Composition of cement
Introduction
Portland cement gets its strength from chemical reactions between the cement and water. The process is known as hydration. This is a complex process that is best understood by first understanding the chemical composition of cement.
Manufacture of cement
Portland cement is manufactured by crushing, milling and proportioning the following materials:
Chemical shorthand
Because of the complex chemical nature of cement, a shorthand form is used to denote the chemical compounds. The shorthand for the basic compounds is:
Chemical composition of clinker
The cement clinker formed has the following typical composition:
Representative weights only. Actual weight varies with type of cement.
Source: Mindess & Young
Properties of cement compounds
These compounds contribute to the properties of cement in different ways
Tricalcium aluminate, C3A:-
It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant.
Tricalcium silicate, C3S:-
This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain.
Dicalcium silicate, C2S:
C2S hydrates and hardens slowly. It is largely responsible for strength gain after one week.
Ferrite, C4AF:
This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000o F to 2,600o F). It hydrates rapidly, but does not contribute much to strength of the cement paste.
By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.
References:
Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 671.
Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.
Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc.,
Introduction
Portland cement gets its strength from chemical reactions between the cement and water. The process is known as hydration. This is a complex process that is best understood by first understanding the chemical composition of cement.
Manufacture of cement
Portland cement is manufactured by crushing, milling and proportioning the following materials:
- Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock
- Silica, SiO2: from sand, old bottles, clay or argillaceous rock
- Alumina, Al2O3: from bauxite, recycled aluminum, clay
- Iron, Fe2O3: from from clay, iron ore, scrap iron and fly ash
- Gypsum, CaSO4.2H20: found together with limestone
Chemical shorthand
Because of the complex chemical nature of cement, a shorthand form is used to denote the chemical compounds. The shorthand for the basic compounds is:
| Calcium oxide (lime) | ||
| Silicon dioxide (silica) | ||
| Aluminum oxide (alumina) | ||
| Iron oxide | ||
| Water | ||
| Sulfate |
The cement clinker formed has the following typical composition:
| Tricalcium aluminate | Ca3Al2O6 | C3A | 10 |
| Tetracalcium aluminoferrite | Ca4Al2Fe2O10 | C4AF | 8 |
| Belite or dicalcium silicate | Ca2SiO5 | C2S | 20 |
| Alite or tricalcium silicate | Ca3SiO4 | C3S | 55 |
| Sodium oxide | Na2O | N | ) )Up to 2 |
| Potassium oxide | K2O | K | |
| Gypsum | CaSO4.2H2O | CSH2 | 5 |
Source: Mindess & Young
Properties of cement compounds
These compounds contribute to the properties of cement in different ways
It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant.
This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain.
C2S hydrates and hardens slowly. It is largely responsible for strength gain after one week.
This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000o F to 2,600o F). It hydrates rapidly, but does not contribute much to strength of the cement paste.
By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.
References:
Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 671.
Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.
Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc.,
What is Hydraulic Cement?
ReplyDeleteHistory and Development of Hydraulic Cement
Chemical Composition of Hydraulic Cement
How Hydraulic Cement Works
When to Use Hydraulic Cement
Hydraulic Cement Uses
How to Apply Hydraulic Cement
Types of Hydraulic Cement
Advantages and Disadvantages of Hydraulic Cement