What process causes cement to lose its placidity?

The correct choice is hydration, which is the process by which cement reacts with water to form a hardened mass. When cement is mixed with water, a series of chemical reactions occur that lead to the formation of various compounds, including calcium silicate hydrate, which provides strength and stability to concrete. This process begins almost immediately after water is added to the cement and continues over time as the mixture cures.

As hydration progresses, the initially fluid cement paste loses its workability or placidity and begins to set, transitioning from a paste to a solid state as it hardens. This is crucial for construction, as it determines how and when the concrete can be shaped and placed. Understanding the hydration process is essential for ensuring proper handling and application of cement-based materials in construction projects.