Hydration of cement is a chemical reaction between cement particles and water. This process occurs primarily through two mechanisms: through-solution and topochemical. In the through-solution process, anhydrous compounds dissolve into their constituents, hydrates form in the solution, and then precipitate from the supersaturated solution. The topochemical process involves solid-state reactions at the cement particle surface. The through-solution process dominates the topochemical process at the early hydration stages.
Aluminates hydrate rapidly, reacting immediately with water to release significant heat. Gypsum is added to slow this reaction by releasing sulfate ions that reduce the solubility of aluminates, thereby controlling the setting rate. The hydration products of aluminates are calcium aluminate hydrate and calcium aluminoferrite hydrate. Calcium silicates hydrate to form calcium-silicate-hydrate (C-S-H) and calcium hydroxide, with C3S hydrating faster than C2S. Complete hydration of tricalcium silicate (C3S) results in 61% calcium-silicate-hydrate (C-S-H) and 39% calcium hydroxide, while hydration of dicalcium silicate (C2S) leads to 82% C-S-H and 18% calcium hydroxide. C-S-H is crucial for cement strength, thus, increasing the C2S content relative to C3S can enhance the ultimate strength of concrete.
