Hydration is the chemical reaction that makes concrete harden.
Specifically, hydration is the process by which Portland cement becomes a firm, hardened mass after the addition of water. This essential reaction transforms a mixture of dry cement powder and water into the durable, solid material we know as concrete.
How Does Hydration Work?
When water is added to Portland cement, a chemical reaction begins between the water molecules and the compounds in the cement powder. This reaction doesn't just dry out the mix; it chemically incorporates the water into new solid compounds.
Here are some key aspects of the hydration process:
- Chemical Transformation: The cement compounds (like alite, belite, aluminate, and ferrite) react with water to form hydration products, primarily Calcium Silicate Hydrates (CSH) and Calcium Hydroxide (CH). The CSH gel is the main binder that gives concrete its strength.
- Binding Action: As hydration progresses, the hydration products grow and intertwine, filling the spaces between the cement particles and binding them together, along with the aggregate (sand and gravel) in the mix.
- Hardening: This process of forming intertwined hydration products leads to the gradual stiffening and hardening of the concrete mix over time.
- Insolubility: The reference states that the hydrated cement is relatively insoluble in water. This is crucial for concrete's durability, allowing it to resist deterioration when exposed to moisture after hardening.
- Heat Evolution: Hydration is an exothermic reaction, meaning it releases heat. During hydration, heat is evolved and temperatures in mass concrete may reach 60 °C. This heat generation, known as the "heat of hydration," can impact the concrete's properties, especially in large structures (mass concrete) where dissipating the heat is difficult.
Practical Insights
Understanding hydration is vital for concrete performance:
- Water-Cement Ratio: The amount of water added significantly impacts hydration. A lower water-cement ratio (within a practical range) generally leads to stronger concrete because it results in a denser structure of hydration products. Too little water, however, won't allow complete hydration.
- Curing: Maintaining adequate moisture and temperature after placement is called curing. Proper curing ensures that hydration continues efficiently, allowing the concrete to develop its intended strength and durability. Drying out too early stops the hydration process prematurely.
- Temperature Effects: Hydration proceeds faster at higher temperatures and slower at lower temperatures. Extreme temperatures (freezing or excessive heat) can be detrimental to the process and the final concrete quality. The heat evolved during hydration itself needs to be managed in large pours to prevent cracking.
In summary, hydration is the fundamental chemical process initiated by adding water to Portland cement, transforming it into a strong, durable, and relatively water-resistant binding material that holds concrete together.