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General ScienceMediumAsked in: NDA · NDA-II 2026, GAT

Which of the following statements is/are correct in respect of Portland cement ? I. Adding more water (higher water-cement ratio) accelerates the initial setting time of cement. II. The addition of a small percentage of gypsum gives the desired retardation. III. Gypsum is added during manufacturing to accelerate the setting time of cement. Select the answer using the code given below :

  1. A.I and II
  2. B.I and III
  3. C.I only
  4. D.II only

Correct answer

D. II only

Explanation

The correct answer is D, II only. Only statement II is correct: a small amount of gypsum is added to cement to slow down its setting.

Portland cement is made by heating limestone and clay to form clinker, which is then ground with about 2 to 3 per cent gypsum (calcium sulphate dihydrate). Without gypsum, the tricalcium aluminate in clinker would react with water almost at once and the cement would set before it could be mixed and placed; this is called flash setting. Gypsum gives masons the working time they need.

Statement I is wrong because adding more water dilutes the paste and delays setting; it also weakens the hardened concrete.

Statement III is wrong because gypsum retards setting; it does not accelerate it.

A and B are wrong because they include statement I, and B also includes statement III.

C is wrong because statement I is false.

Exam tip: gypsum in cement is a retarder; plaster of Paris is made by heating gypsum, CaSO₄·½H₂O.

Practice Questions

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Q1.General ScienceAsked in: UPSC Civil Services · CSE Prelims 2017, GS Paper IMedium

Consider the following pairs : (Commonly used / consumed materials : Unwanted or controversial chemicals likely to be found in them) 1. Lipstick : Lead 2. Soft drinks : Brominated vegetable oils 3. Chinese fast food : Monosodium glutamate Which of the pairs given above is/are correctly matched ?

  1. A.1 only
  2. B.2 and 3 only
  3. C.1 and 3 only
  4. D.1, 2 and 3
Show answer
Correct answer: D. 1, 2 and 3

Each product has been linked to the chemical paired with it. Lead has been found as a trace contaminant in many lipsticks, coming from colour pigments; since lipstick is swallowed in small amounts, regular use raises concern because lead is a toxic heavy metal that harms the nervous system.

Q2.General ScienceAsked in: CDS · CDS-I 2023, GKMedium

Which one of the following is not true for anodizing process?

  1. A.It makes aluminium corrosion resistant.
  2. B.Metals like aluminium, titanium and magnesium can be anodized.
  3. C.Clean aluminium article is the anode and oxygen gas is evolved at the cathode.
  4. D.It is used in aircraft industry.
Show answer
Correct answer: C. Clean aluminium article is the anode and oxygen gas is evolved at the cathode.

In anodizing the clean aluminium article is indeed the anode, but oxygen is released at the anode, not at the cathode; hydrogen is released at the cathode.

Q3.General ScienceAsked in: CDS · CDS-II 2016, GKMedium

Which one of the following statements is not correct?

  1. A.The response of the ear to sound of increasing intensity is approximately logarithmic.
  2. B.The sensitivity of the human ear does not vary with the frequency of the sound.
  3. C.When two or more waves traverse the same medium, the displacement of any element of the medium is the algebraic sum of the displacements due to each wave.
  4. D.Longitudinal waves can travel in all media—solids, liquids and gases.
Show answer
Correct answer: B. The sensitivity of the human ear does not vary with the frequency of the sound.

The statement that the sensitivity of the human ear does not vary with frequency is false, because the ear is far more sensitive to some frequencies than to others.

Q4.General ScienceAsked in: CDS · CDS-I 2017, GKEasy

Kwashiorkor disease in children is caused by

  1. A.sufficient carbohydrates but less fats in diet
  2. B.sufficient carbohydrates and fats but deficient proteins in diet
  3. C.sufficient vitamins but deficient fats in diet
  4. D.sufficient fats but deficient vitamins in diet
Show answer
Correct answer: B. sufficient carbohydrates and fats but deficient proteins in diet

Kwashiorkor is a disease of protein deficiency in a child who still gets enough calories. It usually appears when a child is weaned from breast milk onto a starchy diet with little protein.

Q5.General ScienceAsked in: CDS · CDS-I 2023, GKMedium

Which one of the following is the correct order of pH for the given substances?

  1. A.Coffee < Lemon juice < Milk of magnesia < Blood
  2. B.Milk of magnesia < Blood < Coffee < Lemon juice
  3. C.Lemon juice < Blood < Coffee < Milk of magnesia
  4. D.Lemon juice < Coffee < Blood < Milk of magnesia
Show answer
Correct answer: D. Lemon juice < Coffee < Blood < Milk of magnesia

Arranged from most acidic to most basic, the four substances fall in exactly this order. Lemon juice, rich in citric acid, has a pH of about 2 to 2.5.