OPC 42.5 is characterized by the fact that it attains a minimum strength of 42.5 MPa 28 days after pouring. It is not a chemical difference that separates OPC 42.5 from other popular grades of OPC, such as OPC 52.5, which attains a minimum strength of 52.5 MPa after 28 days, but a size difference. OPC 42.5 is a more coarsely ground product than OPC 52.5, and as a result possesses less strength.

Manufacturing OPC 42.5 N

OPC 42.5 N is manufactured from lime, silica, aluminum and iron. During the manufacturing of OPC these raw materials must be combined in precise quantities. If any of the ingredients is out of proportion, the entire mix is useless. To facilitate correct mix proportions, the raw ingredients are released a little at a time onto a conveyor belt from computer controlled silos, and once mixed together, subjected to extreme analysis.

Once the raw mix has been blended and analyzed to the manufacturer’s satisfaction it is sent into a kiln. Here it is heated to between1400 – 1450 degrees Celsius, which causes melting to take place. As the materials melt, calcium silicates are formed from the lime and silica. Aluminum and iron act as fluxing agents, lowering the temperature at which silicate formation can take place. Without the presence of iron and aluminum, the kiln would have to be heated much higher than 1450 degrees Celsius.

Ultimately, the end result of this firing process are a plethora of hard nodules known as ‘clinker’. Too large to be useful in itself, this clinker is then ground down in a rotating chamber until it attains the correct size, which in the case of OPC 42.5 N is around 40 microns per particle.

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Ordinary Portland cement 42.5 is the standard grade of cement used in the majority of ordinary cement applications, such as pavements, residential buildings, and pipes. OPC 42.5 comes in two sub grades, OPC 42.5 R, and OPC 42.5 N.

OPC 42. 5 R develops high strength early in the curing phase, whereas OPC 42.5 N develops high strength at the normal rate. OPC 42.5 R is suitable for applications where high strength is needed relatively quickly after pouring and setting. Both types of OPC 42.5 will attain a minimum compressive strength of 42.5 MPa on the 28th day after pouring, however OPC 42.5 R will gain strength more quickly than its N rated counterpart.

The Secret of OPC 42.5 R Cement

OPC is a blend of lime, silica, iron and aluminum. These elements must be mined, extracted and crushed so that they can be mixed in the correct proportions. Though made from rough and jagged rocks, precision is the key to making OPC.

Source materials are deposited from separate silos onto a conveyor belt by computer controlled feeds, and the batch, known as ‘raw mix’, is sent to a blending chamber, where it is mixed and analyzed to ensure that the correct amounts of each material are present. This can be either a wet or a dry process, and plants differ greatly from one another as to how they manage this stage of manufacturing.

The R rating of OPC 42.5 R is conferred by blending the base ingredients in certain precise portions. An adjustment of just a few percent of any raw material can lead to large variances in strength and other qualities of the resulting product. This is why chemical analysis at the raw mix stage is carried out so very rigorously.

The bulk of OPC is made up of calcium and silicone, which together form 66% of the total mass of the product. Iron and aluminum act as flux materials, lowering formation temperatures for the calcium silicates which make up OPC.

When analysis is complete, the raw mix is sent to a kiln where it is heated gradually along its length to between 1400 and 1450 degrees Celsius at its hottest point. As the raw mix travels through it, it begins to melt and calcium silicates begin to form. The end result of this firing are small nodules of calcium silicates called ‘clinker’.

Clinker is ground down in a rotating chamber until it becomes a fine powder known as OPC. The size of the final particles determine the grade of OPC. OPC 42.5 is a relatively coarse type of OPC, with the grains of the final product being around 40 microns in diameter.

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When purchasing cement, one quickly finds that the benchmarks by which reputable cements are tested are ASTM Standards. There are over two hundred ASTM standard tests relating to cement and concrete, and in addition to these there are also standard specifications which outline the parameters within which cement must fall in order to deemed to be of good quality.
To the inexperienced eye, these standards may appear to be nothing but a jumble of letters and numbers which make little sense, and it is tempting to ignore these altogether focusing instead on more ostensibly important numbers, such as price. However one should have at least a cursory knowledge of applicable ASTM standards, as understanding what they mean can be imperative when one is committing to purchasing materials which can quite literally make or break a project.
Not all cements are made equal, and not all cements are suitable for all applications. Using the wrong cement can result in disaster, but fortunately ASTM standards ensure that there is a way to verify that the cement you are purchasing is of good quality and equal to the task it must perform.

What is the ASTM?

The American Society for Testing and Materials is an international organization which has been in operation for over a century, and which specializes in developing standards for various commodities and materials. Originally formed in the USA to address problems in the railway industry, where problems with cracking and breaking tracks lead industry professionals to realize that there needed to be standards for steel used in railway tracks, the ASTM soon began to formulate standards for other materials and commodities which suffered from related issues. Today ASTM standards are in place for a plethora of products ranging from adhesives, to cement, to leather, to sports and sport safety equipment, and many many more. In fact, ASTM standards have now been developed for over 60 categories of commonly traded goods.
ASTM standards are developed on a voluntary basis by contributors from around the globe. These contributors are academics, scientists, producers, consumers, and those with interest or expertise in certain fields.

What Is An ASTM Standard?

An ASTM standard is essentially nothing more than a document which outlines methods for testing, and the allowable results which if obtained, will mean that a product meets ASTM standards. Although ASTM has no authority in itself, because of the reliability and global efficacy of ASTM standards, many governing bodies in countries around the world refer to ASTM standards when setting guidelines for materials and products imported into or produced within their own countries.

ASTM Cement Standards

ASTM C 150 – 07 Standard Specification for Portland Cement
This is the ASTM standard which pertains to Portland cement, and outlines requirements for the eight basic types of Portland cement, which possess the following properties.
Type I – Basic general use cement. Can be used in low demand applications which do not come in contact with the ground or ground water, both of which often contain sulfates which deform Type I cement.
Type IA – Air-entraining version of Type I cement.
Type II – General use with added sulfate resistance and medium heat of hydration. This is the most popularly used type of Portland cement, as it is suitable for most construction applications, and can be used in projects which come into contact with the ground and ground water, such as pipes.
Type IIA – Air-entraining version of Type II cement.
Type III – High early strength.
Type IIIA – Air-entraining version of Type III cement.
Type IV – Low heat of hydration.
Type V – High sulfate resistance.

ASTM C 595 – 08 Standard Specification for Blended Hydraulic Cements

This is a general standard for blended hydraulic cements covering the gamut of blended hydraulic cements and including those made with slag, pozzolan, or a blend of the two combined with Portland cement, Portland cement clinker, and also slag with lime.

ASTM C1157 – 03 Standard Performance Specification for Hydraulic Cement

The most recent of the ASTM standards devised specially for hydraulic cements, this is perhaps the most useful type of ASTM standard as it is based entirely on performance testing, and not on chemical composition. Cements are classified under ASTM C1157 – 03 by type, and are based on requirements associated with the following desired properties:
General Use
High Early Strengthened
Sulfate Resistance
Heat of Hydration
Alkali Reactive Aggregate Reaction (Optional)

How Do I Obtain ASTM Standards?

ASTM standards can be purchased directly from ASTM. The bulk of ASTM’s income comes from selling standard documentation, so the information is not usually available freely. ASTM provides documentation on standard specifications, testing methods, and testing conditions so that producers and users seeking to verify the properties of a product can do so in a reliable and replicable fashion.
More information on ASTM standards and testing practices can be found at ASTM.org.

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Aluminous cement is also known as ‘calcium aluminate cement’, ‘high alumina cement’, and by its original French name, ‘ciment fondu’. Unlike traditional cements which are comprised largely of calcium silicates, aluminous cements are made with calcium aluminates. Aluminous cements were invented in France in the early 20th century, and patented in 1908. Originally, aluminous cements were invented as alternatives to existing types of cement which did not have good sulfate resistance, and would often expand and distort when they came into contact with the ground and ground water.

However soon after their invention it was discovered that over and above being resistant to sulfates, aluminous cements also develop strength very quickly, have a high chemical resistance, and some blends maintain integrity at high temperatures as well. Of course, the trade off for these high performance specifications is price, and aluminous cements are a great deal more expensive than ordinary Portland cement.

Uses of Aluminous Cement

Aluminous cement is only used in a very small number of applications, due to its special qualities and its high cost. The sort of construction projects which require the use of aluminous cement include those which will be exposed to high chemical levels, or high temperatures, or applications which require fast high strength development. Aluminous cement is also therefore quite often used as a component of various specialist construction concretes which are used to construct buildings and other structures which will be exposed to concentrated chemicals or extreme temperatures.

Making Aluminous Cements

Aluminous cements are made in a process similar to that by which Portland Cement is made, however instead of using limestone and silica as the primary raw materials, limestone and bauxite are commonly used instead. Refined alumina is also used in the case of white aluminous cement or refractory (high temperature) cements.

At the beginning of the manufacture of aluminous cement, the raw materials are transported from various silos by use of a conveyor belt, which carries them to a raw mill. The purpose of the raw mill is to grind down the chunks of raw material into small pieces which are all of a similar size. It is important that the materials be of similar size so that when they melt in the kiln, they do so evenly.

Once they have been thoroughly ground up in the raw mill, the raw materials are sent on to the blender, where they are mixed together. The blended raw mix is analyzed carefully at this stage to ensure that the correct portions of materials are present. Perhaps the most important aspect of any form of cement production is controlling the relative ratio of raw materials in the raw mix. If the raw materials are present in the wrong proportions when the raw mix enters the kiln, the resulting product will be faulty.

Once blended, the raw mix is then sent through into the kiln where it is melted and where calcium aluminates form in the extreme temperatures. The result of the firing process are small pebble looking nodules of material which are called ‘clinker’.

In order to transform the clinker into cement, it must be ground thoroughly until it is a very fine powder. This is carried out inside a rotating cylinder where the clinker is mixed with hard balls, and gradually broken down to a very fine powder. This powder is aluminous cement, and is ready for use.

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As lower cost alternatives to Portland Cement, blended cements are increasingly being used in a great many construction applications. Blended cements are made by inter grinding Portland cement with another material. Fly ash, blast furnace slag, and condensed silica fume are all common components of blended cements, and each of them results in a different type of blended cement with an unique set of properties.

Blended cements are not a new idea. As far back as Roman times, pozzolanic materials such as volcanic ash were being added to cements in order to create long lasting strong concretes. To see the evidence of how long wearing these cements were, one merely needs to visit Rome and see some ancient cements still standing their ground against time and the elements.

Modern blended cements have been slow to gain popularity for a variety of reasons. For a long time there were no real standards for blended cements, so it was hard to source a reliable product. In many cases the additives which were blended with Portland cement to create blended cements actually weakened the end product and made it less reliable due to unforeseen alkali aggregate reactions.

More recently however, blended cements have come into their own as increasing care has been taken to develop high quality, high performance blended cements which far outperform traditional cements at a far lower price.

Why Blended Cements?

Portland cement is the most widely used cement on the globe, and there are several types of Portland cement which can successfully be used in almost any construction application, so why use blended cements at all?

Here are a few reasons why blended cements are being both developed and used more and more frequently.

Blended cements:

• Are cheaper than Portland cement.
• Are more long lasting and durable than Portland cement.
• Have higher strength tolerances than Portland cement.
• Have less drying shrinkage than Portland cement.
• Can, unlike Portland cement, be made to the buyer’s specifications.
• Are more environmentally friendly than Portland cement, as they are comprised of recycled materials.

Blended cements have been developed to both lower the cost of cement, and also to combat problems which are traditionally encountered with OPC (Ordinary Portland Cement) such as susceptibility to sulfates, cracking, steel corrosion, and susceptibility to freezing failure and carbonation.

Warning: As will be discussed in the section on blended cement standards, there are a myriad of ways in which cements are classified and standardized. There are relatively few blended cements available on the market which meet the specifications of the most recent and reliable standard, ASTM C 1157, but it is recommended that purchasers seek out cements which do meet this standard, as those which do not may be entirely unsatisfactory.

Blended Cement Standards

There are a plethora of cement standards around today, and most of them are wildly confusing to those who are not experts in the field. With three or four different standards, and several subtypes within each standard, it is easy to become quickly confused.

However, finding good quality blended cement is not as difficult as it might seem at first, due to the inception of a new standard specifically designed for blended cements. This is the ASTM C 1157 Standard, and it is recommended that buyers wishing to purchase blended cements check for compliance with this standard first before going any further with their investigations.

Unlike previous standards, which were based on the composition of the cement, ASTM C 1157 is based on performance, making it more reliable and easy to understand than previous standards. There are only two major subtypes within this standard, blended cements which meet the ASTM C 1157 standard, and blended cements which meet the standard and also will not react with alkali-reactive aggregates. These types of cement will be marked with [R].

Various types of cement under the ASTM C 1157 standard are defined by the compressive strength which they attain at the standard one, three, seven and twenty eight day marks.

Fly Ash Pozzolanic Blended Cements

Blended cements made with fly ash are pozzolanic cements. Pozzolanic materials are silica or alumina based materials which are not cements on their own, but which react with calcium hydroxide at room temperature to form cementious compounds. A relatively popular component of blended cements, fly ash in the form of coal ash is often used as an additive in blended cement production, especially in China and the United States.

Blast Furnace Slag Blended Cements

Blast furnace slag is a material which is high in silicates. When silicates are combined with Portland cement they improve the workability of concrete, and create a product which has low permeability, and does not allow alkali aggregate reactions to take place.

Condensed Silica Fume Blended Cements

A waste product from silicone production, condensed silica fume is perhaps the best additive of the three substances commonly used to create blended cements. Silica fume blended cements are high performance cements which have very little permeability and very high strength. Because silica fume is so effective at increasing the performance of cement, small amounts of silica fume may also be added to other types of blended cement as a secondary additive to develop the overall quality of the cement.

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Old Portland cement 52.5 is the highest grade commercial Portland cement widely available on the market. With a minimum compressive strength of 52.5 MPa (or 525 kg/cm2) attained on the 28th day after pouring, OPC 52.5 is suitable for large load bearing structures such as high rise buildings, bridges, and other such projects.

OPC Grades

OPC grades are very straightforward and easy to follow. The numerical rating following the prefix ‘OPC’ is the minimum compressive strength which cement must attain on the 28th day after pouring in order to be classified as being a particular grade. It is worth noting that the 52.5 MPa rating is simply a minimum requirement, and most OPC 52.5 MPa cements exceed that strength.

The two most commonly available Ordinary Portland cement grades sold today are OPC 42.5, and OPC 52.5. There is no difference in chemical composition between the two cement grades, however OPC 52.5 is more finely ground than OPC 42.5, which results in different properties when mixed and poured.

Making OPC 52.5

OPC 52.5 is made according to normal Portland cement standards, and is composed of a minimum of 66% calcium silicates, along with a smaller amount of iron and aluminum The raw material from which OPC is sources usually includes limestone and clay (over 85%), as well as silica sand, iron ore, and other sources of the primary components of Portland Cement. Raw materials normally vary depending on the region where rock is mined, as some areas are rich in the natural resources which are needed for Portland cement production, and others are not, in which case raw materials must be brought in to supplement local supply.

The production of Portland cement begins with the preparation of the raw mix. Because it is so important that each component be present in precise quantities, it is quite common for individual silos containing a single raw material to be positioned over a conveyor belt and to be computer controlled to release the right amount of raw material onto the belt to be carried into the raw mill, where the raw materials are ground together. Cement making is a very precise art, requiring workers to have skill, expertise and dedication to quality control. Even small variances in composition can ruin an entire batch of Portland cement.

The ground up raw materials are referred to as ‘raw mix’, and once created, this raw mix is taken from the raw mill to the blender, where it is thoroughly mixed to ensure that it is a homogeneous batch with no irregularities in it, and chemically analyzed to ensure that the correct portions of raw material are present. Raw mix may either be blended dry, or it may sometimes have water added to it and be blended as a slurry.

Once the blend has been correctly portioned and analyzed it is sent through to the kiln, where it is fired at temperatures exceeding 1400 degrees Celsius. The firing process causes the aluminum and the iron to melt into their liquid states, in which they act as fluxing agents, lowering the temperature at which calcium and silica form calcium silicates which are the main component of Portland cement.

The end product of the firing process is a plethora of small nodules called ‘clinker’. Once produced, clinker can be cooled and taken to storage, or it may be quickly cooled and sent directly through to the final stage of Portland cement production, grinding, where the nodules are put into a revolving grinding chamber and ground to a very fine powder. An additive known as gypsum is also added at this stage in order to control the speed at which the cement will set once poured.

When the grinding process is finished, the result is high quality OPC 52.5, ready to be bagged and sold as an additive in concretes, mortars, and other construction materials.

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Both Portland cement and Blended cements are hydraulic cements, which means that they require the addition of water in order to form the bonds which make them set hard and strong. Once formed, the bonds are not soluble in water, which means that hydraulic cements can be used in situations where they will be exposed to water (such as rain water, drainage water, or sea water) without fear of them breaking down and washing away.

There are six main types of hydraulic cements, and both Portland and Blended cements can be categorized into these types.

Type GU – General Use. Suitable for general applications, except those where ground water may be involved, or where moderate to low heat of hydration is required. Heat of hydration is a term referring to the amount of heat that is given out as bonds are formed within the cement. The reactions which form the bonds in cement are exothermic, which means they give out heat. In some projects, such as large scale dams, it is advantageous to use a cement that has a low heat of hydration, as cements which release a great deal of heat whilst setting and hardening can cause major problems.

Type HE – High Early Strength. High early strength cements set and harden quickly, and can be used as load bearing structures much more quickly than their counterparts. These types of cement are ideal for projects which must be finished quickly.

Type MS – Moderate Sulphate Resistance. Moderate sulphate resistance makes cement suitable for use in applications where the concrete will be exposed to normal levels of sulphates which are present in ground water.

Type HS – High Sulphate Resistance. High sulphate resistance is useful for projects where the cement will be exposed to unusually high levels of sulphates.

Type MH – Moderate Heat of Hydration. Moderate heat of hydration cements are suitable for large applications, such as small bridges, or retaining walls.

Type LH – Low Heat of Hydration. Low heat of hydration cements are designed for relatively rare major construction projects.

Blended Cements

Blended cements are produced by blending two types of materials together to produce a new type of cement. Portland cement, natural pozzolanic compounds, blast furnace slag, silica fume, calcined clay, and fly ash are all common components of blended cements. Blended cements have been developed in response to various needs and market pressures, including the need to find means of recycling materials left over from other processes, such as the production of iron, which leaves blast furnace slag. Blended cements also have certain properties that make them more useful in some cases than ordinary Portland cement, and they quite often reduce the cost of cement as well.

There are five main types of blended cement:
Type IS – Portland blast furnace slag cement, where Portland cement has been ground with blast furnace slag. Blast furnace slag content is usually between twenty five and seventy percent of Type IS cement by mass.

Type IP or P – Portland pozzolan cement, where Portland cement has been ground with pozzolanic material. Pozzolan content is usually between fifteen and fourty percent of total mass.

Type I (PM) – Pozzolan modified Portland cement.

Type S – Slag cement.

Because there are no restrictions on the makeup of blended cements, they can be designed with specific properties in mind to fit specific construction applications.

Expansive Cements

Expansive cements are a type of hydraulic cement which expand as they begin to harden. Expansive cements are designated with an ASTM label of E-1, and come in three categories, K, M, and S, though in certain regions, such as the United States, only K may be available. E-1(K) cement contains lime, calcium sulphate, anhydrous tetracalcium trialuminosulfate, and Portland cement as its main ingredients.

Expansive cements are exceedingly useful in construction applications where structural cracks must not develop. Normal cement types shrink during drying, which can cause problems in some construction applications. Expansive cements compensate for this shrinkage by expanding slightly. There are three main uses for expansive cements the first one being crack resistance, the second being induction of tensile stress in reinforcement applications, and the third being as a means to ensure that dimensions of post tensioned concrete structures remain static.

Portland Cement
There are many different types of cement available on the market today, but by far and away the most popular type of cement is Portland Cement, which comes in a range of specifications to suit various conditions. The basic ingredients of Portland cement are calcium and silicone, along with iron, aluminum, and very small amounts of other substances. Gypsum is also a component of Portland cement, added at the very end of production in order to control the rate at which the cement will set. Gypsum (the lay term for calcium sulfate) has been used since Egyptian times to regulate setting of cement like substances.

ASTM Standards For Portland Cement
As has been mentioned, there are various types of Portland cement which posses different properties which make them suitable for different applications. The American Society for Testing and Materials has classified eight different types of Portland Cement.

Type I Portland Cement
Type I Portland cement is general purpose cement with no particularly special attributes. It is suitable for use in general applications, however it should not be used in applications where the cement will be in contact with the ground, or with ground water, as these substances tend to contain sulphates which can attack the cement and make it expand and distort out of shape.

Type II Portland Cement
Type II Portland cement has some sulphate resistance, and releases less heat during setting and hardening than Type I Portland cement. Type II cement is therefore suitable for construction projects which involve exposure of the cement to ground water, such as pavements and drainage systems. Type II cement can also be used for large scale work such as retaining walls and other such masses.

Type III Portland Cement
Type III Portland cement develops strength much quicker than other types of Portland cement. The trade off is that its final strength tends to be slightly less than other forms of Portland cement which take longer to set and harden. Type III Portland cement is very useful for projects which must be completed quickly.

Type A Portland Cement
Type A Portland cement refers to Type IA, Type IIA, and Type IIIA Portland cements. These cements are simply cement types of the aforementioned categories which have had special additives added to them which cause small pockets of air to be trapped inside the cement in a uniform fashion. This provides greater freeze/thaw resistance to the cement, preventing it from cracking in conditions which experience large variances in temperature.

Type IV Portland Cement
Type IV Portland cement is a relatively rare type of Portland cement which is specially designed to release heat slowly during setting and hardening. Designed for use on very large scale projects, such as dams, Type IV Portland cement can generally only be obtained by special order in very large quantities.

Type V Portland Cement
Type V Portland cement was designed to be used in high sulphate conditions, where Type I and Type II Portland cement cannot successfully be used. It is not typically necessary to use Type V Portland cement, except in extreme conditions. Most applications requiring contact with water are adequately satisfied with Type II Portland cement.

White Portland Cement
Whilst not truly a separate type of cement, white Portland cement is a unique kind of Portland cement that can be produced to meet the specifications of most types of Portland cement. It is different from ordinary Portland cement in that it has a white coloration, instead of a dull grey one. This effect is achieved by limiting the amount of iron and magnesium oxide in the mix during production. White cement can also be colored to achieve various effects, and is quite popular as an alternative to the traditional drab grey coloration of ordinary Portland cement, especially in projects which have a strong aesthetic component.

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OPC 42.5 is a general use grade of ordinary Portland cement. Replacing the once popular OPC 33 in general cement work whilst maintaining its duties as a medium strength cement, OPC 42.5 is commonly used for both construction and pre-cast work. OPC 42.5 can therefore be found in a multitude of locations, from residential buildings to underground pipes.

OPC Grades

There are several different OPC grades, one of which is OPC 42.5. Ordinary Portland cement grades are determined by the compressive strength which is obtained on the 28th day after pouring. OPC 42.5 must have a minimum compressive strength of 42.5 MPa, or 425 kg/cm2 on the 28th day after it is poured in order to qualify as being OPC 42.5. It should be noted that the 42.5 MPa minimum requirement is just that, a minimum requirement, and that most cements exceed this strength.

The two most commonly utilized grades of OPC on the market today are OPC 42.5 and OPC 52.5. There is no chemical difference in composition between these two grades of cement, however OPC 42.5 powder is more coarse than OPC 52.5 powder.

Manufacturing OPC 42.5

OPC 42.5 is made in the same way that all Portland cement is made. First a raw mix of lime, silica, iron, and aluminum is prepared from rock sources which have been crushed and pounded to the size of small pebbles.

Rock is usually mined locally, and any components that are lacking for the making of Portland cement are then added afterwards. Though cement is essentially powdered rock, the various materials must be present in precise quantities in order for high quality Portland cement to be produced. In order to ensure that the correct mix is obtained at the beginning of the cement making process, controlled amounts of individual raw materials are fed from silos onto a conveyor belt, which takes them to the raw mill.

Once in the raw mill, the raw materials are ground up to create what is called the ‘raw mix’. Once the raw mix has been formulated, one of two manufacturing processes can follow. The raw mix is either dried by hot air if a dry process is to be used, or mixed with water to make a raw mix slurry if a wet process is to be used.

Once made, the raw mix is then blended in a blending chamber, and chemically analyzed to ensure that the precise ratios of raw materials are present. It is important that the raw mix is mixed thoroughly to form a completely homogeneous mixture, and that all elements in the mix are as close to being the same size as is possible. Calcium and silicone make up the bulk of the mix because they are the ingredients which later form the calcium silicates that comprises a 66% of Portland cement. Smaller amounts of iron and aluminum are present to act as flux during the firing process, lowering the temperature at which calcium silicates will form.

After blending, the raw mix travels to the kin, a large rotating cylinder which is heated gradually over the length of the kiln, with the temperature rising to between 1400 and 1450 degrees Celsius at its hottest point. As the raw mix grows hotter, aluminum and iron melt, and calcium silicates begin to form. By the end of the firing process, small nodules of an entirely new compound are formed. These nodules are called ‘clinker’.

The clinker is cooled after it leaves the kiln, and then ground with gypsum, an additive which controls the speed at which the cement will set. The grinding process continues until the clinker nodules are a very fine powder. If the clinker is ground relatively coarsely, (relatively coarsely being in the neighborhood of around 40 microns in diameter) the result of this grinding process is OPC 42.5

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Just as there are various types of Portland cement available for different applications there are also various grades of cement existing within ordinary Portland cement. Ordinary Portland cement can be classified as being either 33 grade, 43 grade, or 53 grade, though sometimes the latter grade types are referred to as 42.5 grade or 52.5 grade.

Each grade type corresponds to the compression strength which is attained on the 28th day after pouring, with 53 grade having a minimum compressive strength of 53 MPa, or 530 kg/cm2. It should be noted that the number associated with a particular cement grade is the lowest strength allowable at the 28 day mark. Many cements far exceed the grade number in terms of compression strength. For instance, some 43 Grade OPCs can have a strength over 500 kg/cm2 on the 28th day.

OPC 43 and OPC 53 do not differ in chemical content, OPC 53 is simply ground much finer than OPC 43 during the final grinding process. This creates a product that is much stronger and more durable than less finely ground cements.

33 Grade Ordinary Portland Cement (OPC 33)

33 Grade Ordinary Portland Cement is now very rarely used, as higher strength cements have become increasingly popular. It is suitable for general uses such as general paving and mass concreting, but may be hard to find as it has been phased out in many areas of the world and replaced with either OPC 43 or OPC 53.

43 Grade Ordinary Portland Cement (OPC 43) / (OPC 42.5)

This type of cement is commonly used in construction work, such as residential, commercial, and industrial structures. In addition to this, most pre-cast cement objects such as pipes and tiles are made from OPC 34 cement. Because OPC 33 has now been phased out in most areas of the world, OPC 43 is also quite often used in plastering, flooring and other non structural applications.

Unless a project requires very high strength cement, most experts generally recommend the use of 43 Grade Ordinary Portland Cement. OPC 43 has a set time of around 30 minutes, and develops a minimum compression strength of 230 kg/cm2 after three days, 330 kg/cm2 after seven days, and 430 kg/cm2 after twenty eight days.

53 Grade Ordinary Portland Cement (OPC 53) / (OPC 52.5)

This cement is used for projects where a high level of strength is needed, such as in the construction of skyscrapers, bridges, and other heavy load bearing structures. Not only is this type of cement stronger than other types it is also more durable, and in lower stress applications, can be substituted for lower cement grades. If OPC 53 is substituted for a lower grade cement, overall savings in the amount of cement used can sometimes be made. OPC 53 grade cement often has a compression strength of over 580 kg/cm2 after 28 days.

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The concept of cement has been with humanity virtually since the first attempts at construction began over 600,000 years ago when man moved out from the caves and began to construct his own housing from wood and wattle, and then later on, from mud bricks.

Wood and wattle houses (also known as post houses, because of the wooden posts which were often erected to give them structural integrity) were made by weaving wattle and wood together, and then daubing them with a mixture of materials containing cow dung, lime, and mud. This daub sealed the houses as it dried, due to the reaction of certain chemicals present in the cow dung with the lime in the mixture. The substance produced by this reaction made a strong, solid coating which protected the inhabitants of the building from the elements.

The bricks of mud brick homes were also joined together with a kind of primitive cement, a clay based slurry that dried hard and kept the bricks in place. Both so called ‘cow dung’ and mud brick techniques are still used by some tribal populations when building homes to this very day, and evidence of the relative efficacy of these techniques can be observed in many small tribal villages in Africa.

In many parts of the world however, as mankind evolved from primitive societies towards more technologically advanced ones, better techniques were evolved for the making of bricks. Fired bricks, heat treated in kilns were used in in construction applications in Egyptian and Mesopotamian societies, and as the buildings built by these, and other advanced socities, became larger and more complicated, a need arose for a better kind of binding agent with which to join the bricks together. Various cultures and societies began to experiment with different types of cement like substances to be used as mortar in their buildings. Lime became a popular choice throughout much of the world as an additive which added strength and durability to existing types of cements and mortars. The Egyptians also used gypsum, which is still used in modern cement as an additive which controls how quickly cement sets.

Modern Cement

Modern cement was not invented until the Industrial Revolution which took place in Europe in the late 18th century. Technology was booming during this time, and the need for new and better materials with which to build structures was high. Modern cement was developed in response for a need for stronger concretes, and for cements which could be used in structures which were commonly exposed to water, such as those near the sea, and those in rainy climates.

Prior to this time, stone had been the primary building material, but supplies of stone suitable for building were beginning to run low, and prices were higher than ever before. Some ingenious builders and designers began to build buildings from brick instead, and finished them with a type of stucco that made them appear to be made out of stone. Because these large British buildings had to be completed on strict schedules, there was no time to wait around for weeks on end for cement to dry. In response to this need, a range of quick drying cements were developed.

One of the most popular early quick drying cements was known as ‘Roman’ Cement. It was developed in the 1780′s by a man named James Parker, and was manufactured by burning a specific type of clay deposit known as septaria, a substance which contains both clay and chalk, and grinding the post firing material down into a very fine powder. When combined with sand this cement made for a very quick setting mortar which could dry in just 15 minutes.

The next major development in cement technology was a strong type of hydraulic cement. Hydraulic cement is a type of cement that needs to be mixed with water in order for it to set, and is not soluble in water once it has set and dried.

Prior to the 1820′s, most all cements had been non hydraulic in nature, meaning that they were dry mixtures which had to remain dry in order to develop the necessary strength which was needed for construction applications. This was obviously a problem in many respects, especially in construction projects which involved a great deal of water. The first step towards truly effective hydraulic cements was taken by an English engineer called John Smeaton in 1755 , who was building a lighthouse, and during his investigations into possible construction materials which might be suitable for the building of a lighthouse, discovered that the more clay that was present in the cement, the more hydraulic it was.
For the next seventy years or so, various people worked on the problem of creating a strong cement which could withstand the rigors of being rained on, or washed up against by waves without dissolving away leaving buildings which used it nothing more than untidy piles of bricks.

Portland cement, a hydraulic cement which is still in popular use today was invented by Joseph Aspdin who patented the product in 1824, however it was his son, William Aspdin who developed the strength and durability of Portland cement by adding a compound called ‘alite’ to it, which is still used to strengthen cement today.

William had a difficult time at first convincing people of the usefulness of his product, as it contained much more lime than the original Portland cement, and so needed to be fired at much higher temperatures. Also, due to its hardness, the resulting clinker developed during the firing process caused problems during grinding because it would wear down the millstones much more quickly than other types of cement.

The saving grace of Portland cement was the fact that the end product was a slow setting, high strength cement which could be mixed with concrete to create structures which developed strength quickly, unlike earlier cements which could take weeks to develop enough strength for timber framing to be able to be removed.

Portland cement is now the most popularly used type of cement in the world, although since the 19th century, many different types of cement have been developed for use in different kinds of applications. A few of these cements include expansive cements, which do not shrink during drying like typical hydraulic cements, colored cements, for decorative uses, and masonry cements, which have been developed for used solely in masonry applications and which are not suitable for use in concrete.
Cement continues to evolve, though admittedly at a much slower pace than it did during the boom years of the Industrial revolution. Necessity will always be the mother of invention, and as long as we encounter new needs which are unfulfilled by cement products currently available on the market, it is certain that new types of cement will be developed.

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