What chemical action occurs when water is added to cement and aggregate in concrete preparation?

The correct choice, hydration, is a crucial chemical reaction in the preparation of concrete. When water is mixed with cement and aggregate, it initiates a series of chemical processes that lead to the hardening and strength development of concrete.

Hydration involves the reaction of water molecules with the cement compounds, which results in the formation of calcium silicate hydrate, the primary binding agent in concrete. This process is essential because it transforms the wet mixture into a solid mass, allowing it to gain structural integrity as it cures over time.

Understanding hydration is vital for achieving the desired strength and durability of concrete, as the ratio of water to cement significantly influences these properties. The other terms, such as condensation, evaporation, and oxidation, do not accurately depict the chemical interaction that takes place in concrete preparation. Condensation typically refers to the conversion of vapor into liquid, evaporation involves the transition from liquid to vapor, and oxidation involves the reaction of substances with oxygen, none of which are involved in the core process of cement hardening.