Final answer:
Concrete hardens through the process of setting, which involves a chemical reaction between water and cement.
Step-by-step explanation:
The process through which concrete hardens by reaction between water and cement is known as setting. When water is added to the cement, a chemical reaction called hydration occurs, resulting in the formation of calcium-silicate-hydrate (C-S-H) gel. This gel fills the gaps between the aggregate particles and hardens over time, creating a solid mass.