Question

The average composition of Portland cement is:
(A)- CaO: 40-50%, $Si{{O}_{2}}$: 30-40%, $A{{l}_{2}}{{O}_{3}}$and $F{{e}_{2}}{{O}_{3}}$: 10-20%
(B)- CaO: 50-60%, $Si{{O}_{2}}$: 29-25%, $A{{l}_{2}}{{O}_{3}}$: 5-10%, MgO: 2-3%, $F{{e}_{2}}{{O}_{3}}$: 1-2% and $S{{O}_{2}}$: 1-2%
(C)- $Si{{O}_{2}}$: 40-50%, CaO: 30-40%, $A{{l}_{2}}{{O}_{3}}$: 10-20%
(D)- CaO: 50%, $Si{{O}_{2}}$: 50%

Answer 1

Hint : Portland cement is a grayish coloured finely ground particles of calcareous and argillaceous material. It possesses adhesive and cohesive properties and acts as a binding material for stones, bricks, etc.

Complete step by step solution :
Portland cement is obtained by calcining together a mixture of calcareous (CaO) and argillaceous ($A{{l}_{2}}{{O}_{3}}$ and $Si{{O}_{2}}$) together at a very high temperature of about 1500 $^{o}C$. Portland cement has the property of setting and hardening on addition of water due to certain hydration reactions.
The raw materials used for Portland cement and their corresponding percentages are given below:

Calcareous material- CaO: It is the primary constituent of Portland cement. Source of this material is lime ($CaC{{O}_{3}}$). It is added to 50-60%. Excess of CaO reduces the strength of cement by expansion whereas its low content leads to quick setting of cement.

Argillaceous material- $A{{l}_{2}}{{O}_{3}}$and $Si{{O}_{2}}$: Source of argillaceous material is clay. $A{{l}_{2}}{{O}_{3}}$ added in about 5-10%. It is responsible for quick setting of the cement. Its excess amount weakens the cement. Composition of $Si{{O}_{2}}$ in cement is 29-25%.
Ferric oxide- $F{{e}_{2}}{{O}_{3}}$: It is responsible for the grey colour. It checks the soundness of cement. It is used in about 1-2%.

Magnesium oxide (MgO) : Cements having different concentration of MgO vary in colour and compressive strengths. Excess addition of it leads to expansion of cement.
Sulphur dioxide ($S{{O}_{2}}$): Gypsum ($CaS{{O}_{.4}}.2{{H}_{2}}O$) added to retard or enhances the quick setting of cement is the source of $S{{O}_{2}}$ in Portland cement. It amounts up to 1-2%.

So, the correct answer is “Option B”.

Additional Information:
Four basic components in cement are as follows:
Dicalcium silicate: $2CaO.Si{{O}_{2}}$ or ${{C}_{2}}S$
Tricalcium silicate- $3CaO.Si{{O}_{2}}$ or ${{C}_{3}}S$
Tricalcium aluminate- $3CaO.A{{l}_{2}}{{O}_{3}}$ or ${{C}_{3}}A$
Tetracalcium aluminoferrite- $4CaO.A{{l}_{2}}{{O}_{3}}.F{{e}_{2}}{{O}_{3}}$ or ${{C}_{4}}AF$

Note : We are likely to get confused between the options. So, to avoid confusion keep in mind that in addition to the basic materials like CaO, $Si{{O}_{2}}$ and $A{{l}_{2}}{{O}_{3}}$, small quantities of gypsum, $F{{e}_{2}}{{O}_{3}}$ and MgO are also added to modify and enhance the properties of Portland cement.