MCQs on Cement

Correct Answer: B. C3A

Solution:

Tricalcium Aluminate (C₃A) is the most reactive of the Bogue's compounds. It reacts almost instantaneously with water, releasing a large amount of heat. This rapid reaction is responsible for the initial setting or "flash set" of cement if its activity is not controlled by adding gypsum during manufacturing.

Correct Answer: C. Ferro–Cement

Solution:

Ferro-cement is a composite material consisting of a rich cement mortar mix applied over layers of wire mesh. This technique allows for the construction of very thin (typically 1 to 3 cm), lightweight, yet remarkably hard and strong surfaces, making it suitable for applications like boat hulls, water tanks, and shell roofs.

Correct Answer: A. 3

Solution:

There are generally considered to be three main methods for applying mortar in ferro-cement construction:

1. Hand Plastering: The most common method where mortar is applied manually.
2. Semi-Mechanized Process: A combination of manual application and mechanical tools.
3. Guniting or Shotcreting: Mortar is pneumatically projected at high velocity onto the mesh.

Correct Answer: A. A gap of 0.3 m is to be maintained between cement bag and wall, while storing cement.

Solution:

To prevent moisture from damaging the cement, bags should be stored away from walls. The standard recommended gap is about 30 cm, which is equal to 0.3 m. A gap of 100 cm would be excessive and inefficient use of space. Therefore, statement A is the correct guideline.

Correct Answer: A. Bauxite and limestone

Solution:

High Alumina Cement is manufactured by fusing a mixture of bauxite (which is rich in alumina) and limestone (which provides the lime) at high temperatures. Unlike Portland cement, gypsum is not typically added during the grinding process.

Correct Answer: A. Change in volume of cement after setting

Solution:

The soundness test is crucial to ensure that the cement, once set, does not undergo any appreciable change in volume. Excessive expansion due to the presence of unburnt lime or magnesia can lead to the cracking and disintegration of the hardened concrete. Tests like the Le Chatelier or Autoclave test are used to measure this potential for expansion.

Correct Answer: D. 0.05

Solution:

Cement is highly hygroscopic, meaning it readily absorbs moisture from the atmosphere. If the absorbed moisture content exceeds 5% (or 0.05), the cement undergoes partial hydration, forming lumps and significantly losing its binding properties and strength. At this point, it is considered useless for structural purposes.

Correct Answer: Ambiguous Question

Solution:

Note: This is a flawed question as multiple options are correct benefits of air entrainment. The official commission did not consider any single option correct in its revised answer key.

• (A) is a known side effect, as the entrained air voids can reduce compressive strength.
• (B) is a primary reason for using air entrainment, as it greatly improves freeze-thaw resistance.
• (C) is another key benefit, as the microscopic air bubbles act like ball bearings, improving workability.
• (D) is also a benefit, as the air bubbles improve the cohesion of the mix.

Given that all options describe effects of air entrainment, the question is ambiguous. However, increased resistance to weathering (B) is often cited as the most critical advantage.

Correct Answer: (d) A, B, C and D

Solution:

All the listed specifications are standard for a conventional bag of cement in many regions. These dimensions and weights are used for logistical planning, storage calculations, and mix design by volume.

Correct Answer: B. 45% calcium oxide and 35% silica

Solution:

Blast furnace slag, a by-product of iron manufacturing, has a chemical composition similar to Portland cement. Its primary constituents are calcium oxide (lime) and silica. A typical composition consists of about 45% calcium oxide (CaO) and 35% silica (SiO₂), which gives it its cementitious properties.

Correct Answer: B. Air-permeability method

Solution:

The Air-permeability method (using Blaine's apparatus) is a standard laboratory test to determine the fineness of cement by measuring its specific surface area. The Le-Chatelier apparatus is used for soundness, and the Vicat's apparatus is for consistency and setting times.

Correct Answer: B. Air-permeability method

Solution:

Fineness is a critical property of cement that influences its rate of hydration, and therefore its strength development and heat generation. The Air-permeability method is an essential quality control test to ensure the cement is ground to the correct fineness specification for its type.

Correct Answer: B. rapid hardening cement

Solution:

Rapid hardening cement is preferred for road pavements because it gains strength quickly. This high early strength allows the road to be opened to traffic much sooner than if ordinary Portland cement were used, minimizing disruption and construction time.

Correct Answer: A. C₃A > C₄AF > C₃S > C₂S

Solution:

The rate of hydration determines how quickly cement reacts and gains strength. The order from fastest to slowest is:
1. C₃A (Tricalcium Aluminate): Reacts almost instantly.
2. C₄AF (Tetracalcium Aluminoferrite): Reacts rapidly, but slower than C₃A.
3. C₃S (Tricalcium Silicate): Responsible for early strength (first week).
4. C₂S (Dicalcium Silicate): Reacts the slowest, contributing to long-term strength.

Correct Answer: B. 0.5 mm

Solution:

When testing for the final setting time of cement with the Vicat apparatus, a specific needle with an annular attachment is used. The central needle is designed to project 0.5 mm below the outer circular cutting edge. The final set is considered to be reached when the central needle makes an impression on the cement paste, but the outer edge does not.

Correct Answer: C. sink to the bottom of the bucket

Solution:

Good quality cement is denser than water and should be free of lumps. When a small quantity is thrown into water, its particles should immediately sink to the bottom. Floating particles indicate impurities or that the cement has pre-hydrated due to moisture exposure, reducing its quality.

Correct Answer: B. 1 : 3

Solution:

As per standard testing procedures (e.g., IS 4031), the mortar for creating test cubes and briquettes for strength testing is prepared with a standard proportion of 1 part cement to 3 parts standard sand by weight. This ensures consistency and comparability across different cement tests.

Correct Answer: B. High alumina cement

Solution:

High Alumina Cement offers excellent resistance to chemical attack from chlorides and sulphates, which are prevalent in marine environments. This high resistance makes it the most suitable choice for construction in or near seawater to ensure long-term durability. Blast furnace slag cement also has good resistance but High Alumina Cement is generally considered superior in these conditions.

Correct Answer: C. 30 minutes

Solution:

Although rapid-hardening cement gains strength faster than ordinary Portland cement, its setting times are similar. The Indian Standard specifies a minimum initial setting time of 30 minutes for this type of cement, which is the same as for OPC. This provides sufficient time for mixing and placing the concrete.

Correct Answer: C. Soundness

Solution:

The Le Chatelier's device is specifically used to perform the soundness test on cement. This test determines the expansion of cement due to the presence of excess free lime (CaO), ensuring the cement will not undergo significant volume changes after setting.