Che. of Cement

7/31/2019 Che. of Cement

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CHEMISTRY OF CEMENT

-Mangala.CA8th sem, civil1sj01cv011


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INTRODUCTION OF CEMENT:

Has adhesive and cohesive properties

Capable of bonding mineral fragments

Principle constituents being: - compounds of lime

- calcareous cement

HYDRAULIC CEMENT

The cement having the property of setting & hardening under water

By virtue of chemical reaction is called HYDRAULIC CEMENT.

CONSTITUENTS:

-Silicates

-Aluminates


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MANUFACTURE OF PORTLAND CEMENT:

Raw materials used-calcareous materials and

argillaceous materials.

Process of manufacture-grinding the raw material,

mixing and burning.

Two process of manufacture-wet and dry process.


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Clay water limestone

Wash mill crusher

Storage basinswet grindingstorage basins

mills to make

slurry

blending of slurry to correct composition

storage of corrected slurry

Powdered coal or gas corrected slurry fed to rotary kiln

slurry is converted into clinker

Addition of 2-3% gypsum clinker is ground in ball millcement silospacking plant


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INFLUENCE OF RATE OF COOLING ON

COMPRESSIVE STRENGTH:

Type of cement Cooling conditions Compressive strengthin kg/sq cm

3 day 7 day 28day

Normal cement quick 99 153 259

Moderate 97 210 273

Slow 97 193 239

Very slow 87 187 230

High early strengthcement

quick 102 188 293

moderate 142 267 333

slow 102 210 292

Very slow 91 181 279


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CHEMICAL COMPOSITION :Approximate oxide composition limits of ordinary Portland

cement.

OXIDE % content

CaO 60-70

SIO2 17-25

Al2O3 3.0-8.0

Fe2O3 0.5-6.0

MgO 0.1-4.0

Alkalies

(K2O,Na2O)

0.4-1.3

SO3 1-3


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INDIAN STANDARD SPECIFICATION 269-1976

SPECIFIES THE FOLLOWING CHEMICAL

REQUIREMENT:

Ratio of % of lime to % of silica, alumina and iron oxide;

When calculated by formula-

(CaO-0.7SO3)/(2.8SiO2+1.2Al2O3+0.65Fe2O3)= not greater than

1.02 and not less than 0.66

Ratio of % of alumina to that of iron oxide=not less then 0.66

Weight of insoluble residue= not more then 2%

Weight of magnesia=not more then 6%

Total sulphur content, calculated as sulphuric anhydride

(SO3)=not more then 2.75%

Total loss on ignition=not more then 5%


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The oxide present in the raw materials when subjected

to high clinkering temperature combine with each other

to form complex compounds (BOGUEScompound).

The four compounds regarded as BOGUEScompounds

are listed in table:

Name of compound formula Abbreviated formula

Tricalcium silicate 3 CaO.SiO2 C3S

Dicalcium silicate 2CaO.SiO2 C2S

Tricalcium aluminate 3 CaO.Al2O3 C3A

Tetracalcium

aluminoferrite

4

CaO.Al2O3.Fe2

O3

C4AF


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C-CaO

S-SiO2

A-Al2O3

F-Fe2O3

H-H2O

The equation for calculating the % of major compounds

(as given by Bogues) are:

C3S=4.07 (CaO)-7.60 (SiO2)-6.72 (Al2O3)-1.43 (FE2O3)-2.85

(SO3)

C2S=2.87 (SiO2)-0.754 (3CaO.SiO2)

C3A=2.65 (Al2O3)-1.69 (Fe2O3)

C4AF=3.04 (Fe2O3)


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The oxide composition and the corresponding calculated

compound composition is shown in table below:

Oxidecomposition

% Calculatedcompound

composition

using Bogues

equation

%

CaO 63 C3S 54.1

SiO2 20 C2S 16.6

Al2O3 6 C3A 10.8

Fe2O3 3 C4AF 9.1

MgO 1.5

SO3 2


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HYDRATION OF CEMENT:

Anhydrous cement compounds when mixed with water, react with

each other to form hydrated compounds

Two ways of hydration-through solution and solid stateReaction of cement with water is exothermic-heat of hydration

Pattern of liberation of heat from setting cement isgiven in the next slide.


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Heat liberation from a setting cement


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Fig below shows the rate of hydration of pure compounds:


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CALCIUM SILICATE HYDRATES:

During the course of reaction ofC3S or C2S with water, silicate

hydrate and calcium hydroxide are formed.

The following are the equation showing the reaction of C3S and

C2S with water

2 (3 CaO.SiO2)+6H2O3 CaO.2SiO2 .3H2O+3Ca(OH)2

Similarly,

2C2S + 4H C3S2H3 + Ca(OH)2

C3S reacts with water and producers more heat of hydration.

C2S hydrates slowly and responsible for later strength of concrete.


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Figure shows the development of strength of pure

compounds.


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CALCIUM ALUMINATE HYDRATES

Due to the hydration ofC3A, a calcium aluminatesystem CaO-Al2O3-H2O is formed.

Hydrated aluminates do not contribute anything to the

strength of the paste.

On hydration, C4AF is believed to form a system of the

form CaO-Fe2O3-H2O (this compound is most stable).

CaO.SiO2.H2O of hydration is called tobermorite gel.


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STRUCTURE OF HYDRATED CEMENT

To understand the behavior of concrete, it is necessary to

acquaint ourselves with the structure of hydrated hardened

cement paste.

Two phases: 1) paste phase

2) aggregate phase


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Figure shows the composition of cement paste

at different stages of hydration:


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Figure (a) and (b) shows the structure of hydrated

cement paste:


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WATER REQUIREMENTS FOR

HYDRATION

C3S requires 24% of water by weight of cement andC2S requires 21%.

This 23% of water ofC3S combines with cement and is

called as bound water. A certain quantity of water isimbibed within the gel pores and is called gel water.

It has been further estimated that about 15% by rate of

cement is required to fill up the gel pores.

Therefore 38% of water by weight of cement is

required for the complete chemical reaction and to

occupy the space within gel pores.


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The diagrammatic representation of progress

of hydration is shown in the figure below:


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CURING OF CONCRETE

Concrete derives its strength by hydration of cement

particles.Water/cement ratio of about 0.38 would be required to

hydrate all the particles of cement and also to occupy the

spacing gel pores.

Water used in the concrete evaporates and water

available in the concrete will not be sufficient for

effective hydration to take place especially in the top

layer.

Curing is described as keeping the concrete moist and

warm enough so that the hydration of cement ca continue

under a favorable temperature


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CURING METHODS

Curing methods maybe divided into 4 categories:

a) Water curing satisfies all requirements of curing.

b)Membrane curing it is a method of maintaining a

satisfactory state of wetness in the body of concrete to

promote continuous hydration when W/C ratio is notless than 0.5.

c) Application of heat subjecting the concrete to higher

temperature and maintaining the required wetness canbe achieved by steam curing.

d) Miscellaneous methods of curing by using calcium

chloride as a surface coating.


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WHEN TO START CURING AND HOWLONG TO CURE

Concrete should not be allowed to dry fast in any situation.

Concrete cured for a long time will show superior strength and

develop other good properties. However curing for a long time will

entail cost.

Curing period cannot be prescribed definitely, it varies for

different structures, situation and atmospheric temperature.

For general guidance-concrete must be cured for 7 days.On the other hand for low heat cement curing should be extended

to 21 days.

Che. of Cement

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