Cement and Types of Cements
Cement and Types of Cements
Any substance which bonds materials is normally considered as cement. There are many types of cements. However in construction, the term cement generally refers to bonding agents that are mixed with water or other liquid, or both, to produce a cementing paste. Initially, a mass of particles coated with the paste is in a plastic state which can be formed, or moulded, into various shapes. Such a mixture is normally considered a cementitious material since it can bond other materials together. After a time, due to chemical reactions, the paste sets and the mass harden. When these particles consist of fine aggregate (sand) then mortar is formed. When these particles consist of fine and coarse aggregates then concrete is the result. In its simplest form, concrete is a mixture of paste and aggregates. The paste, composed of portland cement and water, coats the surface of the fine and coarse aggregates. Through the chemical reaction which is called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.
Within this process lies the key to a remarkable trait of concrete is that it is plastic and malleable when newly mixed, strong and durable when hardened. Concrete’s durability, strength and relatively low cost make it the backbone of buildings and infrastructure worldwide houses, schools and hospitals as well as airports, bridges, highways and rail systems. It is the most-produced material on the mother earth.
Even construction professionals sometimes incorrectly use the terms cement and concrete interchangeably. Cement is actually an ingredient of concrete. It is the fine powder which, when mixed with water, sand, and gravel or crushed stone (fine and coarse aggregate), forms the rock-like mass known as concrete.
Though the history of cementing materials is as old as the history of engineering constructions, the modern portland cement industry owes its existence to Joseph Aspdin, a British stone mason, who in 1824, obtained a patent for a cement he produced in his kitchen. Joseph Aspdin heated a mixture of finely ground limestone and clay in his kitchen stove and ground the mixture into a powder create hydraulic cement – one that hardens with the addition of water. Aspdin named the product portland cement since it resembled a stone quarried on the Isle of Portland off the British coast. Portland cement has become one of the most important construction materials since then, primarily because concretes can be used advantageously for so many different purposes.
Further development was influenced significantly by W. Michaelis. He was the first to discuss in 1868 the most favorable composition of the raw materials for portland cement. The first high early strength portland cement was produced in the factory Loruns in Austria in 1912-13 where the clinker was burned at higher temperature and the fineness of the cement was also increased. The first portland cement with improved sulphate resistance was patented by F. Ferrari in Italy in 1919. White portland cement was produced in small quantities already in the 1880s in Heidelberg Germany. The production of oil well cement began around 1930 when the increased well depths required cements with adequately long setting times under high temperature and pressure. The expanding cement appeared around 1920, and, was developed by H. Lossier. The first high alumina cement was produced on the basis of the patent of the French J. Bied. It was used by the French during the World War I for foundations of heavy artilleries.
Particles which become a bonding agent when mixed with water are referred to as hydraulic cements. The term hydraulic cement refers to a powdery material that reacts with water and, as a result, produces a strong as well as water-insoluble solid.
Joseph Aspdin made portland cement by burning powdered limestone and clay in his kitchen stove. With this crude method, he laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water.
Portland cement, the fundamental ingredient in concrete, is calcium silicate cement made with a combination of calcium, silicon, aluminum, and iron. Producing cement which meets specific chemical and physical specification requirements needs careful control of the production process. The first step in the portland cement production process is obtaining raw materials. Generally, raw materials consisting of combinations of limestone, shells or chalk, and shale, clay, sand, or iron ore are mined from a quarry near the plant. After quarrying the rock is crushed. This involves several stages. The first crushing stage reduces the rock to a maximum size of around 150 mm. The rock then goes to secondary crushers or hammer mills for reduction to around 40 mm or smaller.
Once the raw materials arrive at the cement plant, the materials are proportioned to create cement with a specific chemical composition. The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln.
Two different processes namely dry and wet, are used in the production of portland cement. In the dry process, dry raw materials are proportioned, ground to a powder, blended together and fed to the kiln in a dry state. In the wet process, slurry is formed by adding water to the properly proportioned raw materials. The grinding and blending operations are then completed with the materials in slurry form. The most common way to produce portland cement is through a dry method.
After blending, the mixture of the finely ground raw materials is fed into the upper end of a tilted rotating, cylindrical kiln. The mixture passes through the kiln at a rate controlled by the slope and rotational speed of the kiln. At the lower end the kilns are fired with precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft which produces a flame in the kiln.
Kilns are frequently as much as 3 m to 4 m in diameter and around 150 m in length. They are lined with special quality of fireclay refractories. The large kilns are mounted with the axis inclined slightly from the horizontal. As the material moves through the kiln, certain material components are driven off in the form of gases. The remaining elements unite to form a new substance called clinker.
Inside the kiln, raw materials reach temperatures of 1450 deg C to 1650 deg C. At around 1500 deg C, a series of chemical reactions cause the materials to fuse and create cement clinker-grayish-black pellets, often the size of marbles. Clinker is discharged red hot from the lower end of the kiln and transferred to various types of coolers to lower the temperature of clinker to handling temperature. The heated air from the coolers is returned to the kiln, a process which saves fuel and increases burning efficiency.
Cooled clinker is combined with small amount of gypsum and limestone and ground into a fine gray powder. The clinker is ground so fine that nearly all of it passes through a number 200 mesh (75 micron) sieve. This fine gray powder is portland cement. Cement is so fine that 1 kg of cement contains around 300 billion grains. The cement is now packed in air tight bags for delivery to the customers.
Cement plant laboratories check each step in the production of portland cement by frequent chemical and physical tests. The laboratories also analyze and test the finished product to ensure that it complies with the specification requirements of the standards.
Physical and chemical performance requirements of cements
Chemical tests verify the content and composition of cement, while physical tests demonstrate physical criteria.
Chemical testing includes oxide analyses (SiO2, CaO, Al2O3, Fe2O3, etc.) to allow the cement phase composition to be calculated. Type II cements are limited to a maximum of 8 % of tricalcium aluminate (a cement phase, often abbreviated C3A), which impacts the sulphate resistance of the cement. Certain oxides are also themselves limited by specifications. For example, the magnesia (MgO) content is limited to 6 % in portland cements since at higher levels it impacts concrete soundness.
Typical physical requirements for cements are (i) air content, (ii) fineness, (iii) expansion, (iv) strength, (v) heat of hydration, and (vi) setting time. Most of these physical tests are carried out using mortar or paste created with the cement. This testing confirms that the 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 production is relatively similar across the globe, the reference to cement specifications can be different. In addition, test methods can vary as well.
Types of cements
Different types of portland cement are produced to meet various physical and chemical requirements. The properties of cement during hydration vary according to (i) chemical composition, and (ii) degree of fineness. It is possible to produce different types of cements by changing the percentages of the raw materials. The American Society for Testing and Materials (ASTM) Specification C-150 provides for eight types of portland cements (Fig 1). These are as follows.
- Type I portland cement – It is a normal, general purpose cement suitable for all uses. It is used in general construction projects such as buildings, bridges, floors, pavements, and other precast concrete products.
- Type IA portland cement – It is similar to Type I with the addition of air-entraining properties.
- Type II portland cement – It generates less heat at a slower rate and has a moderate resistance to sulphate attack.
- Type IIA portland cement – It is identical to Type II and produces air-entrained concrete.
- Type III portland cement – It is a high early strength cement and causes concrete to set and gain strength rapidly. Type III is chemically and physically similar to Type I, except that its particles have been ground finer.
- Type IIIA portland cement – It is an air-entraining, high early strength cement.
- Type IV portland cement – It has a low heat of hydration and develops strength at a slower rate than other cement types, making it ideal for use in dams and other massive concrete structures where there is little chance for heat to escape.
- Type V portland cement – It is used only in concrete structures which will be exposed to severe sulphate action, principally where concrete is exposed to soil and groundwater with a high sulphate content.
Fig 1 Types of portland cements
Cements can also be categorized as follows.
Ordinary portland cement (OPC)
These are available in many grades, namely 33 grade, 43 grade, 53 grade etc. If 28 day strength is not less than 33 N/sq mm then it is called cement of ?33 grade?. If 28 day strength is not less than 43 N/ sq mm then it is called cement of ?43 grade?. Use of higher grade cement offers many advantages and makes stronger concrete. Although higher grade cement are little costlier than the low grade cement, they offer 10 % to 20 % saving in the cement consumption and they also offer many other hidden advantages. One of the most important advantages is the faster rate of development of the strength.
The cement is used for the ordinary works. This type of cement is used in constructions where there is no exposure to sulphates in the soil or groundwater. The lime saturation factor (LSF) for this cement [(CaO-0.7 SO3}/ (2.8 SiO2 + 1.2 Al2O3 + 0.65 Fe2O3)] is limited between 0.66-1.02. Here each oxide denotes the percentage of the oxide in cement composition. LSF is normally limited to assure that the lime in the raw materials used during the cement production is not high, so as to cause the presence of free lime after the occurrence of chemical equilibrium. While too low a LSF makes the burning in the kiln difficult and the proportion of C3S (tri calcium silicate) in the clinker too low, free lime causes the cement to be unsound. In this cement, the percentage of Al2O3/Fe2O3 is not less than 0.66, insoluble residue not more than 1.5 % and percentage of SO3 is limited to 2.5 % when C3A is 7 % maximum and not more than 3 % when C3A is more than 7 %. The loss of ignition (LOI) is 4 % maximum and percentage of MgO is 5 % maximum. The fineness of OPC is not less than 2250 sq cm per gram.
White portland cement
It is made from raw materials containing very little iron oxide (less than 0.3 % in clinker) and magnesium oxide (which give the OPC its gray colour). China clay (white kaolin) is generally used in its production, together with chalk or limestone, free from specified impurities. Its production needs higher firing temperatures because of the absence of iron element which works as a catalyst in the formation process of the clinker. In some cases, cryolite (sodium-aluminum fluoride) is added as a catalyst. The compounds in this cement are similar to those in OPC, but C4AF (Tetra calcium alumino ferrite) percentage is very low. Contamination of the cement with iron during grinding of clinker is also to be avoided. For this reason, instead of the usual ball mill, the expensive nickel and molybdenum alloy balls are used in a stone or ceramic lined mill. The cost of grinding is thus higher, and this, coupled with the more expensive raw materials, makes white cement rather expensive. It has a slightly lower specific gravity (around 3.05 to 3.1) than OPC. The strength is usually somewhat lower than that of OPC.
Blast furnace slag portland cement
This type of cement consists of an intimate mixture of portland cement and ground granulated blast furnace slag which is generated during the production of hot metal in the blast furnace. Blast furnace slag has a chemical composition of around 42 % CaO, 30 % SiO2, 19 % Al2O3, 5 % MgO, and 1 % alkalis, that is, the same oxides which make up portland cement but not in the same proportions. The maximum percentage of slag used in this type of cement is limited by different standards and may vary in the range of 25 % to 65 %. The early strength of this cement is lower than that of OPC, but their strength is equal at late ages (about 2 months). The requirements for fineness and setting time and soundness are similar for those of OPC (although actually its fineness is higher than that of ordinary cement). The workability is higher than that of OPC. Heat of hydration of this cement is lower than that of OPC. Its sulphate resistance is high. It is used in mass concreting. It is possible to use this cement in constructions which are subjected to sea water (marine constructions). It is not to be used in cold weather concreting.
Rapid hardening cement
As the name indicates, it develops the strength rapidly than OPC. This cement develops at the age of three days, the same strength as that expected from the OPC in seven days. With this cement the initial strength is higher, but they equalize at 2-3 months. Setting time for this type is similar for that of OPC. The rapid rate of development of the strength is due to higher C3S and lower C2S (di calcium silicate) and due to finer grinding of the cement clinker (the minimum fineness is 3250 sq cm per gram. Rate of heat evolution is higher than OPC due to the increase in C3S and C3A, and due to its higher fineness. Chemical composition and soundness requirements are similar to those of OPC. The uses of this cement is indicated where a rapid strength development is desired (to develop high early strength, i.e. its 3 days strength equal that of 7 days OPC, for example (i) when formwork is to be removed for re-use, (ii) where sufficient strength for further construction is wanted as quickly as practicable, such as concrete blocks production, sidewalks and the places which cannot be closed for a long time, and repair works needed to construct quickly, (iii) for construction at low temperatures, to prevent the frost damage of the capillary water, (iv) this type of cement does not use at mass concrete constructions. Special types of rapid hardening portland cement include the following.
- Ultra-high early strength cement in which the rapid strength development is achieved by grinding the cement to a very high fineness (7000 sq cm/gram to 9000 sq cm/gram). Because of this, the gypsum content is to be higher (4 % expressed as SO3). Because of its high fineness, it has a low bulk density. High fineness leads to rapid hydration, and therefore to a high rate of heat generation at early ages and to a rapid strength development (7 days strength of rapid hardening portland cement can be reached at 24 hours when using this type of cement). There is little gain in strength beyond 28 days. It is used in structures where early pre-stressing or putting in service is of importance. This type of cement contains no integral admixtures.
- Extra rapid hardening portland cement which is produced by grinding CaCl2 with rapid hardening portland cement. The percentage of CaCl2 is not to be more than 2 % of the rapid hardening portland cement. By using CaCl2 the properties achieved are (i) the rate of setting and hardening increase (the mixture is preferred to be casted within 20 minutes), (ii) the rate of heat evolution increase in comparison with rapid hardening portland cement, so it is more convenient to be used in cold weather, and (iii) the early strength is higher than for rapid hardening portland cement, but their strength is equal at 90 days. Since CaCl2 is a material that takes the moisture from the atmosphere, care is needed to store this cement at dry place and for a storage period not more than one month so that it does not deteriorate.
Sulphate resisting cement
OPC is susceptible to the sulphate attack. Sulphate reacts with the free calcium hydroxide to form calcium sulphate and hydrate the C3A to form calcium-sulpho-aluminates., the volume of which is approximately 227 % of the volume of the original aluminates. Sulphate resisting cement contains lower percentage of C3A and C4AF which are considered as the most affected compounds by sulphates. This cement has higher percentage of silicates in comparison with OPC. For this type of cement, C2S represents a high proportion of the silicates. Some standards limit the maximum C3A content to 3.5 % minimum and fineness to 2500 sq cm per gram. The cement has low early strength. Its resulted heat of hydration is little higher than that resulted from low heat cement. Its cost is higher than OPC because of the special requirements of material composition, including addition of iron powder to the raw materials.
For the hardened cement, the effects of sulphates are on two types namely (i) hydrated calcium aluminates in their semi-stable hexagonal form (before its transformation to the stable state ? C3AH6 as cubical crystal form ? which have high sulphate resistance) react with sulphates (present in fine aggregate, or soil and ground water), producing hydrated calcium sulfo aluminate, leading to increase in the volume of the reacted materials by about 227 % causing gradual cracking, and (ii) exchange between Ca(OH)2 and sulphates resulting gypsum, and leading to increase in the volume of the reacted materials by around 124 %. The cure of sulphates effect ? is by using sulphate resisting cement. The resultant of reaction C4AF with sulphates is calcium sulfo aluminate and calcium sulfo ferrite, leading to expansion. But an initial layer generally forms which surround the free C3A leading to reduce its affect by sulphates, so C4AF is more resistant to sulphates effect than C3A. Their expansion results in cracks. To remedy this, the use of the cement with the low C3A is recommended. Such cement with low C3A content is known as the sulphate resisting cement. This cement is used under marine conditions, in foundations in soil infested with sulphates, and in concrete used for the foundations for pipes etc.
Quick setting cement
As the name indicates this type of cement sets quickly. This property is brought out by reducing the gypsum content at the time of the clinker grinding. This cement is required to mix, to place and to compact very easily. This cement is generally used for the underwater construction.
Super sulphated cement
Super sulphated cement is produced by grinding together a mixture of 80 % to 85 % of the granulated slag, 10 % to 15 % of the hard burnt gypsum, and 5 % of the portland cement clinker. This cement is high sulphate resistant. Because of this property, it is used for the foundations where chemically aggressive conditions exist.
Low heat cement
Hydration of the cement is exothermic process which liberates high quantity of the heat. This causes the formation of the cracks. A low heat evolution is brought by reducing the C3A and C3S which are the compounds evolving the greater heat of hydration and increasing C2S. Rate of evolution of heat of hydration is therefore less and evolution of heat extends over a large period. The limits for the heat of hydration of this cement are 60 calories per gram at 7 days age and 70 calories per gram at 28 days age. It has lower early strength (half the strength at 7 days age and two third the strength at 28 days age) compared with OPC. Its fineness is not less than 3200 sq cm /gram. Hence in case of low heat cements, the rate of the development of the strength is very low. It is used for the mass construction works. The rise of temperature in mass concrete due to progression in heat of hydration can cause serious cracks. So it is important to limit the rate of heat evolution in this type of construction, by using the low heat cement.
Portland pozzolana cement
Portland pozzolana cement is produced by inter-grinding OPC clinker with 15 % to 40 % of the pozzolana material. Pozzolana is defined as a siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but in finely divided form and in the presence of moisture, chemically reacts with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. Natural pozzolanic materials are volcanic ash while the industrial pozzolanic materials are fired clay, rice husk ash etc. It is essential that pozzolana be in finely divided state as it is only then that silica can combine with calcium hydroxide (produced by the hydrating portland cement) in the presence of water to form stable calcium silicates which have cementitious properties. They are similar to those of portland blast furnace cement.
Portland pozzolana cement produces low heat of hydration and offer greater resistance to the attack of the aggressive water than OPC. It is used in the mass construction works and in marine environments as well as in hydraulic works.
Air entraining cement
This cement is manufactured by mixing small amount of the air entraining agent with the OPC clinker at the time of grinding. At the time of mixing this cement in the concrete, it produces air bubbles in the body of the concrete which modifies the properties of the plastic concrete with respect to the workability, segregation and bleeding.
Coloured cement is prepared by adding special types of pigments to the portland cement. The pigments added to the white cement (2 % to10% of the cement) when needed to obtain light colors, while it added to ordinary portland cement when needed to obtain dark colors. The cement and the pigment are grinded together.
The 28-day compressive strength is required to be not less than 90 % of the strength of a pigment-free control mix, and the water demand is required to be not more than 110 % of the control mix. It is required that pigments are insoluble and not affected by light. They are to be chemically inert and do not contain gypsum which is harmful to the concrete.
This cement is produced by mixing certain materials (stearic acid, oleic acid etc. by 0.1 % to 0.4%) with OPC before grinding, to form water repellent layer around the cement particles. The water repellent film formed around each grain of the cement reduces the deterioration of the cement during the long storage, transportation and unfavourable environment. Water repellent film formed also improves the workability. The water repellent film is removed during the mixing process with water.
Concrete formed using the OPC shrinks during the setting due to the loss of the water. In grouting work, if concrete shrinks the purpose for which the grout is used gets to some extend defeated. A slight expansion with time is advantageous for the grouting work. This type of the cement which does not suffer an overall change in the volume on drying is known as the expansive cement. This cement is produced by using an expansive agent and a stabilizer.
Expansive cements are hydraulic cements which expand slightly during the early hardening period after setting. Expansive cement contains portland cement, anhydrous tetra-calcium-tri-alumino-sulphate, calcium sulphate, and uncombined calcium oxide (lime). Expansive cement is used to make shrinkage-compensating concrete which is used (i) to compensate for volume decrease due to drying shrinkage, (ii) to induce tensile stress in reinforcement, and (iii) to stabilize long term dimensions of post tensioned concrete structures. One of the major advantages of using expansive cement is in the control and reduction of drying shrinkage cracks. In recent years, shrinkage compensating concrete has been of particular interest in bridge deck construction, where crack development is to be minimized.
Anti-bacterial portland cement
It is a portland cement inter-ground with an anti-bacterial agent which prevents microbiological fermentation. This bacterial action is encountered in concrete floors of food processing plants where the leaching out of cement by acids is followed by fermentation caused by bacteria in the presence of moisture.
Blended hydraulic cements
Blended hydraulic cements are produced by intimately blending two or more types of cementitious materials. Primary blending materials are portland cement, ground granulated blast-furnace slag, fly ash, natural pozzolans, and silica fume. These cements are commonly used in the same manner as portland cements. Blended hydraulic cements are of following types. Type IS-portland blast-furnace slag cement, Type IP and Type P-portland pozzolana cement, Type S-slag cement, Type I (PM)-pozzolana modified portland cement, and Type I (SM)-slag modified portland cement. In Type IS the blast furnace slag content is between 25 % and 70 %. The pozzolana content of Types IP and P is between 15 % and 40 % of the blended cement. Type I (PM) contains less than 15 % pozzolana. Type S contains at least 70 % slag. Type I (SM) contains less than 25 % slag. These blended cements are also usually designated as air-entraining, moderate sulphate resistant, or with moderate or low heat of hydration. The blended hydraulic cements also include the following (i) type GU – blended hydraulic cement for general construction, (ii) type HE – high early strength cement, (iii) type MS – moderate sulphate resistant cement, (iv) type HS – high sulphate resistant cement, (v) type MH – moderate heat of hydration cement, and (vi) type LH – low heat of hydration cement. These cements can also be designated for low reactivity with alkali reactive aggregates. There are no restrictions as to the composition of these cements. The producer can optimize ingredients, such as pozzolana and slags, to optimize for particular concrete properties. The most common blended cements available are types IP and IS.