The raw materials used for the manufacture of Portland cement are
- (a)Lime, silica and sulphur dioxide
- (b)Lime, silica and carbon dioxide
- (c)Lime, silica and alumina
- (d)Lime, silica and boric acid
Answer
Why
Correct — C, lime, silica and alumina. Portland cement is made from raw materials that supply lime (CaO, from limestone), silica (SiO₂, from clay and sand) and alumina (Al₂O₃, from clay), along with a little iron oxide; heating them in a kiln forms the calcium silicates and aluminates that make cement set and harden. Sulphur dioxide, carbon dioxide and boric acid are not raw materials of cement.
Why the others are wrong
- (a)Lime, silica and sulphur dioxide — Sulphur dioxide is not a cement ingredient; the third essential oxide is alumina, not sulphur dioxide.
- (b)Lime, silica and carbon dioxide — Carbon dioxide is released from the limestone during manufacture — it is a by-product, not a raw material; the needed oxide is alumina.
- (d)Lime, silica and boric acid — Boric acid has no role in Portland cement; the correct third component is alumina.
Concept
Portland cement is produced by heating a carefully proportioned mixture rich in lime (from limestone) with silica and alumina (from clay) to about 1450 °C, forming a fused 'clinker' of calcium silicates and aluminates; gypsum is then ground in to control the setting time. Lime, silica and alumina are the essential oxides.
Rule out the options that name gases or acids — the essentials are solid oxides that survive the kiln and build the setting compounds. Carbon dioxide leaves during manufacture, so it cannot be a raw material.
Key facts
- Main oxides of Portland cement: lime (CaO), silica (SiO₂) and alumina (Al₂O₃), plus some iron oxide.
- Limestone supplies the lime; clay supplies the silica and alumina.
- The kiln product is called clinker.
- Gypsum is added to the clinker to regulate the setting time.
Study next
Common traps
- Picking carbon dioxide — it is driven off during manufacture, not a raw material.
- Missing alumina as the essential third oxide alongside lime and silica.
Asked as 'raw materials / essential constituents of Portland cement' — the answer set is lime, silica and alumina.
Related PYQs
Which among the following are essential constituents of Portland cement ?
- (a) Sand, lime, clay
- (b) Silica, alumina, lime
- (c) Silica, lime, graphite powder
- (d) Sand, graphene, clay
Answer(b) Silica, alumina, lime
Same fact — the essential constituents of Portland cement are lime, silica and alumina. That NDA item asks it directly; this 2019 question phrases the same trio as 'raw materials'.
Which one of the following is NOT present in cement?
- (a) 2CaO·SiO2
- (b) 3CaO·SiO2
- (c) 4CaO·SiO2
- (d) 3CaO·Al2O3
Answer(c) 4CaO·SiO2
Same topic — the chemistry of cement. That NDA item tests the calcium silicate and aluminate compounds present in set cement; this one tests the raw oxides (lime, silica, alumina) it is made from.
Practice
- practice — not a real PYQ
Which substance is added to cement clinker to regulate its setting time?
- (a)Boric acid
- (b)Gypsum
- (c)Sodium chloride
- (d)Graphite
Answer(b) Gypsum — it is ground with the clinker to slow down setting. - practice — not a real PYQ
The main raw material that supplies lime in Portland cement is
- (a)Clay
- (b)Limestone
- (c)Gypsum
- (d)Sand
Answer(b) Limestone — calcium carbonate that provides the lime (CaO).