Chemical composition of Portland cement
Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout.
The essential raw ingredients of Portland cement are
· Iron Oxide
Excess of clay (Silica + Alumina) causes a cement to "set" quickly, while excess of lime causes it to "set" slowly or imperfectly. Iron oxide gives cement its color. Portland cement is improved in dry storage, as any excess of lime particles are air slaked.
The oxides present in raw materials when subjected to high clinkering temperature (around 1200Oc) combine with each other to form complex compounds.
There are four chief minerals present in a Portland cement grain:
· tricalcium silicate (Ca3SiO5),
· dicalcium silicate (Ca2SiO4),
· tricalcium aluminate (Ca3Al2O5) and
· calcium aluminoferrite (Ca4AlnFe2-nO7).
The formula of each of these minerals can be broken down into the basic calcium, silicon, aluminum and iron oxides
Cement chemists use abbreviated nomenclature based on oxides of various elements to indicate chemical formulae of relevant species, i.e., C = CaO, S = SiO2, A = Al2O3, F = Fe2O3. Hence, traditional cement nomenclature abbreviates each oxide as shown in Table 1.
Tricalcium silicate (alite)
Dicalcium silicate (belite)
The composition of cement is varied depending on the application. A typical example of cement contains
5–15% C4AF, and
3–8% other additives or minerals (such as oxides of calcium and magnesium).
It is the hydration of the calcium silicate, aluminate, and aluminoferrite minerals that causes the hardening, or setting, of cement.
The ratio of C3S to C2S helps to determine how fast the cement will set, with faster setting occurring with higher C3S contents.
The calcium aluminoferrite (C4AF) forms a continuous phase around the other mineral crystallites, as the iron containing species act as a fluxing agent in the rotary kiln during cement production and are the last to solidify around the others. It is worth noting that a given cement grain will not have the same size or even necessarily contain all the same minerals as the next grain. The heterogeneity exists not only within a given particle, but extends from grain to grain, batch-to-batch, plant to plant.
There are three fundamental stages in the production of Portland cement:
1. Preparation of the raw mixture
2. Production of the clinker
3. Preparation of the cement