Process of Hydration

The process of hydration is mainly the formation of minute crystals of calcium and gels fom the solution of cement and water.The hydration process of portland cement is as follows-

Tricalcium silicate hydrates as follows
3(3CaO.SiO2)+6H2O=3CaO.2SiO2.3H2O  + 3Ca(OH)2
                                          Calcium Silicate          LIme

This can be written as -
3C3S+6H2O= C3S2H3+3Ca(OH)2

Dicalcium silicate hydrates as follows
2(2CaO.SiO2)+4H2O=3CaO.2SiO2.3H2O+Ca(OH)2

This can be written as
2C2S+4H2O=C3S2H3+Ca(OH)2

The reaction of triclcium aluminate (C3A) with water very fast and immediately cause stiffening of cement paste which is termed as flash setting.This can be reduced by mixing gypsum(CaSo4.2H2O)
with cement clinker during grinding.Gypsum and C3A reacts to form calcium sulphoaluminate which eventually form calcium sulphoaluminate which eventually form calcium aluminate(hydrate).
3C3A+6H2O+3CA.3Ca3SO4.3H4O=C3A.H2O
 This retards the action of the starting of initial setting.     

Gypsum not only reacts with C3A.These accelerate the hydration of calcium silicate.Gypsum is added very carefully to cement because excess will cause expansion and disintegration of  cement paste and also slower the initial setting.

Hydration

Anhydrous cement does not bind fine and coars aggregate.The cement acquires binding property after mixing with water.The chemical reaction that take place in cement after mixing of water is termed as hydration

Differences Between False Settingand Flash Setting

     False setting                                            Flash setting

1.No heat evolves                           1.Appreciable amount of heat evolves

2.Remixing of cement without          2.Remixing of cement without adding
adding of water regain plasticity.       of water do not regain plasticity.

Setting

Setting refers to the change of cement paste from fluid to rigid.It differs from hardening. Hardening refers to gain of strength after setting. Although cement paste acquire some strength in setting

Aluminium Paint

This consists of very finely grounded aluminum suspended in the medium of slow drying oil or spirit varnish. It is an expensive white paint.It can resist heat to a great extent.

Uses
1.Mainly decorative purpose
2.Also protects steel and iron
3.Used for painting marine pipes, oil tanks, gas tanks,water pipes etc.

Distemper

Distemper is a kind of water paint containing water and oil as thinner agents and glue,starch are used for base and different color pigments for desired color.It is used for interior decoration

Resin

A resin is an amorphous,vitreous,semi liquid organic matter which is soluble in organic liquid and drying oil but non soluble in water.When it is heated it gives a product which is soluble in drying oils.

Preparation of Concrete Surface for Painting

1.Cement and concrete should be treated with benzene to remove grease and oil.

2.Then zinc sulphate should be applied to neutralize lime present in the concrete.

Preparation of Plastered Surface for Painting

1.Plastered surface should be brushed with zinc sulphate before painting.

2.Prior to repainting benzene should be applied to remove grease

Pigments Used in Luminescent Coating

Generally there are two types of pigments used in luminescent coating.

1.Fluorescent Pigments:These are synthetic dyes zinc sulphide or mixture of zinc and cadmium sulphide.

2.Phosphorescent Pigments: These are mainly calcium sulphide or strontium sulphide.

Constituents of Cement

Constituents of cement are mainly two types
1.Mineral constituents , 2.Acid and Alkaline constituents.

Mineral Constituents

       Name                   Oxide compositions   Abbreviation    Percentage(%)

Tricalcium Silicate ~     3CaO.SiO2(*1) ~            C3S(*1) ~              45~55
Dicalcium Silicate ~      2CaO.SiO2(*1) ~            C2S(*1) ~              20~30
Tricalcim Aluminate ~    3CaO.Al2O3 ~                      C3A ~                        9~13
Tetracalcium 
Aluminoferrite ~              4CaO.Al2O3.Fe2O3~             C4AF  ~                 8~20
Calcium Sulphate~          CaSo4  ~                                  ----------        ~            2~6
Other Compounds~        ----------   ~                               ---------   ~                  2~8       

Acid and Alkaline Constituents
        
        Name               Composition                Range of percentage


Calcium oxide     ~     CaO               ~               60~67
Magnesium oxide~     MgO               ~               0.1~4
Silica                    ~     SiO2                  ~                  17~25
Alumina               ~     Al2O3                ~                   3~8
Iron oxide             ~     Fe2O3               ~                  0.5~6
Sulphur trioxide   ~     SO3                    ~                  1~3
Potassium oxide   ~     K2O                   ~                   0.3~1
Sodium oxide       ~      Na2O                ~                   0.4~1.3
Loss on Ignition   ~      --------            ~              1.8~2
Insoluble Resedue ~     --------           ~              0.3~0.5

Harmful Consituents of Brick

The followings are the harmful constituents of brick.

1.Iron pyrites

It causes crystallization and disintegration of brick particles on drying.

2.Alkalies

These are mainly chlorides and sulphates of calcium,magnesium,sodium and potassium causing dark greenish hue on brick surface on drying.It also cause brick to twist,fuse and warp.It absorb moisture which cause efflorescence.

3.Stone particles

It prevents mixing of clay thoroughly and uniformly.It makes brick porous and weak.

4.Vegetation and Organic Matter

These leave pores in brick on burning.

5.Lime

Excess lime cause brick to fuse and hence shape is lost.Lime in the form of lime stone and Kankar nodule very harmful.Because limestone(CaCO3) leaves CO2 in burning and in contact with water it gets hydrated and swells and cause the brick to lose shape.Kankar also cause similar type of damage to brick.but lime is very important in brick.So proper amount of lime should present in brick.

Chemical Composition and It's Function of Brick Clay

The chemical composition of brick clay is as follows-

                                              Silica - 55%

                                              Alumina - 30%

                                              Iron oxide - 8%

                                              Magnesia - 5%

                                               Lime - 1%

                                              Alkali and organic matter-1%
Silica:
Silica is present in brick clay as the chemical composition of alumina and forming silicate of alumina.Sometimes silica is present in pure form termed as flint.
Silica prevents shrinking,cracking and warping of brick clay.High amount cause uniform texture.Too much presence of silica cause brick to be brittle and weak.

Alumina:
Alumina imparts plasticity to brick clay which is very important in molding.It also imparts density.Excess alumina cause brick to crack and warp during drying and very hard when heated.

Iron oxide:
The presence of iron oxide increases impermeability and durability of brick.The color of brick depend upon iron oxide and color changes from light yellow to orange and red as iron oxide goes up to 8%.

Magnesia:
Presence of small quantity of magnesia decreases shrinkage and gives yellowish tint.

Lime:
Lime melts silica in burning and binds the particles of brick together.It also reduces the shrinkage of brick during drying.
Excess of lime cause the brick to fuse and shape will be lost.

Alkali and Organic Matter:
Small quantity assist burning of brick clay.It also reduces the fusion point.If excess amount is present and not burned properly the brick would be porous.

Differences Between Mortar and Plaster

                        Mortar                                          Plaster
                                                                                                                                                               
1.Mortar is the paste made              1.Plaster is thin coat of various
of mixing cementing and inert        composition of mortar which is                                             
 material with water.                       applied both external and internal
                                                          surfaces of walls, ceiling, clolumns
                                                          etc.                                      
                                                                                                         
2.Mortar is the ingredient                2.Plaster is the applied work of mortar.
 of plaster                                                                                          
                                                                                         

Function of Sand in Mortar

1.Sand s mainly used as a inert material to give volume in mortar for economy.

2.It subdivides cementing material into a thin film which is the principle of using all cementing material.

3.It offers requisite surface area for film of cementing material to adhere and spread.

4.Prevents shrinkage and cracking of mortar.

5.Well graded sand provide density.

6.It allows CO2 to enter in some depth in case fat lime mortar and increase setting power.

7.Some chemical reaction take place between silica of sand grain and other cementing material.

Sand

Sand is a very important material in concrete.It plays a vital role in engineering construction. Sand is a form of silica and may be argillaceous(clay like), silicious(silica type) or calcarious(calcium type) according to its composition.

Different Types of Shrinkage Of Concrete

1.Plastic Shrinkage: While the cement is in plastic condition shrinkage occurs during the period of hydration. It is believed that evaporation of water in air of water by aggregate or sub grade cause this shrinkage.

2.Drying Shrinkage: Withdrawal of water from concrete in saturated air cause drying shrinkage, The loss of water contained in hardened concrete does not cause shrinkage. The loss of water from gel pores cause drying shrinkage.

3.Carbonation shrinkage: Carbon-di-oxide(Co2) present in the atmosphere reacts with hydrated cement. As a result calcium hydroxide [Ca(OH)2] decomposes and forms calcium carbonate [CaCO3] and some other compounds of hydrated cement also decomposes.This reaction takes place even at low pressure of CO2 in atmosphere.This reaction is called carbonation. It accompanies with increase of weight and shrinkage of concrete. The shrinkage is due to the production of CaCo3 in the place of Ca(OH)2, where the volume of CaCO3 is less than the volume of of Ca(OH)2.

Problems Of Using Salt Contaminated Sand

Salt contaminated sand absorb moisture from atmosphere causing permanent dampness and efflorescence and work gradually disintegrates. It also contain shells and organic matter which decompose in the body of mortar.Hence strength and life of mortar reduces.

Propertise of Good Sand

Good sand should be pure silica.It should be free from clay, salt,silt and organic matters.It is better wash sand before construction work.

Bulking of Aggregates

The increase of volume of given weight of aggregates due to the presence of moisture is called bulking of aggregates. 5 to 8 percent of moisture of aggregate weight cause 20 to 30 percent volume increase. This because of the creation of thin film of water inside aggregates which cause interlocking air. This are tries to get out and cause volume increase. By adding of water in dry aggregates the volume in crease up to adding of 8 percent of water of aggregate weight. Further adding of water cause water move into the void of aggregates particle. This cause volume to decrease. When aggregates are saturated its volume is same as the dry condition. Five aggregates increase volume more than coarse aggregates.

The bulking is determined by following expression

Bulking, b = Vm/Vd x100

Where Vm is the volume with moist and Vd is the dry or fully saturated volume and b is the bulking percentage.

Alkali Aggregate Reaction

For a long time aggregates are considered to be inert.But now it is proven that they are not inert totally. Some aggregates contain reactive silica which reacts with alkalies,i.e, sodium or potassium present in cement.
The reaction starts by attacking on silicous mineral by alkaline hydroxide derived from alkali. Hence silicate gel of unlimited swelling form inside concrete. This swelling cause pattern of cracks and eventual failure of concrete structure.

Soundness of Aggregates

Soundness of aggregates is the ability of aggregates to resist change of volume due to change of physical condition. These physical conditions include freezing and thawing,temperature change, alternative change of drying and wetting in normal condition and alternative change of drying and wetting in salt water.The aggregates which are weak, porous and containing undesirable materials undergo large volume change in change of those physical conditions.

Importance of Aggregates

As at least three quarters of concrete is occupied by aggregates it is of great importance.

1.Cheaper than cement.Hence it reduces the cost of concrete.

2.Increases the binding capacity of cement.Cement alone can not be strong.

3.The chemical and thermal properties of aggregates effect the properties of concrete.

4.Durability of concrete depends on the characteristics of concrete.

Aggregates

The inert materials which are used in concrete is called aggregates.Such as sand,pebbles,cinder, gravel etc.But aggregates are not always inert. Recent study shows that the chemical and thermal properties of aggregates influence the quality of concrete.

Kinds of aggregates

Two kinds.

1.Fine aggregates:The aggregates which are smaller than 3/16 inch or 4.76 mm is called fine aggregates. Sand is the common fine aggregate.

2.Coarse aggregates:The aggregate which are bigger than 3/16 inch or 4.76 mm is called coarse aggregate. Broken brick,pebbles,gravel etc are coarse aggregates.