CHEMICAL COMPOSITION OF PORTLAND CEMENT

CHEMICAL COMPOSITION OF PORTLAND CEMENT

The raw materials used to manufacture Portland cement are lime, silica, alumina, and iron oxide. These raw materials interact in the kiln, forming complex chemical compounds. Calcination in the kiln restructures the molecular composition, producing four main compounds, as shown in Table 1.

Table 1 Main component of Portland cement

C3S and C2S when hydrated, provide the desired characteristics of the concrete. Alumina and iron, which produce C3A and C4AF are included with the other raw materials to reduce the temperature required to produce C3S from 2000°C to 1350°C (3500°F to 2500°F). This saves energy and reduces the cost of producing the portland cement.

In addition to these main compounds, there are minor compounds, such as magnesium oxide, titanium oxide, manganese oxide, sodium oxide, and potassium oxide. These minor compounds represent a few percent by weight of cement. The term minor compounds refers to their quantity and not to their importance. In fact, two of the minor compounds, sodium oxide(Na2O) and potassium oxide (K2O) are known as alkalis. These alkalis react with the silica in some aggregates causing the disintegration of concrete and affecting the rate of strength development.

Fig 1. Cracks in concrete pavement due to alkali-silica reactivity.