Consider the following oxides: 1.Al₂O₃ 2.CaO 3.SiO₂ Which one of the following is the correct sequence in increasing order of their percentage analysis for an ordinary portland cement?
- A2, 1, 3
- B2, 3, 1
- C3, 2, 1
- D1, 3, 2Correct
Solution
Concept
OPC has lime (CaO) in the largest amount, then silica (SiO₂), then alumina (Al₂O₃).
Step-by-step solution
1. Typical oxide percentages in OPC: CaO = 60–67%, SiO₂ = 17–25%, Al₂O₃ = 3–8%. 2. Increasing order: Al₂O₃ < SiO₂ < CaO. 3. That is 1, 3, 2, so the answer is option D.
Explanation
Lime is the main oxide in cement, about 60–67%. Silica comes next at about 17–25%. Alumina is smaller at about 3–8%.
Why the other options are wrong
A: 2, 1, 3 puts CaO lowest, but CaO is the highest. B: 2, 3, 1 is the decreasing order, not increasing. C: 3, 2, 1 puts Al₂O₃ highest, but it is the lowest of the three.
Shortcut
Remember 'Lime > Silica > Alumina' and reverse it for increasing order.
Common mistake
Writing the decreasing order (2, 3, 1) when the question asks for increasing order.
Correct answer: D) 1, 3, 2