MCQs on Cement

Correct Answer: D. control initial setting time

Solution:

Gypsum (Calcium Sulphate) is a crucial ingredient added to cement clinker during the final grinding process. Its primary function is to act as a retarder, controlling the rapid hydration of Tricalcium Aluminate (C₃A). Without gypsum, the cement would set almost instantly upon contact with water (a phenomenon called "flash set"), making it impossible to work with. Therefore, gypsum is added to control the initial setting time.

Correct Answer: A. Lime

Solution:

Lime (Calcium Oxide, CaO) is the most important constituent of cement, making up 60-67% of its composition. It is the primary component that provides the binding and strength-giving properties after reacting with silica, alumina, and iron oxide to form the complex compounds (C₃S, C₂S, etc.) that hydrate and harden.

Correct Answer: D. retarder

Solution:

Gypsum's main purpose in cement is to act as a retarder. It slows down the hydration of Tricalcium Aluminate (C₃A), which would otherwise cause the cement to set almost instantly (flash set). By retarding this initial reaction, gypsum allows the concrete to remain workable for a sufficient period for mixing, transporting, and placing.

Correct Answer: D. Fineness

Solution:

The air permeability test, commonly known as the Blaine fineness test, is specifically designed to measure the fineness of cement. It determines the total surface area of the cement particles in a given mass. Fineness is a critical property as it affects the rate of hydration, strength development, and workability.

Correct Answer: A. 1.15

Solution:

Cement concrete continues to gain strength over time due to the slow hydration of Dicalcium Silicate (C₂S). While the 28-day strength is considered the characteristic strength, the strength at 3 months (90 days) is typically about 10-20% higher. A ratio of 1.15 is a commonly accepted approximation for this increase.

Correct Answer: B. 10 hours

Solution:

According to Indian Standard IS 8112, the final setting time for Ordinary Portland Cement should not exceed 600 minutes, which is equivalent to 10 hours. The minimum initial setting time is 30 minutes. This ensures the concrete hardens within a reasonable timeframe.

Correct Answer: D. Tri calcium aluminates

Solution:

The initial setting and early hardening of cement paste are primarily driven by the rapid reaction of Tricalcium Aluminate (C₃A) with water. While other compounds like Tricalcium Silicate (C₃S) are responsible for the main strength development, C₃A's immediate hydration and hydrolysis cause the paste to stiffen and set.

Correct Answer: A. Grinding the clinker to a high degree of fineness

Solution:

Rapid hardening cement achieves its properties primarily by being ground much finer than ordinary Portland cement. This high degree of fineness increases the surface area of the cement particles available for hydration, leading to a much faster reaction with water and a quicker development of strength.

Correct Answer: D. 15 kg/cm²

Solution:

According to standard testing procedures for cement, to ensure the reliability of compressive strength results, a set of three specimens is tested. The test is considered valid if the difference between the highest and lowest strength values is not more than 15 kg/cm². If the variation exceeds this limit, the test results are discarded, and the test is repeated.

Correct Answer: A. a rapid rate during the first few days and afterwards it continues to increase at a decreased rate

Solution:

The hardening of cement, which corresponds to its strength gain, is most rapid in the early stages. This is due to the quick hydration of C₃A and C₃S. After about 28 days, the rate of strength gain slows down significantly, but it continues at a much-decreased rate for a long time (months or even years) due to the slow hydration of C₂S.

Correct Answer: B. 10%

Solution:

According to IS 4031 (Part 1), the fineness of Ordinary Portland Cement (OPC) is tested by sieving it through a 90-micron sieve (I.S. Sieve No. 9). For the cement to be of standard quality, the weight of the residue left on the sieve should not be more than 10% of the original weight.

Correct Answer: D. blast furnace slag cement

Solution:

Blast furnace slag cement has a slower rate of strength gain compared to Ordinary Portland Cement, especially in the initial stages. For thin R.C.C. structures, a faster development of strength is often required to ensure stability and allow for the early removal of formwork. Therefore, a slow-hardening cement like blast furnace slag cement is generally avoided.

Correct Answer: A. 30%

Solution:

For producing Portland Pozzolana Cement (PPC), pozzolanic materials are interground with cement clinker. When using pozzolanas of lower reactivity, such as calcined clay containing up to 80% clay, the percentage added is typically limited to ensure adequate performance. A common proportion in this case is around 30%.

Correct Answer: A. Larger proportion of lime grounded finer than normal cement

Solution:

Rapid hardening cement's properties come from two main factors: a higher proportion of lime (which forms more C₃S, the compound responsible for early strength) and being ground to a much higher fineness than ordinary cement. This combination accelerates the hydration process, leading to faster strength development.

Correct Answer: D. 20% to 30%

Solution:

Normal consistency refers to the amount of water, expressed as a percentage of the cement's weight, required to create a standard paste consistency. For most Ordinary Portland Cements, this value typically falls within the range of 20% to 30%. This test is important as the water required for other tests (like setting time) is based on this value.

Correct Answer: A. quality of free lime

Solution:

The soundness test is designed to detect the presence of excess, unburnt free lime (and to a lesser extent, magnesia) in the cement. These unhydrated particles can hydrate slowly after the concrete has set, causing expansion and compromising the structure's integrity. Therefore, the test is a measure of the quality of free lime and the cement's volume stability.

Correct Answer: B. More

Solution:

This is known as the "size effect" in concrete testing. Smaller specimens tend to show higher compressive strength than larger ones. This is because there is a lower probability of finding a critical flaw or weak point in a smaller volume of material. Therefore, a 100 mm cube will generally yield a higher strength value than a 150 mm cube made from the same concrete mix.

Correct Answer: A. magnesium oxide

Solution:

Of the main chemical ingredients in Ordinary Portland Cement, Magnesium Oxide (MgO) is present in the smallest quantity, typically ranging from 1% to 4%. Its content is carefully controlled because an excess can cause unsoundness and unwanted expansion in the long term.

Correct Answer: C. I, III and IV only

Solution:

Let's analyze each statement based on IS specifications for OPC:

• I: Correct. The residue on a 90-micron sieve must not exceed 10%.
• II: Incorrect. The soundness limit by Le Chatelier is a maximum of 10 mm, not a typical range of 5-10 mm.
• III: Correct. The minimum initial setting time is 30 minutes.
• IV: Correct. This corresponds to the 7-day strength requirement for 33 Grade OPC (17.5 MPa ≈ 178 kg/cm²).

Correct Answer: D. all options are correct

Solution:

All the given names are popular commercial brand names for white and colored cements in India. Colocrete and Rainbow Cement are known for colored varieties, while Silvicrete is a well-known brand of white cement. Therefore, all options are correct.