The main constituents of cement are:

Portland cement is the basic component of concrete. Concrete is created when portland cement forms a paste with water that combines with sand and rock to harden.

Cement is made using a strictly controlled chemical combination of calcium, silicon, aluminum, iron, and other ingredients.

Common materials used to make cement include limestone, shells, and chalk or shale combined with shale, clay, shale, blast furnace slag, silica sand, and iron ore. These ingredients, when heated to high temperatures, form a rock-like mass that is ground into a fine powder that we commonly call cement. 

Lime (calcium oxide, $\mathrm{CaO}$), silica (silicon dioxide, ${\mathrm{SiO}}_2$), and alumina (aluminum oxide, ${\mathrm{Al}}_2{\mathrm{O}}_3$) are the main ingredients in Portland cement. 

The other oxides are formed from an argillaceous (clayey) substance, whereas the lime is obtained from a calcareous (lime-containing) raw material.

Hence, the correct option is (A).