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

· Lime

· Silica

· Alumina

· 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 1200^Oc) combine with each other to form complex compounds.

There are four chief minerals present in a Portland cement grain:

(Bogues’s Compounds)

· tricalcium silicate (Ca₃SiO₅),

· dicalcium silicate (Ca₂SiO₄),

· tricalcium aluminate (Ca₃Al₂O₅) and

· calcium aluminoferrite (Ca₄Al_nFe_2-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 inTable 1.

Mineral	Chemical formula	Oxide composition	Abbreviation
Tricalcium silicate (alite)	Ca3SiO5	3CaO.SiO2	C3S
Dicalcium silicate (belite)	Ca2SiO4	2CaO.SiO2	C2S
Tricalcium aluminate	Ca3Al2O4	3CaO.Al2O3	C3A
Tetracalcium aluminoferrite	Ca4AlnFe2-nO7	4CaO.AlnFe2-nO3	C4AF
TABLE 1: Chemical formulae and cement nomenclature for major constituents of Portland cement. Abbreviation notation: C = CaO, S = SiO2, A = Al2O3, F = Fe2O3.  The composition of cement is varied depending on the application. A typical example of cement contains

50–70% C3S,

15–30% C2S,

5–10% C3A,

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. Lower C3A content promotes resistance to sulfates. Higher amounts of ferrite lead to slower hydration. The ferrite phase causes the brownish gray color in cements, so that “white cements” (i.e., those that are low in C4AF) are often used for aesthetic purposes.

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