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
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.
- (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.
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.
- 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.
- 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.
No directly related past PYQ was found.
- 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).