Correct Answer: B. Gypsum
Solution:
Gypsum (calcium sulfate dihydrate) is added to cement clinker during the final grinding stage. Its primary function is to act as a retarder. It controls the hydration of Tricalcium Aluminate (C₃A), which would otherwise react almost instantly with water and cause a "flash set," making the cement unworkable. By slowing this reaction, gypsum ensures a manageable initial setting time.
Correct Answer: A. less amount of gypsum in very fine powdered form
Solution:
Quick setting cement is manufactured by reducing the amount of gypsum (the retarder) and grinding the cement clinker to a much finer powder. A small amount of aluminium sulphate can also be added as an accelerator. The combination of less gypsum and higher fineness causes the cement to set much faster than ordinary cement.
Correct Answer: D. 0.85 P
Solution:
According to standard testing procedures (IS 4031 - Part 5), the amount of water required to prepare a cement paste for testing its setting time is based on the normal consistency (P). The standard specifies that the water to be added should be 0.85 times P. This ensures the paste has the correct consistency for the Vicat apparatus needle to penetrate properly.
Correct Answer: D. All options are correct
Solution:
Portland Pozzolana Cement (PPC) has several advantages that make it suitable for mass concrete works. It has a lower heat of hydration (reducing thermal cracking), improved workability, and enhanced long-term durability and resistance to chemical attack. These properties make it ideal for large structures like dams, massive foundations, and bridge abutments.
Correct Answer: C. 40 mm to 50 mm
Solution:
The Vicat apparatus uses a plunger for the normal consistency test. As per standard specifications, this plunger has a fixed diameter of 10 mm and a length that is typically specified to be in the range of 40 mm to 50 mm.
Correct Answer: B. 25%
Solution:
The percentage of water for normal consistency is always calculated as a percentage of the weight of dry cement, not the weight of the paste. The formula is:
Percentage of Water = (Weight of Water / Weight of Cement) × 100
A cement paste is the combination of dry cement and water. Therefore:
Weight of Cement = Weight of Paste - Weight of Water
Weight of Cement = 1875 g - 1500 g = 375 g
Now, we can calculate the percentage of water:
Percentage of Water = (1500 g / 375 g) × 100 = 400%
Important Note: The question as stated leads to an impossible 400% water content. It likely contains a typo and should have stated "1500g of cement" and "1875g of paste". Let's solve with this correction:
Corrected Calculation:
Weight of Cement = 1500 g
Weight of Water = 1875 g - 1500 g = 375 g
Percentage of Water = (375 g / 1500 g) × 100 = 25%
Correct Answer: C. 2250
Solution:
Specific surface area is a measure of the fineness of cement. For a good quality Ordinary Portland Cement (OPC), the specific surface area, as measured by the Blaine's air permeability test, should not be less than 2250 cm²/g. A higher specific surface indicates a finer cement, which generally hydrates faster.
Correct Answer: A. 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 react with water (hydrate) more quickly. This faster hydration leads to a more rapid gain in early strength.
Correct Answer: A. Premature hardening
Solution:
Flash set is the rapid, premature hardening of the entire cement paste shortly after mixing with water. It is usually caused by the uncontrolled, instantaneous hydration of C₃A in the absence of a retarder like gypsum. This process generates significant heat and makes the concrete unworkable.
Correct Answer: C. Shrinks
Solution:
As concrete made with ordinary cement dries, the water that was not consumed in the hydration process evaporates from the pores. This loss of water causes the volume of the concrete to decrease, a phenomenon known as drying shrinkage. This is a natural property of cement concrete.
Correct Answer: D. All options are correct
Solution:
Hydration is the chemical reaction between cement particles and water. This process involves all four of the main Bogue compounds: Tricalcium silicate (C₃S), Dicalcium silicate (C₂S), Tricalcium aluminate (C₃A), and Tetracalcium aluminoferrite (C₄AF). Each compound hydrates at a different rate and contributes to the overall properties of the hardened concrete.
Correct Answer: D. 1400⁰ and 1500⁰
Solution:
The manufacturing process of cement involves heating the raw materials in a rotary kiln to a very high temperature. The critical phase is the clinkering zone, where the temperature is maintained between 1400°C and 1500°C. At this point, the materials fuse to form hard, rounded nodules called cement clinker, which is then ground to produce cement powder.
Correct Answer: B. Tri-calcium aluminate
Solution:
Tri-calcium aluminate (C₃A) is the most reactive of the Bogue's compounds. It hydrates almost instantaneously upon contact with water, which causes the initial setting of the cement paste. This reaction is so rapid that it must be controlled by adding gypsum during the manufacturing process.
Correct Answer: A. Rapid hardening cement
Solution:
Rapid hardening cement is designed to gain strength much faster than other types of cement. Because it achieves its design strength in a shorter period, the required curing period to ensure proper hydration and durability is also minimized. This allows for earlier removal of formwork and faster construction progress.
Correct Answer: D. Alkalies
Solution:
Efflorescence is the whitish, powdery deposit that can appear on the surface of concrete or masonry. It is caused by soluble salts within the material being transported to the surface by water. The primary culprits for this in cement are alkalies (such as sodium and potassium oxides), which are soluble and can easily migrate with moisture.