hydration

Hydration is the chemical reaction between Portland cement clinker and water that transforms a powder into a solid structure. When water contacts cement particles, calcium silicates and aluminates dissolve slightly and recrystallize as hydration products, primarily calcium silicate hydrate (C-S-H), which binds aggregate particles together. This is not merely drying; it is an exothermic reaction that releases heat, typically 300 to 500 joules per gram of cement over the first 28 days.

The rate of hydration follows a predictable curve. An initial rapid reaction occurs in the first minutes, then slows dramatically as a protective layer of hydration products forms around each cement grain, slowing water access. This dormant period typically lasts 1 to 3 hours and is crucial for placing and finishing concrete. After this induction phase, hydration accelerates again, with the concrete developing most of its strength in the first 7 days, though full hydration can take months or years depending on cement type and curing conditions.

Hydration and concrete performance

Adequate water and temperature are essential for complete hydration. Concrete cured at 20°C (68°F) with sufficient moisture will hydrate normally; freezing temperatures dramatically slow the reaction, while heat accelerates it. Conversely, insufficient water during curing leaves internal cement particles unhydrated, creating weak points and reducing ultimate strength. This is why wet curing or sealed curing extends strength development and improves durability.

Different cement types hydrate at different rates. Type I Portland cement is standard; Type III develops strength faster through finer grinding; Type IV hydrates slowly and generates less heat, valuable for massive pours or hot climates. Supplementary cementitious materials like fly ash or slag also participate in hydration reactions, but typically require higher temperatures or longer timeframes than Portland cement alone.

Incomplete hydration weakens concrete and increases permeability, allowing water and chlorides to penetrate and corrode reinforcement. Differential hydration can also occur, where concrete cores remain less hydrated than outer layers if internal temperature or moisture conditions are unfavorable. Understanding and controlling hydration is therefore fundamental to designing concrete mixes and specifying curing regimes that meet structural and durability requirements.