Problem 166 - A Complete Resource Book in Chemistry for JEE Main

Question

In curing cement plasters, water is sprinkled from time to time. This helps in \(\quad\) [2003] (a) hydrating sand gravel mixed with cement (b) keeping it cool (c) converting sand into silicic acid (d) developing interlocking needle like crystals of hydrated silicates

Step-by-Step Solution

Answer

The correct option is (d): developing interlocking needle-like crystals of hydrated silicates.

Step 1: Understanding the Context

When water is sprinkled on cement plasters, it assists in the chemical process of curing. This process is essential to ensure the cement hardens properly and gains strength over time.

Step 2: Analyzing the Options

Let's review each option: (a) refers to hydration of sand gravel, which is incorrect as sand does not hydrate. (b) suggests keeping the plaster cool, which is a side effect but not the main purpose. (c) involves converting sand into silicic acid, which does not occur in this context. (d) pertains to the formation of needle-like crystals, which is a part of the curing process.

Step 3: Identifying the Correct Option

The correct process involves the formation of interlocking long needle-like crystals of hydrated silicates. This growth happens when water reacts with cement, contributing to the structure and strength of the cured cement.

Step 4: Conclusion

Based on our analysis, option (d) best describes the purpose of sprinkling water on cement plasters, as it contributes to the development of interlocking crystals that reinforce the cement's structure.

Key Concepts

Hydration of Cement

Hydration in cement is a critical chemical reaction. When water is added to cement, it starts to react with the compounds present, mainly the silicates and aluminates. This process is known as hydration.

• Hydration initiates upon contact with water, beginning a complex series of reactions.
• The initial reaction creates a paste, binding the components as it hardens over time.
• This reaction is exothermic, releasing heat as cement transitions from a fluid to a rigid mass.

The hydration process does not happen instantly; it occurs over a period, depending on the specific type of cement, conditions, and amount of water.
This gradual reaction contributes to the cement gaining strength progressively rather than instantaneously.

Crystal Formation in Cement

One fascinating aspect of cement curing is the formation of crystals. As the hydration process continues, various compounds begin crystallizing in the mixed paste. These crystals grow and form a tightly packed structure. - The most well-known are the calcium silicate hydrates (C-S-H), which are needle-like crystals. - These crystals significantly contribute to the strength and stability of the cement. - Interlocking of these crystals forms a hardened and solid structure that we see in concrete and plasters. The nature of these crystals ensures that the cement matrix is dense and robust, preventing easy wear and providing durability. This is why water is crucial early in the cement hydration, as it allows for complete crystal development.

Cement Strength Development

As cement undergoes curing, its strength development is essential. This process ensures the material can withstand loads over time. - Initially, after mixing with water, cement gains a minor amount of strength. - Over time, with continual hydration, the strength increases, generally reaching around 70-80% of its full potential in about 28 days. - Factors affecting strength development include the water-cement ratio, temperature, and curing conditions. There is typically a direct relationship between time and strength development: the longer the cement cures with adequate moisture, the stronger it gets. Efficient curing practices, such as sprinkling water, ensure that hydration continues for crystal growth and results in long-lasting, durable concrete structures.