Question

Portland cement does not contain

• A. \(CaSiO_4\)
• B. \(CaSiO_3\)
• C. \(Ca_3Al_2O_6\)
• D. \(Ca_3(PO_4)_2\)
• E. Both (C) and (D)

Hint:

The approximate composition of Portland cement is \(CaO\) (60-67%), \(SiO_2\) (17-25%), and \(Al_2O_3\) (3-8%). Phosphates are never intentionally added.

Answer 1

The Correct Option is D

Concept: Portland cement is a complex mixture of silicates and aluminates of calcium. The primary raw materials are limestone (source of \(CaO\)) and clay (source of \(SiO_2, Al_2O_3, \text{and } Fe_2O_3\)).

Step 1: Identify major components.
The main chemical compounds in Portland cement (often expressed as oxides) are:
• Tricalcium silicate (\(3CaO \cdot SiO_2\) or \(Ca_3SiO_5\))
• Dicalcium silicate (\(2CaO \cdot SiO_2\) or \(Ca_2SiO_4\))
• Tricalcium aluminate (\(3CaO \cdot Al_2O_3\) or \(Ca_3Al_2O_6\))

Step 2: Evaluate the options.
Options (A), (B), and (C) represent various silicates and aluminates of calcium which are standard components or phases formed during the clinkering process.

Step 3: Identify the foreign substance.
Calcium phosphate [\(Ca_3(PO_4)_2\)] is not a constituent of Portland cement. In fact, phosphorus compounds are generally considered impurities in cement production because they can significantly retard the setting time and reduce the ultimate strength of the concrete.