MCQs on Cement

Correct Answer: A. 7.05 cm

Solution:

For testing the compressive strength of cement, a mortar cube is used, not a concrete cube. The standard size for this mould is 7.06 cm, which provides a face area of 50 cm². The value 7.05 cm is the closest practical dimension given in the options. The 15 cm (or 150 mm) cube is the standard size for testing the compressive strength of concrete.

Correct Answer: A. C₃S and C₂S

Solution:

The most important constituents of cement in terms of strength are Tricalcium Silicate (C₃S) and Dicalcium Silicate (C₂S). C₃S is responsible for early strength gain, while C₂S contributes to the long-term, ultimate strength. Together, they make up the bulk of the cement and are the primary strength-giving compounds.

Correct Answer: D. IS 90 μ sieve where at least 90% (by weight) should pass

Solution:

The sieve test is a simple and cheap method for checking the fineness of cement on-site. According to Indian Standards for Ordinary Portland Cement, when tested with a 90-micron IS sieve, the residue by weight should not exceed 10%. This means at least 90% of the cement should pass through the sieve.

Correct Answer: D. 0.85 p

Solution:

The standard procedure for the setting time test (both initial and final) requires a cement paste of a specific consistency. This is achieved by mixing the cement with a quantity of water equal to 0.85 times P, where 'P' is the percentage of water required for normal consistency.

Correct Answer: B. 3.15

Solution:

The specific gravity of Ordinary Portland Cement (OPC) is a standard value used in various mix design calculations. While it can vary slightly, the generally accepted and standard value is 3.15.

Correct Answer: C. Both (i) and (ii)

Solution:

Tricalcium Silicate (C₃S) is responsible for early strength gain and generates a high amount of heat during hydration. Therefore, a high percentage of C₃S leads to rapid hardening and high early strength with high heat generation. Conversely, Dicalcium Silicate (C₂S) hydrates slowly, contributing to long-term strength and better chemical resistance. A low percentage of C₂S would result in less resistance to chemical attack.

Correct Answer: A. C₃A

Solution:

The heat of hydration is the heat released when cement reacts with water. The compound that liberates the maximum heat is Tricalcium Aluminate (C₃A), which is also the fastest to react. The approximate heat generated is: C₃A (865 J/g) > C₃S (500 J/g) > C₄AF (420 J/g) > C₂S (260 J/g).

Correct Answer: C. C₃A

Solution:

Tricalcium Aluminate (C₃A) is the most reactive of the four Bogue's compounds. It reacts almost instantaneously with water, causing the initial setting and early hardening of the cement paste. This rapid reaction is what necessitates the use of gypsum as a retarder.

Correct Answer: A. 23 grade

Solution:

The Bureau of Indian Standards (BIS) specifies three grades of Ordinary Portland Cement based on their 28-day compressive strength: 33 Grade, 43 Grade, and 53 Grade. While a 23 Grade may have existed in the past or in other standards, it is not a currently available or specified grade in the Indian market.

Correct Answer: D. Both heat is generated and chemical reaction is initiated

Solution:

When water is added to cement, an exothermic chemical reaction known as hydration begins. This reaction is responsible for the setting and hardening of cement. As it is an exothermic process, heat is generated, which is referred to as the heat of hydration. Therefore, both events occur simultaneously.