Which one of the following is NOT present in cement?

Portland cement is a complex mixture of silicates and aluminates of calcium.

The main constituents of Portland cement are:
Dicalcium silicate (2CaO \cdot SiO_2$)<br>Tricalcium silicate (3CaO \cdot SiO_2$)
Tricalcium aluminate (3CaO \cdot Al_2O_3$)<br>Tetracalcium aluminoferrite (4CaO \cdot Al_2O_3 \cdot Fe_2O_3$)

Tetracalcium silicate (4CaO \cdot SiO_2$) is not a constituent of cement.<br><br>Answer: 4CaO \cdot SiO_2$