When water is added to cement, a chemical reaction, called hydration, takes place and the paste passes from a fluid and workable state to a solid one that binds aggregates together. During this reaction, cement and water are consumed to form several hydrated compounds, having the appearance of a rods network structure, which envelops aggregates and is responsible for the final strength of concrete. Cement is highly reactive with water, hence it might hydrate and then harden also with air relative humidity. This is the reason why cement shouldn’t be stocked for long periods before use, especially in moist climate.
