Composition of cement. Pcd цемент


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Cement | building material | Britannica.com

Cement, in general, adhesive substances of all kinds, but, in a narrower sense, the binding materials used in building and civil engineering construction. Cements of this kind are finely ground powders that, when mixed with water, set to a hard mass. Setting and hardening result from hydration, which is a chemical combination of the cement compounds with water that yields submicroscopic crystals or a gel-like material with a high surface area. Because of their hydrating properties, constructional cements, which will even set and harden under water, are often called hydraulic cements. The most important of these is portland cement.

The cement-making process, from crushing and grinding of raw materials, through roasting of the ground and mixed ingredients, to final cooling and storing of the finished product.Encyclopædia Britannica, Inc.

This article surveys the historical development of cement, its manufacture from raw materials, its composition and properties, and the testing of those properties. The focus is on portland cement, but attention also is given to other types, such as slag-containing cement and high-alumina cement. Construction cements share certain chemical constituents and processing techniques with ceramic products such as brick and tile, abrasives, and refractories. For detailed description of one of the principal applications of cement, see the article building construction.

Applications of cement

Cements may be used alone (i.e., “neat,” as grouting materials), but the normal use is in mortar and concrete in which the cement is mixed with inert material known as aggregate. Mortar is cement mixed with sand or crushed stone that must be less than approximately 5 mm (3/16 inch) in size. Concrete is a mixture of cement, sand or other fine aggregate, and a coarse aggregate that for most purposes is up to 19 to 25 mm (3/4 to 1 inch) in size, but the coarse aggregate may also be as large as 150 mm (6 inches) when concrete is placed in large masses such as dams. Mortars are used for binding bricks, blocks, and stone in walls or as surface renderings. Concrete is used for a large variety of constructional purposes. Mixtures of soil and portland cement are used as a base for roads. Portland cement also is used in the manufacture of bricks, tiles, shingles, pipes, beams, railroad ties, and various extruded products. The products are prefabricated in factories and supplied ready for installation.

Because concrete is the most widely used of all construction materials in the world today, the manufacture of cement is widespread. Each year almost one ton of concrete is poured per capita in the developed countries.

History of cement

The origin of hydraulic cements goes back to ancient Greece and Rome. The materials used were lime and a volcanic ash that slowly reacted with it in the presence of water to form a hard mass. This formed the cementing material of the Roman mortars and concretes of 2,000 years ago and of subsequent construction work in western Europe. Volcanic ash mined near what is now the city of Pozzuoli, Italy, was particularly rich in essential aluminosilicate minerals, giving rise to the classic pozzolana cement of the Roman era. To this day the term pozzolana, or pozzolan, refers either to the cement itself or to any finely divided aluminosilicate that reacts with lime in water to form cement. (The term cement, meanwhile, derives from the Latin word caementum, which meant stone chippings such as were used in Roman mortar—not the binding material itself.)

Portland cement is a successor to a hydraulic lime that was first developed by John Smeaton in 1756 when he was called in to erect the Eddystone Lighthouse off the coast of Plymouth, Devon, England. The next development, taking place about 1800 in England and France, was a material obtained by burning nodules of clayey limestone. Soon afterward in the United States, a similar material was obtained by burning a naturally occurring substance called “cement rock.” These materials belong to a class known as natural cement, allied to portland cement but more lightly burned and not of controlled composition.

The invention of portland cement usually is attributed to Joseph Aspdin of Leeds, Yorkshire, England, who in 1824 took out a patent for a material that was produced from a synthetic mixture of limestone and clay. He called the product “portland cement” because of a fancied resemblance of the material, when set, to portland stone, a limestone used for building in England. Aspdin’s product may well have been too lightly burned to be a true portland cement, and the real prototype was perhaps that produced by Isaac Charles Johnson in southeastern England about 1850. The manufacture of portland cement rapidly spread to other European countries and North America. During the 20th century, cement manufacture spread worldwide. By the early 21st century, China and India had become the world leaders in cement production, followed by the United States, Brazil, Turkey, and Iran.

Raw materials

Composition

Portland cement consists essentially of compounds of lime (calcium oxide, CaO) mixed with silica (silicon dioxide, SiO2) and alumina (aluminum oxide, Al2O3). The lime is obtained from a calcareous (lime-containing) raw material, and the other oxides are derived from an argillaceous (clayey) material. Additional raw materials such as silica sand, iron oxide (Fe2O3), and bauxite—containing hydrated aluminum, Al(OH)3—may be used in smaller quantities to get the desired composition.

The commonest calcareous raw materials are limestone and chalk, but others, such as coral or shell deposits, also are used. Clays, shales, slates, and estuarine muds are the common argillaceous raw materials. Marl, a compact calcareous clay, and cement rock contain both the calcareous and argillaceous components in proportions that sometimes approximate cement compositions. Another raw material is blast-furnace slag, which consists mainly of lime, silica, and alumina and is mixed with a calcareous material of high lime content. Kaolin, a white clay that contains little iron oxide, is used as the argillaceous component for white portland cement. Industrial wastes, such as fly ash and calcium carbonate from chemical manufacture, are other possible raw materials, but their use is small compared with that of the natural materials.

The magnesia (magnesium oxide, MgO) content of raw materials must be low because the permissible limit in portland cement is 4 to 5 percent. Other impurities in raw materials that must be strictly limited are fluorine compounds, phosphates, metal oxides and sulfides, and excessive alkalies.

Another essential raw material is gypsum, some 5 percent of which is added to the burned cement clinker during grinding to control the setting time of the cement. Portland cement also can be made in a combined process with sulfuric acid using calcium sulfate or anhydrite in place of calcium carbonate. The sulfur dioxide produced in the flue gases on burning is converted to sulfuric acid by normal processes.

The percentage compositions of some of the typical raw materials used for the manufacture of portland cement are shown in the table.

Raw materials used in the manufacture of portland cement(percentage composition)limestone chalk cement rock clay slag
52 3 1 0.5 0.5 42
54 1 0.5 0.2 0.3 43
43 11 3 1 2 36
1 57 16 7 1 14
42 34 15 1 4 0

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Cement Types

Construction documents often specify a cement type based on the required performance of the concrete or the placement conditions. Certain cement manufacturing plants only produce certain types of portland cement. What are the differences in these cement types and how are they tested, produced, and identified in practice? 

In the most general sense, portland cement is produced by heating sources of lime, iron, silica, and alumina to clinkering temperature (2,500 to 2,800 degrees Fahrenheit) in a rotating kiln, then grinding the clinker to a fine powder. The heating that occurs in the kiln transforms the raw materials into new chemical compounds. Therefore, the chemical composition of the cement is defined by the mass percentages and composition of the raw sources of lime, iron, silica, and alumina as well as the temperature and duration of heating. It is this variation in raw materials source and the plant-specific characteristics, as well as the finishing processes (i.e. grinding and possible blending with gypsum, limestone, or supplementary cementing materials), that define the cement produced.

Standards?

To ensure a level of consistency between cement-producing plants, certain chemical and physical limits are placed on cements. These chemical limits are defined by a variety of standards and specifications. For instance, portland cements and blended hydraulic cements for concrete in the U.S. conform to the American Society for Testing and Materials (ASTM) C150 (Standard Specification for Portland Cement), C595 (Standard Specification for Blended Hydraulic Cement) or C1157 (Performance Specification for Hydraulic Cements).

Some state agencies refer to very similar specifications:  AASHTO M 85 for portland cement and M 240 for blended cements. These specifications refer to standard test methods to assure that the testing is performed in the same manner. For example, ASTM C109 (Standard Test Method for Compressive Strength for Hydraulic Cement Mortars using 2-inch Cube Specimens), describes in detail how to fabricate and test mortar cubes for compressive strength testing in a standardized fashion.

Nomenclature Differences

In the US, three separate standards may apply depending on the category of cement. For portland cement types, ASTM C150 describes:

Cement Type          DescriptionType I                      NormalType II                     Moderate Sulfate ResistanceType II (MH)             Moderate Heat of Hydration (and Moderate Sulfate Resistance)                            Type III                    High Early StrengthType IV                    Low Heat HydrationType V                     High Sulfate Resistance

For blended hydraulic cements – specified by ASTM C595 – the following nomenclature is used:

Cement Type           DescriptionType IL                    Portland-Limestone CementType IS                    Portland-Slag CementType IP                    Portland-Pozzonlan Cement Type IT                    Ternary Blended Cement

In addition, some blended cements have special performance properties verified by additional testing. These are designated by letters in parentheses following the cement type. For example Type IP(MS) is a portland-pozzolan cement with moderate sulfate resistance properties. Other special properties are designated by (HS), for high sulfate resistance; (A), for air-entraining cements; (MH) for moderate heat of hydration; and (LH) for low heat of hydration. Refer to ASTM C595 for more detail.

However, with an interest in the industry for performance-based specifications, ASTM C1157 describes cements by their performance attributes: 

Cement Type         DescriptionType GU                  General UseType HE                  High Early-StrengthType MS                 Moderate Sulfate ResistanceType HS                  High Sulfate ResistanceType MH                  Moderate Heat of HydrationType LH                   Low Heat of Hydration

Note: For a thorough review of US cement types and their characteristics see PCA’s Design and Control of Concrete Mixtures, EB001 or Effect of Cement Characteristics on Concrete Properties, EB226.  

Physical and Chemical Performance Requirements

Chemical tests verify the content and composition of cement,while physical testing demonstrates physical criteria. 

In C150/M 85 and C595/M 240, both chemical and physical properties are limited. In C1157, the limits are almost entirely physical requirements.

Chemical testing includes oxide analyses (SiO2, CaO, Al2O3, Fe2O3, etc.) to allow the cement phase composition to be calculated. Type II cements are limited in C150/M 85 to a maximum of 8 percent by mass of tricalcium aluminate (a cement phase, often abbreviated C3A), which impacts a cement’s sulfate resistance. Certain oxides are also themselves limited by specifications:  For example, the magnesia (MgO) content which is limited to 6 percent maximum by weight for portland cements, because it can impact soundness at higher levels.

Typical physical requirements for cements are: air content, fineness, expansion, strength, heat of hydration, and setting time. Most of these physical tests are carried out using mortar or paste created from the cement. This testing confirms that a cement has the ability to perform well in concrete; however, the performance of concrete in the field is determined by all of the concrete ingredients, their quantity, as well as the environment, and the handling and placing procedures used. 

Although the process for cement manufacture is relatively similar across North America and much of the globe, the reference to cement specifications can be different depending on the jurisdiction. In addition, test methods can vary as well, so that compressive strength requirements (for example) in Europe don’t ‘translate’ directly to those in North America. When ordering concrete for construction projects, work with a local concrete producer to verify that cement meeting the requirements for the project environment and application is used, and one that meets the appropriate cement specification.

 

www.cement.org

Types of Cement and their Uses, Composition, Advantages in Construction

There are various types of cement used in building and construction works for various purposes. Thus, it is important to understand composition, properties, uses and advantages of each types of cement.

Types of Cement and their Uses

  1. Rapid Hardening Cement
  2. Quick setting cement
  3. Low Heat Cement
  4. Sulphates resisting cement
  5. Blast Furnace Slag Cement
  6. High Alumina Cement
  7. White Cement
  8. Coloured cement
  9. Pozzolanic Cement
  10. Air Entraining Cement
  11. Expansive cement
  12. Hydrographic cement

1. Rapid Hardening Cement

Rapid hardening cement attains high strength in early days it is used in concrete where formworks are removed at an early stage and is similar to ordinary portland cement (OPC). This cement has increased lime content and contains higher c3s content and finer grinding which gives greater strength development than OPC at an early stage.

The strength of rapid hardening cement at the 3 days is similar to 7 days strength of OPC with the same water-cement ratio. Thus, advantage of this cement is that formwork can be removed earlier which increases the rate of construction and decreases cost of construction by saving formwork cost.

Rapid hardening cement is used in prefabricated concrete construction, road works, etc.

2. Quick setting cement

The difference between the quick setting cement and rapid hardening cement is that quick setting cement sets earlier while rate of gain of strength is similar to Ordinary Portland Cement, while rapid hardening cement gains strength quickly. Formworks in both cases can be removed earlier.

Quick setting cement is used where works is to be completed in very short period and for concreting in static or running water.

3. Low Heat Cement

Low heat cement is prepared by maintaining the percentage of tricalcium aluminate below 6% by increasing the proportion of C2S. This makes the concrete to produce low heat of hydration and thus is used in mass concrete construction like gravity dams, as the low heat of hydration prevents the cracking of concrete due to heat.

This cement has increased power against sulphates and is less reactive and initial setting time is greater than OPC.

4. Sulphates Resisting Cement

Sulfate resisting cement is used to reduce the risk of sulphate attack on concrete and thus is used in construction of foundations where soil has high sulphate content. This cement has reduced contents of C3A and C4AF.

Sulfate resisting cement is used in construction exposed to severe sulphate action by water and soil in places like canals linings, culverts, retaining walls, siphons etc.

5. Blast Furnace Slag Cement

Blast furnace slag cement is obtained by grinding the clinkers with about 60% slag and resembles more or less in properties of Portland cement. It can be used for works economic considerations is predominant.

6. High Alumina Cement

High alumina cement is obtained by melting mixture of bauxite and lime and grinding with the clinker. It is a rapid hardening cement with initial and final setting time of about 3.5 and 5 hours respectively.

The compressive strength of this cement is very high and more workable than ordinary portland cement and is used in works where concrete is subjected to high temperatures, frost, and acidic action.

7. White Cement

It is prepared from raw materials free from Iron oxide and is a type of ordinary portland cement which is white in color. It is costlier and is used for architectural purposes such as precast curtain wall and facing panels, terrazzo surface etc. and for interior and exterior decorative work like external renderings of buildings, facing slabs, floorings, ornamental concrete products, paths of gardens, swimming pools etc.

8. Colored cement

It is produced by mixing 5- 10% mineral pigments with ordinary cement. They are widely used for decorative works in floors.

9. Portland Pozzolana Cement

Portland pozzolana cement is prepared by grinding pozzolanic clinker with Portland cement. It is also produced by adding pozzolana with the addition of gypsum or calcium sulfate or by intimately and uniformly blending portland cement and fine pozzolana.

This cement has high resistance to various chemical attacks on concrete compared with ordinary portland cement and thus it is widely used. It is used in marine structures, sewage works, sewage works and for laying concrete under water such as bridges, piers, dams and mass concrete works etc.

10. Air Entraining Cement

Air entraining cement is produced by adding indigenous air entraining agents such as resins, glues, sodium salts of sulphates etc. during the grinding of clinker.

This type of cement is especially suited to improve the workability with smaller water cement ratio and to improve frost resistance of concrete.

11. Expansive Cement

Expansive cement expands slightly with time and does not shrink during and after the time of hardening . This  cement is mainly used for grouting anchor bolts and prestressed concrete ducts.

12. Hydrographic cement

Hydrographic cement is prepared by mixing water repelling chemicals and has high workability and strength. It has the property of repelling water and is unaffected during monsoon or rains. Hydrophobic cement is mainly used for the construction of water structures such dams, water tanks, spillways, water retaining structures etc.

Read More:

Applications of Different Cement Types for Concrete Construction

Tests on Cement at Construction Site To Check Quality of Cement

Soil Cement – Types, Composition, Mix, Applications and Advantages

Manufacture of Cement- Materials and Manufacturing Process of Portland Cement

theconstructor.org

Composition of cement

Composition of cement

Introduction Portland cement gets its strength from chemical reactions between the cement and water. The process is known as hydration. This is a complex process that is best understood by first understanding the chemical composition of cement.

Manufacture of cement Portland cement is manufactured by crushing, milling and proportioning the following materials:

    • Lime or calcium oxide, CaO: from limestone, chalk, shells, shale or calcareous rock
    • Silica, SiO2: from sand, old bottles, clay or argillaceous rock
    • Alumina, Al2O3: from bauxite, recycled aluminum, clay
    • Iron, Fe2O3: from from clay, iron ore, scrap iron and fly ash
    • Gypsum, CaSO4.2h30: found together with limestone
The materials, without the gypsum, are proportioned to produce a mixture with the desired chemical composition and then ground and blended by one of two processes - dry process or wet process. The materials are then fed through a kiln at 2,600º F to produce grayish-black pellets known as clinker. The alumina and iron act as fluxing agents which lower the melting point of silica from 3,000 to 2600º F. After this stage, the clinker is cooled, pulverized and gypsum added to regulate setting time. It is then ground extremely fine to produce cement.

Chemical shorthand Because of the complex chemical nature of cement, a shorthand form is used to denote the chemical compounds. The shorthand for the basic compounds is:  

Compound Formula Shorthand form
Calcium oxide (lime) Ca0 C
Silicon dioxide (silica) SiO2 S
Aluminum oxide (alumina) Al2O3 A
Iron oxide Fe2O3 F
Water  h3O H
Sulfate SO3 S
 Chemical composition of clinker The cement clinker formed has the following typical composition:  
Compound Formula Shorthand form % by weight1
Tricalcium aluminate Ca3Al2O6 C3A 10
Tetracalcium aluminoferrite Ca4Al2Fe2O10 C4AF 8
Belite or dicalcium silicate Ca2SiO5 C2S 20
Alite or tricalcium silicate Ca3SiO4 C3S 55
Sodium oxide Na2O N

)Up to 2

Potassium oxide K2O K
Gypsum CaSO4.2h3O CSh3 5
Representative weights only. Actual weight varies with type of cement. Source: Mindess & Young

Properties of cement compounds These compounds contribute to the properties of cement in different ways  

  • Tricalcium aluminate, C3A:-
  • It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant.  
  • Tricalcium silicate, C3S:-
  • This compound hydrates and hardens rapidly. It is largely responsible for portland cements initial set and early strength gain.  
  • Dicalcium silicate, C2S:
  • C2S hydrates and hardens slowly. It is largely responsible for strength gain after one week.  
  • Ferrite, C4AF:
  • This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000o F to 2,600o F). It hydrates rapidly, but does not contribute much to strength of the cement paste.

    By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.

    References: Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 671.

    Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.

    Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc.,

    www.engr.psu.edu


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