CEMENT AND CEMENT CONCRETE

PORTLAND CEMENTDefinition : Cement, can be described as a materials

with an adhesive and cohesive properties which make a capable of bounding materials fragment in to a compact whole.

It can be seen that it is made primary from a combination of calcareous materials such as limestone or chalk, silica and alumina found as clay or shale.

Row Materials :The raw materials required for the manufactured of

cement are :

1. Limestone : CaCO3 ---- CaO + O2 (lime oxide)

2. Clay containing : Silica ---- SiO2 (Silica Oxide)

Alumina ---- Al2O3 (Alumina oxide)

Ferro/iron ---- Fe2O3 (Ferro oxide)

3. Gypsum : is added during the manufacturing of cement to reduce its setting time.

CaO, SiO2, Al2O3 and Fe2O3 are called main oxide of cement or cement oxide

Manufactured :

The process of manufacture consists essentially of grinding the raw materials in to a very fine powder, mixing them intimately in predetermined proportion and burning in a large rotary kiln at a temperature of about 1400 ºC or 2550 ºF when the material sinters and partially fuses in to clinker. The clinker is cooled and ground to a fine powder with some gypsum added, and the resulting product is a commercial product Portland Cement used throughout the world.

Basic Chemistry of Cement :

The raw materials use in the manufactured of Portland Cement consist mainly of : - lime oxide (CaO), - silica oxide (SiO2),

- alumina oxide (Al2O3) and

- iron oxide (Fe2O3).

These cement oxide after burned at about 1400 ºC interacted whit one another in the kiln to form a series of more complex product.

These four compounds are regarded as the major constituents of cement, they are listed bellow together whit their abbreviated symbols :

No.7

Name of Compound

Oxide Composition Abbre-viation

1 Tricalcium Silicate 3CaO.SiO2 C3S

2 Dicalcium Silicate 2CaO.SiO2 C2S

3 Tricalcium Alumanate

3CaO.Al2O3 C3A

4 Tetracalcium Aluminoferrite

4CaO.Al2O3.Fe2O3 C4AF

1. The silicates, C3S and C2S are the most important compounds, which are responsible for the strength.

2. The presence of C3A in cement is undesirable, it contributes little or nothing to the strength of cement except at early ages.

3. C4AF is also present in cement in small quantities, and, if compared with the other three compound, it does not effect the behavior significantly.

4. In addition to the four main compounds listed above, there exist minor compounds, such as MgO, Mn2O3, K2O and Na2O, they usually amount to not more then a few per cent of the mass of cement.

Grafik hubungan antara kekuatan dengan umur setelah proses pengikatan dari dari Cement compounds:

Kuat tekan [N/mm2]

Waktu/umur [Hari]

The general idea of the composition of cement can be as follow, which gave the oxide composition limit of Portland cements.

Oxide Content ( % )

CaOSiO2Al2O3Fe2O3MgO

AlkalisSO3

60 – 70 17 – 25 3 – 8 0.5 – 6.0 0.1 – 4.0 0.2 – 1.3 1 – 3

Type of Portland Cement :

So far, we have considered Portland cement as a generic materials. However, when hydrated, cement differing in chemical composition my exhibit different properties. It should thus be possible to select mixtures of raw materials for production of cement with various desired properties. In fact, several type of Portland cement are available commercially, and additional specials cement can be produced for special uses.

The following table lists the main type of Portland cement together with the appropriate BS (British Standard) and ASTM (American Society for Testing and Materials) Standard, and the next table gave the average values of compound composition. BRIITSH CLASIFICATION AMERICAN CLASIFICATION

Discription BS Discription ASTMOrdinary PC (OPC) 12 : 1991 Type – I C 150 – 92

Rapid-hardening PC. 12 : 1991 Type – III C 150 – 92

Low-heat PC. 1370 : 1979 Type – IV C 150 – 92

Modified PC. - Type – II C 150 – 92

Sulphate-resisting PC. 4027 : 1991 Type – V C 150 – 92

Portland blast furnace (slug cement)

146 : 1991 Type ISType IS (MS)

C 595 – 93

Low-heat PC.blastfurnace 4246 : 1991 - -

White Portland Cement 12 : 1989 - -

Type IP C 150 – 92

Portland Pozzolan 6588 : 19853892 : 1993

Type PType I (PM)

C 595 – 93

1. Ordinary Portland cement (Type I cement) :This is by far the most common cement used in general concrete construction, when there is no exposure to sulphates the soil or in the groundwater. Ordinary Portland cement or Type I cement is an excellent general cement and is the cement most widely used.

2. Rapid-hardening Portland cement (Type III cement) :This cement is similar to Type I cement, and is covered by the same standard. As the name implies, the strength of this cement develops rapidly, because this cement have a higher C3S content and higher fineness. The principal reason for the use of Type III cement is when formwork is to be remove early for re-use, or where sufficient strength for further construction is required quickly. Rapid-hardening Portland cement should not be use in mass concrete construction or in large structural section, because of its higher rate of heat development

3. Low-heat Portland cement (Type IV cement) : This cement has a low heat of hydration, so it’s a very low early strength. It because this cement developed in the US for use in

large gravity dams construction.

4. Modified Portland cement (Type II cement) : In some application a very low early strength may be a disadvantage, and for this reason a modified cement was developed in the US. This cement has a higher rate of heat development than that of Type IV cement, and a rate of gain of strength similar to that of Type I cement. Type II cement is recommended for structures where a moderately low heat generation is desirable or where moderate sulphate attack may occur. This cement is not available in the United Kingdom.

5. Sulphate-resisting Portland cement (Type V cement) : This cement has a low C3A content, so as to avoid sulphate attack from out side the concrete, otherwise the formation of calcium sulphoaluminate and gypsum would cause disruption of the concrete due to an increased volume of the resultant compound.

6. Portland Pozzolan CementThis type of cement is produced by grinding together portland cement clinker with pozzolan.Portland-pozzolans have lower heat hidration than type-I

portlands. Although initial strength is lower, but with proper curing they may develop a comparable final strength if good pozzolan is used. However, resistance against sulphate bearing water is higher.Portland pozzolan cement can be used for corrossive works (as it has advantage of resisting the corrossive action of saline

solutions and sea water much better than portland cement), also in mass concrete dams, and for protection against alkali- aggregate reaction.

7. Portland Composite Cement

This type of cement is produced by grinding together portland cement clinker with pozzolan and mixing high fineness hydrate lime.Similar as PPC, PCC has generally lower heat liberation than type-I portlands. Although initial strength is lower, but with proper curing they usually may develop a comparable final strength. Besides, resistance against sulphate bearing water is higher.

8. Blast Furnace Slag Cement

It is made by intergrinding ordinary portland cement clinker with selected blast furnace slag.Blast furnace slag is a biproduct of pig iron manufacture in the blast furnace, and is formed by the combination of the earthy constituents of the iron ore with the limestone flux. It contains normally the same oxides as are present in portland cement.The physical properties of blast furnace cement are similar to ordinary portland cement, usually with greater fineness, slower rate of hardening, lower heat of hydration, lower initial compressive strength, and better resistance against sulphate attack.This type of cement can be used for general building work, including water retaining structures, and precast concrete.

9. White Portland Cement

This type of cement is extensively used for visual effect in white or colored concretes which are to be left exposed and also in white or colored mortars for masonry and rendering.It has the same properties as ordinary portland cement, but is manufactured from special raw materials being substantially free from colour forming compounds such as the iron oxides which give other cements a characteristically grey or grey brown colour. In general, the materials used in Indonesia are pure chalk and white china clay.For this type of cement, manufacturing methods are also modified so that discoloring materials are not included during firing and grinding.

II. AGGREGATE Aggregate was originally viewed as an inert,

inexpensive material dispersed throughout the cement paste so as to produced a large volume of the concrete.

Natural aggregates are formed by the process of weathering and abrasion (gravel or stone takes from river) , or by artificially crushing a large parent mass (mountain rock).

2.1 Classification of Aggregate : Aggregate can be classified according to :

1. Geological Origin2. Particle Shape and Texture 3. Size 4. Unit weight

1. Aggregates Classification According to Geological Origin :

1. Natural Aggregates :Natural aggregates are form by the process of weathering and abrasion, such as sand, gravel or stone takes from river, or by artificial crushing a large parent mass of rock.

2. Artificial Aggregates :The artificial aggregates usually used for specials concrete, such as Light-weight concrete, Heavy-weight concrete or High-density concrete and Ultra-high Strength concrete.

2. Aggregates Classification According to Particle Shape :

The external characteristic of aggregate, in particular the particle shape and surface texture are of importance with regard to the properties fresh and hardened concrete.

Particle shape classification of aggregates according to BS 812 : part 1: 1975 are as follow : - Rounded- Irregular- Flaky- Angular- Allongated- Flaky and Alongated

3. Aggregates Classification According to Size :

Concrete is made with aggregate particle covering a range of size up to a maximum size which usually lies between 10 mm and 20 mm and 50 mm is typical. The typical size distribution is called grading.The alternative, very much more common, and always used in the manufacture of good quality concrete, is to obtain the aggregate in at least two separate lots :1. Fine aggregate (sand) : have a lower size limit of about 0.075 mm and maximum size limit 5 mm2. Coarse aggregate : have a lower size limit of

about 5 mm

4. Aggregates Classification According to Unit Weight :

1. Normal weight aggregate :Specific gravity = 2.3 – 2.6 ton/m³

2. Heavy weight aggregate : Specific gravity = 2.8 – 2.9 ton/m³

3. Light weight aggregate : Specific gravity = 1.0 – 1.2 ton/m³

2.2 Properties of Aggregate :

Mechanical Properties : - Bond- Strength- Toughness, resistance of aggregate to failure by the impact - Hardness, resistance of aggregate to failure by the wear- Modulus of Elastisity

Physical Properties :- Specific gravity and absorption- Density- Moisture content - Porosity- Voids- Shrinkage- Gradation and Fineness Modulus

PROPERTIES OF CONCRETE

I. FRESH CONCRETE :

The properties of the fresh (or plastic) concrete should be adjusted production equipment, process and transportation. And the importance properties of fresh concrete that have a influences to the construction is workability. The strict definition of workability is the amount of useful internal work necessary to produced full compaction.

Workability is generally measured with a slump test. The result from this test called slump, is also used as an indirect measure of the characteristics of workability. The slump test is made immediately after the concrete has been discharged from the mixer or transit truck, and the slump is read promptly after the metal cone is lifted.

II. BEHAVIOR DURING SETTING :

Evaporation of the free water from the concrete will continue after the concrete has been placed and compacted and is accompanied by

shrinkage. As a long as the concrete is sufficient fluid, it will be free to shrink.

III. CHEMICAL ADMIXTURES :

Materials that are added in small amounts to the concrete (usually less than 1 % by weight of the cement) and dissolved in the mixing water are known generically as chemical admixtures.

The main influence of chemical admixtures is on the properties of the fresh concrete.

1. Water-reducing Admixtures (WRA) :

An alternative use of the WRA is to reduce the water content of the mix while keeping the flow properties (slump) constant.

These Admixtures are used for three purposes :1. To achieve the higher strength by decreasing the

w/c ratio at the same workability as an admixture-free mix.

2. To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.

3. To increase the workability so as to ease placing in an accessible locations.The use of WRA increase the slump of the concrete, but does not necessarily reduce the rate of slump loss.

2. Set- retarding Admixtures :These Admixtures delay the setting of the concrete by slowing down the early hydration reaction and the rate of early strength development.The Admixture are use in hot weather condition to offset the acceleration of setting cause by the higher temperature.

3. Set- accelerating Admixtures :These Admixtures accelerate the rate of setting of the concrete by accelerating the rates of early hydration reaction. This effect influences not only the hydration reaction during the setting period, but it lasts also into the hardening stage, thus leading to higher early strength gain.

4. Air- entraining Admixtures :

These Admixtures as their name implies, are also surfactant which act at the air-water interface. Air-entraining Admixtures also improves the workability of the concrete by the generating a mix with improve flow properties (higher slump) and reduced segregation.

The presence of the air, however, leads to some reduction in strength which my be in range of 10 % to 20 %, depending on the content and dispersion of the air bubbles.

IV. HARDENED CONCRETE :

The strength of the concrete is its most important engineering property because it not only reflect its mechanical quality, but also provides an indication of long-term performance. Stronger concrete is usually denser, less permeable and thus better resistant to deleterious environmental influences. There are four kind of strength :

- Compressive strength- Flexural strength- Tensile strength- Shear strength

Concrete is a brittle materials, like most ceramic, and is much stronger in compression tan in tensionor flexural. The strength of the concrete would depend on the w/c ratio, as well as on the period of the moist curing in water and also depend on the temperature of curing.

Stress-Strain Curve : The aggregates in normal strength concrete have only a small influence on strength, yet their presence substantially affects the stress-strain curve of the concrete. The curve is curvilinear over the whole range of applied stress, although at low stress level the it is very nearly linear.

Modulus of Elasticity

References :

1. J. Francis Young, Sidney Mindess, Robert J. Gray, Arnon Bentur

“The Science and Technology of Cicil Engineering Materials”

2. Shan Somayaji

“Civil Engineering Materials”

3. AM Neville

“Concrete Technology”