Structure III

Lecture 1

Contents

• Cement• Cement Ingredients• Chemical constituents of Portland Cement• Types of cement• Cement hydration• Structural properties of cement

Cement

• Cement may be defined as “an adhesive substances capable of uniting fragments or masses of solid matter to a compact whole”.

• Cement may also be defined as “a hydraulic binder, which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes and which, after hardening, retains its strength and stability even under water”.

Contd.

• Cement was first of all introduced in the year 1824 by Joseph Aspdin, a brick layerer of Leeds, England. He called it Portland cement, because after hardening it resembled in colour to the stone quarried near Portland in England.

Cement Ingredients• The basic ingredients of an Ordinary Portland Cement are

Lime in the form of calcium oxide and alumina.• The following two raw materials are required for the

manufacture of cement:-1. Calcareous materials- The materials which contains

calcium or lime as their major constituent are called calcareous materials such as lime stone, chalk or shells etc.

2. Argillaceous materials- The materials which contains alumina as their major constituent are called argillaceous materials such as shale, laterite etc.

Review questions

• What is cement ?• What are the main ingredients of cement ?

Chemical constituents of Portland Cement

S. No. Ingredients Approx. Proportion Common proportion

1. Lime (CaO) 60 to 67% 63%2. Silica (SiO2 ) 17 to 25% 22%3. Alumina (Al2O2 ) 3 to 8% 6%4. Iron oxide (Fe2O3) 0.5 to 6% 3%5. Magnesium oxide (MgO) 0.1 to 4% 2.5%6. Sulphur trioxide (SO2) 1 to 3% 1.75%7. Insoluble residue Not more than 1.5% 0.25%

8. Loss of ignition Not more than 4% 1.5%

Chemical constituents of Portland Cement

• Lime- This is the main ingredient of cement, its percentage being about 60 per cent of the total ingredients. Deficiency of lime causes decrease in the strength of cement and excess free lime causes unnecessary expansion and disintegration of the cement.

• Silica- During the manufacturing of cement, silica undergoes chemical reaction with calcium to form dicalcium and tricalcium silicates which are responsible for imparting strength to the cement. Excess of silica adds strength to the cement but its setting time is prolonged.

Contd.

• Alumina- Alumina forms complex compounds with silica and calcium to impart setting property to the cement. Larger quantity of alumina quickens setting but weakens the cement.

• Iron oxide- This ingredient is mainly responsible to impart colour to the cement. It also helps in adding strength and hardness to the cement to a certain extent.

Contd.

• Magnesium oxide- This ingredient is mainly responsible to impart colour to the cement. It also helps in adding strength and hardness to the cement to a certain extent.

• Sulphur trioxide- It makes cement sound if present in very small quantity. Excess quantity of sulphur trioxide also makes cement unsound.

Contd.

• Alkalies- Large quantity of alkalies (sodium and potassium oxides) are carried away by flue gases during clinkering. However, a small quantity is left behind, excess of which causes efflorescence.

• Gypsum- During the grinding process a small quantity of gypsum is added. The presence of gypsum retards the setting time of cement so that there is time for transporting and placing concrete/mortar for which the cement is used.

Review questions

• What are the main chemical constituents of cement ?

• What is the role of gypsum in cement ?• Which constituent in cement is mainly

responsible for the strength?

Types of cement

• Main types of cement are-1. OPC2. Rapid hardening cement3. Clinker cement4. Sulphate resisting cement5. Oil well cement6. Portland blast furnace slag cement7. Portland pozzolana cement8. White cement

Contd.• Ordinary Portland Cement (OPC): Also referred to as grey cement or OPC,

it is of much use in ordinary concrete construction. In the production of this type of cement in India, Iron (Fe2O3), Magnesium (MgO), Silica (SiO2), Alumina (AL2O3), and Sulphur trioxide (SO3) components are used.

• Rapid Hardening Portland Cement: The texture of this cement type is quite similar to that of OPC. But, it is bit more fine than OPC and possesses immense compressible strength, which makes casting work easy.

• Portland Pozzolona Cement (PPC): As it prevents cracks, it is useful in the casting work of huge volumes of concrete. The rate of hydration heat is lower in this cement type. Fly ash, coal waste or burnt clay is used in the production of this category of cement. It can be availed at low cost in comparison to OPC.

Contd.• Sulphate Resisting Portland Cement: This cement is beneficial in the areas

where concrete has an exposure to seacoast or sea water or soil or ground water. Under any such instances, the concrete is vulnerable to sulphates attack in large amounts and can cause damage to the structure. Hence, by using this cement one can reduce the impact of damage to the structure. This cement has high demand in India.

• Portland Blast Furnace Slag Cement (PBFSC): The rate of hydration heat is found lower in this cement type in comparison to PPC. It is most useful in massive construction projects, for example - dams.

• White cement: It is a kind of Ordinary Portland Cement. The content of iron oxide is maintained below 0.4% to secure whiteness. White cement is largely used to increase the aesthetic value of a construction. It is preferred for tiles and flooring works. This cement costs more than grey cement.

Contd.

• Clinker Cement: Produced at the temperature of about 1400 to1450 degree Celsius, clinker cement is needed in the construction work of complexes, houses and bridges. The ingredients for this cement comprise iron, quartz, clay, limestone and bauxite.

• Oil Well Cement: Made of iron, coke, limestone and iron scrap, Oil Well Cement is used in constructing or fixing oil wells. This is applied on both the off-shore and on-shore of the wells.

Review questions

• List various types of cement ?• In what condition, they used as a main

binder ?

Cement hydration

When water is added to cement, what happens?• Dissolution of cement grains• Growing ionic concentration in “water” (now a solution)• Formation of compounds in solution i.e. tricalcium

silicate (Ca3Si), dicalcium silicate (Ca2S), tricalcium aluminate (Ca3A), tetracalcium aluminium ferrite (Ca4AlF) etc.

• After reaching a saturation concentration, compounds precipitate out as solids (“hydration products”)

Contd.• Aluminates, being highly soluble in plain water, react

immediately and rapidly deposition of calcium aluminate crystals takes place.

• This rapid deposition of calcium aluminate crystals is responsible for initial setting of cement.

• Whereas, C3S (Tri-calcium silicate) reacts slower than C3A (tri-calcium aluminates), but faster than C2S (Di-calcium silicate) and is responsible for first 28 days strength.

• C2S (Di-calcium silicate) reacts slowly and it continues for two to three years and contributes to the development of later strength.

Contd.C3S Tricalcium silicate Hydrates & hardens rapidly,

Responsible for initial set and early strengthC2S Dicalcium silicate Hydrates & hardens slowly,

Contributes to later age strength (beyond 7 days)

C3A Tricalcium aluminate Liberates a large amount of heat during first few Days, Contributes slightly to early strength development,Cements with low %-ages are more resistant to Sulfates,

C4AF Tetracalcium aluminoferrite

Reduces clinkering temperature,Hydrates rapidly but contributes little to strength,Colour of hydrated cement (gray) due to ferrite hydrates

Structural properties of cement

• The strength developed by portland cement depends on its composition and the fineness to which it is ground.

• The C3S (Tri-calcium silicate) is mainly responsible for the strength developed in the first week of hardening.

• The C2S (Di-calcium silicate) for the subsequent increase in strength.

• The alumina and iron compounds that are present only in lesser amounts make little direct contribution to strength.

Contd.• Set cement and concrete can suffer deterioration from attack by some

natural or artificial chemical agents. • The alumina compound is the most vulnerable to chemical attack in

soils containing sulfate salts or in seawater, while the iron compound and the two calcium silicates are more resistant.

• Calcium hydroxide released during the hydration of the calcium silicates is also vulnerable to attack. Because cement liberates heat when it hydrates, concrete placed in large masses, as in dams, can cause the temperature inside the mass to rise as much as 40 °C (70 °F) above the outside temperature.

• Subsequent cooling can be a cause of cracking. The highest heat of hydration is shown by C3A, followed in descending order by C3S, C4AF, and C2S.

Review questions

• What do you understand by ‘cement hydration’ ?

• Name the compounds those forms during cement hydration process and what is their role for gain in strength in cement ?

• What do you understand by structural properties of cement ?