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Ordinary Portland Cement Ordinary Portland cement (OPC) is the most important type of cement. The OPC was classified into three grades, namely 33 grade, 43 grade and53 grade depending upon the strength

of the cement at 28 days when tested as per IS 4031-1988. If the 28 days strength is not less than 33N/mm2, it is called 33 grade cement, if the strength is not less than 43N/mm2, it is called 43 grade cement, and if the strength is not less than 53 N/mm2, it is called 53 grade cement.

But the actual strength obtained by these cements at the factory are much higher than the specifications. The manufacture of OPC is decreasing all over the world in view of the popularity of blended cement on account

of lower energy consumption, environmental pollution, economic and other technical reasons. In the years to come the use of OPC may still come down, but all the same the OPC will remain as an important

type for general construction. 

Rapid Hardening Cement This cement is similar to ordinary Portland cement. As the name indicates it develops strength rapidly and as

such it may be more appropriate to call it as high early strength cement. Rapid hardening cement which develops higher rate of development of strength should not be confused with

quick-setting cement which only sets quickly. Rapid hardening cement develops at the age of three days, the same strength as that is expected of ordinary

Portland cement at seven days. The rapid rate of development of strength is attributed to the higher fineness of grinding and higher C3S and

lower C2S content. The higher fineness of cement particles expose greater surface area for action of water and also higher

proportion of C3S results in quicker hydration. Therefore, rapid hardening cement should not be used in mass concrete construction.

Uses:

In pre-fabricated concrete construction. Where formwork is required to be removed early for reuse. Road repair works. In cold weather concrete where the rapid rate of development of strength reduces the vulnerability of concrete

to the frost damage. 

Extra Rapid Hardening Cement Extra rapid hardening cement is obtained by intergrinding calcium chloride with rapid hardening Portland

cement. The normal addition of calcium chloride should not exceed 2 percent by weight of the rapid hardening cement. It is necessary that the concrete made by using extra rapid hardening cement should be transported, placed and

compacted and finished within about 20 minutes. It is also necessary that this cement should not be stored for more than a month. Extra rapid hardening cement accelerates the setting and hardening process. A large quantity of heat is evolved in a very short time after placing. The acceleration of setting, hardening and evolution of this large quantity of heat in the early period of

hydration makes the cement very suitable for concreting in cold weather. The strength of extra rapid hardening cement is about 25 per cent higher than that of rapid hardening cement at

one or two days and 10–20 per cent higher at 7 days.

The gain of strength will disappear with age and at 90 days the strength of extra rapid hardening cement or the ordinary portland cement may be nearly the same.

There is small amount of initial corrosion of reinforcement when extra rapid hardening cement is used, but in general, this effect does not appear to be progressive and as such there is no harm in using extra rapid hardening cement in reinforced concrete work. However, its use in prestress concrete construction is prohibited. 

Sulphate Resisting Cement o Ordinary Portland cement is susceptible to the attack of sulphates, in particular to the action of magnesium sulphate. Sulphates react both with

the free calcium hydroxide in set cement to form calcium sulphate and with hydrate of calcium aluminate to form calcium sulphoaluminate, the volume of which is approximately 227% of the volume of the original aluminates.

o Their expansion within the frame work of hardened cement paste results in cracks and subsequent disruption.

o Solid sulphate do not attack the cement compound. Sulphates in solution permeate into hardened concrete and attack calcium hydroxide, hydrated calcium aluminate and even hydrated silicates.

o The above is known as sulphate attack. Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying which normally takes place in marine structures in the zone of tidal variations.

o To remedy the sulphate attack, the use of cement with low C3A content is found to be effective. Such cement with low C3A and comparatively low C4AF content is known as Sulphate Resisting Cement. In other words, this cement has a high silicate content. The specification generally limits the C3A content to 5 per cent.

o Tetracalcium Alumino Ferrite (C3AF) varies in Normal Portland Cement between to 6 to 12%. Since it is often not feasible to reduce the Al2O3 content of the raw material, Fe2O3 may be added to the mix so that the C4AF content increases at the expense of C3A. IS code limits the total content of C4AF and C3A, as follows.

2C3A + C4AF should not exceed 25%

Uses : 

o Concrete to be used in marine condition;

o Concrete to be used in foundation and basement, where soil is infested with sulphates;

o Concrete used for fabrication of pipes which are likely to be buried in marshy region or sulphate bearing soils;

o Concrete to be used in the construction of sewage treatment works. 

Quick Setting Cement This cement as the name indicates sets very early. The early setting property is brought out by reducing the gypsum content at the time of clinker grinding. This cement is required to be mixed, placed and compacted very early. It is used mostly in under water construction where pumping is involved. Use of quick setting cement in such conditions reduces the pumping time and makes it economical. Quick setting

cement may also find its use in some typical grouting operations. 

Super Sulphated Cement Super sulphated cement is manufactured by grinding together a mixture of 80-85 percent granulated slag, 10-15

per cent hard burnt gypsum, and about 5 per cent Portland cement clinker. The product is ground finer than that of Portland cement.

Specific surface must not be less than 4000 sq.cm per gm. This cement is rather more sensitive to deterioration during storage than Portland cement. This cement has high sulphate resistance. Because of this property this cement is particularly recommended for

use in foundation, where chemically aggressive conditions exist. As super-sulphated cement has more resistance than Portland blast furnace slag cement to attack by sea water,

it is also used in the marine works. Other areas where super-sulphated cement is recommended include the fabrication of reinforced concrete pipes which are likely to be buried in sulphate bearing soils. The substitution of granulated slag is responsible for better resistance to sulphate attack.

When we use super sulphated cement the water/ cement ratio should not be less than0.5. A mix leaner than about 1:6 is also not recommended.

  Low Heat Cement It is well known that hydration of cement is anexothermic action which produces large quantity of heat during

hydration. Formation of cracks in large bodyof concrete due to heat of hydration has focussed the attention of the concrete

technologists to produce a kind of cement which produces less heat or the same amount of heat, at a low rate during the hydration process.

Cement having this property was developed in U.S.A. during 1930 fo r use in mass concrete construction, such as dams, where temperature rise by the heat of hydration can become excessively large.

A low-heat evolution is achieved by reducing the contents of C3S and C3A which are the compounds evolving the maximum heat of hydration and increasing C2S.

A reduction of temperature will retard the chemical action of hardening and so further restrict the rate of evolution of heat. The rate of evolution of heat will, therefore, be less and evolution of heat will extend over a longer period.

The specific surface of low heat cement as found out by air-permeability method is not less than 3200 sq. cm/gm. The 7 days strength of low heat cement is not less than 16 MPa in contrast to 22 MPa in the case of ordinary Portland cement. Other properties, such as setting time and soundness are same as that of ordinary Portland cement. 

Portland Pozzolana Cement The history of pozzolanic material goes back to Roman’s time. The descriptions and details of pozzolanic

material will be dealt separately under the chapter ‘Admixtures’. Portland Pozzolana cement (PPC) is manufactured by the intergrinding of OPC clinker with 10 to 25 per cent of

pozzolanic material (as per the latest amendment, it is 15 to 35%). A pozzolanic material is essentially a silicious or aluminous material which while in itself possessing no

cementitious properties, which will, in finely divided form and in the presence of water, react with calcium hydroxide, liberated in the hydration process, at ordinary temperature, to form compounds possessing cementitious properties.

The pozzolanic materials generally used for manufacture of PPC are calcined clay or fly ash. The pozzolanic action is shown below:

Calcium hydroxide + Pozzolana + water ----> C – S – H (gel)

Portland pozzolana cement produces less heat of hydration and offers greater resistance to the attack of aggressive waters than ordinary Portland cement. Moreover, it reduces the leaching of calcium hydroxide when used in hydraulic structures. It is particularly useful in marine and hydraulic construction and other mass concrete constructions.Uses:

For hydraulic structures; For mass concrete structures like dam, bridge piers and thick foundation; For marine structures; For sewers and sewage disposal works.

Air-Entraining Cement Air-Entraining Cement is made by mixing a small amount of an air-entraining agent with ordinary Portland

cement clinker at the time of grinding.

The following types of air-entraining agents could be used:1. Alkali salts of wood resins.2. Synthetic detergents of the alkyl-aryl sulphonate type.3. Calcium lignosulphate derived from the sulphite process in paper making.4. Calcium salts of glues and other proteins obtained in the treatment of animal hides. These agents in powder, or in liquid forms are added to the extent of 0.025–0.1 per cent by weight of cement

clinker. There are other additives including animal and vegetable fats, oil and their acids could be used. Wetting agents, aluminium powder, hydrogen peroxide could also be used. Air-entraining cement will produce

at the time of mixing, tough, tiny, discrete non-coalesceing air bubbles in the body of the concrete which will modify the properties of plastic concrete with respect to workability, segregation and bleeding. It will modify the properties of hardened concrete with respect to its resistance to frost action. Airentraining agent can also be added at the time of mixing ordinary Portland cement with rest of the ingredients. 

Coloured Cement For manufacturing various colored cements, either white cement or grey Portland cement is used as a base. The use of white cement as a base is costly. With the use of grey cement, only red or brown cement can be

produced. Coloured cement consists of Portland cement with 5-10% of pigment. The pigment cannot be satisfactorily distributed throughout the cement by mixing, and hence, it is usual to grind

the cement & pigment together. Chromium oxide – green colour Cobalt – blue colour Iron oxide – brown colour The raw materials used for white cement are:1. High purity limestone (96% CaCO3 & less than 0.07% iron oxide)2. China clay (0.72-0.8% of iron oxide, silica sand, fluorspar as flux and selenite as retarder) Grey colour of OPC is due to the iron oxide present.

  Hydrophobic Cement Hydrophobic cement is obtained by grinding ordinary Portland cement clinker with water repellant film-forming

substance such as oleic acid, and stearic acid. The water-repellant film formed around each grain of cement, reduces the rate of deterioration of the cement

during long storage, transport, or under unfavourable conditions. The film is broken out when the cement and aggregate are mixed together at the mixer exposing the cement

particles for normal hydration. The film forming water-repellant material will entrain certain amount of air in the body of the concrete which incidentally will improve the workability of concrete.

Some places get plenty of rainfall in the rainy season and have high humidity in other seasons.The transportation and storage of cement in such places cause deterioration in the quality of cement. In such far off places with poor communication system, cement perforce requires to be stored for long time.

Ordinary cement gets deteriorated and loses some if its strength, whereas the hydrophobic cement which does not lose strength is an answer for such situations.

The properties of hydrophobic cement is nearly the same as that ordinary Portlandcement except that it entrains a small quantity of air bubbles. The hydrophobic cement is made actually from ordinary Portland cement clinker. After grinding, the cement particle is sprayed in one direction and film forming materials such as oleic acid, or stearic acid, or pentachlorophenol, or calcium oleate are sprayed from another direction such that every particle of cement is coated with a very fine film of this water repellant material which

protects them from the bad effect of moisture during storage and transporation. The cost of this cement is nominally higher than ordinary Portland cement. 

Masonry Cement Ordinary cement mortar, though good when compared to lime mortar with respect to strength and setting

properties, is inferior to lime mortar with respect to workability, water retentivity,shrinkage property and extensibility.

Masonry cement is a type of cement which is particularly made with such combination of materials, which when used for making mortar, incorporates all the good properties of lime mortar and discards all the not so ideal properties of cement mortar.

This kind of cement is mostly used, as the name indicates, for masonry construction. It contains certain amount of air-entraining agent and mineral admixtures to improve the plasticity and water

retentivity. 

Expansive Cement Concrete made with ordinary Portland cement shrinks while setting due to loss of free water. Concrete also

shrinks continuously for long time. This is known as drying shrinkage. Cement used for grouting anchor bolts or grouting machine foundations or the cement used in grouting the

prestress concrete ducts, if shrinks, the purpose for which the grout is used will be to some extent defeated. There has been a search for such type of cement which will not shrink while hardening and thereafter. As a matter of fact, a slight expansion with time will prove to be advantageous for grouting purpose. This type of cement which suffers no overall change in volume on drying is known as expansive cement.

Cement of this type has been developed by using an expanding agent and a stabilizer very carefully. Proper material and controlled proportioning are necessary in order to obtain the desired expansion.

Generally, about 8-20 parts of the sulphoaluminate clinker are mixed with 100 parts of thePortland cement and 15 parts of the stabilizer. Since expansion takes place only so long as concrete is moist, curing must be carefully controlled. The use of expanding cement requires skill and experience.

One type of expansive cement is known as shrinkage compensating cement. This cement when used in concrete, with restrained expansion, induces compressive stresses which approximately offset the tensile stress induced by shrinkage.

Another similar type of cement is known as Self Stressing cement. This cement when used in concrete induces significant compressive stresses after the drying shrinkage has occurred. The induced compressive stresses not only compensate the shrinkage but also give some sort of prestressing effects in the tensile zone of a flexural member.

  IRS-T 40 Special Grade Cement IRS-T-40 special grade cement is manufactured as per specification laid down by ministry of Railways under

IRST40:1985. It is a very finely ground cement with high C3S content designed to develop high early strength required for

manufacture of concrete sleeper for Indian Railways. This cement can also be used with advantage fo r o ther applications where high early strength concrete is required. This cement can be used for prestressed concrete elements, high rise buildings, high strength concrete.

  Oil-Well Cement Oil-wells are drilled through stratified sedimentary rocks through a great depth in search of oil. It is likely that if

oil is struck, oil or gas may escape through the space between the steel casing and rock formation. Cement slurry

is used to seal off the annular space between steel casing and rock strata and also to seal off any other fissures or cavities in the sedimentary rock layer. The cement slurry has to be pumped into position, at considerable depth where the prevailing temperature may be upto 175°C. The pressure required may go upto 1300 kg/ cm2. The slurry should remain sufficiently mobile to be able to flow under these conditions for periods upto several hours and then hardened fairly rapidly. It may also have to resist corrosive conditions from sulphur gases or waters containing dissolved salts. The type of cement suitable for the above conditions is known as Oil-well cement.

The desired properties of Oil-well cement can be obtained in two ways: by adjusting the compound composition of cement or by adding retarders to ordinary Portland cement. Many admixtures have been patented as retarders. The commonest agents are starches or cellulose products or acids. These retarding agents prevent quick setting and retains the slurry in mobile condition to facilitate penetration to all fissures and cavities. Sometimes workability agents are also added to this cement to increase the mobility.

  Rediset Cement Acclerating the setting and hardening of concrete by the use of admixtures is a common knowledge. Calcium

chloride, lignosulfonates, and cellulose products form the base of some of admixtures. The limitations on the use of admixtures and the factors influencing the end properties are also fairly well known.

High alumina cement, though good for early strengths, shows retrogression of strength when exposed to hot and humid conditions. A new product was needed for use in the precast concrete industry, for rapid repairs of concrete roads and pavements, and slip-forming. In brief, for all jobs where the time and strength relationship was important. In the PCA laboratories of USA, investigations were conducted for developing a cement which could yield high strengths in a matter of hours, without showing any retrogression. Regset cement was the result of investigation. Associated Cement Company of India have developed an equivalent cement by name “REDISET” Cement. 

High Alumina Cement High alumina cement is obtained by fusing or sintering a mixture, in suitable proportions, of alumina and

calcareous materials and grinding the resultant product to a fine powder. The raw materials used for the manufacture of high alumina cement are limestone and bauxite.

These raw materials with the required proportion of coke were charged into the furnace. The furnace is fired with pulverised coal or oil with a hot air blast. The fusion takes place at a temperature of about 1550-1600°C. The cement is maintained in a liquid state in the furnace. Afterwards the molten cement is run into moulds and cooled. These castings are known as pigs. After cooling the cement mass resembles a dark, fine gey compact rock resembling the structure and hardeness of basalt rock.

The pigs of fused cement, after cooling are crushed and then ground in tube mills to a finess of about 3000 sq. cm/gm.

  High Early Strength Cement Development of high early strength becomes an important factor, sometimes, for repair and emergency work.

Research has been carried out in the recent past to develop rapid setting and hardening cement to give materials of very high early strength.

Lithium salts have been effectively used as accelerators in high alumina cement. This has resulted in very high early strength in cement and a marginal reduction in later strength. Strength as high as 4 MPa has been obtained within 1 hour and 27 MPa has been obtained within 3 hours time and 49 MPa in one day. 

Types of cement Ordinary Portland Cement Rapid Hardening Cement Extra Rapid Hardening Cement Sulphate Resisting Cement Quick Setting Cement Super Sulphated Cement Low Heat Cement Portland Pozzolana Cement Air-Entraining Cement Coloured Cement Hydrophobic cement Masonry Cement Expansive Cement IRS-T 40 Special Grade Cement Oil-Well Cement Rediset Cement High Alumina Cement High Early Strength cement

TYPES OF CONCRETE

Light weight concrete

One of the main advantages of conventional concrete is the self weight of concrete. Density of normal concrete is of the order of 2200 to 2600. This self weight will make it to some extend an uneconomical structural material.

1. Self weight of light weight concrete varies from 300 to 1850 kg/m3. 2. It helps reduce the dead load, increase the progress of building and lowers the

hauling and handling cost. 3. The weight of building on foundation is an important factor in the design ,

particularly in case of weak soil and tall structures. In framed structure , the beam and column have to carry load of wall and floor. If these wall and floor are made of light weight concrete it will result in considerable economy.

4. Light weight concrete have low thermal conductivity.( In extreme climatic condition where air condition is to installed the use of light weight concrete with low thermal conductivity is advantageous from the point of thermal comfort and low power consumption.

5. Only method for making concrete light by inclusion of air. This is achieved by a) replacing original mineral aggregate by light weight aggregate, b) By introducing gas or air bubble in mortar c) By omitting sand fraction from concrete. This is called no – fine concrete.

6. Light weight aggregate include pumice, saw dust rice husk, thermocole beads, formed slag. Etc

7. Light weight concrete aggregate exhibit high fire resistance. 8. Structural lightweight aggregate’s cellular structure provides internal curing

through water entrainment which is especially beneficial for high-performance concrete

9.   lightweight aggregate has better thermal properties, better fire ratings, reduced shrinkage, excellent freezing and thawing durability , improved contact between aggregate and cement matrix, less micro-cracking as a result of better elastic compatibility, more blast resistant, and has better shock and sound absorption, High-Performance lightweight aggregate concrete also has less cracking, improved skid resistance and is readily placed by the concrete pumping method

1. Aerated concrete is made by introducing air or gas into a slurry composed of Portland cement.

2. No fine concrete is made up of only coarse aggregate , cement and water.These type of concrete is used for load bearing cast in situ external walls for building. They are also used for temporary structures because of low initial cost and can be reused as aggregate.

High density concrete

1. The density of high density concrete varies from 3360 kg/m3 to 3840 kg/m3.They can however be produced with density upto 5820 kg/m3 using iron as both fine and coarse aggregate.

2. Heavyweight concrete uses heavy natural aggregates such as barites or magnetite or manufactured aggregates such as iron or lead shot. The density achieved will depend on the type of aggregate used. Typically using barites the density will be in the region of 3,500kg/m 3 , which is 45% greater than that of normal concrete, while with magnetite the density will be 3,900kg/m 3 , or 60% greater than normal concrete.

Very heavy concretes can be achieved with iron or lead shot as aggregate, is 5,900kg/m 3 and 8,900kg/m 3 respectively.

1. They are mainly used in the construction of radiation shields (medical or nuclear). Offshore, heavyweight concrete is used for ballasting for pipelines and similar structures

2. The ideal property of normal and high density concrete are high modulus of elasticity , low thermal expansion , and creep deformation

3. Because of high density of concrete there will be tendency for segregation. To avoid this pre placed aggregate method of concreting is adopted.

4. High Modulus of Elasticity, Low thermal Expansion ,Low elasticity and creep deformation are ideal properties.

5. The high density. Concrete is used in construction of radiation shields. They are effective and economic construction material for permanent shielding purpose.

6. Most of the aggregate specific gravity is more than 3.5

Mass concrete

Mass concrete is defined in ACI as “any volume of concrete with dimensions large enough to require that measures be taken to cope with generation of heat from hydration of the cement and attendant volume change to minimize cracking.” The design of mass concrete structures is generally based on durability, economy, and thermal action, with strength often being a secondary, rather than a primary, concern. The one characteristic that distinguishes mass concrete from other concrete work is thermal behavior. Because the cement-water reaction is exothermic by nature, the temperature rise within a large concrete mass, where the heat is not quickly dissipated, can be quite high. Significant tensile stresses and strains may result from the restrained volume change associated with a decline in temperature as heat of hydration is dissipated. Measures should be taken where cracking due to thermal behavior may cause a loss of structural integrity and monolithic action, excessive seepage and shortening of the service life of the structure, or be aesthetically objectionable. Many of the principles in mass concrete practice can also be applied to general concrete work, whereby economic and other benefits may be realized. Mass concreting practices were developed largely from concrete dam construction, where temperature-related cracking was first identified. Temperature-related cracking has also been experienced in other thick-section concrete structures, including mat foundations, pile caps, bridge piers, thick walls, and tunnel linings

Ready-mix Concrete Ready-mix concrete has cement, aggregates, water     and     other     ingredients,     which     are weigh-batched     at a     centrally located     plant. This is     then   delivered     to the     construction site in truck mounted transit mixers and can be used straight away without any further treatment. This results in a precise mixture, allowing specialty concrete mixtures to be developed and implemented on construction sites. Ready-mix concrete is sometimes preferred over on-site concrete mixing because of the precision of the mixture and reduced worksite confusion. However, using a pre-determined concrete mixture reduces flexibility, both in the supply chain and in the actual components of the concrete. Ready Mixed Concrete, or RMC as it is popularly called, refers to concrete that is specifically manufactured for delivery to the customer’s construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of Portland cement, water and aggregates comprising sand and gravel or crushed stone. In traditional work sites, each of these materials is procured separately and mixed in specified proportions at site to make concrete. Ready Mixed Concrete is bought and sold by volume – usually expressed in cubic meters. Ready Mixed Concrete is manufactured under computer-controlled operations and transported and placed at site using sophisticated equipment and methods. RMC assures its customers numerous benefits.

Advantages of Ready mix Concrete over Site mix Concrete

A centralised concrete batching plant can serve a wide area. The plants are located in areas zoned for industrial use, and yet the delivery

trucks can service residential districts or inner cities. Better quality concrete is produced. Elimination of storage space for basic materials at site. Elimination of procurement / hiring of plant and machinery Wastage of basic materials is avoided. Labor associated with production of concrete is eliminated. Time required is greatly reduced. Noise and dust pollution at site is reduced.

Disadvantages of Ready-Mix Concrete

The materials are batched at a central plant, and the mixing begins at that plant, so the traveling time from the plant to the site is critical over longer distances. Some sites are just too far away, though this is usually a commercial rather than technical issue.

Access roads and site access have to be able to carry the weight of the truck and load. Concrete is approx. 2.5tonne per m². This problem can be overcome by utilizing so-called ‘minimix’ companies, using smaller 4m³ capacity mixers able to access more restricted sites.

Concrete’s limited time span between mixing and going-off means that ready- mix should be placed within 2 hours of batching at the plant. Concrete is still usable after this point but may not conform to relevant specifications.

Polymer concrete

Concrete is porous. The porosity is due to air voids , water voids or due to inherent property of gel structures. On account of porosity strength of concrete is reduced , reduction of porosity result in increase in strength of concrete. The impregnation of monomer and subsequent polymerization is the latest technique adopted to reduce inherent porosity of concrete and increase strength and other properties of concrete

There are mainly 4 types of polymer concrete

1. Polymer impregnated concrete

2. Polymer cement concrete

3. Polymer concrete

4. Partially impregnated and surface coated polymer concrete.

Polymer impregnated concrete

It is a precast conventional concrete cured and dried in oven or by dielectric heating from which the air in the open cell is removed by vacuum. Then a low viscosity monomer is diffused through the open cell and polymerized by using radiation, application of heat or by chemical initiation.

Mainly the following type of monomers are used

Methyl methacrlylate(MMA)

1. Acrylonitrile

2. t- butyl styrene

3. Other thermoplastic monomer

4. The amount of monomer that can be loaded into a concrete specimen is limited by the amount of water and air that has occupied the total void space.

5. PIC require cast in situ structures

Polymer cement concrete

Polymer cement concrete is made by mixing cement, aggregate, water and monomer. Such plastic mixture is cast in moulds , cured dried and polymerized. The monomer that are used in PCC are

1. Polyster- styrene

2. Epoxy-styrene

3. Furans

4. Vinyldene chloride

PCC produced in this way have been disappointing. In many cases material poorer than ordinary concrete is obtained.This is because organic material are incompatable with aqueous systems and some times interfere with the alkaline cement hydration process. Russians developed a superior polymer by incorporation of furfuryl alcohol and aniline hydrochloride in the wet mix. This material is dense and non shrinking and to have high corrosion resistance, low permeability and high resistance to vibration and axial extension .PCC can be cast in situ for field application.

Polymer concrete

Polymer concrete is an aggregate bound with a polymer binder instead of Portland cement as in conventional concrete. The main technique in producing PC is to minimize void volume in the aggregate mass so as to reduce the quantity of polymer needed for binding the aggregate. This is achieved by properly grading and mixing the aggregate to attain maximum density and minimum voids

Shotcrete

It is defined as a mortar conveyed through a hose and pneumatically projected at high velocity on to a surface. There are mainly two different methods namely wet

mix and dry mix process. In wet mix process the material is conveyed after mixing with water.

Pre packed concrete

In constructions where the reinforcement is very complicated or where certain arrangements like pipe, opening or other arrangements are incorporated this type of concreting is adopted. One of the methods is concrete process in which mortar is made in a high speed double drum and grouting is done by pouring on prepacked aggregate. This is mainly adopted for pavement slabs

Vacuum concrete

Concrete poured into a framework that is fitted with a vacuum mat to remove water not required for setting of the cement; in this framework, concrete attains its 28-day strength in 10 days and has a 25% higher crushing strength. The elastic and shrinkage deformations are considerably greater than for normal-weight concrete.

Pumped concrete

Pumped concrete must be designed to that it can be easily conveyed by pressure through a rigid pipe of flexible hose for discharge directly into the desired area.   Pozzocrete use can greatly improve concrete flow characteristics making it much easier to pump , while enhancing the quality of the concrete and controlling costs.

Mix Homogeneity

The designer must be aware of the need to improve the grade and maintain uniformity of the various materials used in the pumped mix in order to achieve greater homogeneity of the total mix.   Three mix proportioning methods frequently used to produce pump able concrete are :

Maximum Density of Combined Materials

Maximum Density – Least Voids

Minimum Voids – Minimum Area

Mixes must be designed with several factors in mind:

1. Pumped concrete must be more fluid with enough fine material and water to fill internal voids.

2. Since the surface area and void content of fine material below 300 microns control the liquid under pressure, there must be a high quantity of fine material in a normal mix.   Generally speaking, the finer the material, the greater the control.

3. Coarse aggregate grading should be continuous, and often the sand content must be increased by up to five percent at the expense of the coarser aggregate so as to balance the 500 micron fraction against the finer solids.

Pozzocrete Effective

Unfortunately, adding extra water and fine aggregate leads to a weaker concrete. The usual remedies for this are either to increase the cement content, which is costly, or to use chemical admixtures , which can also be costly and may lead to segregation in marginal mixes. There is another and far more effective alternative:

POZZOCRETE

There are many advantages to including POZZOCRETE in concrete mixes to be pumped. Among them are :

1. Particle Size. Pozzocrete meets IS 3812 Specification with 66% passing the 325 (45-micron) sieve and these fine particles are ideal for void filling.   Just a small deficiency in the mix fines can often prevent successful pumping.

2. Particle Shape. Microscopic examination shows most Pozzocrete particles are spherical and act like miniature ball bearings aiding the movement of the concrete by reducing frictional losses in the pump and pining.   Studies have shown that Pozzocrete can be twice as effective as cement in improving workability and, therefore, improve pumping characteristics.

Pozzolanic Activity:

his chemical reaction combines the Pozzocrete particles with the calcium hydroxide liberated through the hydration of cement to form additional cementitious compounds which increase concrete strength.

Water Requirement:

Excess water in pumped mixes resulting in over six inch slumps will often cause material segregation and result in line blockage.   As in conventionally placed mixes, pumped concrete mixes with excessive water also contribute to lower strength, increased bleeding and shrinkage. The use of Pozzocrete in pumped or conventionally placed mixes can reduce the water requirement by 2% to 10% for any given slump.

Sand/Coarse Aggregate Ratio:

In pumped mixes, the inclusion of liberal quantities of coarse aggregate can be very beneficial because it reduces the total aggregate surface area, thereby increasing the effectiveness of the available cementitious paste.   This approach is in keeping with the “minimum voids, minimum area” proportioning method.   As aggregate size increases, so does the optimum quantity of coarse aggregate.   Unfortunately, this process is frequently reversed in pump mixes, and sand would be substituted for coarse aggregate to make pumping easier.   When that happens, there is a need to increase costly cementitious material to compensate for strength loss.   However, if Pozzocrete is utilized, its unique workability and pump ability properties permit a better balance of sand to coarse aggregate resulting in a more economical, pump able concrete.

SHOTCRETE

Shotcrete is a process where concrete is projected or "shot" under pressure using a feeder or "gun" onto a surface to form structural shapes including walls, floors, and roofs. The surface can be wood, steel, polystyrene, or any other surface that concrete can be projected onto. The surface can be trowelled smooth while the concrete is still wet.

Benefits

Shotcrete has high strength, durability, low permeability, excellent bond and limitless shape possibilities. These properties allow shotcrete to be used in most cases as a structural material. Although the hardened properties of shotcrete are similar to conventional cast-in-place concrete, the nature of the placement process provides additional benefits, such as excellent bond with most substrates and instant or rapid capabilities, particularly on complex forms or shapes. In addition to building homes, shotcrete can also be used to build pools

Methods of Application

Wet Mix – All ingredients, including water, are thoroughly mixed and introduced into the delivery equipment. Wet material is pumped to the nozzle where compressed air is added to provide high velocity for placement and consolidation of the material onto the receiving surface.

Dry Mix – Pre-blended dry or damp materials are placed into the delivery equipment. Compressed air conveys material through a hose at high velocity to the nozzle, where water is added. Material is consolidated on the receiving surface by the high-impact velocity.

Features

The properties of both wet and dry process shotcrete can be further enhanced through the addition of many different additives or admixtures such as:

Silica Fume – Provides reduced permeability, increased compressive and flexural strength, increased resistance to alkali and chemical attack, improved resistance to water washout, reduced rebound levels and allows for thicker single pass applications.

Air-Entraining Admixtures – Improve pumpability and adhesion in wet-process shotcrete and freeze-thaw durability in both wet and dry processes.

Fibers – Control cracking, increase toughness values and improve impact resistance and energy absorption.

Accelerators – Improve placement characteristics in adverse conditions, allow for thicker single pass applications, increase production capabilities and reduce the occurrence of fallouts on structures subjected to vibration.

Extract from:A study of limestone quarrying at Llanymynech

Written in 1984 by Harvey Kynaston

THE HOFFMAN KILNSTRUCTURE, LOCATION AND FUNCTION

STRUCTURE OF THE KILN

At the turn of the century there was a demand for finer lime than the familiar inverted bottle shaped kilns could produce. It was decided to build a superior type of kiln at Llanymynech which could provide lime by firing with less coal therefore producing a finer lime for industry. The Hoffman kiln today is very much overgrown. Plants such as ferns, mosses, nettle and ivy have taken a firm hold on it and there are even a number of sizeable lime trees on and around the site.

Despite this ever encroaching vegetation, the condition of the kiln is remarkably good. Its oval shape with steep sloping sides has a circumference of approximately 110 metres and each long side has a length of 40 metres. At one time the entire kiln was covered by a curved galvanised roof but this has long since disappeared, leaving a total height from the present day floor of about 4 metres or 40 layers of bricks.  While counting these layers it was interesting to note that each layer alternates, having bricks lying end on then side on and so on.

At one end of the kiln (nearest the canal) stands a tall square section chimney approximately 30 metres high which took the smoke away from the area and probably served the purpose of "drawing" the draught through. Over the years its double skin of bricks has weakened and now it is supported by an iron brace.

Along each side of the kiln are 6 arches 1.25 metres wide and 2.20 metres high and they provide entrance to the formidable interior in which stands a long central pillar to support the roof. In the roof can be seen small square holes spaced out at regular intervals. The function of these will be discussed later.

To the left hand side of each arch is a draught hole 90 centimetres wide and 80 centimetres high leading into the kiln and also beneath its floor.

Approaching the site from the A483 one can still catch glimpses of the sleepers and rails of the tramways which served the kiln with trucks carrying limestone or coal.

It has already been mentioned that this structure was intended to produce a finer lime at the turn of this century. It is one of only a few in the country, if not Europe, and by looking at the 1.70 metre thickness of its walls, it was surely built to last. And yet the Hoffman kiln closed down in 1914 and was therefore only in production for about 20 years. Why this was so is one of many unanswered questions. Perhaps one or all of the following possible reasons led to its closure:

Complaints of smokeBetter quality limestone at PorthywaenOutbreak of first world warPoor maintenance and deterioration of the canalThe Hoffman kiln never performed well

INSIDE THE HOFFMAN KILN

SKETCH MAP SHOWING LOCATION OF HOFFMAN KILN

FUNCTIONING OF THE KILN

There are several speculations as to how the kiln actually worked. It is certainly different from the “inverted bottle” type kiln in that limestone is loaded through its arches and not from above.

Coal (presumably a slow burning anthracite) arrived by horse drawn barge on the nearby Montgomery canal, now a branch of the Shropshire Union canal, and was poured into the kiln through holes in its roof by the firers.

There are several holes to each section of the kiln but before coal was poured into them iron rods were held in position through the holes so that the packers beneath could skilfully build a stack of limestone rocks around them. Occasionally, a rough built stack would collapse and the entire structure would have to be built again. The limestone loaded trucks which descend on an incline from the rock face, arrived in pairs and were probably pushed around the kiln until they arrived at the correct arch. There they would be swivelled through 90°, run on temporary rails into the kiln and unloaded. From the scanty evidence of existing trucks it is difficult to say exactly how they were unloaded. The type of truck can be found under the rock face and also the type of tipping mechanism employed. However these trucks were relatively small and it could be that a larger rectangular truck which had one side left open was used for travelling up and down the incline.

It is known that each section through its respective arch was packed and fired in succession rather

than every section packed and the whole kiln fired. In this way there was a sort of rotation - as one kiln was smouldering, another was being packed and another was being raked out. Their timing was such that by the time the workers had completed one “lap” the first section would be ready to rake out. One wonders how long the whole process took, - twelve hours, a day, three days? Again many questions still remain unanswered.

Surveying the now overgrown kiln shrouded in its green canopy it is difficult to get an impression of the working conditions that the men must have endured. The first factor that springs to mind when considering a modern busy industrial concern is noise. But would it have been noisy working at this kiln or indeed any other? What sounds would one hear? A steam engine from the adjacent kiln, the rumble of trucks descending the incline, the occasional steam train, the odd blast from Llanymynech rock but certainly little mechanical noise.

There must have been more uncomfortable aggravations than noise. Dust for instance, heat certainly, and although the tall adjoining chimney must have carried much of the smoke away, there was probably a constant stench of burning to anyone in the vicinity.

It is hard to believe, walking round the kiln, that men actually worked here. Conversely, I am sure it would have been difficult to convince a worker in the heyday of the kiln that in seventy years time, cows would be meandering in and out of it and trees would be growing on top.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The following diagram and text are from an unidentified document. The description relates to the use of the Hoffman kiln for brick making but the principle of a ‘continuous rotary burn’ applies to the lime kiln.

Hoffman kiln

This is a continuous kiln, that is, the fire never goes out as it is transferred from one chamber to another. To facilitate loading and unloading, the kiln is divided into a number of separate chambers, usually 16, having 8 on each side with dampers between them which may be opened for the fire to pass from one chamber to the next. All the chambers in one kiln are connected to a single chimney shaft. As the fire passes round the kiln so the chambers in front of the actual firing zone are gradually warmed, and the chambers behind cool off slowly. Although the burning time is only about 3 days, the bricks are in the kiln for about 10 days to allow for raising the temperature and, after burning, subsequent lowering of the temperature before unloading the chambers

Contents [hide]

1 Mix design 2 Regular concrete 3 High-strength concrete 4 Stamped concrete 5 High-performance concrete 6 Ultra-High-performance concrete 7 Self-consolidating concretes 8 Vacuum concretes 9 Shotcrete 10 Pervious concrete

o 10.1 Installation o 10.2 Characteristics o 10.3 Cellular concrete o 10.4 Cork-cement composites o 10.5 Roller-compacted concrete o 10.6 Glass concrete o 10.7 Asphalt concrete o 10.8 Rapid strength concrete o 10.9 Rubberized concrete o 10.10 Polymer concrete o 10.11 Geopolymer or green concrete o 10.12 Limecrete o 10.13 Refractory Cement o 10.14 Concrete cloth o 10.15 Innovative mixtures

11 Gypsum concrete 12 References

[edit] Mix design

Modern concrete mix designs can be complex. The design of a concrete, or the way the weights of the components of a concrete is determined, is specified by the requirements of the project and the various local building codes and regulations.

The design begins by determining the "durability" requirements of the concrete. These requirements take into consideration the weather conditions that the concrete will be exposed to in service, and the required design strength. The compressive strength of a concrete is determined by taking standard molded, standard-cured cylinder samples.

Many factors need to be taken into account, from the cost of the various additives and aggregates, to the trade offs between, the "slump" for easy mixing and placement and ultimate performance.

A mix is then designed using cement (Portland or other cementitious material), coarse and fine aggregates, water and chemical admixtures. The method of mixing will also be specified, as well as conditions that it may be used in.

This allows a user of the concrete to be confident that the structure will perform properly.

Various types of concrete have been developed for specialist application and have become known by these names..

Concrete mixes can also be designed using software programs. Such softwares provide the user an opportunity to select their preferred method of mix design and enter the material data to arrive at proper mix designs.

[edit] Regular concrete

Regular concrete is the lay term describing concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. This concrete can be produced to yield a varying strength from about 10 MPa (1450 psi) to about 40 MPa (5800 psi), depending on the purpose, ranging from blinding to structural concrete respectively. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.

Typically, a batch of concrete can be made by using 1 part Portland cement, 2 parts dry sand, 3 parts dry stone, 1/2 part water. The parts are in terms of weight – not volume. For example, 1-cubic-foot (0.028 m3) of concrete would be made using 22 lb (10.0 kg) cement, 10 lb (4.5 kg) water, 41 lb (19 kg) dry sand, 70 lb (32 kg) dry stone (1/2" to 3/4" stone). This would make 1-cubic-foot (0.028 m3) of concrete and would weigh about 143 lb (65 kg). The sand should be mortar or brick sand (washed and filtered if possible) and the stone should be washed if possible. Organic materials (leaves, twigs, etc) should be removed from the sand and stone to ensure the highest strength.

[edit] High-strength concrete

High-strength concrete has a compressive strength generally greater than 6,000 pounds per square inch (40 MPa = 5800 psi). High-strength concrete is made by lowering the water-cement (W/C) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.

Low W/C ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.

In some applications of high-strength concrete the design criterion is the elastic modulus rather than the ultimate compressive strength.

[edit] Stamped concrete

Stamped concrete is an architectural concrete which has a superior surface finish. After a concrete floor has been laid, floor hardeners (can be pigmented) are impregnated on the surface and a mold which may be textured to replicate a stone / brick or even wood is stamped on to give an attractive textured surface finish. After sufficient hardening the surface is cleaned and generally sealed to give a protection. The wear resistance of stamped concrete is generally excellent and hence found in applications like parking lots, pavements, walkways etc.

[edit] High-performance concrete

High-performance concrete (HPC) is a relatively new term used to describe concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength. Some examples of such standards currently used in relation to HPC are:

Ease of placement Compaction without segregation Early age strength Long-term mechanical properties Permeability Density Heat of hydration Toughness Volume stability Long life in severe environments Depending on its implementation, environmental [1]

[edit] Ultra-High-performance concrete

Ultra-High-performance concrete is a new type of concrete that is being developed by agencies concerned with infrastructure protection. UHPC is characterized by being a steel fibre-reinforced cement composite material with compressive strengths in excess of 150 MPa, upto and possibly exceeding 250 MPa[2]. UHPC is also characterized by its constituent material make-up: typically fine-grained sand, silica fume, small steel fibers, and special blends of high-strength portland cement. Note that there is no large aggregate. The current types in production (Ductal, Taktl, etc.) differ from normal concrete in compression by their strain hardening, followed by sudden brittle failure. Ongoing research into UHPC failure via tensile and shear failure is being conducted by multiple government agencies and universities around the world.

[edit] Self-consolidating concretesFor more details on this topic, see Self Leveling Concrete.

During the 1980s a number of countries including Japan, Sweden and France developed concretes that are self-compacting, known as self-consolidating concrete in the United States. This self-consolidating concrete (SCCs) is characterized by:

extreme fluidity as measured by flow, typically between 650–750 mm on a flow table, rather than slump(height)

no need for vibrators to compact the concrete placement being easier. no bleed water, or aggregate segregation Increased Liquid Head Pressure, Can be detrimental to Safety and workmanship

SCC can save up to 50% in labor costs due to 80% faster pouring and reduced wear and tear on formwork.

As of 2005, self-consolidating concretes account for 10-15% of concrete sales in some European countries. In the US precast concrete industry, SCC represents over 75% of concrete production. 38 departments of transportation in the US accept the use of SCC for road and bridge projects.

This emerging technology is made possible by the use of polycarboxylates plasticizer instead of older naphthalene based polymers, and viscosity modifiers to address aggregate segregation.

[edit] Vacuum concretes

The use of steam to produce a vacuum inside of concrete mixing truck to release air bubbles inside the concrete is being researched. The idea is that the steam displaces the air normally over the concrete. When the steam condenses into water it will create a low pressure over the concrete that will pull air from the concrete. This will make the concrete stronger due to there being less air in the mixture. Obviously this needs to be done in a sealed container.

[edit] ShotcreteMain article: Shotcrete

Shotcrete (also known by the trade name Gunite) uses compressed air to shoot concrete onto (or into) a frame or structure. The greatest advantage of the process is that shotcrete can be applied overhead or on vertical surfaces without forming. It is often used for concrete repairs or placement on bridges, dams, pools, and on other applications where forming is costly or material handling and installation is difficult. Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminates the need for formwork. It is sometimes used for rock support, especially in tunneling. Shotcrete is also used for applications where seepage is an issue to limit the amount of water entering a construction site due to a high water table or other subterranean sources. This type of concrete is often used as a quick fix for weathering for loose soil types in construction zones.

There are two application methods for shotcrete.

dry-mix – the dry mixture of cement and aggregates is filled into the machine and conveyed with compressed air through the hoses. The water needed for the hydration is added at the nozzle.

wet-mix – the mixes are prepared with all necessary water for hydration. The mixes are pumped through the hoses. At the nozzle compressed air is added for spraying.

For both methods additives such as accelerators and fiber reinforcement may be used.[3]

[edit] Pervious concrete

Pervious concrete contains a network of holes or voids, to allow air or water to move through the concrete.

This allows water to drain naturally through it, and can both remove the normal surface-water drainage infrastructure, and allow replenishment of groundwater when conventional concrete does not.

It is formed by leaving out some or all of the fine aggregate (fines). The remaining large aggregate then is bound by a relatively small amount of Portland cement. When set, typically between 15 % and 25 % of the concrete volume is voids, allowing water to drain at around 5 gal/ft²/ min (70 L/m²/min) through the concrete.

[edit] Installation

Pervious concrete is installed by being poured into forms, then screeded off, to level (not smooth) the surface, then packed or tamped into place. Due to the low water content and air permeability, within 5–15 minutes of tamping, the concrete must be covered with a 6-mil poly plastic, or it will dry out prematurely and not properly hydrate and cure.

[edit] Characteristics

Pervious concrete can significantly reduce noise, by allowing air to be squeezed between vehicle tires and the roadway to escape. This product cannot be used on major U.S. state highways currently due to the high psi ratings required by most states. Pervious concrete has been tested up to 4500  psi so far.

[edit] Cellular concrete

Aerated concrete produced by the addition of an air-entraining agent to the concrete (or a lightweight aggregate like expanded clay pellets or cork granules and vermiculite) is sometimes called cellular concrete, lightweight aerated concrete, variable density concrete, foamed concrete and lightweight or ultra-lightweight concrete,[4][5] not to be confused with aerated autoclaved concrete, which is manufactured off-site using an entirely different method.

In the 1977 work on A Pattern Language: Towns, Buildings and Construction, architect Christopher Alexander wrote in pattern 209 on Good Materials:

"Regular concrete is too dense. It is heavy and hard to work. After it sets one cannot cut into it, or nail into it. And it's [sic] surface is ugly, cold, and hard in feeling unless covered by expensive finishes not integral to the structure.

And yet concrete, in some form, is a fascinating material. It is fluid, strong, and relatively cheap. It is available in almost every part of the world. A University of California professor of engineering sciences, P. Kumar Mehta, has even just recently found a way of converting abandoned rice husks into Portland cement.

Is there any way of combining all these good qualities of concrete and also having a material which is light in weight, easy to work, with a pleasant finish? There is. It is possible to use a whole range of ultra-lightweight concretes which have a density and compressive strength very similar to that of wood. They are easy to work with, can be nailed with ordinary nails, cut with a saw, drilled with wood-working tools, easily repaired.

We believe that ultra-lightweight concrete is one of the most fundamental bulk materials of the future."

The variable density is normally described in kg per m³, where regular concrete is 2400 kg/m³. Variable density can be as low as 300 kg/m³,[6] although at this density it would have no structural integrity at all and would function as a filler or insulation use only. The variable density reduces strength[7] to increase thermal[8] and acoustical insulation by replacing the dense heavy concrete with air or a light material such as clay, cork granules and vermiculite. There are many competing products that use a foaming agent that resembles shaving cream to mix air bubbles in with the concrete. All accomplish the same outcome: to displace concrete with air.

[edit] Cork-cement composites

Waste Cork granules are obtained during production of bottle stoppers from the treated bark of Cork oak.[9] These granules have a density of about 300 kg/m³, lower than most lightweight aggregates used for making lightweight concrete. Cork granules do not significantly influence cement hydration, but cork dust may.[10] Cork cement composites have several advantages over standard concrete, such as lower thermal conductivities, lower densities and good energy absorption characteristics. These composites can be made of density from 400 to 1500 kg/m³, compressive strength from 1 to 26 MPa, and flexural strength from 0.5 to 4.0 MPa.

[edit] Roller-compacted concreteMain article: Roller-compacted concrete

Roller-compacted concrete, sometimes called rollcrete, is a low-cement-content stiff concrete placed using techniques borrowed from earthmoving and paving work. The concrete is placed on the surface to be covered, and is compacted in place using large heavy rollers typically used in earthwork. The concrete mix achieves a high density and cures over time into a strong monolithic block.[11] Roller-compacted concrete is typically used for concrete pavement, but has also been used to build concrete dams, as the low cement content causes less heat to be generated while curing than typical for conventionally placed massive concrete pours.

[edit] Glass concrete

The use of recycled glass as aggregate in concrete has become popular in modern times, with large scale research being carried out at Columbia University in New York. This greatly enhances the aesthetic

appeal of the concrete. Recent research findings have shown that concrete made with recycled glass aggregates have shown better long term strength and better thermal insulation due to its better thermal properties of the glass aggregates.[12]

[edit] Asphalt concrete

Strictly speaking, asphalt is a form of concrete as well, with bituminous materials replacing cement as the binder.

[edit] Rapid strength concrete

This type of concrete is able to develop high resistance within few hours after being manufactured. This feature has advantages such as removing the formwork early and to move forward in the building process at record time, repair road surfaces that become fully operational in just a few hours.

[edit] Rubberized concrete

While "rubberized asphalt concrete" is common, rubberized Portland cement concrete ("rubberized PCC") is still undergoing experimental tests, as of 2009.[13] [14] [15][16]

[edit] Polymer concrete

Polymer concrete is concrete which uses polymers to bind the aggregate. Polymer concrete can gain a lot of strength in a short amount of time. For example, a polymer mix may reach 5000 psi in only four hours. Polymer concrete is generally more expensive than conventional concretes.

[edit] Geopolymer or green concrete

Geopolymer concrete is a greener alternative to ordinary Portland cement made from inorganic aluminosilicate (Al-Si) polymer compounds that can utilise 100% recycled industrial waste (e.g. fly ash and slag) as the manufacturing inputs resulting in up to 80% lower carbon dioxide emissions. Greater chemical and thermal resistance, and better mechanical properties, are said to be achieved by the manufacturer at both atmospheric and extreme conditions.[17]

Similar concretes have not only been used in Ancient Rome (see Roman concrete) as mentioned but also in the former Soviet Union in the 1950s and 1960s. Buildings in Ukraine are still standing after 45 years so that this kind of formulation has a sound track record.[18]

[edit] Limecrete

Limecrete or lime concrete is concrete where cement is replaced by lime.[19] One successful formula was developed in the mid 1800s by Dr. John E. Park[20]

[edit] Refractory Cement

High-temperature applications, such as masonry ovens and the like, generally require the use of a refractory cement; concretes based on Portland cement can be damaged or destroyed by elevated temperatures, but refractory concretes are better able to withstand such conditions.

[edit] Concrete cloth

A recent innovation is the concrete cloth. It consists of a three-dimensional fiber matrix, containing a specially formulated dry concrete mix. A PVC backing on one surface of the cloth ensures the material is completely waterproof, while hydrophilic fibers on the opposite surface aid hydration by drawing water into the cement. Concrete cloth can be used to rapidly create waterproof, fireproof, fiber-reinforced thin concrete forms across a wide range of applications: rapid trackway or landing surfaces, structural reinforcement, ground stabilization, ballistic protection and sterile concrete shelters.[21]

[edit] Innovative mixtures

On-going research into alternative mixtures and constituents has identified potential mixtures that promise radically different properties and characteristics.

One university has identified a mixture with much smaller crack propagation that does not suffer the usual cracking and subsequent loss of strength at high levels of tensile . Researchers have been able to take mixtures beyond 3 percent strain, past the more typical 0.1% point at which failure occurs.[22]

Other institutions have identified magnesium silicate (talc) as an alternative ingredient to replace Portland cement in the mix. This avoids the usual high-temperature production process that is very energy and greenhouse gas intensive and actually absorbs carbon dioxide while it cures.[23][24]

[edit] Gypsum concreteMain article: Gypsum concrete

Gypsum concrete is a building material used as a floor underlayment [25] used in wood-frame and concrete construction for fire ratings,[25] sound reduction,[25] radiant heating,[26] and floor leveling. It is a mixture of gypsum, Portland cement, and sand.[25]

[edit] References

The rotary kiln

General layout of a rotary kiln

The rotary kiln consists of a tube made from steel plate, and lined with firebrick. The tube slopes slightly (1–4°) and slowly rotates on its axis at between 30 and 250 revolutions per hour. Rawmix is fed in at the upper end, and the rotation of the kiln causes it gradually to move downhill to the other end of the kiln. At the other end fuel, in the form of gas, oil, or pulverized solid fuel, is blown in through the "burner pipe", producing a large concentric flame in the lower part of the kiln tube. As material moves under the flame, it reaches its peak temperature, before dropping out of the kiln tube into the cooler. Air is drawn first through the cooler and then through the kiln for combustion of the fuel. In the cooler the air is heated by the cooling clinker, so that it may be 400 to 800 °C before it enters the kiln, thus causing intense and rapid combustion of the fuel.

The earliest successful rotary kilns were developed in Pennsylvania around 1890, and were about 1.5 m in diameter and 15 m in length. Such a kiln made about 20 tonnes of clinker per day. The fuel, initially, was oil, which was readily available in Pennsylvania at the time. It was particularly easy to get a good flame with this fuel. Within the next 10 years, the technique of firing by blowing in pulverized coal was developed, allowing the use of the cheapest available fuel. By 1905, the largest kilns were 2.7 x 60 m in size, and made 190 tonnes per day. At that date, after only 15 years of development, rotary kilns accounted for half of world production. Since then, the capacity of kilns has increased steadily, and the largest kilns today produce around 10,000 tonnes per day. In contrast to static kilns, the material passes through quickly: it takes from 3 hours (in some old wet process kilns) to as little as 10 minutes (in short precalciner kilns). Rotary kilns run 24 hours a day, and are typically stopped only for a few days once or twice a year for essential maintenance. This is an important discipline, because heating up and cooling down are long, wasteful and damaging processes. Uninterrupted runs as long as 18 months have been achieved.

[edit] The wet process and the dry process

% of North American Capacity using Wet Process

Mean Fuel Energy used in North American Kilns

From the earliest times, two different methods of rawmix preparation were used: the mineral components were either dry-ground to form a flour-like powder, or were wet-ground with added water to produce a fine slurry with the consistency of paint, and with a typical water content of 40–45%[4].

The wet process suffered the obvious disadvantage that, when the slurry was introduced into the kiln, a large amount of extra fuel was used in evaporating the water. Furthermore, a larger kiln was needed for a given clinker output, because much of the kiln's length was used up for the drying process. On the other hand, the wet process had a number of advantages. Wet grinding of hard minerals is usually much more efficient than dry grinding. When slurry is dried in the kiln, it forms a granular crumble that is ideal for subsequent heating in the kiln. In the dry process, it is very difficult to keep the fine powder rawmix in the kiln, because the fast-flowing combustion gases tend to blow it back out again. It became a practice to spray water into dry kilns in order to "damp down" the dry mix, and thus, for many years there was little difference in efficiency between the two processes, and the overwhelming majority of kilns used the wet process. By 1950, a typical large, wet process kiln, fitted with drying-zone heat exchangers, was 3.3 x 120 m in size, made 680 tonnes per day, and used about 0.25–0.30 tonnes of coal fuel for every tonne of clinker produced. Before the energy crisis of the 1970s put an end to new wet-process installations, kilns as large as 5.8 x 225 m in size were making 3000 tonnes per day.

An interesting footnote on the wet process history is that some manufacturers have in fact made very old wet process facilities profitable through the use of waste fuels. Plants that burn waste fuels enjoy a negative fuel cost (they are paid by industries needing to dispose of materials that have energy content and can be safely disposed of in the cement kiln thanks to its high temperatures and longer retention times). As a result the inefficiency of the wet process is an advantage—to the manufacturer. By locating waste burning operations at older wet process locations, higher fuel consumption actually equates to higher profits for the manufacturer, although it produces correspondingly greater emission of CO2. Manufacturers who think such emissions should be reduced are abandoning the use of wet process.

[edit] Preheaters

In the 1930s, significantly, in Germany, the first attempts were made to redesign the kiln system to minimize waste of fuel[5]. This led to two significant developments:

the grate preheater the gas-suspension preheater.

[edit] Grate preheaters

The grate preheater consists of a chamber containing a chain-like high-temperature steel moving grate, attached to the cold end of the rotary kiln[6]. A dry-powder rawmix is turned into a hard pellets of 10–20 mm diameter in a nodulizing pan, with the addition of 10-15% water. The pellets are loaded onto the moving grate, and the hot combustion gases from the rear of the kiln are passed through the bed of pellets from beneath. This dries and partially calcines the rawmix very efficiently. The pellets then drop into the kiln. Very little powdery material is blown out of the kiln. Because the rawmix is damped in order to make pellets, this is referred to as a "semi-dry" process. The grate preheater is also applicable to the "semi-wet" process, in which the rawmix is made as a slurry, which is first de-watered with a high-pressure filter, and the resulting "filter-cake" is extruded into pellets, which are fed to the grate. In this case, the water content of the pellets is 17-20%. Grate preheaters were most popular in the 1950s and 60s, when a typical system would have a grate 28 m long and 4 m wide, and a rotary kiln of 3.9 x 60 m, making 1050 tonnes per day, using about 0.11-0.13 tonnes of coal fuel for every tonne of clinker produced. Systems up to 3000 tonnes per day were installed.

[edit] Gas-suspension preheaters

Cutaway view of cyclone showing air path

The key component of the gas-suspension preheater is the cyclone. A cyclone is a conical vessel into which a dust-bearing gas-stream is passed tangentially. This produces a vortex within the vessel. The gas leaves the vessel through a co-axial "vortex-finder". The solids are thrown to the outside edge of the vessel by centrifugal action, and leave through a valve in the vertex of the cone. Cyclones were originally used to clean up the dust-laden gases leaving simple dry process kilns. If, instead, the entire feed of rawmix is encouraged to pass through the cyclone, it is found that a very efficient heat exchange takes place: the gas is efficiently cooled, hence producing less waste of heat to the atmosphere, and the rawmix is efficiently heated. This efficiency is further increased if a number of cyclones are connected in series.

4-Stage preheater, showing path of feed

The number of cyclones stages used in practice varies from 1 to 6. Energy, in the form of fan-power, is required to draw the gases through the string of cyclones, and at a string of 6 cyclones, the cost of the added fan-power needed for an extra cyclone exceeds the efficiency advantage gained. It is normal to use the warm exhaust gas to dry the raw materials in the rawmill, and if the raw materials are wet, hot gas from a less efficient preheater is desirable. For this reason, the most commonly encountered suspension preheaters have 4 cyclones. The hot feed that leaves the base of the preheater string is typically 20% calcined, so the kiln has less subsequent processing to do, and can therefore achieve a higher specific output. Typical large systems installed in the early 1970s had cyclones 6 m in diameter, a rotary kiln of 5 x 75 m, making 2500 tonnes per day, using about 0.11-0.12 tonnes of coal fuel for every tonne of clinker produced.

A penalty paid for the efficiency of suspension preheaters is their tendency to block up. Salts, such as the sulfate and chloride of sodium and potassium, tend to evaporate in the burning zone of the kiln. They are carried back in vapor form, and re-condense when a sufficiently low temperature is encountered. Because these salts re-circulate back into the rawmix and re-enter the burning zone, a recirculation cycle establishes itself. A kiln with 0.1% chloride in the rawmix and clinker may have 5% chloride in the mid-kiln material. Condensation usually occurs in the preheater, and a sticky deposit of liquid salts glues dusty rawmix into a hard deposit, typically on surfaces against which the gas-flow is impacting. This can choke the preheater to the point that air-flow can no longer be maintained in the kiln. It then becomes necessary to manually break the build-up away. Modern installations often have automatic devices installed at vulnerable points to knock out build-up regularly. An alternative approach is to "bleed off" some of the kiln exhaust at the kiln inlet where the salts are still in the vapor phase, and remove and discard the solids in this. This is usually termed an "alkali bleed" and it breaks the recirculation cycle. It can also be of advantage for cement quality reasons, since it reduces the alkali content of the clinker. However, hot gas is run to waste so the process is inefficient and increases kiln fuel consumption.

[edit] Precalciners

% of North American Capacity using Precalciners

Mean Daily Output (tonnes) of North American Kilns

In the 1970s the precalciner was pioneered in Japan, and has subsequently become the equipment of choice for new large installations worldwide[7]. The precalciner is a development of the suspension preheater. The philosophy is this: the amount of fuel that can be burned in the kiln is directly related to the size of the kiln. If part of the fuel necessary to burn the rawmix is burned outside the kiln, the output of the system can be increased for a given kiln size. Users of suspension preheaters found that output could be increased by injecting extra fuel into the base of the preheater. The logical development was to install a specially designed combustion chamber at the base of the preheater, into which pulverized coal is injected. This is referred to as an "air-through" precalciner, because the combustion air for both the kiln fuel and the calciner fuel all passes through the kiln. This kind of precalciner can burn up to 30% (typically 20%) of its fuel in the calciner. If more fuel were injected in the calciner, the extra amount of air drawn through the kiln would cool the kiln flame excessively. The feed is 40-60% calcined before it enters the rotary kiln.

The ultimate development is the "air-separate" precalciner, in which the hot combustion air for the calciner arrives in a duct directly from the cooler, bypassing the kiln. Typically, 60-75% of the fuel is burned in the precalciner. In these systems, the feed entering the rotary kiln is 100% calcined. The kiln has only to raise the feed to sintering temperature. In theory the maximum efficiency would be achieved if all the fuel were burned in the preheater, but the sintering operation involves partial melting and nodulization to make clinker, and the rolling action of the rotary kiln remains the most efficient way of doing this. Large modern installations typically have two parallel strings of 4 or 5 cyclones, with one attached to the kiln and the other attached to the precalciner chamber. A rotary kiln of 6 x 100 m makes 8,000–10,000 tonnes per day, using about 0.10-0.11 tonnes of coal fuel for every tonne of clinker produced. The kiln is dwarfed by the massive preheater tower and cooler in these installations. Such a kiln produces 3 million tonnes of clinker per year, and consumes 300,000 tonnes of coal. A diameter of 6 m appears to be the limit of size of rotary kilns, because the flexibility of the steel shell becomes unmanageable at or above this size, and the firebrick lining tends to fail when the kiln flexes.

A particular advantage of the air-separate precalciner is that a large proportion, or even 100%, of the alkali-laden kiln exhaust gas can be taken off as alkali bleed (see above). Because this accounts for only 40% of the system heat input, it can be done with lower heat wastage than in a simple suspension preheater bleed. Because of this, air-separate precalciners are now always prescribed when only high-alkali raw materials are available at a cement plant.

The accompanying figures show the movement towards the use of the more efficient processes in North America (for which data is readily available). But the average output per kiln in, for example, Thailand is twice that in North America.

[edit] Ancillary equipment

Essential equipment in addition to the kiln tube and the preheater are:

Cooler Fuel mills Fans Exhaust gas cleaning equipment.

[edit] Coolers

A pair of kilns with satellite coolers in Ashaka, Nigeria Sysy

Early systems used rotary coolers, which were rotating cylinders similar to the kiln, into which the hot clinker dropped[8]. The combustion air was drawn up through the cooler as the clinker moved down, cascading through the air stream. In the 1920s, satellite coolers became common and remained in use until recently. These consist of a set (typically 7–9) of tubes attached to the kiln tube. They have the advantage that they are sealed to the kiln, and require no separate drive. From about 1930, the grate cooler was developed. This consists of a perforated grate through which cold air is blown, enclosed in a rectangular chamber. A bed of clinker up to 0.5 m deep moves along the grate. These coolers have two main advantages: they cool the clinker rapidly, which is desirable from a quality point of view (to avoid that alite, thermodynamically unstable below 1250°C, revert to belite and free CaO on slow cooling), and, because they do not rotate, hot air can be ducted out of them for use in fuel drying, or for use as precalciner combustion air. The latter advantage means that they have become the only type used in modern systems.

[edit] Fuel mills

Fuel systems are divided into two categories[9]:

Direct firing Indirect firing

In direct firing, the fuel is fed at a controlled rate to the fuel mill, and the fine product is immediately blown into the kiln. The advantage of this system is that it is not necessary to store the hazardous ground fuel: it is used as soon as it is made. For this reason it was the system of choice for older kilns. A disadvantage is that the fuel mill has to run all the time: if it breaks down, the kiln has to stop if no backup system is available.

In indirect firing, the fuel is ground by an intermittently run mill, and the fine product is stored in a silo of sufficient size to supply the kiln though fuel mill stoppage periods. The fine fuel is metered out of the silo at a controlled rate and blown into the kiln. This method is now favoured for precalciner systems, because both the kiln and the precalciner can be fed with fuel from the same system. Special techniques are required to store the fine fuel safely, and coals with high volatiles are normally milled in an inert atmosphere (e.g. CO2).

[edit] Fans

A large volume of gases has to be moved through the kiln system[10]. Particularly in suspension preheater systems a high degree of suction has to be developed at the exit of the system to drive this. Fans are also used to force air through the cooler bed, and to propel the fuel into the kiln. Fans account for most of the electric power consumed in the system, typically amounting to 10–15 kW·h per tonne of clinker.

[edit] Gas cleaning

The exhaust gases from a modern kiln typically amount to 2 tonnes (or 1500 cubic metres at STP) per tonne of clinker made[11]. The gases carry a large amount of dust—typically 30 grams per cubic metre. Environmental regulations specific to different countries require that this be reduced to (typically) 0.1 gram per cubic metre, so dust capture needs to be at least 99.7% efficient. Methods of capture include electrostatic precipitators and bag-filters. See also cement kiln emissions.

[edit] Kiln fuels

Used tires being fed mid-kiln to a pair of long kilns

Fuels that have been used for primary firing include coal, petroleum coke, heavy fuel oil, natural gas, landfill off-gas and oil refinery flare gas[12]. High carbon fuels such as coal are preferred for kiln firing, because they yield a luminous flame. The clinker is brought to its peak temperature mainly by radiant heat transfer, and a bright (i.e. high emissivity) and hot flame is essential for this. In favorable circumstances, high-rank bituminous coal can produce a flame at 2050 °C. Natural gas can only produce a flame of, at best 1950 °C, and this is also less luminous, so it tends to result in lower kiln output.

In addition to these primary fuels, various combustible waste materials have been fed to kilns, notably used tires, which are very difficult to dispose of by other means. In theory, cement kilns are an attractive way of disposing of hazardous materials, because of:

the temperatures in the kiln, which are much higher than in other combustion systems (e.g. incinerators),

the alkaline conditions in the kiln, afforded by the high-calcium rawmix, which can absorb acidic combustion products,

the ability of the clinker to absorb heavy metals into its structure.

Whole tires are commonly introduced in the kiln, by rolling them into the upper end of a preheater kiln, or by dropping them through a slot midway along a long wet kiln. In either case, the high gas temperatures (1000–1200 °C) cause almost instantaneous, complete and smokeless combustion of the

tire. Alternatively, tires are chopped into 5–10 mm chips, in which form they can be injected into a precalciner combustion chamber. The steel and zinc in the tires become chemically incorporated into the clinker.

Other wastes have included solvents and clinical wastes. A very high level of monitoring of both the fuel and its combustion products is necessary to maintain safe operation.

For maximum kiln efficiency, high quality conventional fuels are the best choice. When using waste materials, in order to avoid prohibited emissions (e.g. of dioxins) it is necessary to control the kiln system in a manner that is non-optimal for efficiency and output, and coarse combustibles such as tires can cause major product quality problems.

[edit] Kiln control

Online X-ray diffraction with automatic sample feed for free calcium oxide measurement

The objective of kiln operation is to make clinker with the required chemical and physical properties, at the maximum rate that the size of kiln will allow, while meeting environmental standards, at the lowest possible operating cost[13]. The kiln is very sensitive to control strategies, and a poorly run kiln can easily double cement plant operating costs[14].

Formation of the desired clinker minerals involves heating the rawmix through the temperature stages mentioned above. The finishing transformation that takes place in the hottest part of the kiln, under the flame, is the reaction of belite (Ca2SiO4) with calcium oxide to form alite (Ca3O·SiO4):

Ca2SiO4 + CaO → Ca3SiO5

Also abbreviated in the cement chemist notation (CCN) as:

C2S + C → C3S

Tricalcium silicate is thermodynamically unstable below 1250°C, but can be preserved in a metastable state at room temperature by fast cooling: on slow cooling it tends to revert to belite (Ca2SiO4) and CaO.

If the reaction is incomplete, excessive amounts of free calcium oxide remain in the clinker. Regular measurement of the free CaO content is used as a means of tracking the clinker quality. As a parameter in kiln control, free CaO data is somewhat ineffective because, even with fast automated sampling and analysis, the data, when it arrives, may be 10 minutes "out of date", and more immediate data must be used for minute-to-minute control.

Conversion of belite to alite requires partial melting, the resulting liquid being the solvent in which the reaction takes place. The amount of liquid, and hence the speed of the finishing reaction, is related to temperature. To meet the clinker quality objective, the most obvious control is that the clinker should reach a peak temperature such that the finishing reaction takes place to the required degree. A further reason to maintain constant liquid formation in the hot end of the kiln is that the sintering material forms a dam that prevents the cooler upstream feed from flooding out of the kiln. The feed in the calcining zone, because it is a powder evolving carbon dioxide, is extremely fluid. Cooling of the burning zone, and loss of unburned material into the cooler, is called "flushing", and in addition to causing lost production can cause massive damage.

However, for efficient operation, steady conditions need to be maintained throughout the whole kiln system. The feed at each stage must be at a temperature such that it is "ready" for processing in the next stage. To ensure this, the temperature of both feed and gas must be optimized and maintained at every point. The external controls available to achieve this are few:

Feed rate: this defines the kiln output Rotary kiln speed: this controls the rate at which the feed moves through the kiln tube Fuel injection rate: this controls the rate at which the "hot end" of the system is heated Exhaust fan speed or power: this controls gas flow, and the rate at which heat is drawn from the "hot

end" of the system to the "cold end"

In the case of precalciner kilns, further controls are available:

Independent control of fuel to kiln and calciner Independent fan controls where there are multiple preheater strings.

The independent use of fan speed and fuel rate is constrained by the fact that there must always be sufficient oxygen available to burn the fuel, and in particular, to burn carbon to carbon dioxide. If carbon monoxide is formed, this represents a waste of fuel, and also indicates reducing conditions within the kiln which must be avoided at all costs since it causes destruction of the clinker mineral structure. For this reason, the exhaust gas is continually analyzed for O2, CO, NO and SO2.

The assessment of the clinker peak temperature has always been problematic. Contact temperature measurement is impossible because of the chemically aggressive and abrasive nature of the hot clinker, and optical methods such as infrared pyrometry are difficult because of the dust and fume-laden atmosphere in the burning zone. The traditional method of assessment was to view the bed of clinker and deduce the amount of liquid formation by experience. As more liquid forms, the clinker becomes stickier, and the bed of material climbs higher up the rising side of the kiln. It is usually also possible to assess the length of the zone of liquid formation, beyond which powdery "fresh" feed can be seen. Cameras, with or without infrared measurement capability, are mounted on the kiln hood to facilitate this. On many kilns, the same information can be inferred from the kiln motor power drawn, since sticky

feed riding high on the kiln wall increases the eccentric turning load of the kiln. Further information can be obtained from the exhaust gas analyzers. The formation of NO from nitrogen and oxygen takes place only at high temperatures, and so the NO level gives an indication of the combined feed and flame temperature. SO2 is formed by thermal decomposition of calcium sulfate in the clinker, and so also gives in indication of clinker temperature. Modern computer control systems usually make a "calculated" temperature, using contributions from all these information sources, and then set about controlling it.

As an exercise in process control, kiln control is extremely challenging, because of multiple inter-related variables, non-linear responses, and variable process lags. Computer control systems were first tried in the early 1960s, initially with poor results due mainly to poor process measurements. Since 1990, complex high level supervisory control systems have been standard on new installations. These operate using expert system strategies, that maintain a "just sufficient" burning zone temperature, below which the kiln's operating condition will deteriorate catastrophically, thus requiring rapid-response, "knife-edge" control.

[edit] Cement kiln emissions

Emissions from cement works are determined both by continuous and discontinuous measuring methods, which are described in corresponding national guidelines and standards. Continuous measurement is primarily used for dust, NOx and SO2, while the remaining parameters relevant pursuant to ambient pollution legislation are usually determined discontinuously by individual measurements.

The following descriptions of emissions refer to modern kiln plants based on dry process technology.

[edit] Carbon dioxide

During the clinker burning process CO2 is emitted. CO2 accounts for the main share of these gases. CO2 emissions are both raw material-related and energy-related. Raw material-related emissions are produced during limestone decarbonation (CaCO3) and account for about 60 % of total CO2 emissions.

[edit] Dust

To manufacture 1 t of Portland cement, about 1.5 to 1.7 t raw materials, 0.1 t coal and 1 t clinker (besides other cement constituents and sulfate agents) must be ground to dust fineness during production. In this process, the steps of raw material processing, fuel preparation, clinker burning and cement grinding constitute major emission sources for particulate components. While particulate emissions of up to 3,000 mg/m3 were measured leaving the stack of cement rotary kiln plants as recently as in the 1950s, legal limits are typically 30 mg/m3 today, and much lower levels are achievable.

[edit] Nitrogen oxides (NOx)

The clinker burning process is a high-temperature process resulting in the formation of nitrogen oxides (NOx). The amount formed is directly related to the main flame temperature (typically 1850–2000 °C). Nitrogen monoxide (NO) accounts for about 95 %, and nitrogen dioxide (NO2) for about 5 % of this compound present in the exhaust gas of rotary kiln plants. As most of the NO is converted to NO2 in the atmosphere, emissions are given as NO2 per cubic metre exhaust gas.

Without reduction measures, process-related NOx contents in the exhaust gas of rotary kiln plants would in most cases considerably exceed the specifications of e.g. European legislation for waste burning plants (0.50 g/m3 for new plants and 0.80 g/m3 for existing plants). Reduction measures are aimed at smoothing and optimising plant operation. Technically, staged combustion and Selective Non-Catalytic NO Reduction (SNCR) are applied to cope with the emission limit values.

High process temperatures are required to convert the raw material mix to Portland cement clinker. Kiln charge temperatures in the sintering zone of rotary kilns range at around 1450 °C. To reach these, flame temperatures of about 2000 °C are necessary. For reasons of clinker quality the burning process takes place under oxidising conditions, under which the partial oxidation of the molecular nitrogen in the combustion air resulting in the formation of nitrogen monoxide (NO) dominates. This reaction is also called thermal NO formation. At the lower temperatures prevailing in a precalciner, however, thermal NO formation is negligible: here, the nitrogen bound in the fuel can result in the formation of what is known as fuel-related NO. Staged combustion is used to reduce NO: calciner fuel is added with insufficient combustion air. This causes CO to form.The CO then reduces the NO into molecular nitrogen:

2 CO + 2 NO → 2 CO2 + N2.

Hot tertiary air is then added to oxidize the remaining CO.

[edit] Sulfur dioxide (SO2)

Sulfur is input into the clinker burning process via raw materials and fuels. Depending on their origin, the raw materials may contain sulfur bound as sulfide or sulfate. Higher SO2 emissions by rotary kiln systems in the cement industry are often attributable to the sulfides contained in the raw material, which become oxidised to form SO2 at the temperatures between 370 °C and 420 °C prevailing in the kiln preheater. Most of the sulfides are pyrite or marcasite contained in the raw materials. Given the sulfide concentrations found e.g. in German raw material deposits, SO2 emission concentrations can total up to 1.2 g/m3 depending on the site location. In some cases, injected calcium hydroxide is used to lower SO2 emissions.

The sulfur input with the fuels is completely converted to SO2 during combustion in the rotary kiln. In the preheater and the kiln, this SO2 reacts to form alkali sulfates, which are bound in the clinker, provided that oxidizing conditions are maintained in the kiln.

[edit] Carbon monoxide (CO) and total carbon

The exhaust gas concentrations of CO and organically bound carbon are a yardstick for the burn-out rate of the fuels utilised in energy conversion plants, such as power stations. By contrast, the clinker burning process is a material conversion process that must always be operated with excess air for reasons of clinker quality. In concert with long residence times in the high-temperature range, this leads to complete fuel burn-up.

The emissions of CO and organically bound carbon during the clinker burning process are caused by the small quantities of organic constituents input via the natural raw materials (remnants of organisms and

plants incorporated in the rock in the course of geological history). These are converted during kiln feed preheating and become oxidized to form CO and CO2. In this process, small portions of organic trace gases (total organic carbon) are formed as well. In case of the clinker burning process, the content of CO and organic trace gases in the clean gas therefore may not be directly related to combustion conditions.

[edit] Dioxins and furans (PCDD/F)

Rotary kilns of the cement industry and classic incineration plants mainly differ in terms of the combustion conditions prevailing during clinker burning. Kiln feed and rotary kiln exhaust gases are conveyed in counter-flow and mixed thoroughly. Thus, temperature distribution and residence time in rotary kilns afford particularly favourable conditions for organic compounds, introduced either via fuels or derived from them, to be completely destroyed. For that reason, only very low concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (colloquially "dioxins and furans") can be found in the exhaust gas from cement rotary kilns.

[edit] Polychlorinated biphenyls (PCB)

The emission behaviour of PCB is comparable to that of dioxins and furans. PCB may be introduced into the process via alternative raw materials and fuels. The rotary kiln systems of the cement industry destroy these trace components virtually completely.[citation needed]

[edit] Polycyclic aromatic hydrocarbons (PAH)

PAHs (according to EPA 610) in the exhaust gas of rotary kilns usually appear at a distribution dominated by naphthalene, which accounts for a share of more than 90 % by mass. The rotary kiln systems of the cement industry destroy virtually completely the PAHs input via fuels. Emissions are generated from organic constituents in the raw material.

[edit] Benzene, toluene, ethylbenzene, xylene (BTEX)

As a rule benzene, toluene, ethylbenzene and xylene are present in the exhaust gas of rotary kilns in a characteristic ratio. BTEX is formed during the thermal decomposition of organic raw material constituents in the preheater.

[edit] Gaseous inorganic chlorine compounds (HCl)

Chlorides are minor additional constituents contained in the raw materials and fuels of the clinker burning process. They are released when the fuels are burnt or the kiln feed is heated, and primarily react with the alkalis from the kiln feed to form alkali chlorides. These compounds, which are initially vaporous, condense on the kiln feed or the kiln dust, at temperatures between 700 °C and 900 °C, subsequently re-enter the rotary kiln system and evaporate again. This cycle in the area between the rotary kiln and the preheater can result in coating formation. A bypass at the kiln inlet allows effective reduction of alkali chloride cycles and to diminish coating build-up problems. During the clinker burning process, gaseous inorganic chlorine compounds are either not emitted at all or in very small quantities only.

[edit] Gaseous inorganic fluorine compounds (HF)

Of the fluorine present in rotary kilns, 90 to 95 % is bound in the clinker, and the remainder is bound with dust in the form of calcium fluoride stable under the conditions of the burning process. Ultra-fine dust fractions that pass through the measuring gas filter may give the impression of low contents of gaseous fluorine compounds in rotary kiln systems of the cement industry.

[edit] Trace elements

The emission behaviour of the individual elements in the clinker burning process is determined by the input scenario, the behaviour in the plant and the precipitation efficiency of the dust collection device. The trace elements introduced into the burning process via the raw materials and fuels may evaporate completely or partially in the hot zones of the preheater and/or rotary kiln depending on their volatility, react with the constituents present in the gas phase, and condense on the kiln feed in the cooler sections of the kiln system. Depending on the volatility and the operating conditions, this may result in the formation of cycles that are either restricted to the kiln and the preheater or include the combined drying and grinding plant as well. Trace elements from the fuels initially enter the combustion gases, but are emitted to an extremely small extent only owing to the retention capacity of the kiln and the preheater

Under the conditions prevailing in the clinker burning process, non-volatile elements (e.g. arsenic, vanadium, nickel) are completely bound in the clinker.

Elements such as lead and cadmium preferentially react with the excess chlorides and sulfates in the section between the rotary kiln and the preheater, forming volatile compounds. Owing to the large surface area available, these compounds condense on the kiln feed particles at temperatures between 700 °C and 900 °C. In this way, the volatile elements accumulated in the kiln-preheater system are precipitated again in the cyclone preheater, remaining almost completely in the clinker.

Thallium (as the chloride) condenses in the upper zone of the cyclone preheater at temperatures between 450 °C and 500 °C. As a consequence, a cycle can be formed between preheater, raw material drying and exhaust gas purification.

Mercury and its compounds are not precipitated in the kiln and the preheater. They condense on the exhaust gas route due to the cooling of the gas and are partially adsorbed by the raw material particles. This portion is precipitated in the kiln exhaust gas filter.

Owing to trace element behaviour during the clinker burning process and the high precipitation efficiency of the dust collection devices trace element emission concentrations are on a low overall level.

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