Hydration
The chemical reaction between cement and water that hardens concrete. It is a reaction, not evaporation, which is why concrete cures underwater and why keeping it wet makes it stronger.
What does hydration mean?
Hydration begins as soon as water touches cement and continues for months or years as long as moisture and unreacted cement remain. It generates heat, which is helpful in cold weather and a problem in massive placements. The single most damaging misunderstanding in concrete work is treating hardening as drying, which leads crews to let slabs dry out exactly when they most need moisture.
In practice
This is the reason a slab you keep wet for a week ends up harder than the identical slab left to bake in the sun.