1. Initial setting time of ordinary Portland cement: Not more than 30 minutes
2. Initial setting time of sulphate resistant Portland cement: Not less than 45 minutes
3. Final setting time of sulphate resistant Portland cement: Not more than 600 minutes
Correct Answer: D. Only 3
Solution:
Let's check the IS specifications for setting times:
- 1. Initial setting time of OPC: According to IS specifications, the initial setting time for Ordinary Portland Cement should be not less than 30 minutes. The statement says "not more than", which is incorrect.
- 2. Initial setting time of sulphate resistant cement: The initial setting time for this cement is also specified as not less than 30 minutes, not 45 minutes. So, this statement is incorrect.
- 3. Final setting time of sulphate resistant Portland cement: The final setting time for most types of Portland cement, including sulphate resistant and OPC, is specified as not more than 600 minutes (10 hours). This statement is correct.
Therefore, only the third statement is correct.
i. Use of gypsum in making cement avoids flash set of cement.
ii. The manufacture of cement by the wet process requires less fuel when compared to that required by the dry process.
iii. When the temperature in a rotary kiln reaches 750°C, clinker formation takes place.
Correct Answer: C. Only i
Solution:
Let's analyze each statement:
- (i) is correct: Gypsum is added to cement clinker during grinding to control the setting time. It slows down the rapid hydration of C₃A, thus preventing a "flash set".
- (ii) is incorrect: The dry process is more fuel-efficient than the wet process because it doesn't require energy to evaporate the large amount of water present in the slurry.
- (iii) is incorrect: Clinker formation (sintering) occurs at a much higher temperature, typically between 1400°C and 1500°C. At 750°C, the process of calcination is still occurring.
Therefore, only the first statement is correct.
Correct Answer: C. Vicat apparatus
Solution:
The Vicat apparatus is the standard equipment used to determine both the initial and final setting times of cement by measuring the penetration of a needle into a cement paste of standard consistency.
Correct Answer: D. CaO
Solution:
The main constituent of Ordinary Portland Cement is Lime (Calcium Oxide, CaO), which typically makes up 60-67% of the total composition. Silica (SiO₂) is the second most abundant, usually around 17-25%.
Correct Answer: A. Increased surface area for hydration
Solution:
Finer cement particles have a higher specific surface area. This provides a larger area for the chemical reaction with water (hydration) to occur, which leads to a faster rate of strength gain and heat development.
Correct Answer: C. Directly proportional
Solution:
The fineness of cement is directly proportional to the rate of strength development. Finer cement particles have a larger surface area, which allows them to hydrate more quickly. This faster hydration leads to a more rapid gain in compressive strength, especially in the early stages.
Correct Answer: A. Lime
Solution:
Rapid hardening cement achieves its properties through finer grinding and a higher proportion of Tricalcium Silicate (C₃S). Since C₃S is formed from a higher concentration of Lime (CaO) in the raw mix, lime is the correct answer. This higher C₃S content is what primarily drives the rapid strength gain.
Correct Answer: B. Tricalcium aluminate
Solution:
For mass concrete projects like dams, it's crucial to minimize the heat of hydration to prevent thermal cracking. Tricalcium aluminate (C₃A) has the highest heat of hydration among the Bogue compounds. Therefore, in low-heat cement, the percentage of C₃A is deliberately kept low.
Correct Answer: D. 30 minutes and 10 hours
Solution:
According to Indian Standards for Ordinary Portland Cement (OPC), the initial setting time should not be less than 30 minutes, and the final setting time should not be more than 600 minutes (10 hours).
Correct Answer: A. Dicalcium silicate
Solution:
Dicalcium silicate (C₂S) is responsible for the long-term or progressive strength gain in cement. It hydrates and hardens slowly, releasing a small amount of heat over a long period. This slow reaction contributes to the strength of the concrete at later stages (after 7 days).
Correct Answer: B. Density bottle method
Solution:
The fineness of cement is tested using the Sieve method, the Air permeability method (Blaine's apparatus), and the Wagner Turbidimeter method. The Density bottle method is used to determine the specific gravity of cement, not its fineness.
Correct Answer: D. 30 mm
Solution:
The Le-Chatelier apparatus consists of a small split brass cylinder. According to standard specifications, this cylinder has an internal diameter of 30 mm and a height of 30 mm. It is used to measure the soundness of cement, which indicates its resistance to volume change after setting.
Correct Answer: B. Gypsum
Solution:
Gypsum is added to cement to act as a retarder, slowing down the initial setting time. To produce Quick Setting Cement, the amount of gypsum added during the clinker grinding process is significantly reduced. This allows the cement to set much faster than ordinary cement.
Correct Answer: C. C₃A
Solution:
Tricalcium aluminate (C₃A) is the Bogue compound most susceptible to sulphate attack. When sulphates react with C₃A, they form expansive products that cause cracking and deterioration of the concrete. Therefore, in sulphate-resisting cement, the percentage of C₃A is kept to a minimum (typically below 5%).
Correct Answer: B. 17-25%
Solution:
In Ordinary Portland Cement, the main chemical constituents have typical percentage ranges. Silicon dioxide (Silica, SiO₂) is the second most abundant component after lime, and its approximate composition is between 17% and 25%.
Correct Answer: A. 28 day compressive strength
Solution:
The grade of Ordinary Portland Cement (e.g., 33, 43, 53) directly corresponds to its minimum compressive strength achieved after 28 days of curing, measured in N/mm² (or MPa). For example, 53 Grade OPC should have a minimum 28-day compressive strength of 53 N/mm².
Correct Answer: B. C₃S
Solution:
Tricalcium Silicate (C₃S) is the primary compound responsible for the early strength of cement, particularly within the first 7 days. It hydrates quickly and contributes significantly to the initial hardening of the concrete.
Correct Answer: D. 5 to 7 mm
Solution:
For a cement paste to be of standard consistency, the Vicat plunger (10 mm diameter) should penetrate to a point 5 to 7 mm from the bottom of the Vicat mould. This corresponds to a penetration depth of 33 to 35 mm from the top surface of the paste.