Cementmaterils

Applied Chemistry CODE NO: 07 I B.TECH CIVIL ENGINEERING Unit No: VIII Nos. of slides:

Applied Chemistry INORGANIC CEMENTING MATERIALS Term: 2008-09 Unit-VIII Power Point Presentations Text Books:  A text book of Engineering Chemistry by Jain & Jain,  Chemistry of Engineering Materials by C.P. Murthy, C.V. Agarwal and A. Naidu

INDEX UNIT-VIII PPTS Srl. No.

PPT

Module as per

Lecture

Session Planner No. Slide No. -----------------------------------------------------------------------------------------------4. Introduction L-1 L1-1 to L1-19 2. Hardness & Units L-2 L2-1 to L2-28 3. Estimation of Hardness by EDTA L-3 L3-1 to L3-18 4. Problems on Hardness L-4 L4-1 to L4-18 5. Analysis of water L-5 L5-1 to L5-19 6. Methods for treatment of water (Domestic) L-6,7 L6,7-1 to L6,7-33

The manufacture of cement consists of the following steps.

Mixing of raw materials or slurry: An intimate mixture of finely ground limestone and clay (3:1) is made into thin slurry with water by any of the following methods. Dry process Wet process

Dry process: If the lime stone and clay are hard, then the dry process is used. in this process the lime stone is first broken into small pieces. it is then mixed with clay in the proper proportion (3:1) and finally pulverized to such a finesse that 90-95% passes through a 100 mesh sieve. Then the raw mix is fed to a rotary kiln.

Wet process: If lime stone and clay are soft, the wet process is preffered. In this process, the clay is washed with water in wash mill to remove the foreign materials, organic matters etc. the powdered limestone is then mixed with the clay paste in the proper proportion (3:1) and the two ingredients are finely ground and homogenized. in this process, the slurry contains about 40% of water. Now the slurry can be fed to rotary Kiln.

Burning:

The dry pulverized raw mixture or slurry is introduced into a rotary kiln which consists of an inclined steel rotating cylinder. 150-200 feet long and 10 feet in diameter lined with fire bricks. The water evaporates at the upper ends of kiln by means of hot gases. The Kiln rotates on its axis at the rate of ½ to 1 revolution per minute. As the Kiln rotates the charge slowly moves down-wards due to the rotary motion of the Kiln. Now the charge is heated by blast of air charged with coal dust is admitted. this produces a temperature range of 1500 to 1700 0C in step wise process as:

At 750 0C the moisture is completely eliminated. • b. At 10000C the limestone is completely decomposed to CaO. c. At 1500 to 17000C the mixture is partly fused and sintered and chemical combinations between lime, alumina, ferric oxide and silica. The charge takes 2 to 3 hrs. to covers the journey in the Kiln. Chemical reactions: in the rotary Kiln the chemical reactions can be divided into the following parts. •

a. Drying zone: In this zone temperature raises to maximum 750 0C so that entire moisture in the slurry gets evaporated. The clay is broken into Al2O3, SiO2 and Fe2O3. Al2O3 2SiO2 Fe2O3 2H2O  Al2O3 + 2SiO2 + Fe2O3 + 2H2O

Calcinations zone: When the temperature raises at 1000 0C, the limestone is completely decomposed into CaO. CaCO3  CaO + CO2

Reaction zone (Clinkering zone): when the temperature reaches about 16000C, the mixture is partly fused and chemical combinations between lime, alumina, ferric oxide and silica, resulting in the formation of calcium aluminates and silicates occur.

2CaO + SiO2  2CaO.SiO2 (di calcium silicate) 3CaO + SiO2  3CaO.SiO2 (Tri calcium silicate) 2CaO + Al2O3  2CaO. Al2O3(di calcium Aluminate) 3CaO + Al2O3  3CaO. Al2O3(tri calcium Aluminate) 4CaO + Al2O3 + Fe2O3 4CaO. Al2O3 Fe2O3(Tetra calcium Aluminate)

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The resulting product is known as cement clinkers and as it comes out into the cooler. The clinkers are very hot (10000C. The clinkers have the appearance of small greenish black or grey colored.

3. Mixing of cement clinkers with gypsum: 

The cooled clinker is ground and almost 3% of gypsum is mixed with it in order to reduce the rate of setting. After the initial setting, Al2O3 which is a fast setting constituent of clinker reacts with gypsum to form the crystals of calcium sulpho aluminate.

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3CaO. Al2O3 3(CaSO4.2H2O) + 2H2O  3CaO. Al2O3 3CaSO42H2O + 6H2O

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At the initial setting, gypsum removes the fast setting elements and hence the process of setting cement gets retarded and the results in better strength of the mass which sets.

Packing: 

The ground cement is stored in silos, from which it is fed to automatic packing machines. Each bag, usually, contains 50 kg of cement.

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2CaO + SiO2  2CaO.SiO2 (di calcium silicate) 3CaO + SiO2  3CaO.SiO2 (Tri calcium silicate) 2CaO + Al2O3  2CaO. Al2O3(di calcium Aluminate) 3CaO + Al2O3  3CaO. Al2O3(tri calcium Aluminate) 4CaO + Al2O3 + Fe2O3 4CaO. Al2O3 Fe2O3(Tetra calcium Aluminate)

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The resulting product is known as cement clinkers and as it comes out into the cooler. The clinkers are very hot (10000C. The clinkers have the appearance of small greenish black or grey colored.

3. Mixing of cement clinkers with gypsum: 

The cooled clinker is ground and almost 3% of gypsum is mixed with it in order to reduce the rate of setting. After the initial setting, Al2O3 which is a fast setting constituent of clinker reacts with gypsum to form the crystals of calcium sulpho aluminate.

3CaO. Al2O3 3(CaSO4.2H2O) + 2H2O 3CaO. Al2O3 3CaSO42H2O + 6H2O

At the initial setting, gypsum removes the fast setting elements and hence the process of setting cement gets retarded and the results in better strength of the mass which sets.

Packing: 

The ground cement is stored in silos, from which it is fed to automatic packing machines. Each bag, usually, contains 50 kg of cement.

Calcareous materials (Lime stone)

Argillaceous materials (clay or Shale)

Crushing and powdering

Washing

Proportioning

Grinding

Water

Slurry

Rotary Kiln

Cement clinkers

Cooler

Gypsum

Clinkers grinding elevators

Cement soils

Pulverized coal

Ch emic al composit ion of

cem en t

Chem ical composi ti on of cement:

• The essential constituents are lime, silica and alumina. For a good quality of cement the following ratio should be maintained. • %SiO2 = 2.5 to 4.0 • %Al2O3 • _____%CaO - %SO3____ = 1.9 to 2. • %SiO2 + %Al2O3 + Fe2O3

• The ratio of silica to alumina lies between 2.5 and 4. • The ratio of calcium oxide to silica + alumina + ferric oxide should be close to 2.5 and 4. • Total percentage of magnesia should be < 6% • Total sulphur content should not be more than 2.75% • The ratio of Alumina to Iron oxide should be close to 0.65 • Total loss on ignition shall not exceed 4%.

Setti ng an d Har deni ng of Cem en t:

• Cement has the property of setting to hard mass after being mixed with water which is called cement paste. After mixing with water, hydration reaction starts and the mass becomes hard and very resistant to pressure. This is called as the setting of cement. The first setting takes place within 24 hours. The subsequent hardening requires about a fortnight. Setting is defined as stiffening of the original plastic mass and hardening is development of strength due to crystallization. Both these processes are due to hydration and hydrolysis reactions.

Initia l se ttin g: • primarily the reactions involved are the hydration of calcium aluminates and calcium silicates which change into their colloidal gel. At the same time some calcium hydroxide and aluminium hydroxide are formed as precipitates due to hydrolysis. • 3CaO Al2O3 + 6H 2O  3CaO . Al2O3 .6H2 O + 880 Kj/ kg • 4CaO Al2O3 F e2O3 + 7H2O  3CaO . Al2O 3 .6H2O + 3Ca O F e2O3 H2O + 420 Kj/ kg • 3CaO Al2O3 + 3( CaS O4 + 2H 2O  3CaO Al2O3 3CaS O4 2H2O • Addition of gypsum removes the fast setting elements. • Reaction takes place between 1 and 7 days: • Hydrolysis reaction: 3Ca OS iO2 + H2O  Ca (OH )2 + 2Ca OS iO2

Fin al se ttin g ( between 7 to 28 days): • begins to hydrate and forms the hydrated colloidal gel of the composition. • 2CaO SiO 2 + X H2O  2CaO SiO 2 XH 2O + 500K j/kg

Sequence of chemi cal r eacti ons during setting har de ning of c ement :

and

• when water is added to cement, its various constituents undergo hydration and crystallization at different rates. • At first, hydration of tricalcium aluminate and tetra calcium aluminoferrite takes place. • Next, the hydration of tricalcium silicate begins within 24 hours and gets completed in 7 days.

Sequence of chemi cal r eacti ons during setting har de ning of c ement :

and

• The gel of aluminate begins to crystallize and at the same time, dicalcium silicate begins to hydrate in 7 to 28 days. Thus, the initial set of cement is due to the hydration of aluminate. The development of earlystrength, between 1 to 7 days, is due to the hydration of tricalcium silicate and the further hydration of aluminate. The increase of strength, between 7 to 28 days, is due to hydration of dicalcium silicate and continued hydration of tricalcium silicates.