Alumina, silica, lime and iron oxide are the basic raw material for the manufacture of portland cement. The component of Portland cement which first hardens is

ANS:A - 5CaO . 3 Al₂O₃

The component of Portland cement which first hardens is 3CaO · Al2O3, also known as tricalcium aluminate (C3A). Tricalcium aluminate (C3A) reacts rapidly with water to form calcium aluminate hydrates, which contribute to the early strength development of cementitious systems. This rapid reaction is responsible for the initial setting and hardening of Portland cement.