Correct Answer: D. Dry process
Solution:
The dry process is the most commonly used method in modern cement plants. It is more energy-efficient and environmentally friendly compared to the wet process, as it requires significantly less water and fuel for heating the raw materials in the kiln. This leads to lower production costs and reduced emissions, making it the preferred choice for new and upgraded facilities.
Correct Answer: C. CaSO₄ . 1/2H₂O
Solution:
Plaster of Paris is chemically known as calcium sulphate hemihydrate. It is formed by heating gypsum (CaSO₄ . 2H₂O), which causes it to lose water molecules. Its correct chemical formula is CaSO₄ . 1/2H₂O.
Correct Answer: B. Blane's air permeability
Solution:
The fineness of cement, which affects the rate of hydration, is determined by measuring its specific surface area. The Blane's air permeability apparatus is a standard method used for this purpose. It measures the resistance of a bed of cement to the flow of air, which is related to the fineness of the cement particles.
- The slump cone measures the workability of fresh concrete.
- A universal testing machine is used for strength tests (compressive, tensile).
- The Le Chatelier apparatus is used to determine the soundness of cement.
Correct Answer: B. Sulphate-resisting cement
Solution:
Sulphate attack is a chemical reaction that occurs when concrete is exposed to sulphate ions, commonly found in some soils and groundwater. This attack can cause expansion, cracking, and deterioration of the concrete. Sulphate-resisting cement is specially formulated with a low content of tricalcium aluminate (C3A) to resist this type of chemical attack, making it ideal for structures like sewage treatment plants and foundations in sulphate-rich environments.
Correct Answer: C. 10
Solution:
The soundness test using the Le Chatelier apparatus measures the expansion of cement due to the presence of uncombined lime. According to Indian Standard IS 8112 for 43-grade and IS 12269 for 53-grade Ordinary Portland Cement, the expansion should not exceed 10 mm. This ensures the long-term volume stability of the cement paste.
Correct Answer: A. C₃S
Solution:
In Ordinary Portland Cement (OPC), Tricalcium Silicate (C₃S) is the compound primarily responsible for early strength development (within the first 7 days) and rapid hardening. It also generates a significant amount of heat during hydration. C₃A reacts faster but contributes less to strength, while C₂S is responsible for long-term strength gain.
Correct Answer: A. C₃S
Solution:
In Ordinary Portland Cement (OPC), Tricalcium Silicate (C₃S) is the compound primarily responsible for early strength development (within the first 7 days) and rapid hardening. It also generates a significant amount of heat during hydration. C₃A reacts faster but contributes less to strength, while C₂S is responsible for long-term strength gain.
Correct Answer: B. 90 µm
Solution:
The sieve test for determining the fineness of cement is conducted using an IS Sieve with an aperture size of 90 microns (µm). According to IS 4031 (Part 1), the residue of cement left on the sieve should not exceed 10% by weight for Ordinary Portland Cement.
Correct Answer: C. Massive dams
Solution:
Rapid-hardening cement gains strength quickly and generates a high heat of hydration. This is advantageous for road repairs, cold weather concreting, and prefabrication. However, for massive concrete structures like dams, this high heat is a major disadvantage as it can lead to thermal stress and cracking. For such structures, low-heat cement is preferred.
Correct Answer: C. Vicat's apparatus
Solution:
The Vicat's apparatus is the standard equipment used to determine the setting times of cement. By measuring the penetration of a specific needle into a standard cement paste over time, both the initial and final setting times can be accurately determined.
Correct Answer: B. PPC produces more heat of hydration than ordinary Portland cement.
Solution:
The incorrect statement is that PPC produces more heat of hydration. In fact, Portland Pozzolana Cement (PPC) produces less heat of hydration than Ordinary Portland Cement (OPC). This is because the pozzolanic material (like fly ash) replaces some of the cement clinker, and the pozzolanic reaction is slower and generates less heat, making PPC suitable for mass concrete works.
Correct Answer: D. C₂S
Solution:
Dicalcium Silicate (C₂S) hydrates and hardens slowly, contributing mainly to the ultimate or long-term strength of cement. It has a low heat of hydration and provides good resistance to chemical attack, making cements with higher C₂S content more durable in aggressive environments.
Correct Answer: D. Dry process
Solution:
The dry process of cement manufacturing involves grinding the raw materials (limestone and clay) into a fine powder, which is then blended and fed directly into the rotary kiln in a dry state. This contrasts with the wet process, where the materials are mixed with water to form a slurry before being fed into the kiln.
Correct Answer: B. fineness
Solution:
The Blaine air permeability apparatus is used to determine the fineness of cement by measuring the specific surface area of the cement particles. Fineness is a crucial property as it influences the rate of hydration and strength gain.
Correct Answer: D. two
Solution:
There are two primary methods of cement manufacturing: the Dry Process and the Wet Process. The main difference is whether the raw materials are ground and mixed in a dry state or with water to form a slurry.
Correct Answer: B. The unit of the fineness of cement is cm³/kg.
Solution:
The fineness of cement is a measure of its surface area per unit mass. Therefore, its unit is expressed as area per mass, such as cm²/g or m²/kg. The unit cm³/kg represents volume per mass, which is incorrect for fineness.
Correct Answer: D. iron oxide
Solution:
Iron oxide (Fe₂O₃) is the component responsible for imparting the characteristic grey color to Ordinary Portland Cement. It also contributes to the hardness and strength of the cement.
Correct Answer: A. Rapid hardening Portland cement
Solution:
Rapid Hardening Portland Cement is manufactured to gain strength more quickly than Ordinary Portland Cement (OPC). It achieves the same strength in 1 day that OPC achieves in 3 days. This is due to its higher fineness and increased C₃S content.
Correct Answer: C. Vicat's apparatus
Solution:
The Vicat's apparatus is used to determine the standard consistency of cement paste, which is the percentage of water required to produce a paste of a specific resistance to penetration. This value is then used for other tests like setting time and soundness.
Statement 1: When you insert your hand in a bag of cement, it should give you a hot feeling.
Statement 2: In adulteration test, take a pinch of cement and feel (rub) between the fingers; it should give a smooth feeling and not a gritty feeling.
Correct Answer: C. Statement 1 is false and statement 2 is true
Solution:
For a field test of good quality cement:
- Statement 1 is false: A bag of fresh cement should feel cool to the touch, not hot. A hot feeling indicates that hydration has already started, which is a sign of old or poorly stored cement.
- Statement 2 is true: Rubbing cement between the fingers should feel smooth. A gritty feeling indicates adulteration with sand or other coarse materials.