For a good quality cement, the ratio of silica to alumina is found to be :

Solution

Chemistry ma’am · 3 steps

Recall the NCERT window: silica to alumina must be between 2.5 and 4, and only 3 sits inside it.

1. What cement contains. Portland cement is mainly lime (CaO, about 50 to 60%), silica (SiO₂, about 20 to 25%) and alumina (Al₂O₃, about 5 to 10%). Small amounts of MgO, Fe₂O₃ and SO₃ are also present.
2. The quality rule. For good quality cement, the ratio SiO₂ : Al₂O₃ should lie between 2.5 and 4. A second rule says the ratio of CaO to the total oxides of Si, Al and Fe should be as close as possible to 2.

   Option	Silica : alumina	Inside 2.5 to 4?
   (a)	1.5	No, too low
   (b)	2	No, too low
   (c)	3	Yes
   (d)	4.5	No, too high

3. So. Only a silica to alumina ratio of 3 falls in the required range, so the answer is (c).

The idea to keep. Cement has two ratio rules: SiO₂/Al₂O₃ between 2.5 and 4, and CaO to the oxides of Si, Al and Fe close to 2. Keep them apart.

Answer from the NTA final answer key. Solution by PrepWiser.