Thursday, September 9, 2010

Chemical Composition of Cement

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 1200Oc) 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 (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.

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

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