4.2.2. Hydration Products

Calcium Silicate Hydrate (C–S–H):

• Main product of hydration.
• Provides most of the strength.
• Amorphous in nature, forms a dense gel.

Calcium Silicate Hydrate, often referred to as C–S–H, is the primary product formed during the hydration of cement. When cement reacts chemically with water, C–S–H forms and binds the aggregates together within concrete. This binding is crucial as it provides the strength necessary for concrete structures. The unique property of C–S–H is its amorphous nature, meaning it does not have a fixed structure but rather forms a dense gel-like substance. This characteristic contributes to the overall strength of concrete over time.

Think of C–S–H like the glue that holds together the pieces of a puzzle. When you add water to cement, it’s like adding glue; the C–S–H fills in the gaps between the puzzle pieces (aggregates) and solidifies the whole structure, making it strong and robust.

Calcium Hydroxide (Ca(OH)₂):

• Also known as lime.
• Does not contribute to strength.
• Makes the concrete alkaline (pH ~12.5), which helps prevent corrosion of steel.

Calcium Hydroxide, commonly known as lime, is another product of cement hydration. While it does not contribute directly to the strength of concrete, it plays an important role in other aspects of concrete performance. One crucial benefit of Ca(OH)₂ is that it increases the pH of concrete to around 12.5. This high pH level is essential as it helps protect the steel reinforcement bars embedded in concrete from corrosion, thus increasing the longevity and durability of concrete structures.

Imagine diving into a pool of water that has a pH balance. If the pH is too low (acidic), it can not only be uncomfortable for swimmers but also cause damage to the pool’s structure. Similarly, in concrete, the high pH created by calcium hydroxide protects the structure and is vital for maintaining the integrity of the reinforcing steel.

Ettringite:

• Formed from the reaction between C₃A and gypsum.
• Controls the setting time.
• Excess ettringite later can lead to expansion (delayed ettringite formation).

Ettringite is a compound that forms during the hydration process, particularly from the interaction between Tricalcium Aluminate (C₃A) and gypsum. It plays a crucial role in controlling the setting time of concrete, allowing for a workable period during which the concrete can be mixed and placed. However, it’s important to manage the amount of ettringite formed, as excessive amounts can lead to delayed ettringite formation, resulting in expansion and potentially causing cracks in the concrete over time.

Consider ettringite like a timer in a cooking recipe. It helps control the right moments to add ingredients or change the cooking method. If you add too much of an ingredient that releases gas (like too much ettringite), it can cause the dish to expand beyond its intended size, leading to an overflow or mess. In concrete, managing ettringite ensures the mixture sets correctly without unwanted expansion.

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