The chemical reaction between the cement and water which turns cement into the binding material with strong adhesive property is called hydration of cement.The following are the approximate equations showing the reactions of `C_3S` and `C_2S `with water.

alt="Explain the Hydration of cement."

The reaction of pure `C_3A` with water is very fast and this may lead to flash set (Rapid and early loss of workability which cannot be restored). To prevent this flash set, gypsum is added at the time of grinding the cement clinker.

`2C_3A+6H = 2C_3AH_6`

Hydration of `C_3A` in presence of gypsum:
`C_3A+32H + 3CaSO_4 = C_6AS_3H_(32)` (tri-sulphate hydrate - ?ettringite?)
`C_3A+ 18H + CaSO_4 = C_4ASH_(18)` (mono-sulphate hydrate)

Addition of gypsum in cement control the flash set reaction of C3A. In presence of gypsum it
reacts with 32 molecule of water resulting large volume change. As this reaction takes place
in green stage of concrete, volume change will not be the problem. But if this takes place in
harden stage of concrete due to external sulphate environment or presence of excess of
gypsum in cement, the large volume change cause crack & detoriation in concrete called
sulphate attack.

`C_4AF + 2CH + 10H = C_3AH_6 + C_3FH_6`

The hydration products of `C_4AF` are similar to those of `C_3A`. However, the reaction rate of `C_4AF` is slower than that of `C_3A`.

The quantity of heat developed upon the complete hydration at the given temperature is called heat of hydration. The heat of hydration follows following pattern:

`C_3A > C_3S > C_4AF > C_2S`

The chemical reaction between the cement and water which turns cement into the binding material with strong adhesive property is called hydration of cement.The following are the approximate equations showing the reactions of `C_3S` and `C_2S `with water.

alt="Explain the Hydration of cement."

The reaction of pure `C_3A` with water is very fast and this may lead to flash set (Rapid and early loss of workability which cannot be restored). To prevent this flash set, gypsum is added at the time of grinding the cement clinker.

`2C_3A+6H = 2C_3AH_6`

Hydration of `C_3A` in presence of gypsum:
`C_3A+32H + 3CaSO_4 = C_6AS_3H_(32)` (tri-sulphate hydrate - ?ettringite?)
`C_3A+ 18H + CaSO_4 = C_4ASH_(18)` (mono-sulphate hydrate)

Addition of gypsum in cement control the flash set reaction of C3A. In presence of gypsum it
reacts with 32 molecule of water resulting large volume change. As this reaction takes place
in green stage of concrete, volume change will not be the problem. But if this takes place in
harden stage of concrete due to external sulphate environment or presence of excess of
gypsum in cement, the large volume change cause crack & detoriation in concrete called
sulphate attack.

`C_4AF + 2CH + 10H = C_3AH_6 + C_3FH_6`

The hydration products of `C_4AF` are similar to those of `C_3A`. However, the reaction rate of `C_4AF` is slower than that of `C_3A`.

The quantity of heat developed upon the complete hydration at the given temperature is called heat of hydration. The heat of hydration follows following pattern:

`C_3A > C_3S > C_4AF > C_2S`

Concrete is a heterogeneous, multi-phase composite material composed of coarse granular material (the aggregate or filler) embedded in a hard matrix of material
(the cement or binder) that fills the space among the aggregate particles and glues them together. The simplest definition
of concrete can be written as,

concrete =Aggregate + Binding material + Admixtures

The ingredient of Golden age Concrete are:

1. Hydraulic lime to provide self cementing properties
   
2. Volcanic Ash to make concrete set under water
   
3. Horse hair to import reinforcement so as to make concrete less liable to crack
   
4. Aggregate likes sand,stones etc.

The ingredient of Modern age Concrete are:

1. Cement (10 - 15 % )as a binding material
   
2. Aggregate ( 70- 75 % ) like sand, stones etc.
   
3. Admixtures as per the requirements.
4. Water ( 15 - 25 % ) for hydration and to increase the workability.
   

Concrete is a heterogeneous, multi-phase composite material composed of coarse granular material (the aggregate or filler) embedded in a hard matrix of material
(the cement or binder) that fills the space among the aggregate particles and glues them together. The simplest definition
of concrete can be written as,

concrete =Aggregate + Binding material + Admixtures

The ingredient of Golden age Concrete are:

1. Hydraulic lime to provide self cementing properties
   
2. Volcanic Ash to make concrete set under water
   
3. Horse hair to import reinforcement so as to make concrete less liable to crack
   
4. Aggregate likes sand,stones etc.

The ingredient of Modern age Concrete are:

1. Cement (10 - 15 % )as a binding material
   
2. Aggregate ( 70- 75 % ) like sand, stones etc.
   
3. Admixtures as per the requirements.
4. Water ( 15 - 25 % ) for hydration and to increase the workability.
   

Admixture is defined as a material, other than cement, water and aggregates, that is used as an ingredient of concrete and is added to the batch immediately before or during mixing to modify one or more properties of concrete in plastic or hardened state.

Function of Admixtures:

1. To increase the strength, durability and workability of concrete.
2. To increase the resistance to chemical attack, freezing and thawing effects.
3. To accelerate or retard the rate of hydration of cement.
4. To reduce the permeability of concrete.
5. To reduce the segregation and bleeding of concrete.
6. To reduce the shrinkage during setting of concrete.
7. To control the alkali-aggregate reactions.

Classifications of Admixtures:

1. Chemical Admixtures
2. Mineral Admixtures

CHEMICAL ADMIXTURES:

1. Accelerators
2. Retarders
3. Plasticizers
4. Super Plasticizers
5. Colouring Admixture
6. Air entraining Admixture
7. Water Proofing Admixture

Accelerators:

Accelerating admixtures are added to concrete to increase the rate of early strength
development in concrete to

1. permit earlier removal of formwork;
2. reduce the required period of curing;
3. advance the time that a structure can be placed in service
4. partially compensate for the retarding effect of low temperature during cold weather
   concreting
5. in the emergency repair work.

Calcium chlorides, caustic soda, caustic potash, etc are commonly used accelerators.

Retarders:

A retarder is an admixture that slows down the chemical process of hydration so that concrete remains plastic and workable for a longer time than concrete without the retarder. Calcium sulphate (gypsum), calcium borate, glucose etc are commonly used retarders.

Plasticizers (Water Reducers):

The organic substances or combinations of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water content, are termed as plasticizing admixtures. The basic products constituting plasticizers are as follows:

1. Anionic surfactants such as lignosulphonates and their modifications and derivatives, salts of sulphonates hydrocarbons.
2. Nonionic surfactants, such as polyglycol esters, acid of hydroxylated carboxylic acids and their modifications and derivatives.
3. Other products, such as carbohydrates etc.

Lignosulphonic acid is  most commonly used plasticizers.

When plasticizers are used, they get adsorbed on the cement particles. The adsorption of charged polymer on the particles of cement creates particle-to-particle repulsive forces which overcome the attractive forces. This repulsive force is called Zeta Potential, which depends on the base, solid content, quantity of plasticizer used. The overall result is that the cement particles are deflocculated and dispersed. When cement particles are deflocculated, the water trapped inside the flocs gets released and now available to fluidify the mix.

Plasticizers are of two types:

1. Accelerating Plasticizers
2. Retarding Plasticizers

Superplasticizers:

Superplasticizers, also known as High Range Water Reducers, are the improved version of plasticizer used for production of flowing, self levelling, self
compacting and for the production of high strength and high performance concrete which permit the reduction of water to the extent upto 30 percent without reducing workability in contrast to the possible reduction up to 15 percent in case of plasticizers..Superplasticizers can produce:

1. at the same w/c ratio much more workable concrete than the plain ones,
2. for the same workability, it permits the use of lower w/c ratio,
3. as a consequence of increased strength with lower w/c ratio, it also permits a reduction of cement content.
4. The superplasticizers also produce a homogeneous, cohesive concrete generally without any tendency for segregation and bleeding.

Carboxylic acrylic ester (CAE) and multicarboxylatether (MCE) are generally used as superplasticizers.

Air Entraining Admixtures:

Air entrainment will effect directly the following three properties of concrete:

1.  Increased resistance to freezing and thawing.
2. Improvement in workability.
3. Reduction in strength.

The common air entraining agents are Vinsol resin, Darex, N Tair, Airalon, Orvus, Teepol, Petrosan and Cheecol.

Water Proofing Admixtures:

It consists of water repellent materials such as calcium chlorides, zinc sulphates, aluminium sulphates etc. and is generally used in hydraulic structures.

POZZOLONIC OR MINERAL ADMIXTURE:

Pozzolanic materials are siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value, but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to form compounds, possessing cementitious properties. Pozzolanic materials can be divided into two groups: natural pozzolana and artificial pozzolana.

Natural Pozzolans

1.  Clay and Shales
2. Opalinc Cherts
3. Diatomaceous Earth
4. Volcanic Tuffs and Pumicites.

Artificial Pozzolans

1. Fly ash
2. Blast Furnace Slag
3. Silica Fume
4. Rice Husk ash
5. Metakaoline
6. Surkhi.

Fly Ash:

Fly ash is finely divided pozzolanic material resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator. Use of right quality fly ash, results in reduction of water demand for desired slump. With the reduction of unit water content, bleeding and drying shrinkage will also be reduced.

Silica Fume:

Silica fume, also referred to as microsilica or condensed silica fume, is another material that is used as an artificial pozzolanic admixture. It is a product resulting from reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. It is reported that fresh concrete containing microsilica is vulnerable to plastic shrinkage cracking and, therefore, sheet or mat curing should be considered. However, Hardened Concrete containing microsilica showed outstanding characteristics in the development of strength and durability.

Rice Husk Ash:

Rice husk ash, is obtained by burning rice husk in a controlled manner without causing environmental pollution. When properly burnt it has high `SiO_2` content and can be used as a concrete admixture. Rice husk ash exhibits high pozzolanic characteristics and contributes to high strength and high impermeability of concrete.

Surkhi:

Surkhi is an artificial pozzolana made by powdering bricks or burnt clay balls. It is used as a water proofing agent. It is not commonly used nowadays.

Metakaolin:

Unpurified materials like activated ordinary clay and kaolintic clay are terned as Metakaolin. Highly reactive metakaolin is made by water processing to remove unreactive impurities to make 100% reactive pozzolan. Such a product, white or cream in colour, purified, thermally activated is called High Reactive Metakaolin (HRM). The high reactive metakaolin is having the potential to compete with silica fume.

Local Materials as Admixtures in Nepal:

Present possibilities:

1. Rice husk ash
2. Calcined clay pozzolona (Brick dust / Burned clay dust)
3. Stone dust

Future possibilities with industrial development:

• Silica fume
• Fly ash
• Blast furnace slag
• Various chemical admixtures.

Admixture is defined as a material, other than cement, water and aggregates, that is used as an ingredient of concrete and is added to the batch immediately before or during mixing to modify one or more properties of concrete in plastic or hardened state.

Function of Admixtures:

1. To increase the strength, durability and workability of concrete.
2. To increase the resistance to chemical attack, freezing and thawing effects.
3. To accelerate or retard the rate of hydration of cement.
4. To reduce the permeability of concrete.
5. To reduce the segregation and bleeding of concrete.
6. To reduce the shrinkage during setting of concrete.
7. To control the alkali-aggregate reactions.

Classifications of Admixtures:

1. Chemical Admixtures
2. Mineral Admixtures

CHEMICAL ADMIXTURES:

1. Accelerators
2. Retarders
3. Plasticizers
4. Super Plasticizers
5. Colouring Admixture
6. Air entraining Admixture
7. Water Proofing Admixture

Accelerators:

Accelerating admixtures are added to concrete to increase the rate of early strength
development in concrete to

1. permit earlier removal of formwork;
2. reduce the required period of curing;
3. advance the time that a structure can be placed in service
4. partially compensate for the retarding effect of low temperature during cold weather
   concreting
5. in the emergency repair work.

Calcium chlorides, caustic soda, caustic potash, etc are commonly used accelerators.

Retarders:

A retarder is an admixture that slows down the chemical process of hydration so that concrete remains plastic and workable for a longer time than concrete without the retarder. Calcium sulphate (gypsum), calcium borate, glucose etc are commonly used retarders.

Plasticizers (Water Reducers):

The organic substances or combinations of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water content, are termed as plasticizing admixtures. The basic products constituting plasticizers are as follows:

1. Anionic surfactants such as lignosulphonates and their modifications and derivatives, salts of sulphonates hydrocarbons.
2. Nonionic surfactants, such as polyglycol esters, acid of hydroxylated carboxylic acids and their modifications and derivatives.
3. Other products, such as carbohydrates etc.

Lignosulphonic acid is  most commonly used plasticizers.

When plasticizers are used, they get adsorbed on the cement particles. The adsorption of charged polymer on the particles of cement creates particle-to-particle repulsive forces which overcome the attractive forces. This repulsive force is called Zeta Potential, which depends on the base, solid content, quantity of plasticizer used. The overall result is that the cement particles are deflocculated and dispersed. When cement particles are deflocculated, the water trapped inside the flocs gets released and now available to fluidify the mix.

Plasticizers are of two types:

1. Accelerating Plasticizers
2. Retarding Plasticizers

Superplasticizers:

Superplasticizers, also known as High Range Water Reducers, are the improved version of plasticizer used for production of flowing, self levelling, self
compacting and for the production of high strength and high performance concrete which permit the reduction of water to the extent upto 30 percent without reducing workability in contrast to the possible reduction up to 15 percent in case of plasticizers..Superplasticizers can produce:

1. at the same w/c ratio much more workable concrete than the plain ones,
2. for the same workability, it permits the use of lower w/c ratio,
3. as a consequence of increased strength with lower w/c ratio, it also permits a reduction of cement content.
4. The superplasticizers also produce a homogeneous, cohesive concrete generally without any tendency for segregation and bleeding.

Carboxylic acrylic ester (CAE) and multicarboxylatether (MCE) are generally used as superplasticizers.

Air Entraining Admixtures:

Air entrainment will effect directly the following three properties of concrete:

1.  Increased resistance to freezing and thawing.
2. Improvement in workability.
3. Reduction in strength.

The common air entraining agents are Vinsol resin, Darex, N Tair, Airalon, Orvus, Teepol, Petrosan and Cheecol.

Water Proofing Admixtures:

It consists of water repellent materials such as calcium chlorides, zinc sulphates, aluminium sulphates etc. and is generally used in hydraulic structures.

POZZOLONIC OR MINERAL ADMIXTURE:

Pozzolanic materials are siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value, but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to form compounds, possessing cementitious properties. Pozzolanic materials can be divided into two groups: natural pozzolana and artificial pozzolana.

Natural Pozzolans

1.  Clay and Shales
2. Opalinc Cherts
3. Diatomaceous Earth
4. Volcanic Tuffs and Pumicites.

Artificial Pozzolans

1. Fly ash
2. Blast Furnace Slag
3. Silica Fume
4. Rice Husk ash
5. Metakaoline
6. Surkhi.

Fly Ash:

Fly ash is finely divided pozzolanic material resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator. Use of right quality fly ash, results in reduction of water demand for desired slump. With the reduction of unit water content, bleeding and drying shrinkage will also be reduced.

Silica Fume:

Silica fume, also referred to as microsilica or condensed silica fume, is another material that is used as an artificial pozzolanic admixture. It is a product resulting from reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. It is reported that fresh concrete containing microsilica is vulnerable to plastic shrinkage cracking and, therefore, sheet or mat curing should be considered. However, Hardened Concrete containing microsilica showed outstanding characteristics in the development of strength and durability.

Rice Husk Ash:

Rice husk ash, is obtained by burning rice husk in a controlled manner without causing environmental pollution. When properly burnt it has high `SiO_2` content and can be used as a concrete admixture. Rice husk ash exhibits high pozzolanic characteristics and contributes to high strength and high impermeability of concrete.

Surkhi:

Surkhi is an artificial pozzolana made by powdering bricks or burnt clay balls. It is used as a water proofing agent. It is not commonly used nowadays.

Metakaolin:

Unpurified materials like activated ordinary clay and kaolintic clay are terned as Metakaolin. Highly reactive metakaolin is made by water processing to remove unreactive impurities to make 100% reactive pozzolan. Such a product, white or cream in colour, purified, thermally activated is called High Reactive Metakaolin (HRM). The high reactive metakaolin is having the potential to compete with silica fume.

Local Materials as Admixtures in Nepal:

Present possibilities:

1. Rice husk ash
2. Calcined clay pozzolona (Brick dust / Burned clay dust)
3. Stone dust

Future possibilities with industrial development:

• Silica fume
• Fly ash
• Blast furnace slag
• Various chemical admixtures.

The Requirements of Coarse and Fine Aggregates to be used in building construction are as follows:

1. For normal concrete used for structural members such as beams and columns, the maximum size of coarse aggregate is about 25 mm. For mass concrete used for dams or deep foundations, the maximum size can be as large as 150 mm. But in any case not greater than one-fourth of the minimum thickness of the member.
2. The maximum percentage of Absorption allowed by the standard Specification is 5.0 Percent and applies to aggregate class AP, AS,A,B, and C only of Coarse Aggregate. The absorption quality requirement applies only to Coarse Aggregate.
   
3. Void Content ranges about 30% to 45%  (35% average )for coarse aggregate to about 40 to 50% (43% average) for fine aggregate.
4. Flat and elongated particles should be avoided or at least limited to about 15% by mass of total aggregate for both coarse and fine aggregate.
5. Fine aggregate should be 25 to 35% by volume of total aggregate.
6. Less Porous Aggregates are to be used in construction sites.
7. The bulk density of fine aggregate is generally 1.44 and that of Coarse aggregate is 1.60.
8. High Specific gravity aggregates indicates good quality materials.
9. The soundness (average loss of weight after 5 cycles) is about 10 when tested with `Na_2SO_4` and 15 when tested with `MgSO_4` for fine aggregates and 12 when tested with `Na_2SO_4` and 18 when tested with `MgSO_4` for Coarse aggregates.
10. It should have rough surface as the high bonding is possible due to more contact areas but should have sufficient workability.

The Requirements of Coarse and Fine Aggregates to be used in building construction are as follows:

1. For normal concrete used for structural members such as beams and columns, the maximum size of coarse aggregate is about 25 mm. For mass concrete used for dams or deep foundations, the maximum size can be as large as 150 mm. But in any case not greater than one-fourth of the minimum thickness of the member.
2. The maximum percentage of Absorption allowed by the standard Specification is 5.0 Percent and applies to aggregate class AP, AS,A,B, and C only of Coarse Aggregate. The absorption quality requirement applies only to Coarse Aggregate.
   
3. Void Content ranges about 30% to 45%  (35% average )for coarse aggregate to about 40 to 50% (43% average) for fine aggregate.
4. Flat and elongated particles should be avoided or at least limited to about 15% by mass of total aggregate for both coarse and fine aggregate.
5. Fine aggregate should be 25 to 35% by volume of total aggregate.
6. Less Porous Aggregates are to be used in construction sites.
7. The bulk density of fine aggregate is generally 1.44 and that of Coarse aggregate is 1.60.
8. High Specific gravity aggregates indicates good quality materials.
9. The soundness (average loss of weight after 5 cycles) is about 10 when tested with `Na_2SO_4` and 15 when tested with `MgSO_4` for fine aggregates and 12 when tested with `Na_2SO_4` and 18 when tested with `MgSO_4` for Coarse aggregates.
10. It should have rough surface as the high bonding is possible due to more contact areas but should have sufficient workability.

Cement is defined as a pulverized binding material possessing both adhesive and cohesive properties which sets, hardens and adheres to the other materials to bind them together. The raw materials required for manufacture of Portland cement are calcareous materials,
such as limestone or chalk, and argillaceous material such as shale or clay. The process of manufacture of cement consists of grinding the raw materials, mixing them intimately in certain proportions depending upon their purity and composition and burning them in a kiln at a temperature of about 1300 to 1500�C, at which temperature, the material sinters and partially fuses to form nodular shaped clinker. The clinker is cooled and ground to fine powder with addition of about 3 to 5% of gypsum. The product formed by using this procedure is Portland cement. Manufacture of cement can be carried out with two method (depending upon mixing & grinding of raw material) :

Wet process:

In wet process, limestone from quarries crushed to smaller fragments & is then taken into ball mill where it is mixed with clay or shale & converted to slurry
(creamy liquid) with addition of 35-50% water. Chemical composition of slurry is then checked & corrected. The slurry is sprayed to rotary kiln against hot surface of hanging chain. The slurry gets dry & undergoes series of chemical change. The material gets fused at high temperature & converted into 3-20mm nodular form called clinker. The clinker so formed is dropped in to cooler & stored in silo/bins. The clinker is then ground to required fineness in ball-mill in addition of 3-5% gypsum.

• Energy consumption is high in this process.
• Produces good quality products.
• Dust emission is low.
  

Dry Process:

In dry process, raw materials are proportioned & mixing/grinding is carried out in dry condition. Then is blended/corrected for its composition. By addition of about 12% of water blended mass is converted in to pellets which permit air flow & exchange of heat in rotary kiln. The mass is then charged into kiln (1300-
1500 deg C) to form clinker. The clinker so formed is dropped in to cooler & stored in silo/bins. The clinker is then ground to required fineness in ball-mill in addition of 3- 5% gypsum.

• Energy consumption is less.
• Comparatively low quality product is obtained.
• Economical
  
  

Cement is defined as a pulverized binding material possessing both adhesive and cohesive properties which sets, hardens and adheres to the other materials to bind them together. The raw materials required for manufacture of Portland cement are calcareous materials,
such as limestone or chalk, and argillaceous material such as shale or clay. The process of manufacture of cement consists of grinding the raw materials, mixing them intimately in certain proportions depending upon their purity and composition and burning them in a kiln at a temperature of about 1300 to 1500�C, at which temperature, the material sinters and partially fuses to form nodular shaped clinker. The clinker is cooled and ground to fine powder with addition of about 3 to 5% of gypsum. The product formed by using this procedure is Portland cement. Manufacture of cement can be carried out with two method (depending upon mixing & grinding of raw material) :

Wet process:

In wet process, limestone from quarries crushed to smaller fragments & is then taken into ball mill where it is mixed with clay or shale & converted to slurry
(creamy liquid) with addition of 35-50% water. Chemical composition of slurry is then checked & corrected. The slurry is sprayed to rotary kiln against hot surface of hanging chain. The slurry gets dry & undergoes series of chemical change. The material gets fused at high temperature & converted into 3-20mm nodular form called clinker. The clinker so formed is dropped in to cooler & stored in silo/bins. The clinker is then ground to required fineness in ball-mill in addition of 3-5% gypsum.

• Energy consumption is high in this process.
• Produces good quality products.
• Dust emission is low.
  

Dry Process:

In dry process, raw materials are proportioned & mixing/grinding is carried out in dry condition. Then is blended/corrected for its composition. By addition of about 12% of water blended mass is converted in to pellets which permit air flow & exchange of heat in rotary kiln. The mass is then charged into kiln (1300-
1500 deg C) to form clinker. The clinker so formed is dropped in to cooler & stored in silo/bins. The clinker is then ground to required fineness in ball-mill in addition of 3- 5% gypsum.

• Energy consumption is less.
• Comparatively low quality product is obtained.
• Economical
  
  

1. Concrete Structures are found more safer than steel structures because Concrete Structures can resist high temperatures and offer excellent resistant to explosion.
2. Concrete is relatively less expensive and easily available than that of steel.
3. Working with concrete is almost twice as fast as that of with steel.
4. Concrete Structures are more resistant to Corrosive action than Steel Structures.
5. Steel Superstructures are easy to recycle than Concrete Structures.
6. Concrete Structures produces almost four times solid waste as that of Steel Structures.
7. Concrete Structures produces more toxic waste.
8. Concrete Structures consumes almost six times as much natural resources than that of Steel.

1. Concrete Structures are found more safer than steel structures because Concrete Structures can resist high temperatures and offer excellent resistant to explosion.
2. Concrete is relatively less expensive and easily available than that of steel.
3. Working with concrete is almost twice as fast as that of with steel.
4. Concrete Structures are more resistant to Corrosive action than Steel Structures.
5. Steel Superstructures are easy to recycle than Concrete Structures.
6. Concrete Structures produces almost four times solid waste as that of Steel Structures.
7. Concrete Structures produces more toxic waste.
8. Concrete Structures consumes almost six times as much natural resources than that of Steel.

Concrete are classified as follows on various basics:

1. Vacuum Concrete where Concrete is initially placed with higher W/C ratio & immediately after placing, excess
   water is removed through  vacuum process.
   
2. Aerated Concrete where Aluminium is used as a mixture in the concrete.
   
3. Fire Resistance Concrete which is mostly used in structures directly exposed to fireworks or high temperatures.
   
4. Self Compacted Concrete which flows under its own weight with very high degree of workability.
   
5. Fibre reinforced concrete where fine fibrous structure is used for reinforcement purpose.
6. Sulphur infiltrated concrete which involves the impregnation of sulphur into concrete by various methods.
7. Polymer Concrete which involves the impregnation of polymer into concrete or by mixing it in green stage.
8. Ferrocement: Ferrocement is a mixture consisting of cement sandmortar & wiremesh with cement-sand ratio 1:2 to 1:3 and
   W/C ratio 0.40 to 0.45 and wiremesh 0.5 to 1.0mm dia @ 5 ? 10mm spacing.
9. Guniting / Shotcreting: Concreting process in which concrete is pneumatically
   projected to surface at high velocity is called Guniting which is  Normally used in tunnel lining, refractory
   concreting, slope stabilization & numerous other application where placing of formwork
   is difficult.
   
10. Based on Unit weight, Concrete are classified as:
• Normal weight concrete: Concrete having unit weight in between 1800 ? 3200 kg/`m^3` is
  treated as normal weight concrete.
• Light weight concrete: Concrete having unit wt. in between 300 ? 1800kg`m^3`.
• Heavy weight concrete: Concrete having unit wt. > 3200 kg`m^3` Heavyweight concrete is used to build some special
  structures, such as laboratories, hospital examination rooms, and nuclear plant, where radioactive
  protection is needed to minimize its influence on people?s health.
11. Based on Compressive Strength:
• Low Strength: fck < 20 Mpa
• Medium Strength: fck = 20 ? 40 Mpa
• High Strength: fck >40 Mpa
  Where, fck = 28days characteristic strength of concrete.

Concrete are classified as follows on various basics:

1. Vacuum Concrete where Concrete is initially placed with higher W/C ratio & immediately after placing, excess
   water is removed through  vacuum process.
   
2. Aerated Concrete where Aluminium is used as a mixture in the concrete.
   
3. Fire Resistance Concrete which is mostly used in structures directly exposed to fireworks or high temperatures.
   
4. Self Compacted Concrete which flows under its own weight with very high degree of workability.
   
5. Fibre reinforced concrete where fine fibrous structure is used for reinforcement purpose.
6. Sulphur infiltrated concrete which involves the impregnation of sulphur into concrete by various methods.
7. Polymer Concrete which involves the impregnation of polymer into concrete or by mixing it in green stage.
8. Ferrocement: Ferrocement is a mixture consisting of cement sandmortar & wiremesh with cement-sand ratio 1:2 to 1:3 and
   W/C ratio 0.40 to 0.45 and wiremesh 0.5 to 1.0mm dia @ 5 ? 10mm spacing.
9. Guniting / Shotcreting: Concreting process in which concrete is pneumatically
   projected to surface at high velocity is called Guniting which is  Normally used in tunnel lining, refractory
   concreting, slope stabilization & numerous other application where placing of formwork
   is difficult.
   
10. Based on Unit weight, Concrete are classified as:
• Normal weight concrete: Concrete having unit weight in between 1800 ? 3200 kg/`m^3` is
  treated as normal weight concrete.
• Light weight concrete: Concrete having unit wt. in between 300 ? 1800kg`m^3`.
• Heavy weight concrete: Concrete having unit wt. > 3200 kg`m^3` Heavyweight concrete is used to build some special
  structures, such as laboratories, hospital examination rooms, and nuclear plant, where radioactive
  protection is needed to minimize its influence on people?s health.
11. Based on Compressive Strength:
• Low Strength: fck < 20 Mpa
• Medium Strength: fck = 20 ? 40 Mpa
• High Strength: fck >40 Mpa
  Where, fck = 28days characteristic strength of concrete.

The grading of aggregate can be measured by following methods:

1. Specific surface and surface index
   
2. Fineness Modulus
3. Sieve Analysis

Specific surface and surface index:

The surface area per unit weight of the material is termed as specific surface. This is an indirect measure of the aggregate grading. Consideration of specific surface gives a somewhat misleading picture of the workability to be expected. To overcome this difficulty Murdock has suggested the use of ?Surface Index? which is an empirical number related to the specific surface of the particle with more weightage given to the finer fractions.The total surface index (fx) of a mixture of aggregate is calculated by multiplying the percentage of material retained on its sieve by the corresponding surface index and to their sum is added a constant of 330 and the result is divided by 1000.

Similarly, surface index can be calculated for standard grading curve, and this value of surface index can be taken as the desirable surface index of the combined aggregate. This parameter of surface index can be made use of for finding out the proportion of fine aggregate to coarse aggregate available in the field to obtain specified or desirable surface index in the following way.
Let x = surface index of fine aggregate
y = surface index of coarse aggregate
z = surface index of combined aggregate
a = proportion of fine to coarse aggregate

Then ,`a =( z -y )/( x- z )`

Fineness Modulus:

Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 micron and dividing this sum by an arbitrary number 100. The larger the figure, the coarser is the material.

The F.M of aggregates is shown below:

1. Fine aggregate:2-4
2. Intermediate Aggregate: 4-6.5
   
3. Coarse Aggregate: 6.5-8

To find fineness modulus of fine aggregate, we need sieve sizes of 0.15mm,0.3mm, 0.6 mm,1.18mm, 2.36 mm, 4.75mm.

The fineness Modulus value of an aggregate is 2.75 means that the average value of fine aggregate is in between 2nd and 3rd sieve i.e, in between 0.3mm and  0.6 mm. A sand having a fineness modulus more than 3.2 will be unsuitable for making satisfactory concrete as a fine aggregate. The various values of F.M. for different sands are listed below:

1. For finer sand, F.M= 2.2-2.6
2. For intermediate Sand, F.M= 2.6-2.9
3. For Coarse Sand, F.M=  2.9-3.2
   

To find fineness modulus of coarse, we need sieve sizes of 0.15mm,0.3mm, 0.6 mm,1.18mm, 2.36 mm, 4.75mm,10mm, 20 mm, 40 mm, 80mm.

The fineness Modulus value of an aggregate is 7.17 means that the average size of particle of given coarse aggregate sample is between 7th and 8th sieves,i.e, between 10 mm and 20 mm.
The F.M. for the following maximum size of coarse aggregate are:

1. for 20mm maximum size of coarse aggregate, f.m=6.0-6.9
2. For 40 mm maximum size of coarse aggregate, F.M.= 6.9-7.5
3. For 75 mm maximum size of coarse aggregate, F.M= 7.5-8.0
4. For 150 mm maximum size of coarse aggregate,F.M= 8.0-8.5
   

The grading of aggregate can be measured by following methods:

1. Specific surface and surface index
   
2. Fineness Modulus
3. Sieve Analysis

Specific surface and surface index:

The surface area per unit weight of the material is termed as specific surface. This is an indirect measure of the aggregate grading. Consideration of specific surface gives a somewhat misleading picture of the workability to be expected. To overcome this difficulty Murdock has suggested the use of ?Surface Index? which is an empirical number related to the specific surface of the particle with more weightage given to the finer fractions.The total surface index (fx) of a mixture of aggregate is calculated by multiplying the percentage of material retained on its sieve by the corresponding surface index and to their sum is added a constant of 330 and the result is divided by 1000.

Similarly, surface index can be calculated for standard grading curve, and this value of surface index can be taken as the desirable surface index of the combined aggregate. This parameter of surface index can be made use of for finding out the proportion of fine aggregate to coarse aggregate available in the field to obtain specified or desirable surface index in the following way.
Let x = surface index of fine aggregate
y = surface index of coarse aggregate
z = surface index of combined aggregate
a = proportion of fine to coarse aggregate

Then ,`a =( z -y )/( x- z )`

Fineness Modulus:

Fineness modulus is an empirical factor obtained by adding the cumulative percentages of aggregate retained on each of the standard sieves ranging from 80 mm to 150 micron and dividing this sum by an arbitrary number 100. The larger the figure, the coarser is the material.

The F.M of aggregates is shown below:

1. Fine aggregate:2-4
2. Intermediate Aggregate: 4-6.5
   
3. Coarse Aggregate: 6.5-8

To find fineness modulus of fine aggregate, we need sieve sizes of 0.15mm,0.3mm, 0.6 mm,1.18mm, 2.36 mm, 4.75mm.

The fineness Modulus value of an aggregate is 2.75 means that the average value of fine aggregate is in between 2nd and 3rd sieve i.e, in between 0.3mm and  0.6 mm. A sand having a fineness modulus more than 3.2 will be unsuitable for making satisfactory concrete as a fine aggregate. The various values of F.M. for different sands are listed below:

1. For finer sand, F.M= 2.2-2.6
2. For intermediate Sand, F.M= 2.6-2.9
3. For Coarse Sand, F.M=  2.9-3.2
   

To find fineness modulus of coarse, we need sieve sizes of 0.15mm,0.3mm, 0.6 mm,1.18mm, 2.36 mm, 4.75mm,10mm, 20 mm, 40 mm, 80mm.

The fineness Modulus value of an aggregate is 7.17 means that the average size of particle of given coarse aggregate sample is between 7th and 8th sieves,i.e, between 10 mm and 20 mm.
The F.M. for the following maximum size of coarse aggregate are:

1. for 20mm maximum size of coarse aggregate, f.m=6.0-6.9
2. For 40 mm maximum size of coarse aggregate, F.M.= 6.9-7.5
3. For 75 mm maximum size of coarse aggregate, F.M= 7.5-8.0
4. For 150 mm maximum size of coarse aggregate,F.M= 8.0-8.5
   

The major compounds of ordinary Portland cement i.e, Tricalcium silicate, Dicalcium silicate, Tricalcium aluminate, and Tetracalcium aluminoferrite are known as Bogues Compounds.

The Percentage of these major Compounds in a cement can be found using Bogues equation as:

`%C_3S= 4.071 C ? 7.600 S ? 6.718 A ? 1.430 F ? 2.850 S`
`%C_ S= 2.867 S ? 0.754 C 3 S`
`%C_3A= 2.650 A ? 1.692 F`
`%C_4AF= 3.043 F`

Where,C ? CaO
S - `SiO_2`
A ? `Al_2O_3`
F ? `Fe_2O_3`
S ? `SO_3`

The properties and reactivity of these compounds are as follows:

Tricalcium Silicate:

1. Hydrates and hardens rapidly.
2. Generates more heat of hydration.
3. Less resistant to sulphate attack.
4. Develops early strength in cement.

Dicalcium Silicate:

1. Hydrates and hardens slowly.
2. Generates less heat of hydration.
3. More resistant to sulphate attack.
4. Responsible for ultimate final strength of cement.

Tricalcium Aluminate:

1. Hydrates rapidly. So, gypsum is added to slow the rate of reaction that takes place in the cement due to this compound.
   
2. Generates high heat of hydration.
3. Less resistant to sulphate attack.
4. Contributes a very little to strength of cement.

Calcium Alumina Ferrite:

1. Hydrates rapidly.
2. Generates high heat of hydration.
3. Less resistant to sulphate attack and infact contributes to the sulphate attack.
4. No effect on strength of cement.

The major compounds of ordinary Portland cement i.e, Tricalcium silicate, Dicalcium silicate, Tricalcium aluminate, and Tetracalcium aluminoferrite are known as Bogues Compounds.

The Percentage of these major Compounds in a cement can be found using Bogues equation as:

`%C_3S= 4.071 C ? 7.600 S ? 6.718 A ? 1.430 F ? 2.850 S`
`%C_ S= 2.867 S ? 0.754 C 3 S`
`%C_3A= 2.650 A ? 1.692 F`
`%C_4AF= 3.043 F`

Where,C ? CaO
S - `SiO_2`
A ? `Al_2O_3`
F ? `Fe_2O_3`
S ? `SO_3`

The properties and reactivity of these compounds are as follows:

Tricalcium Silicate:

1. Hydrates and hardens rapidly.
2. Generates more heat of hydration.
3. Less resistant to sulphate attack.
4. Develops early strength in cement.

Dicalcium Silicate:

1. Hydrates and hardens slowly.
2. Generates less heat of hydration.
3. More resistant to sulphate attack.
4. Responsible for ultimate final strength of cement.

Tricalcium Aluminate:

1. Hydrates rapidly. So, gypsum is added to slow the rate of reaction that takes place in the cement due to this compound.
   
2. Generates high heat of hydration.
3. Less resistant to sulphate attack.
4. Contributes a very little to strength of cement.

Calcium Alumina Ferrite:

1. Hydrates rapidly.
2. Generates high heat of hydration.
3. Less resistant to sulphate attack and infact contributes to the sulphate attack.
4. No effect on strength of cement.

Concrete is used as a structural materials because of the following reasons:

1. It is able to resist the effect of high temperature.
2. It can bear high compressive force.
3. It has excellent resistant to water and is less prone to weathering and corrosive effect.
4. Structures made using Concrete are highly durable and fire proof.
5. It requires less maintenance and can be recycled after a useful service life.
6. The cost of production of concrete is low compared with other engineered
   construction materials.

Concrete is used as a structural materials because of the following reasons:

1. It is able to resist the effect of high temperature.
2. It can bear high compressive force.
3. It has excellent resistant to water and is less prone to weathering and corrosive effect.
4. Structures made using Concrete are highly durable and fire proof.
5. It requires less maintenance and can be recycled after a useful service life.
6. The cost of production of concrete is low compared with other engineered
   construction materials.