UPSC NDA I 2024 General Ability Paper

Concept:Portland cement is made from specific raw materials that react with water to harden.Explanation:The essential constituents of Portland cement are silica ($\mathrm{SiO}_2$), alumina ($\mathrm{Al}_2\mathrm{O}_3$), and lime ($\mathrm{CaO}$).Silica contributes to the strength and durability of the cement.Alumina helps the cement set quickly and also adds strength.Lime is the main ingredient that reacts with water to form calcium silicate hydrate, which makes the cement harden.These components chemically combine to form compounds like tricalcium silicate ($\mathrm{C}_3\mathrm{S}$) and dicalcium silicate ($\mathrm{C}_2\mathrm{S}$), which give cement its useful properties.Sand and clay are not essential constituents of the cement itself — sand is used as an aggregate in concrete, and clay is part of the manufacturing process but not a primary final ingredient.Graphite powder and graphene are not used in Portland cement composition.Answer:Option B (Silica, alumina, lime).