The setting time of cement is lowered by adding
- (a)oxides of aluminium
- (b)gypsum
- (c)oxides of magnesium
- (d)silica
Correct — D, silica. Silica is one of the two acidic oxides that a cement works actually adjusts to control how the product behaves — it enters the kiln as silica-bearing clay alongside limestone, and it ends up in the calcium silicate phases that are what harden when cement meets water. The chemistry line this paper follows treats silica as the oxide used to bring the setting time down, and the elimination confirms it: gypsum has a fixed and opposite job, magnesia is not a setting control at all, and the key marks silica rather than alumina.
- (a)oxides of aluminium — Alumina is the other setting-related oxide in the mix, and the tricalcium aluminate phase it forms is the one that would set almost instantly if it were not held back. Several Indian building-materials texts consequently describe excess alumina as making cement quick-setting, so this option has a genuine following. The official key nonetheless marks silica, and the tension is set out in the note below.
- (b)gypsum — The exact opposite of what the stem asks, and the most instructive wrong answer in the set. Gypsum is ground into cement clinker specifically as an inhibitor to prevent flash setting — it slows the aluminate phase down so that concrete can be mixed, carried and placed. Gypsum therefore raises the setting time, and can never be the answer to a question about lowering it.
- (c)oxides of magnesium — Magnesia is treated as an impurity in cement, not a control. Its content is capped by specification because free magnesia hydrates slowly long after the concrete has hardened and makes it expand and crack, a defect called unsoundness. It has no useful role in speeding up or slowing down the set.
Portland cement is made by burning limestone with silica-bearing clay in a kiln at around 1450 °C. The limestone loses carbon dioxide and becomes lime, which then combines with the silica, alumina and iron oxide to form the clinker phases — chiefly tricalcium silicate and dicalcium silicate, plus tricalcium aluminate and a ferrite phase. When water is added, these phases hydrate, the paste stiffens, and it goes on gaining strength for weeks. Setting time is the interval between mixing and that stiffening, and it is governed by which phases are present in what proportion, plus what is ground in with the clinker.
Be aware of a genuine disagreement in the textbooks here, because it will show up if you read widely. Several Indian civil-engineering books state the reverse of this key — that excess silica makes cement slow-setting while excess alumina makes it quick-setting — which would point to option (a). The official UPSC key for this paper marks silica, so that is the answer for this question, and this card is authored to the key rather than around it. What is not in dispute, and what is far more likely to be asked again, is the gypsum fact: gypsum is a retarder, added to stop flash setting. If you carry only one thing forward from this item, carry that.
- Portland cement is produced by burning limestone with silica-bearing clay; calcination converts the limestone to lime, which is also why cement manufacture is a large source of carbon dioxide.
- The clinker phases that set and harden are mainly the calcium silicates, together with tricalcium aluminate and a ferrite phase.
- Gypsum is interground with clinker as an inhibitor to prevent flash or quick setting, so it lengthens the setting time.
- Magnesia content is limited by specification because free magnesia causes delayed expansion and cracking, known as unsoundness.
- Initial set for ordinary Portland cement is conventionally required not to be earlier than about 30 minutes, and final set not later than about 10 hours.
- Marking gypsum because it is the compound most associated with cement — its role is to slow setting, which is the opposite of what the stem asks.
- Assuming magnesia behaves like the other oxides in the list; it is a limited impurity, not a control.
- Reading 'setting time is lowered' as 'setting is slowed' — a lower setting time means the cement sets sooner.
Cement reaches NDA either as this composition-and-setting item or as a manufacturing question, and the gypsum-as-retarder point is the single fact that recurs most often across both.
Consider the following statements: Statement I: Studies indicate that carbon dioxide emissions from cement industry account for more than 5% of global carbon emissions. Statement II: Silica-bearing clay is mixed with limestone while manufacturing cement. Statement III: Limestone is converted into lime during clinker production for cement manufacturing. Which one of the following is correct in respect of the above statements?
- (a) Both Statement II and Statement III are correct and both of them explain Statement I
- (b) Both Statement II and Statement III are correct but only one of them explains Statement I
- (c) Only one of the Statements II and III is correct and that explains Statement I
- (d) Neither Statement II nor Statement III is correct
Answer(b) Both Statement II and Statement III are correct but only one of them explains Statement I
The same raw mix, examined nine years later — it states outright that silica-bearing clay goes in with the limestone and that calcination turns the limestone to lime, which is the manufacturing background this NDA item assumes.
Consider the following statements: 1. Banking soda is used in fire extinguishers. 2. Quicklime is used in the manufacture of glass. 3. Gypsum is used in the manufacture of Plaster of Paris. Which of the statements given above is/are correct?
- (a) 1 and 2
- (b) 2 and 3
- (c) 1 only
- (d) 1, 2 and 3
Answer(d) 1, 2 and 3
Places gypsum and lime in their wider industrial settings, which helps keep gypsum's role straight — it is a raw material for plaster of Paris and a retarder in cement, never an accelerator.
Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)₂) is an example of
- (a) Displacement reaction.
- (b) Endothermic reaction.
- (c) Decomposition reaction.
- (d) Exothermic reaction.
Answer(d) Exothermic reaction.
The lime chemistry that sits underneath cement setting — hydration of the lime-bearing phases releases heat, which is why large concrete pours have to be managed for temperature.
- practice — not a real PYQ
Gypsum is added to cement clinker during grinding mainly in order to
- (a)increase the strength of the concrete
- (b)prevent flash setting of the cement
- (c)give the cement its grey colour
- (d)reduce the amount of water needed
Answer(b) prevent flash setting of the cement — gypsum acts as a retarder on the aluminate phase, giving enough working time to mix and place the concrete.
- practice — not a real PYQ
The two principal raw materials burnt together in a rotary kiln to produce Portland cement clinker are
- (a)limestone and silica-bearing clay
- (b)gypsum and bauxite
- (c)dolomite and quartz sand
- (d)fly ash and slaked lime
Answer(a) limestone and silica-bearing clay — the limestone supplies lime after calcination and the clay supplies silica, alumina and iron oxide.