NDA (I) 2024 General Ability Test · Question 65 of 146

Correct — B, silica, alumina and lime. Ordinary Portland cement is manufactured by heating a calcareous material (limestone, which supplies lime, CaO) together with an argillaceous material (clay or shale, which supplies silica, SiO2, and alumina, Al2O3), plus a small amount of iron oxide. Lime is the largest ingredient at roughly three-fifths of the mass, silica about one-fifth and alumina a few per cent; these oxides combine on firing into the calcium silicates and calcium aluminates that give cement its binding and setting power. Silica, alumina and lime are therefore its essential constituents.

• (a)Sand, lime, clay — Lime is genuinely an ingredient, but 'sand' and 'clay' are loose raw-material words rather than the recognised constituent oxides. The standard list names silica and alumina (which clay actually supplies); plain sand is not counted as an essential constituent, so this pairing is imprecise and never names alumina.
• (c)Silica, lime, graphite powder — Silica and lime are correct, but graphite (a form of carbon) has no place in cement — it provides no cementing action and would not help the mix set or harden.
• (d)Sand, graphene, clay — Graphene is a modern one-atom-thick carbon material studied only as an experimental additive, not an essential constituent of ordinary cement. This option also omits lime, the single largest and most important ingredient.

Ordinary Portland cement is a hydraulic binder made by heating limestone with clay or shale to about 1450 degrees C to form 'clinker', which is then ground with a little gypsum. Expressed as oxides its essential constituents are lime (CaO), silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3), with lime dominating the composition.

The trap is to reach for everyday material names such as 'sand' and 'clay' instead of the constituent oxides that chemistry texts list. Reason it out from what makes cement work: strength comes from calcium silicates and calcium aluminates, so the constituents must include lime together with silica and alumina, while carbon forms like graphite or graphene play no cementing role at all.

• Approximate composition is lime 60-65 per cent, silica 17-25 per cent, alumina 3-8 per cent, with a few per cent each of iron oxide and magnesia.
• Raw materials are a calcareous source (limestone, chalk) for lime and an argillaceous source (clay, shale) for silica and alumina.
• The fired intermediate is clinker; grinding it with about 3-5 per cent gypsum controls the setting time.
• The main strength-giving compounds are tricalcium silicate, dicalcium silicate and tricalcium aluminate.

• Choosing informal words like 'sand' and 'clay' instead of the oxide constituents silica, alumina and lime.
• Including a carbon material (graphite or graphene) as if it were a cement ingredient.
• Forgetting that lime is the single largest constituent, not a minor one.

NDA and other exams ask for the essential constituents of cement or the role of gypsum in it — anchor the answer to the oxide trio lime, silica and alumina.

No directly related past PYQ was found.

• practice — not a real PYQ

  Gypsum is added to cement clinker mainly to

  • (a)increase its strength
  • (b)slow down the setting time
  • (c)give it a grey colour
  • (d)make it waterproof

  Answer(b) slow down the setting time — without gypsum the cement would set almost instantly (flash set), so a few per cent is ground in to regulate it.

• practice — not a real PYQ

  The chief raw material that supplies lime in the manufacture of Portland cement is

  • (a)clay
  • (b)gypsum
  • (c)limestone
  • (d)silica sand

  Answer(c) limestone — the calcareous raw material that provides calcium oxide (lime), the largest constituent of cement.