Correct Answer: D. Quick setting cement
Solution:
Quick Setting Cement is the most suitable choice for underwater construction. Its key property is that it sets very rapidly once mixed with water. This is crucial in underwater or running water conditions because the mortar or concrete must harden before it can be washed away by the water. This rapid setting is achieved by reducing the amount of gypsum (which acts as a retarder) and grinding the cement clinker to a finer consistency.
Correct Answer: D. Clinker
Solution:
The calcined product, which is formed by heating the raw materials (like limestone and clay) at very high temperatures (around 1450°C) in a kiln, is known as clinker. This hard, nodular material is the primary component of cement. It is then cooled and ground with a small amount of gypsum to produce the final cement powder.
Correct Answer: B. Tricalcium aluminate
Solution:
In cement chemistry notation, C represents CaO (Calcium Oxide), A represents Al₂O₃ (Aluminum Oxide), and S represents SiO₂ (Silicon Dioxide). Therefore, C3A stands for 3CaO·Al₂O₃, which is Tricalcium Aluminate. This compound is responsible for the initial setting and high heat of hydration in cement.
Correct Answer: A. Dicalcium silicate
Solution:
The ultimate or long-term strength of cement is primarily provided by Dicalcium silicate (C₂S). While Tricalcium silicate (C₃S) is responsible for the early strength (in the first 7 to 28 days), C₂S hydrates and hardens slowly, contributing to the progressive increase in strength over a longer period, such as months or even years.
Correct Answer: A. Portland-pozzolana cement
Solution:
Portland-pozzolana cement (PPC) is highly resistant to chemical attacks, particularly from sulphates found in seawater. The pozzolanic material reacts with calcium hydroxide (a byproduct of cement hydration) to form additional stable compounds, making the concrete denser, less permeable, and more durable in aggressive marine environments.
Correct Answer: D. 25 cm
Solution:
The floor of a cement warehouse needs to be strong and thick enough to support the heavy loads of stacked cement bags and movement of equipment. A thickness of 25 cm is a common recommendation to ensure adequate load-bearing capacity and durability for such industrial applications.
Correct Answer: D. All option are correct
Solution:
Calcareous materials are those rich in calcium carbonate (CaCO₃). All the listed options are sources of calcium carbonate used in cement manufacturing. Limestone is the most common, but cement rock (a naturally occurring limestone with clay) and marine shells (like oyster shells) are also used as the primary calcareous component.
Correct Answer: A. 30 minutes
Solution:
The initial setting time of Ordinary Portland Cement is typically a minimum of 30 minutes. This is the crucial window during which the concrete remains plastic and workable. Therefore, all operations—mixing, transporting, placing, and compacting—must be completed within this timeframe to avoid cold joints and ensure the concrete sets properly as a monolithic structure.
Correct Answer: C. Tricalcium aluminate
Solution:
Tricalcium aluminate (C₃A) is the most reactive of the four main Bogue compounds in Portland cement. It reacts almost instantly with water, generating a significant amount of heat and contributing to the initial set (stiffening) of the cement paste. Its rapid reaction is controlled by the addition of gypsum during the grinding process.
Correct Answer: D. Strength per unit mass is low
Solution:
This statement is incorrect. Ferro-cement is a type of thin-wall reinforced concrete known for its very high strength per unit mass (high strength-to-weight ratio). Its other properties include being impervious, having high shock resistance, and often not requiring formwork for construction, making it suitable for structures like boats and water tanks.
Correct Answer: D. All option are correct.
Solution:
The method of voids is a principle used in concrete mix design. It works on the hierarchical assumption that the total volume is made up of solid particles and the voids between them. The core assumptions are that the cement paste fills the voids in the fine aggregate, and the resulting mortar then fills the voids in the coarse aggregate. To ensure all voids are filled, a slight excess (often 10%) is added. Therefore, all the given statements are part of the assumptions made in this method.
Correct Answer: C. Pressure compaction of the bags on lower layers
Solution:
When many cement bags are placed on top of one bag in a warehouse, compaction starts to occur in the lower bag due to the weight of the cement placed on top of it. This action is called "warehouse pack" of cement. To prevent this, cement should not be stacked higher than 15 bags, and bags should be kept with a clear space (Gap) of 25 to 45 cm from the walls.
Correct Answer: C. 28 days
Solution:
The standard curing period for Ordinary Portland Cement (OPC) to reach its characteristic compressive strength is 28 days. By this time, the hydration process has progressed significantly, allowing the concrete to achieve approximately 99% of its ultimate (long-term) strength. While strength gain continues slowly for a long time, the 28-day mark is the industry standard for design and testing purposes.
Correct Answer: B. 20–25%
Solution:
For the complete hydration of Portland cement, two types of water are needed: bound water for chemical reactions and gel water to fill the pores in the hydration product. The water required purely for the chemical reactions (to form hydrated compounds) is approximately 20% to 25% of the weight of the cement. Another 15% is needed to fill the gel pores, leading to a total water requirement of about 35-40% for full hydration and workability.
Correct Answer: C. To make the plaster non-shrinkable
Solution:
Lime is added to cement plaster to improve its properties. While it enhances workability, its primary role in the top coat is to reduce shrinkage. Lime has high water retentivity, which slows down the drying process, minimizing the formation of shrinkage cracks and resulting in a smoother, more durable finish.
Correct Answer: A. Low-heat Portland cement
Solution:
Low-heat Portland cement is specifically designed to have a slower rate of hydration. This is achieved by reducing the proportions of Tricalcium Silicate (C₃S) and Tricalcium Aluminate (C₃A). As a result, it gains strength more slowly at early ages compared to OPC, but its ultimate strength is comparable or even higher. This slow heat generation makes it ideal for mass concrete structures.
Correct Answer: A. Reacts fast with water
Solution:
Tricalcium aluminate (C₃A) is the most reactive compound in cement. It reacts almost instantaneously with water, a process that generates a large amount of heat. This rapid reaction is primarily responsible for the initial setting (stiffening) of the cement paste. While it contributes to very early strength, its contribution to the ultimate strength is minimal.
Correct Answer: B. 0.3
Solution:
Normal (or standard) consistency of cement is the percentage of water required to produce a standard paste, as determined by the Vicat apparatus. For most types of Ordinary Portland Cement, this value typically falls in the range of 26% to 33%. Therefore, 0.3 (or 30%) is the most representative value.
Correct Answer: B. 0.05
Solution:
Rapid Hardening Cement is ground finer than Ordinary Portland Cement to achieve faster strength gain. According to Indian Standards, when tested for fineness using the I.S. Sieve No. 9 (90-micron sieve), the weight of the residue should not exceed 5% of the original weight. This corresponds to a value of 0.05.
Correct Answer: C. Fineness test
Solution:
The air permeability method, most commonly performed using Blaine's air permeability apparatus, is a standard procedure for conducting a fineness test on cement. This test measures the specific surface area of the cement particles, which is a direct indicator of how finely the cement has been ground.