Material Investigation

MATERIAL INVESTIGATION

3.1 GENERAL

Water, cement, fine aggregate, polystyrene beads are the various materials used in this project. Before casting the specimen in various test of materials has been conducted.

3.2 CEMENT

             Figure 3.1 Cement

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

3.3 Manufacture of cement
            Portland cement is manufactured by crushing, milling and proportioning the following materials:

• Lime or calcium oxide, C_aO: 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

·         The materials, without the gypsum, are proportioned to produce a mixture with the desired chemical composition and then ground and blended by one of two processes - dry process or wet process. The materials are then fed through a kiln at 2,600º F to produce grayish-black pellets known as clinker. The alumina and iron act as fluxing agents which lower the melting point of silica from 3,000 to 2600º F. After this stage, the clinker is cooled, pulverized and gypsum added to regulate setting time. It is then ground extremely fine to produce cement.

Compound	Formula	Shorthand form
Calcium oxide (lime)	Ca0	C
Silicon dioxide (silica)	SiO2	S
Aluminum oxide (alumina)	Al2O3	A
Iron oxide	Fe2O3	F
Water	H2O	H
Sulfate	SO3	S

3.4 Chemical shorthand
         Because of the complex chemical nature of cement, a shorthand form is used to denote the chemical compounds.

TABLE 3.1 The shorthand for the basic compounds is
 

3.5 Chemical composition of clinker

TABLE 3.2 The cement clinker formed has the following typical composition

Compound	Formula	Shorthand form	% by weight1
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

TABLE 3.3 The cement clinker formed has the following typical composition:
 

Compound	Formula	Shorthand form	% by weight1
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

Representative weights only. Actual weight varies with type of cement.
Source: Mindess & Young

3.6 Properties of cement compounds

These compounds contribute to the properties of cement in different ways

3.6.1 Tri calcium 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. 

3.6.2 Tricalcium silicate, C₃S:-
          This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain.

3.6.3 Dicalcium silicate, C₂S:
          C₂S hydrates and hardens slowly. It is largely responsible for strength gain after one week. 

3.6.4 Ferrite, C₄AF:
           This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000^o F to 2,600^o 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