3S in a highly alkaline medium had no significant effect on the mechanical development of the material, whereas in C2S hydration, that medium hastened hydration substantially, impacting setting and hardening times. This finding has technological implications, given the low early-age reactivity of dicalcium silicate under normal hydration conditions.