For a good quality cement, the ratio of silica to alumina should be between

Cement is a product obtained by combining a material rich in lime, CaO with other material such as clay which contains silica, SiO₂ along with the oxides of aluminium, iron and magnesium

The average composition of Portland cement is : CaO, 50- 60%; SiO₂, 20-25%; Al₂O₃, 5-10%; MgO, 2- 3%; Fe₂O₃, 1-2% and SO₃, 1-2%. For a good quality cement, the ratio of silica (SiO₂) to alumina (Al₂O₃) should be between 2.5 and 4