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 (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

(d) Developing interlocking needle-like crystals of hydrated silicates.

Step 1: Understanding Cement Curing

When cement-based plasters are applied, they don't simply dry; rather, they undergo a chemical process called 'curing,' which is crucial to developing the desired hardness and strength of the material.

Step 2: Exploring Water's Role in Curing

Water is essential in curing because it reacts chemically with the cement particles through a process known as hydration. This chemical reaction is necessary for the cement's strength and durability.

Step 3: Examining the Options

Consider each option: - (a) Hydrating sand gravel mixed with cement: Sand and gravel do not hydrate; it is the cement that does. - (b) Keeping it cool: While water can cool the material, this is not the primary intention in curing. - (c) Converting sand into silicic acid: Sand does not convert into silicic acid during curing. - (d) Developing interlocking needle-like crystals of hydrated silicates: This is a key part of the hydration process in cement, as these crystals contribute to the strength and structure of the set material.

Step 4: Choosing the Correct Answer

The correct option is (d) because water facilitates the development of interlocking needle-like crystals of hydrated silicates which are critical in cement plaster strength and hardness.

Key Concepts

Hydration Process

The hydration process in cement is a critical chemical reaction that transforms the cement powder into a hard, solid mass. Unlike just drying, hydration involves the combination of cement particles with water.
This reaction results in a series of changes, turning the mix into a rock-like material. Once water is added, it starts to react with different compounds in the cement, primarily the silicates.
These reactions produce hydrated compounds that slowly form strong crystals, which bind the mixture together. Without this process, cement would not develop the necessary sturdiness and longevity.

• Initiation: Begins immediately as water is added.
• Formation: Hydrated compounds form over hours and days.
• Strengthening: Continues to harden and strengthen over weeks.

Curing with water is integral to maintaining optimal moisture, ensuring that the reaction reaches full potential without drying out too quickly.

Chemical Reaction in Cement

Cement hardens due to a fascinating chemical reaction, rather than mere drying. At its core, this chemical reaction is the combination of water with cement to form new compounds.
Two main ingredients in this reaction are tricalcium silicate and dicalcium silicate. Once mixed with water, these silicates hydrate to produce calcium silicate hydrate and calcium hydroxide.
The crystals formed through hydration characterize the strength that we associate with concrete or cement-based structures.

• Initial Mix: Water penetrates the cement particles.
• Hydration: Silicates react, causing the mixture to harden.
• Heat Release: Exothermic nature of the reaction releases heat.

This process does not only make the mix solid but also creates the chemical bonds that contribute significantly to the durability of most construction materials.

Interlocking Crystals of Hydrated Silicates

When we talk about cement's strength and structural integrity, the focus falls on interlocking needle-like crystals of hydrated silicates. These crystals form throughout the curing process.
As the hydration reaction unfolds, these crystals grow and interlock with each other, creating a dense network.
This network is what gives the cement its remarkable strength and stability. The interlocking nature ensures that even tiny spaces within the cement are filled, making it exceptionally robust.

• Growth: Crystals extend as hydration progresses.
• Interlock: Crystals weave together for strength.
• Network: Fills gaps for overall rigidity.

Ultimately, these interlocked crystals are essential, as they directly improve the tensile and compressive strength of the finished cement structure. They form the backbone of durable construction, ensuring the longevity of buildings, roads, and infrastructure.