Question

In curing cement plasters, water is sprinkled from time to time. This helps in: (a) Hydrated sand gravel mixed with cement (b) Keeping it cool (c) Converting sand into silicic acid (d) Developing interlocking needle like crystals of hydrated silicates

Short answer

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

Step-by-step solution

Understanding Cement Curing

Cement curing involves the process of maintaining adequate moisture, temperature, and time to allow the hydraulic cement to achieve its desired properties. The main goal is to keep the cement surface from drying too quickly, which can cause cracks and weak spots.

Importance of Water in Curing

Water plays a crucial role in the hydration process during curing. When water is sprinkled on curing cement plasters, it prevents the premature drying of the surface and facilitates the chemical reaction necessary for the cement to bond properly.

Result of Proper Curing

Proper curing leads to the development of the internal structure of cement, which includes the formation of interlocking crystals. These crystals are primarily made up of hydrated silicates, which significantly enhance the strength and stability of the cement matrix.

Evaluating Options

Among the given options, the sprinkling of water on curing cement helps in the development of interlocking needle-like crystals of hydrated silicates. This option (d) directly relates to the fundamental goal of proper cement curing, which ensures strength and durability.

Key concepts

Hydration Process

The hydration process is a critical phase in the curing of cement. It occurs when water is added to cement, initiating a series of chemical reactions. These reactions transform the dry cement into a solid mass that binds aggregate particles, like sand and gravel, into concrete.

During hydration, the main compounds in cement, primarily calcium silicates, react with water to form new compounds. One of the key products formed during this stage is calcium silicate hydrate, often abbreviated as C-S-H.

• C-S-H is the primary phase that imparts strength to the set cement.
• This reaction generates heat, known as the heat of hydration, which is crucial for the curing process.
• Proper hydration requires the right amount of water; too much or too little can affect cement strength.

These complex reactions are what harden the mixture into a strong, stone-like substance, integral for the structural integrity of buildings.

Interlocking Crystals

As the cement cures, interlocking crystals begin to form. These are needle-like structures that develop from the compounds created during the hydration process.

The interlocking nature of these crystals plays a crucial role in the cement's structural properties. Here’s why they’re important:

• They enhance the mechanical strength of the cement by linking the particles together.
• The interlocking crystals help distribute internal stresses, allowing the cement to withstand external forces more effectively.
• This crystalline network minimizes shrinkage and cracking as the cement continues to cure.

Interlocking crystals contribute significantly to the rigidity and durability of the final concrete structure, ensuring that it remains stable even under pressure.

Hydrated Silicates

Hydrated silicates, like calcium silicate hydrate (C-S-H), are crucial components in the curing cement. They form when the main compounds in Portland cement react with water during the hydration process.

• These compounds give the cement its characteristic high strength and durability.
• The C-S-H phase is the glue that holds everything together within the hardened cement matrix.

Hydrated silicates fill the microscopic pores between the grains of cement, significantly reducing permeability. This low permeability prevents water ingress, protecting the concrete from harmful chemicals and ensuring longevity.
Additionally, as hydrated silicates form and settle, they contribute to decreasing porosity, which is vital for the structural integrity and dense packing of the cement paste.