Post on 19-Nov-2014
Cement historyThroughout history, cementing materials have played a vital role. They were used widely in the ancient world. The Egyptians used calcined gypsum as a cement. The Greeks and Romans used lime made by heating limestone and added sand to make mortar, with coarser stones for concrete.
The Romans found that a cement could be made which set under water and this was used for the construction of harbours. The cement was made by adding crushed volcanic ash to lime and was later called a "pozzolanic" cement, named after the village of Pozzuoli near Vesuvius.
In places such as Britain, where volcanic ash was scarce, crushed brick or tile was used instead. The Romans were therefore probably the first to manipulate the properties of cementitious materials for specific applications and situations.
Hadrian's Wall, England, a few miles east of Housesteads.
Marcus Vitruvius Pollio, a Roman architect and engineer in the 1st century BC wrote his "Ten books of Architecture" - a revealing historical insight into ancient technology. Writing about concrete floors, for example:
"First I shall begin with the concrete flooring, which is the most important of the polished finishings, observing that great pains and the utmost precaution must be taken to ensure its durability".
"On this, lay the nucleus, consisting of pounded tile mixed with lime in the proportions of three parts to one, and forming a layer not less than six digits thick."
And on pozzolana:
"There is also a kind of powder from which natural causes produces astonishing results. This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers are constructed of it in the sea, they set hard under water."
(Vitruvius, "The Ten Books of Architecture," Dover Publications, 1960.)
His "Ten books of Architecture" are a real historical gem bringing together history and technology. Anyone wishing to follow his instructions might first need to find a thousand or so slaves to dig, saw, pound and polish...
After the Romans, there was a general loss in building skills in Europe, particularly with regard to cement. Mortars hardened mainly by carbonation of lime, a slow process. The use of pozzolana was rediscovered in the late Middle Ages.
The great mediaeval cathedrals, such as Durham, Lincoln and Rochester in England and Chartres and Rheims in France, were clearly built by highly skilled masons. Despite this, it would probably be fair to say they did not have the technology to manipulate the properties of cementitious materials in the way the Romans had done a thousand years earlier.
The Renaissance and Age of Enlightenment brought new ways of thinking, which for better or worse, led to the industrial revolution. In eighteenth century Britain, the interests of industry and empire coincided, with the need to build lighthouses on exposed rocks to prevent shipping losses. The constant loss of merchant ships and warships drove cement technology forwards.
Smeaton, building the third Eddystone lighthouse (1759) off the coast of Cornwall in Southwestern England, found that a mix of lime, clay and crushed slag from iron-making produced a mortar which hardened under water. Joseph Aspdin took out a patent in 1824 for "Portland Cement," a material he produced by firing finely-ground clay and limestone until the limestone was calcined. He called it Portland Cement because the concrete made from it looked like Portland stone, a widely-used building stone in England.
While Aspdin is usually regarded as the inventor of Portland cement, Aspdin's cement was not produced at a high-enough temperature to be the real forerunner of modern Portland Cement. Nevertheless, his was a major innovation and subsequent progress could be viewed as mere development.
A ship carrying barrels of Aspdin's cement sank off the Isle of Sheppey in Kent, England, and the barrels of set cement, minus the wooden staves, were later incorporated into a pub in Sheerness and are still there now.
A few years later, in 1845, Isaac Johnson made the first modern Portland Cement by firing a mixture of chalk and clay at much higher temperatures, similar to those used today. At these temperatures (1400C-1500C), clinkering occurs and minerals form which are very reactive and more strongly cementitious.
While Johnson used the same materials to make Portland cement as we use now, three important developments in the manufacturing process lead to modern Portland cement:
- Development of rotary kilns
- Addition of gypsum to control setting
- Use of ball mills to grind clinker and raw materials
Rotary kilns gradually replaced the original vertical shaft kilns used for making lime from the 1890s. Rotary kilns heat the clinker mainly by radiative heat transfer and this is more efficient at higher temperatures, enabling higher burning temperatures to be achieved. Also, because the clinker is constantly moving within the kiln, a fairly uniform clinkering temperature is achieved in the hottest part of the kiln, the burning zone.
The two other principal technical developments, gypsum addition to control setting and the use of ball mills to grind the clinker, were also introduced at around the end of the 19th century.
Some basic definitions used in cement and concrete
A few useful basic definitions follow, since the meanings of the words 'cement' and 'concrete' are rather blurred in general use.
Portland Cement: Material made by heating a mixture of limestone and clay in a kiln at about 1450 C, then grinding to a fine powder with a small addition of gypsum. Portland Cement, the main subject of this site, is the most common type of cement - 'basic cement', if you like. In particular, ordinary Portland cement is the normal, grey, cement with which most people are familiar. Other types of Portland cement include White Portland Cement and Sulfate Resisting Portland Cement (SRPC).
Clinker: Portland cement is made by grinding clinker and a little added gypsum. Clinker is a nodular material before it is ground up. The nodules can be anything from 1mm to 25mm in diameter.
Cement: Usually taken to mean Portland Cement, but could mean any other type of
cement, depending on the context.
Difference between Cement (left) and clinker (right). The coin is a UKone-pound coin about 23mm across.
Aggregate: Cobbles, pebbles, gravel, sand and silt - the 'rock' component of all particle sizes in concrete.
Concrete: Synthetic rock made using cement (usually, but not necessarily, Portland cement) mixed with aggregate and water.
Mortar: Mixture of cement and fine aggregate, mainly sand. Used typically to bond bricks and building stone.
Grout: Mixture of cement (possibly of various types) and other fine material such as fine sand. Used in a wide range of applications from filling the gaps between bathroom tiles to oil wells.
Composite cements: Some types of cement are mixtures of Portland cement with other material, such as blastfurnace slag from iron production and pulverised fuel ash from coal-fired electricity power stations. These widely-used mixtures are called 'composite' cements.
Non-Portland cements
Of course, there are other types of cement apart from Portland cement. Important examples include:
Calcium aluminate cements
Lime concrete/mortar
Expansive cements
Calcium aluminate cements (CACs)
These may also be termed 'Ciment Fondu' and used to be called 'high alumina cements.' They are made from lime or limestone mixed with bauxite (aluminium ore) or other high-alumina material.
Concretes made with CACs develop strength quickly and are resistant to chemical attack. CACs have a wide range of compositions, mainly with different ratios of lime to alumina; strictly, ‘Ciment Fondu’ is only one part of this compositional range. CACs are generally brown or grey-black, but can be white if made from pure alumina.
As well as being used in concrete, CACs are also used in grouts and other specialised applications, often mixed with Portland cement and other materials such as gypsum.
Lime concrete and mortar
Lime mortar and concrete have been used for thousands of years (see history of cement) so, historically, lime is probably the most basic cementitious material of all. Today, lime mortar and concrete are used mainly in the rebuilding or repair of historic or ancient buildings, although in the UK there has been some recent use of lime mortar in the construction of new buildings. There are several advantages in using lime mortar:
Cracks that develop in lime mortar tend to heal themselves, unlike conventional mortar made with Portland Cement.
Lime mortar is usually weaker than mortars made with Portland Cement and so can be removed from the brick or stone at the end of the useful life of the building. Particularly in the case of bricks, this means that they can be recycled, saving a lot of energy otherwise needed to make newbricks. Bricks used with mortar made with Portland Cement generally can’t be re-used as it is difficult to detach the mortar from the brick without damaging it.
Lime is produced at a lower temperature than Portland Cement, so other things being equal, it takes less energy to produce a lime mortar compared with a mortar containing Portland Cement.
Expansive cements
These are special cements designed to exert an expansive force on their surroundings after the cement has set. (With most cements, manufacturers go to a lot of trouble to make sure the cement is not expansive). Expansive cements are used mainly in demolition and also in mining.
Clinker: Cement chemistry notation
Cement chemists use a form of notation which, at first sight - and maybe second or third sight - may seem a little odd.
Oxides are referred to by their first letter: 'C' represents CaO; 'M' is MgO and so on, for all the oxides likely to be encountered in cementitious systems, as shown below.
'Normal chemists' unfamiliar with this notation may find it strange to use 'C' to represent calcium oxide, rather than carbon, but there is a point to all this. It shortens what are otherwise very long names, for example:
Alite: Ca3SiO5 in terms of its oxides is 3CaO.SiO2. The CaO term is shortened to C and the SiO2 to S. The compound thus becomes C3S.
Belite: Similarly, Ca2SiO4 is 2CaO.SiO2, which is shortened to C2S.
Tricalcium aluminate: Ca3Al2O6 is 3CaO.Al2O3. The Al2O3 term is shortened to A and the compound becomes C3A.
Tetracalcium aluminoferrite: 2(Ca2AlFeO5) is 4CaO.Al2O3.Fe2O3. Fe2O3 is shortened to F and the compound becomes C4AF.
(With long names like this last one, the need for a shorthand description is all-too clear.)
In other words, for each of the clinker main minerals, we now have at least three possible descriptions, as below, as well as the full chemical formulae for the pure compounds.
Alite or tricalcium silicate or C3S
Belite, or dicalcium silicate or C2S
Tricalcium aluminate (or the 'aluminate phase') or C3A
Calcium alumino-ferrite (or the ‘ferrite’ phase) or tetracalcium aluminoferrite or C4AF
Although strictly, these do not mean the same thing, they are frequently used indiscriminately. This lax use means that names like ‘C3A’ should not usually be taken to signify a definite composition in the sense of a pure compound, unless this is indicated by the context .
To take another example, strictly speaking tricalcium silicate is a pure compound, while alite is a mineral composed largely of tricalcium silicate but also with a significant quantity of impurities, mainly magnesium, iron and aluminium.
The symbols for carbonate and sulfate are referred to as 'C bar' and 'S bar,' for obvious reasons. These last two are used less than they once were, perhaps because they are tricky to write using a word processor.
This notation is widely used in cement scientific literature and also in analyses and other information provided by cement manufacturers.
A few basic chemistry notesBemused by basic chemistry? Read on...
Some very basic chemistry will help a lot with understanding how cement is made and how it works. If you missed out on chemistry at school, or it just seems a long time ago, the following short notes might be useful.
Basic Chemistry 1: Chemical symbols
Chemical symbols are abbreviated forms of the names of chemical elements, eg: Ca-calcium; Si-silicon; K-potassium.
Symbols are usually based on the Latin names, so they don't always resemble the English names especially for elements known in antiquity (eg: Au-gold, aurum).
Some useful elemental symbols in the context of cement follow, with their approximate atomic weights:
Element Symbol Atomic weightHydrogen H 1Carbon C 12Oxygen O 16Nitrogen N 18Sodium Na 23Magnesium Mg 26Aluminium Al 27Silicon Si 28Phosphorus P 31Sulfur S 32Chlorine Cl 35.5Potassium K 39Calcium Ca 40Titanium Ti 48Chromium Cr 52Manganese Mn 55Iron Fe 56
Basic Chemistry 2: Chemical formulae
A chemical formula is more than just a convenient short form of the name of a chemical - they indicate its composition.
For example, common table salt is sodium chloride, NaCl. One molecule of sodium chloride contains one atom of sodium and one of chlorine.
Calcium carbonate, CaCO3, contains one atom of calcium, one of carbon and three of oxygen.
Basic Chemistry 3: Atomic weights (also known as relative atomic masses)
In the old days, atomic weights were based on hydrogen with an atomic weight of 1. This meant that calcium, with an atomic weight of 40, is 40 times as massive for the same number of atoms as hydrogen.
In other words, if you have 1 gram of hydrogen and 40 grams of calcium, there would be the same number of atoms in each.
These days, atomic weights are not based on hydrogen, but on one-twelfth of carbon-12. There have been other definitions as well, but they are all the same to within about 1%. Although called atomic weights, they are not really weights because they are ratios and therefore dimensionless but the term atomic weight is kept for historical reasons.
The basic point is that the atomic weight tells you the relative masses of atoms. A sodium atom is 23 times as massive as a hydrogen atom and a sulfur atom is twice as massive as an oxygen atom (see the Table above and you'll get the idea).
This is useful because with this knowledge we can calculate how much, by weight, of each element is present in compounds. It also means we can weigh stuff out in the right proportions to make different compounds.
Sodium chloride again: the atomic ratio of sodium to chlorine is 1:1. There is one atom of sodium and one of chlorine. The proportions by weight are different. The weight (relative atomic mass) of sodium is 23 and of chlorine is 35.5. If we add these together we get the formula weight of a sodium chloride molecule: 23+35.5=58.5.
So, the proportion of sodium in sodium chloride is: 23/58.5 x 100% = 39.3% by mass.
The proportion of chlorine in sodium chloride is: 35.5/58.5 x 100% = 60.7% by mass.
Let's do a slightly more complicated one. The formula for calcium carbonate is CaCO3 and from the Table above, the relative atomic mass of calcium is 40, of carbon is 12 and of oxygen is 16.
In calcium carbonate, we have one atom of calcium, one of carbon and three of oxygen. So, the formula weight of calcium carbonate is: 40+12+48=100.
So, calcium carbonate contains 40% calcium, 12% carbon and 48% oxygen by mass.
A little pedantry...
The term 'formula weight' has been replaced by 'molar mass'. Understanding the formula weight is easy, it is just the sum of the atomic weights of all the atoms in the formula for a compound. The molar mass is numerically the same, multiplied by 1gram/mol. The reason for this is to make the expression dimensionally correct. Atomic weights aren't weights, they are ratios; for calcium carbonate, the molar mass is 100 grams per mol.
'Mol' is the symbol for 'mole' and is linked to the number of atoms in 12 grams of carbon-12, the same as atomic weights. The mole can be applied to elements or compounds.
So, one mole of calcium weighs 40 grams, one mole of iron weighs 56 grams and one mole of calcium carbonate weighs 100 grams.
Basic Chemistry 4: Anions and cations
Atoms are normally electrically neutral because they have the same number of protons (positively charged) as electrons (negatively charged). However, if it gains or loses one or more electrons, an atom becomes electrically charged. For reasons to do with the number of electrons in the outer shell of the atom, some atoms easily gain one or more electrons and others lose one or more electrons.
Those that gain electrons become negatively charged and are called anions. Those that lose an electron become positively charged and are called cations.
If you want more detail, get a book, or go online, and look up valence electrons.
However for now, and to keep it simple, just learn the following:
In normal cement-related compounds, cations and anions have the following charges (valencies):
Hydrogen +1Carbon +4Nitrogen +4Sodium, potassium +1Calcium, magnesium +2Aluminium +3Silicon +4Phosphorus +5Oxygen -2Chlorine -1Hydroxide (OH) -1Chromium +3 or +6Manganese +2 or +3 (can have others)Iron +2 or +3Sulfate (SO3) -2
It may be a bit tedious learning these but it is worth the effort. If you remember the formulae for some compounds as well, you can work out valencies you have forgotten because the charges have to balance.
For example, everyone knows the formula of water is H2O. If you remember hydrogen has a charge of +1 but you forgot about oxygen, water has to be electrically neutral, so the oxygen anion must have a charge of -2.
If you remember that the sodium cation has a charge of +1 and the hydroxide anion -1, you can work out that the formula for sodium hydroxide must be NaOH; it cannot be, for example, Na2OH because it would then be electrically charged by +1.
If you find these basic chemistry short notes useful let us know using the Contact Form and we'll expand them.
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Cement manufacturing:components of a cement plant
This page and the linked pages below summarize the cement manufacturing process from the perspective of the individual components of a cement plant - the kiln, the cement mill etc..
For View of a cement kiln (the long nearly-horizontal cylinder) and preheater tower. (Picture courtesy Castle Cement.)
Summary of production process
Cement is typically made from limestone and clay or shale. These raw materials are extracted from the quarry crushed to a very fine powder and then blended in the correct proportions.
This blended raw material is called the 'raw feed' or 'kiln feed' and is heated in a rotary kiln where it reaches a temperature of about 1400 C to 1500 C. In its simplest form, the rotary kiln is a tube up to 200 metres long and perhaps 6 metres in diameter, with a long flame at one end. The raw feed enters the kiln at the cool end and gradually passes down to the hot end, then falls out of the kiln and cools down.
The material formed in the kiln is described as 'clinker' and is typically composed of rounded nodules between 1mm and 25mm across.
After cooling, the clinker may be stored temporarily in a clinker store, or it may pass directly to the cement mill.
The cement mill grinds the clinker to a fine powder. A small amount of gypsum - a form of calcium sulfate - is normally ground up with the clinker. The gypsum controls the
setting properties of the cement when water is added.
The basic components of the cement production process.
The following pages (see links below) have more detail on each of the different stages of cement production. Also, see the ' Clinker ' pages for information on raw materials, the chemical reactions in the kiln, cement clinker and cement chemistry.
Cement manufacturing - raw materials
If you happen to be a geologist, the raw materials quarry is probably the most interesting part of a cement works, maybe unless you view the clinkering process as igneous rocks in the making.
The most common raw rock types used in cement production are:
- Limestone (supplies the bulk of the lime)
- Clay, marl or shale (supplies the bulk of the silica, alumina and ferric oxide)
- Other supplementary materials such as sand, pulverised fuel ash (PFA), or ironstone to achieve the desired bulk composition
Quarry management is an art. Most quarries will probably have "good material" from which cement can easily be made. They may also have some material that is not as good; this might be harder to grind, or be of less convenient composition.
Limestone blocks being taken away for crushing. (Picture courtesy Castle Cement.)
If the 'good stuff' is all used up first, it may be difficult to make cement out of what is left. Careful selection on a day-to-day basis is needed to make the best use of all the materials available.
Raw materials are extracted from the quarry, then crushed and ground as necessary to provide a fine material for blending. Most of the material is usually ground finer than 90 microns - the fineness is often expressed in terms of the percentage retained on a 90 micron sieve.
Once the the raw materials are ground fine enough, they are blended in the proportions required to produce clinker of the desired composition.
The blended raw materials are stored in a silo before being fed into the kiln. The silo stores several days' supply of material to provide a buffer against any glitches in the supply of raw material from the quarry.
Technically, a cement producer can have almost complete control over clinker composition by blending raw materials of different compositions to produce the desired result. In practice, however, clinker composition is largely determined by the compositions of the locally-available raw materials which make up the bulk of the raw meal.
Supplementary materials are used to adjust the composition of the raw meal but cost and availability are likely to determine the extent to which they are used. Transport
costs in particular become significant in view of the large quantities of materials used in making cement.
Manufacturing - the cement kilnMost cement is made in a rotary kiln. Basically, this is a long cylinder rotating about its axis once every minute or two. The axis is inclined at a slight angle, the end with the burner being lower.
The rotation causes the raw meal to gradually pass along from where it enters at the cool end, to the hot end where it eventually drops out and cools. They were introduced in the 1890s and became widespread in the early part of the 20th century and were a great improvement on the earlier shaft kilns, giving continuous production and a more uniform product in larger quantities.
For Principle of a basic wet-process kiln.
Wet process kilns
The original rotary cement kilns were called 'wet process' kilns. In their basic form they were relatively simple compared with modern developments. The raw meal was supplied at ambient temperature in the form of a slurry.
A wet process kiln may be up to 200m long and 6m in diameter. It has to be long because a lot of water has to be evaporated and the process of heat transfer is not very efficient.
The slurry may contain about 40% water. This takes a lot of energy to evaporate and various developments of the wet process were aimed at reducing the water content of the raw meal. An example of this is the 'filter press' (imagine a musical accordion 10-20 metres long and several metres across) - such adaptions were described as 'semi-wet' processes.
The wet process has survived for over a century because many raw materials are suited to blending as a slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately.
Quite a few wet process kilns are still in operation, usually now with higher-tech bits bolted on. However, new cement kilns are of the 'dry process' type.
Dry process kilns
In a modern works, the blended raw material enters the kiln via the pre-heater tower. Here, hot gases from the kiln, and probably the cooled clinker at the far end of the kiln, are used to heat the raw meal. As a result, the raw meal is already hot before it enters the kiln.
The dry process is much more thermally efficient than the wet process.
Firstly, and most obviously, this is because the meal is a dry powder and there is little or no water that has to be evaporated.
Secondly, and less obviously, the process of transferring heat is much more efficient in a dry process kiln.
An integral part of the process is a heat exchanger called a 'suspension preheater'. This is a tower with a series of cyclones in which fast-moving hot gases keep the meal powder suspended in air. All the time, the meal gets hotter and the gas gets cooler until the meal is at almost the same temperature as the gas.
The basic dry process system consists of the kiln and a suspension preheater. The raw materials, limestone and shale for example, are ground finely and blended to produce the raw meal. The raw meal is fed in at the top of the preheater tower and passes through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air from the clinker cooler are blown through the cyclones. Heat is transferred efficiently from the hot gases to the raw meal.
The heating process is efficient because the meal particles have a very high surface area in relation to their size and because of the large difference in
temperature between the hot gas and the cooler meal. Typically, 30%-40% of
the meal is decarbonated before enterinCement manufacturing -
brief description of a cement millCement clinker is usually ground using a ball mill. This is essentially a large rotating drum containing grinding media - normally steel balls. As the drum rotates, the motion of the balls crushes the clinker. The drum rotates approximately once every couple of seconds.
The drum is generally divided into two or three chambers, with different size grinding media. As the clinker particles are ground down, smaller media are more efficient at reducing the particle size still further.
Grinding systems are either 'open circuit' or 'closed circuit.' In an open circuit system, the feed rate of incoming clinker is adjusted to achieve the desired fineness of the product. In a closed circuit system, coarse particles are separated from the finer product and returned for further grinding.
Gypsum is interground with the clinker in order to control the setting properties of the cement. Clinker grinding uses a lot of energy and the cement becomes hot - this can result in the gypsum becoming dehydrated, with potentially undesirable results - see
the link at the bottom of this page for more information.
Inside a (stationary!) cement mill. The part-ground clinker and steel grinding media are clearly visible.(Picture courtesy Castle Cement).
This is the end of the descriptions of the main components of a cement plant. The next step is to look at cement-making from a materials point of view and the reactions that occur as they are converted into clinker. To continue
g the kiln.
A development of this process is the 'precalciner' kiln. Most new cement plant is of this type. The principle is similar to that of the dry process preheater system but with the major addition of another burner, or precalciner. With the additional heat, about 85%-
95% of the meal is decarbonated before it enters the kiln.
Basic principle of precalciner kiln.
Since meal enters the kiln at about 900 C, (compared with about 20 C in the wet process), the kiln can be shorter and of smaller diameter for the same output. This
reduces the capital costs of a new cement plant. A dry process kiln might be only 70m long and 6m wide but produce a similar quantity of clinker (usually measured in tonnes per day) as a wet process kiln of the same diameter but 200m in length. For the same output, a dry process kiln without a precalciner would be shorter than a wet process kiln
but longer than a dry process kiln with a precalciner.
Kiln and preheater tower: raw meal passes down the tower while hot gases rise up, heating the raw meal. At 'A,' the raw meal largely decarbonates; at 'B,' the temperature is 1000 C - 1200 C and intermediate compounds are forming and at 'C,' the burning zone, clinker nodules and the final clinker minerals form. A preheater tower is likely to have 4-6 stages, not the three shown here. Many designs are more complex but this diagram illustrates the principle. See the 'Clinker' pages for more information on reactions in the kiln.
The kiln is made of a steel casing lined with refractory bricks. There are many different types of refractory brick and they have to withstand not only the high temperatures in the kiln but reactions with the meal and gases in the kiln, abrasion and mechanical stresses induced by deformation of the kiln shell as it rotates.
Bricks in the burning zone are in a more aggressive environment compared with those at the cooler end of the kiln (the 'back end'), so different parts of the kiln are lined with different types of brick.
Periodically, the brick lining, or part of it, has to be replaced. Refractory life is reduced by severe changes in temperature, such as occur if the kiln has to be stopped. As the cost of refractories is a major expense in operating a cement plant, kiln stoppages are avoided as far as possible.
As the meal passes through the burning zone, it reaches clinkering temperatures of about 1400 C - 1500 C. Nodules form as the burning zone is approached. When the clinker has passed the burning zone, it starts to cool, slowly at first, then much more quickly as it passes over the 'nose ring' at the end of the kiln and drops out into the
cooler.
There are various types of cooler - we will consider only one, the 'grate cooler'.
Cooler - red-hot clinker falls onto the grate, cooled by air blown from beneath. The clinker is moving towards the front of the picture. (Picture courtesy Cemex UK Cement)
The purpose of a cooler is, obviously, to cool the clinker. This is important for a several reasons:
From an engineering viewpoint, cooling is necessary to prevent damage to clinker handling equipment such as conveyors.
From both a process and chemical viewpoint, it is beneficial to minimise clinker temperature as it enters the clinker mill. The clinker gets hot in the mill and excessive mill temperatures are undesirable. It is clearly helpful, therefore, if the clinker is cool as it enters the mill.
From an environmental and a cost viewpoint, the cooler reduces energy consumption by extracting heat from the clinker, enabling it to be used to heat the raw materials.
From a cement performance viewpoint, faster cooling of the clinker enhances silicate reactivity.
The cooled clinker is then conveyed either to the clinker store or directly to the clinker mill. The clinker store is usually capable of holding several weeks' supply of clinker, so that deliveries to customers can be maintained when the kiln is not operating.
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Portland cement clinker - overview
Portland cement clinker is a dark grey nodular material made by heating ground limestone and clay at a temperature of about 1400 C-1500 C. The nodules are ground
up to a fine powder to produce cement, with a small amount of gypsum added to control the setting properties.
This page gives a thumbnail sketch. For more information see the following pages - links
at the bottom of this page.
Polished section of nodule (scanning electron microscope image). Most of the nodule is alite (light grey) - some clusters of belite are visible (arrowed). Aluminate and ferrite are present but not visible at this relatively low magnification.
Nodules range in size from 1mm to 25mm or more and are composed mainly of calcium silicates, typically 70%-80%. The strength of concrete is mainly due to the reaction of these calcium silicates with water.
Portland cement clinker contains four main minerals:
Alite: approximately tricalcium silicate (typically about 65% of the total)
Belite: approximately dicalcium silicate (typically about 15% of the total)
Aluminate: very approximately tricalcium aluminate (typically about 7% of the total)
Ferrite: very approximately tetracalcium aluminoferrite (typically about 8% of the total)
The balance is made up of alkali sulfates and minor impurities. The typical mineral
contents shown are subject to wide variation.
Optical microscope image (polished section) of nodule. Brown crystals are alite, blue crystals are belite, bright interstitial material is mainly ferrite, with small dark inclusions of aluminate. Grey material is the epoxy resin used to make the specimen. NB: Alite is not actually brown and belite is not actually blue - they appear brown and blue here because the polished section has been etched to show the crystals more clearly.
A clinker chemical analysis is normally given in oxide form - a typical example might be:
Example of a typical clinker analysis (oxide weight%).SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total21.5 5.2 2.8 66.6 1.0 0.6 0.2 1.0 1.5 0.5 98.9
Free lime = 1.0% CaO
Balance is typically due to small amounts of oxides of titanium, manganese, phosphorus and chromium.
From the chemical analysis, the quantity of each of the four main minerals can be calculated using the 'Bogue' calculation (click on the link below for more information.)
Clinker: reactions in the kiln
This page reviews the reactions which take place as the feed passes through the kiln.
The blended, milled raw materials go to a silo and from there to the kiln.
The reactions which take place in the kiln can be considered under three broad headings:
Decomposition of raw materials - reactions at temperatures up to about 1300 C.
Alite formation and other reactions at 1300 C-1450 C in the burning zone.
Cooling of the clinker.
Decomposition of raw materials - reactions at temperatures up to about 1300 C
This includes:
i. Water evaporation in the raw feed, if any.ii. Loss of carbon dioxide from the limestone (ie: calcining).iii. Decomposition of the siliceous and aluminosilicate fractions of the feed.iv. Formation of a sulfate melt phase.
The decomposition products react with lime to form intermediate compounds which in turn form other compounds as clinkering proceeds.
Water evaporationIn wet-process kilns, and their derivatives, water must first be driven off. In a wet-process kiln, calcining takes place after the water has been driven off, about a third of the way down the kiln. In the more modern pre-calciner kilns, the feed is calcined prior to entering the kiln.
CalciningIn isolation, decarbonation of calcium carbonate at 1 atmosphere takes place at 894 C. This temperature is reduced to 500 C-600 C if the reaction takes place in contact with quartz or the decomposition products of clay minerals, which react with the calcium oxide as it forms.
In a wet-process or preheater system without a pre-calciner, most of the calcination takes place in the rotary kiln within a moving mass of feed. This situation is not ideal for calcination because heat transfer has to take place through a large mass of material and CO2 has to escape outwards as heat moves inwards.
A pre-calciner calcines the raw material much more efficiently than a wet-process kiln. Raw meal is dispersed in the hot gas and calcination takes place in seconds, rather than the half an hour or so inside a kiln at the same temperature.
Formation of early and intermediate compoundsDuring calcination, the lime produced starts to react with other components of the raw feed. The initial silicate product is belite. Some calcium aluminate and ferrite phases also start to form.
A number of phases are formed in the clinker feed before the burning zone proper is reached. These intermediate phases dissociate in the burning zone and are not therefore found in clinker but assist in forming the final clinker minerals.
Sulfate melt phaseAt intermediate temperatures, sulfates combined with calcium and alkalis form a liquid phase. This is separate from the aluminate and aluminoferrite-based liquid formed in the burning zone - the two liquids are immiscible.
As with the main liquid phase, the sulfate liquid phase contributes to ion mobility and promotes combination.
Alite formation and other reactions at 1300 C-1450 C in the burning zone
In the burning zone, above about 1300 C, reactions take place quickly. The clinker is in the burning zone for perhaps 10-20 minutes but in this time a lot happens:
i. The proportion of clinker liquid increases and nodules form.ii. Intermediate phases dissociate to form liquid and belite.iii. Belite reacts with free lime to form alite.iv. Some volatile phases evaporate.
Clinker liquid and nodule formationAbove about 1300 C the proportion of liquid starts to increase - by 1450 C, perhaps 20-30% of the mix is liquid. The liquid forms from melting ferrite and aluminate phases and some belite. The liquid content is more than the sum of the aluminate and ferrite phases in the cooled clinker because of the dissolved lime and silica.
The additional liquid causes coalescence of clinker particles, leading to the formation of nodules.
Dissociation of intermediate phasesThe intermediate phases dissociate to form mainly aluminate phase, which then becomes part of the liquid, and belite.
Alite formationAlite forms by the transition of some of the belite to alite and also directly from free lime and silica to alite. These reactions occur rapidly once the clinker temperature is above about 1400 C.
Evaporation of volatilesVolatile phases in the cement kiln are principally alkali sulfates, with a much smaller proportion of alkali chlorides. As the part-burned feed approaches the burning zone, these volatile phases are in liquid form and a proportion volatilizes, the remainder passing out of the kiln in the clinker as inclusions within the pores.
The volatilized material passes back down the kiln, where it condenses on the relatively cool incoming feed. It again becomes part of the sulfate melt phase, promoting reactions, and is once again carried within the clinker towards the burning zone.
This recirculating load of alkali and sulfate can occasionally become excessively high. Large quantities of condensing volatiles can then cause blockages in the kiln or in the preheater as the condensed liquid sticks feed particles together, forming accretions.
Cooling of the clinker
As the clinker cools, the main liquid phase crystallizes to form aluminate phase, ferrite and a little belite.
Fast cooling of clinker is advantageous - it makes for more hydraulically-reactive silicates and lots of small, intergrown, aluminate and ferrite crystals.
Slow cooling gives less hydraulically-reactive silicates and produces coarse crystals of aluminate and ferrite - over-large aluminate crystals can lead to erratic cement setting characteristics. Very slow cooling allows alite to decompose to belite and free lime.
Clinker: compositional parametersParameters based on the oxide composition are very useful in describing clinker characteristics. The following parameters are widely used (chemical formulae represent weight percentages):
Lime Saturation Factor
The LSF is a ratio of CaO to the other three main oxides. Applied to clinker, it is calculated as:
LSF=CaO/(2.8SiO2 + 1.2Al2O3 + 0.65Fe2O3)
Often, this is referred to as a percentage and therefore multiplied by 100.
The LSF controls the ratio of alite to belite in the clinker. A clinker with a higher LSF will have a higher proportion of alite to belite than will a clinker with a low LSF.
Typical LSF values in modern clinkers are 0.92-0.98, or 92%-98%.
Values above 1.0 indicate that free lime is likely to be present in the clinker. This is because, in principle, at LSF=1.0 all the free lime should have combined with belite to form alite. If the LSF is higher than 1.0, the surplus free lime has nothing with which to combine and will remain as free lime.
In practice, the mixing of raw materials is never perfect and there are always regions within the clinker where the LSF is locally a little above, or a little below, the target for the clinker as a whole. This means that there is almost always some residual free lime, even where the LSF is considerably below 1.0. It also means that to convert virtually all the belite to alite, an LSF slightly above 1.0 is needed.
The LSF calculation can also be applied to Portland cement containing clinker and gypsum if (0.7 x SO3) is subtracted from the CaO content. (NB: This calculation (ie: 0.7 x SO3) does not account for sulfate present as clinker sulfate in the form of potassium and sodium sulfates) and this will introduce a slight error. More particularly, it does not account for fine limestone or other material such as slag or fly ash in the cement. If these materials are present, calculation of the original clinker LSF becomes more complex. Limestone can be quantified by measuring the CO2 content and the formula adjusted accordingly, but if slag or fly ash are present, calculation of the original clinker LSF may not be conveniently practicable.)
Silica Ratio (SR)
The silica ratio (also known as the Silica Modulus) is defined as:
SR = SiO2/(Al2O3 + Fe2O3)
A high silica ratio means that more calcium silicates are present in the clinker and less aluminate and ferrite. SR is typically between 2.0 and 3.0.
The silica ratio is sometimes called the ‘silica modulus.’
Alumina Ratio (AR)
The alumina ratio is defined as:
AR=(Al2O3/(Fe2O3)
This determines the potential relative proportions of aluminate and ferrite phase in the clinker.
An increase in clinker AR (also sometimes written as A/F) means there will be proportionally more aluminate and less ferrite in the clinker. In ordinary Portland cement clinker, the AR is usually between 1 and 4.
The above three parameters are those most commonly used. A fourth, the 'Lime Combination Factor' (LCF) is the same as the LSF parameter, but with the clinker free lime content subtracted from the total CaO content. With an LCF=1.0, therefore, the
maximum amount of silica is
Clinker: combinability of mixes
The ease of combination ("combinability", or "burnability") are about how easily the raw materials react with each other to produce the clinker minerals.
Clinker composition is evidently one of the key factors which determine cement quality. Composition is controlled mainly by suitable blending of raw materials, but there are limitations to what can be achieved.
Before considering these limitations, a summary of the clinkering process, and of the role of the liquid phase, may be useful.
The essential reactions in making Portland cement are the calcination of limestone to produce lime (calcium oxide) and the combination of this lime with silica to make belite and, especially, alite.
Importance of the liquid phase in clinkering
During clinkering, the clinker contains solid phases and a liquid phase. The bulk of the clinker remains as a solid. At the highest temperatures reached by the clinker, perhaps only about 25% of the clinker is a liquid. The solid phases are mainly alite, belite and free lime.
The liquid is vital in that it acts a flux, promoting reactions by ion transfer; without the liquid phase, combinability would be poor and it would be very difficult to make cement.
The liquid phase is composed largely of oxides of calcium, iron and aluminium, with some silicon and other minor elements. As the clinker leaves the kiln and cools, crystals of aluminate and ferrite form from the liquid.
Combination
The combinability of a raw mix will depend largely on:
The fineness of the raw materials - fine material will evidently react more readily than will coarser material, so finer material makes for better combinability.
Lime Saturation Factor - higher LSF mixes are more difficult to combine than are lower LSF mixes, so a higher LSF makes for poorer combinability.
Silica Ratio - mixes of higher SR are more difficult to combine because there is less liquid flux present, so a higher SR makes for poorer combinability.
Alumina Ratio - mixes of AR approximately equal to 1.4 will be easier to burn than if the AR is higher or lower. This is because at an AR of about 1.4, there is more clinker liquid at a lower temperature and combinability is optimised. (Minor constituents such as MgO can alter this optimum AR).
The intrinsic reactivity of the raw materials - some types of silica, for example, will react more easily than will others.
Ideally, a cement producer would like to control all three clinker compositional parameters, LSF, SR and AR. That would define the approximate proportions of the four main minerals in the clinker.
Blending and proportioning
Suppose the cement producer has a source of limestone and a source of clay and that he knows the chemical composition of each.
He can blend the limestone and clay in the correct proportions to give whatever value for LSF he likes, say 98%. However, the SR and AR will then be fixed by whatever the composition of the raw materials determines them to be. Although there will probably be some SiO2, Al2O3 and Fe2O3 in the limestone, these oxides will be mainly contributed by the clay. In this example, therefore, it is the clay composition which will largely determine SR and AR.
In general terms, two types of raw material, such as limestone and clay, can be proportioned to fix any one parameter only, say the LSF.
To fix x parameters, x+1 materials of suitable composition are needed, so to control all three parameters, LSF, SR and AR, a cement works needs to blend four different materials of suitable composition. On a coal-fired works, the composition of the coal ash also needs to be allowed for, since the ash falls onto the part-reacted feed and combines with it.
In practice, a works may have 5 or 6 raw materials in order to control composition.
Alite is the clinker mineral that contributes most to strength in concrete, especially earlier strengths. Therefore, where high early strengths are important, the cement producer may want to maximise the alite content; it might appear logical that he would want all the silicates to be present as alite, with no belite present in the clinker. This
may be so but often it isn't quite that simple.
Optimum burning regime
For a given mix, there will be an optimum burning regime. Under-burning will not combine most of the lime to make alite. However, over-burned clinker is likely to contain silicates that are less hydraulically reactive - they react more slowly with water. Harder burning, at a higher temperature or a longer period of time or both, may therefore combine more free lime but at the expense of silicate reactivity.
If the manufacturer tries to increase the alite content too far, he may produce a clinker that has more alite, but less-reactive alite. Overall, the clinker may produce better strengths with a slightly lower proportion of more reactive alite.
Effect of coal ash
Where coal is the fuel for the kiln, the raw mix composition has also to take into account the effect of coal ash, as much of the ash will become incorporated into the clinker. The quantity of ash is enough to have a significant effect on clinker composition - ash may represent perhaps 2%-3%, or more, of the clinker.
Portland cement clinker: the Bogue calculation
The Bogue calculation is used to calculate the approximate proportions of the four main minerals in Portland cement clinker.
The standard Bogue calculation refers to cement clinker, rather than cement, but it can be adjusted for use with cement. Although the result is only approximate, the calculation is an extremely useful and widely-used calculation in the cement industry.
The calculation assumes that the four main clinker minerals are pure minerals with compositions:
Alite: C3S, or tricalcium silicate
Belite: C2S, or dicalcium silicate
Aluminate phase: C3A, or tricalcium aluminate
Ferrite phase: C4AF, or tetracalcium aluminoferrite
It is important to remember that these assumed compositions are only approximations to the actual compositions of the minerals.
Clinker is made by combining lime and silica and also lime with alumina and iron. If some of the lime remains uncombined, (which it almost certainly will) we need to subtract this from the total lime content before we do the calculation in order to get the best estimate of the proportions of the four main clinker minerals present. For this reason, a clinker analysis normally gives a figure for uncombined free lime.
(NB: If it is desired only to calculate the potential mineral proportions in a clinker, the correction for uncombined free lime can be ignored; the calculation will then give the clinker mineral proportions assuming that all the lime has combined).
The calculation is simple in principle:
Firstly, according to the assumed mineral compositions, ferrite phase is the only mineral to contain iron. The iron content of the clinker therefore fixes the ferrite content.
Secondly, the aluminate content is fixed by the total alumina content of the clinker, minus the alumina in the ferrite phase. This can now be calculated, since the amount of ferrite phase has been calculated.
Thirdly, it is assumed that all the silica is present as belite and the next calculation determines how much lime is needed to form belite from the total silica content of the clinker. There will be a surplus of lime.
Fourthly, the lime surplus is allocated to the belite, converting some of it to alite.
In practice, the above process of allocating the oxides can be reduced to the following equations, in which the oxides represent the weight percentages of the oxides in the clinker:
BOGUE CALCULATION
C3S = 4.0710CaO-7.6024SiO2-1.4297Fe2O3-6.7187Al2O3
C2S = 8.6024SiO2+1.0785Fe2O3+5.0683Al2O3-3.0710CaO
C3A = 2.6504Al2O3-1.6920Fe2O3
C4AF = 3.0432Fe2O3
Clinker analysisSiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O SO3 LOI IR Total21.5 5.2 2.8 66.6 1.0 0.6 0.2 1.0 1.5 0.5 98.9Free lime = 1.0% CaO
Using the above analysis, the calculation is as follows:
Worked example of a Bogue calculation:
Combined CaO = (66.6% - 1.0% free lime) = 65.6%
This is the figure we use for CaO in the calculation.
From the analysis, we have:
CaO=65.6%; SiO2=21.5%; Al2O3=5.2% and Fe2O3=2.8%
The Bogue calculation is therefore:
C3S = 4.0710CaO-7.6024SiO2-1.4297Fe2O3-6.7187Al2O3
C2S = 8.6024SiO2+1.1Fe2O3+5.0683Al2O3-3.0710CaO
C3A = 2.6504Al2O3-1.6920Fe2O3
C4AF = 3.0432Fe2O3
Therefore:
C3S = (4.0710 x 65.6)-(7.6024 x 21.5)-(1.4297 x 2.8)-(6.718 x 5.2)
C2S = (8.6024 x 21.5)+(1.0785 x 2.8)+(5.0683 x 5.2)-(3.0710 x 65.6)
C3A = (2.6504 x 5.2)-(1.6920 x 2.8)
C4AF = 3.0432 x 2.8
So:
C3S = 64.7%
C2S = 12.9%
C3A = 9.0%
C4AF = 8.5%
It should be stressed that the Bogue calculation does not give the 'true' amounts of the four main clinker phases present, although this is sometimes forgotten. The results of the Bogue calculation differ from the 'true' amounts (often called the phase proportions) principally because the actual mineral compositions differ - often only slightly, but occasionally more so, particularly in the case of the ferrite phase - from those assumed in the calculation.
To adjust the calculation for use with Portland cement, it is necessary to consider first what other materials may be present in the cement. If the cement is a mixture of clinker and gypsum only, the calcium bound with the gypsum can be allowed for approximately by deducting (0.7 x SO3) from the total CaO. Note that this does not allow for any clinker sulfate present as potassium or sodium sulfate and a small error will therefore be introduced. A
similar adjustment can be carried out for limestone; the limestone content can be estimated by determining the CO2 content of the cement and calculating the coresponding CaO. If either slag or fly ash is present, in principle the formula could be adjusted to take it into account, but the slag or ash composition would need to be known accurately and in practice this is not an adjustment normally made.
Cement MillingCement milling is usually carried out using ball mills with two or more separate chambers containing different sizes of grinding media (steel balls).
Grinding clinker requires a lot of energy. How easy a particular clinker is to grind ("grindability") is difficult to predict, but rapid cooling of the clinker is thought to improve grindability due to the presence of microcracks in alite and to the finer crystal size of the flux phases. It is frequently observed that belite crystals, which have a characteristic round shape, tend to separate and form single crystal grains during grinding.
As part of the grinding process, calcium sulfate is added as a set regulator, usually in the form of gypsum (CaSO4.2H2O). Natural anhydrite may also be added to discourage lumpiness of the gypsum due to its water content.
Since the clinker gets hot in the mill due to the heat generated by grinding, gypsum can be partly dehydrated. It then forms hemihydrate, or plaster of Paris - 2CaSO4.H2O. On further heating, hemihydrate dehydrates further to a form of calcium sulfate known as soluble anhydrite (~CaSO4). This has a similar solubility in water to hemihydrate, which in turn has a higher solubility than either gypsum or natural anhydrite.
Cement mills need to be cooled to limit the temperature rise of the cement. This is done by a mixture of both air-cooling and water-cooling, including spraying water inside the mill.
The relative proportions and different solubilities of these various types of calcium sulfate are of importance in controlling the rate the rate of C3A hydration and consequently of cement set retardation. Problems associated with setting and strength characteristics of concrete can often be traced to changes in the quantity of gypsum and hemihydrate, or with variations in cooling rate of the clinker in the kiln and subsequent changes in the proportions or size of the C3A crystals.
For set regulation, the most important feature of aluminate is not necessarily the absolute amount present, but the amount of surface which is available to water for reaction. This will be governed by many factors, such as the surface area of the cement, the grinding characteristics of the different phases and also the size of the aluminate crystals. Over-large crystals can lead to erratic setting characteristics.
Cement hydration
By the process of hydration (reaction with water) Portland cement mixed with sand gravel and water produces the synthetic rock we call concrete. Concrete is as essential a part of the modern world as are electricity or computers.
Other pages on this web site describe how PC is made and what is in it. Here, we will discuss what happens when it is mixed with water.
Clinker is anhydrous (without water) having come from a hot kiln. Cement powder is also anhydrous if we ignore the small amount of water in any gypsum added at the clinker grinding stage.
The reaction with water is termed "hydration". This involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.
The hydration process: reactions
In the anhydrous state, four main types of minerals are normally present: alite, belite, aluminate (C3A) and a ferrite phase (C4AF). For more information on the composition of clinker, see the clinker pages. Also present are small amounts of clinker sulfate (sulfates of sodium, potassium and calcium) and also gypsum, which was added when the clinker was ground up to produce the familiar grey powder.
When water is added, the reactions which occur are mostly exothermic, that is, the reactions generate heat. We can get an indication of the rate at which the minerals are reacting by monitoring the rate at which heat is evolved using a technique called conduction calorimetry. An illustrative example of the heat evolution curve produced is shown below.
Three principal reactions occur:
Almost immediately on adding water some of the clinker sulphates and gypsum dissolve producing an alkaline, sulfate-rich, solution.
Soon after mixing, the (C3A) phase (the most reactive of the four main clinker minerals) reacts with the water to form an aluminate-rich gel (Stage I on the heat evolution curve above). The gel reacts with sulfate in solution to form small rod-like crystals of ettringite. (C3A) reaction is with water is strongly exothermic but does not last long, typically only a few minutes, and is followed by a period of a few hours of relatively low heat evolution. This is called the dormant, or induction period (Stage II).
The first part of the dormant period, up to perhaps half-way through, corresponds to when concrete can be placed. As the dormant period progresses, the paste becomes too stiff to be workable.
At the end of the dormant period, the alite and belite in the cement start to react, with the formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the main period of hydration (Stage III), during which time concrete strengths increase. The individual grains react from the surface inwards, and the anhydrous particles become smaller. (C3A) hydration also continues, as fresh crystals become accessible to water.
The period of maximum heat evolution occurs typically between about 10 and 20 hours after mixing and then gradually tails off. In a mix containing PC only, most of the strength gain has occurred within about a month. Where PC has been partly-replaced by other materials, such as fly ash, strength growth may occur more slowly and continue for several months or even a year.
Ferrite reaction also starts quickly as water is added, but then slows down, probably because a layer of iron hydroxide gel forms, coating the ferrite and acting as a barrier, preventing further reaction.
Hydration products
The products of the reaction between cement and water are termed "hydration products." In concrete (or mortar or other cementitious materials) there are typically four main types:
Calcium silicate hydrate: this is the main reaction product and is the main source of concrete strength. It is often abbreviated, using cement chemists' notation, to "C-S-H," the dashes indicating that no strict ratio of SiO2 to CaO is inferred. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50.
Calcium hydroxide - Ca(OH)2: often abbreviated to 'CH.' CH is formed mainly from alite hydration. Alite has a Ca:Si ratio of 3:1 and C-S-H has a Ca/Si ratio of approximately 2:1, so excess lime is available; this produces CH.
Ettringite: ettringite is present as rod-like crystals in the early stages of reaction. The chemical formula for ettringite is [Ca3Al(OH)6.12H2O]2.2H2O] or, mixing notations, C3A.3CaSO4.32H2O.
Monosulfate: monosulfate tends to occur in the later stages of reaction, after a few days. Usually, it replaces ettringite, either fully or partly. The chemical formula for monosulfate is C3A.CaSO4.12H2O. Both ettringite and monosulfate are compounds of C3A, CaSO4 (anhydrite) and water, in different proportions.
AFm and AFt phases: monosulfate is one of a group of minerals called "AFm" phases. Ettringite is a member of a group known as AFt phases. The general definitions of these phases are somewhat technical, but ettringite is an AFt phase because it contains three (t-tri) molecules of anhydrite when written as C3A.3CaSO4.32H2O and monosulfate is an AFm phase because it contains one (m-mono) molecule of anhydrite when written as C3A.CaSO4.12H2O.
Important points to note about AFm and AFt phases are that:
They contain a lot of water, especially the AFt phases. They contain different ratios of sulfur to aluminium. The aluminium can be partly-replaced by iron in both AFm and AFt phases. The sulfate ion in AFm phases can be replaced by other anions; a one-for-one
substitution if the anion is doubly-charged(eg: carbonate, CO22-) or one-for-two if the substituent anion is singly-charged (eg: hydroxyl, OH- or chloride, Cl-). The sulfate in ettringite can be replaced by carbonate or, probably, partly replaced by two hydroxyl ions.
Monosulfate gradually replaces ettringite in many concretes because the ratio of available alumina to sulfate increases with continued cement hydration. On first contact with water, most of the sulfate is readily available to dissolve, but much of the C3A is contained inside cement grains with no initial access to water. Continued hydration gradually releases alumina and the proportion of ettringite decreases as that of monosulfate increases.
If there is eventually more alumina than sulfate available, all the sulfate will be as monosulfate, with the additional alumina present as hydroxyl-substituted AFm phase. If there is an excess of sulfate, the cement paste will contain a mixture of monosulfate and ettringite.
If fine limestone is present (interground with the cement or as fine aggregate), monocarbonate may form; the proportion of monosulfate will then decrease, with displaced sulfate forming additional ettringite. See the UC e-book for more information.
Near the concrete surface, carbonation will release sulfate as carbonate ions replace sulfate in the ettringite and monosulfate phases.
Hydrogarnet: hydrogarnet forms mainly as the result of ferrite or C3A hydration. Hydrogarnets have a range of compositions, of which C3AH6 is the main phase forming from normal cement hydration and then only in small amounts. A wider range of hydrogarnet compositions can be found in autoclaved cement products.
Concrete strengthMany factors influence the rate at which the strength of concrete increases after mixing. Some of these are discussed below. First, though a couple of definitions will be useful:
The process of strength growth is called 'hardening.' This is often confused with 'setting' but setting and hardening are not the same.
Setting is the stiffening of the concrete after it has been placed. A concrete can be 'set' in that it is no longer fluid, but it may still be very weak; you may not be able to walk on it, for example. Setting is due to early-stage calcium silicate hydrate formation and to ettringite formation. The terms 'initial set' and 'final set' are arbitrary definitions of early and later set; there are laboratory procedures for determining these using weighted needles penetrating into cement paste.
Hardening is the process of strength growth and may continue for weeks or months after the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement continues to hydrate.
The rate at which concrete sets is independent of the rate at which it hardens. Rapid-hardening cement may have similar setting times to ordinary Portland cement.
Autoclaved aerated concrete (AAC, Aircrete)
Introduction
Autoclaved aerated concrete is a versatile lightweight construction material and usually used as blocks. Compared with normal (ie: “dense” concrete) aircrete has a low density and excellent insulation properties.
The low density is achieved by the formation of air voids to produce a cellular structure. These voids are typically 1mm - 5mm across and give the material its characteristic appearance. Blocks typically have strengths ranging from 3-9 Nmm-2 (when tested in accordance with BS EN 771-1:2000). Densities range from about 460 to 750 kg m-3; for comparison, medium density concrete blocks have a typical density range of 1350-1500 kg m-3 and dense concrete blocks a range of 2300-2500 kg m-3.
Autoclaved aerated concrete block with a sawn surface to show the cellular pore structure(Picture courtesy H+H UK Ltd.)
Detailed view of cellular pore structure in an aircrete block.
Autoclaved aerated concrete blocks are excellent thermal insulators and are typically used to form the inner leaf of a cavity wall. They are also used in the outer leaf, when they are usually rendered, and in foundations. It is possible to construct virtually an entire house from autoclaved aerated concrete, including walls, floors - using reinforced aircrete beams, ceilings and the roof. Autoclaved aerated concrete is easily cut to any required shape.
Aircrete also has good acoustic properties and it is durable, with good resistance to sulfate attack and to damage by fire and frost.
Production
Autoclaved aerated concrete is cured in an autoclave - a large pressure vessel. In aircrete production the autoclave is normally a steel tube some 3 metres in diameter and 45 metres long. Steam is fed into the autoclave at high pressure, typically reaching a pressure of 800 kPa and a temperature of 180 °C.
Autoclaved aerated concrete can be produced using a wide range of cementitous materials, commonly:
Portland cement, lime and pulverised fuel ash (PFA)
or
Portland cement, lime and fine silica sand. The sand is usually milled to achieve adequate fineness.
A small amount of anhydrite or gypsum is also often added.
Autoclaved aerated concrete is quite different from dense concrete (ie: “normal concrete”) in both the way it is produced and in the composition of the final product.
Dense concrete is typically a mixture of cement and water, often with slag or PFA, and fine and coarse aggregate. It gains strength as the cement hydrates, reaching 50% of its final strength after perhaps about 2 days and most of its final strength after a month.
In contrast, autoclaved aerated concrete is of much lower density than dense concrete. The chemical reactions forming the hydration products go virtually to completion during autoclaving and so when removed from the autoclave and cooled, the blocks are ready for use.
Autoclaved aerated concrete does not contain any aggregate; all the main mix components are reactive, even milled sand where it is used. The sand, inert when used in dense concrete, behaves as a pozzolan in the autoclave due to the high temperature and pressure.
The autoclaved aerated concrete production process differs slightly between individual production plants but the principles are similar. We will assume a mix that contains cement, lime and sand; these are mixed to form a slurry. Also present in the slurry is fine aluminium powder - this is added to produce the cellular structure. The density of the final block can be varied by changing the amount of aluminium powder in the mix.
The slurry is poured into moulds that resemble small railway wagons with drop-down sides. Over a period of several hours, two processes occur simultaneously:
The cement hydrates normally to produce ettringite and calcium silicate hydrates and the mix gradually stiffens to form what is termed a "green cake".
The green cake rises in the mould due to the evolution of hydrogen gas formed from the reaction between the fine aluminium particles and the alkaline liquid. These gas bubbles give the material its cellular structure.
Slurry being poured into moulds (Picture courtesy H+H UK Ltd.)
Green cake rising in mould (Picture courtesy H+H UK Ltd.)
There are some parallels between autoclaved aerated concrete production and bread-making. In bread, the dough contains yeast and is mixed, then left to rise as the yeast converts sugars to carbon dioxide.
The dough must have the right consistency; too hard and the bubbles of carbon dioxide cannot 'stretch' the dough to make it rise, but if the dough is too sloppy, the carbon dioxide bubbles rise to the surface and are lost and the dough collapses. With the right consistency, the dough is sufficiently elastic to stretch and expand, but strong enough to retain the gas so that the dough does not collapse. When risen, the dough is placed in the oven.
Although a much more complex process, Aircrete production conditions are precisely-controlled for, in part, somewhat similar reasons. The mix proportions and the initial mix temperature must be correct and the aluminium powder must be present in the
required amount and with the appropriate reactivity an an alkaline environment. All of the materials be be of suitable fineness. A complicating factor is that the temperature of the green cake increases due to the exothermic reactions as the lime and the cement hydrate, so the reactions proceed faster.
When the cake has risen to the required height, the mould moves along a track to where the cake is cut to the required block size. Depending on the actual production process, the cake may be demoulded entirely onto a trolley before cutting, or it may be cut in the mould after the sides are removed.
The cake is cut by passing through a series of cutting wires.
Green cake being cut by wires (Picture courtesy H+H UK Ltd.)
At the cutting stage, the blocks are still green - only a few hours has passed since the mix was poured into the mould and they are soft and easily damaged. However, if they are too soft, the cut blocks may either fall apart or stick together; if they are too hard, the wires will not cut them - here too, the process has to be carefully controlled to achieve the necessary consistency.
The cut blocks are then loaded into the autoclave. It takes a couple of hours for the autoclave to reach maximum temperature and pressure, which is held for perhaps 8-10 hours, or longer for high density/high strength aircrete.
"Green" blocks being loaded into an autoclave (Picture courtesy H+H UK Ltd.)
When removed from the autoclave and cooled, the blocks have achieved their full strength and are packed ready for transport.
AAC Composition
The essence of aircrete production is that lime from the cement and lime in the mix reacts with silica to form 1.1 nm tobermorite.
NB: Cement chemistry notation is used below. If you are not familiar with this, see ourcement chemistry notation explained page.
During the green stage, the cement is hydrating at normal temperatures and the hydration products are initially similar to those in dense concrete - C-S-H, CH and ettringite and/or monosulfate. After autoclaving, tobermorite is normally the principal final reaction product due to the high temperature and pressure.
Small amounts of other hydrated phases will also be present in the final product. Additionally, hydrated phases form in the autoclave as intermediate products, principally C-S-H(I). This is a more crystalline form of calcium silicate hydrate than occurs in dense concrete; it can have a ratio of calcium to silicon of (0.8<Ca/Si<1.5) but 0.8 to 1.0 is desirable as this ratio facilitates the formation of 1.1 nm tobermorite.
The compositions of the hydration products in aircrete are therefore quite different from those in dense concrete cured at normal temperatures (ie: calcium silicate hydrate (C-S-H), calcium hydroxide (CH), ettringite and monosulfate. See the “hydration” page for more information).
Looking at this in a little more detail from when the green blocks enter the autoclave, the main reactions that occur are broadly as follows:
Over 2 hours or so, as the pressure and temperature increase, the normal cement hydration products that formed in the green state progressively disappear and the sand becomes reactive.
C-S-H(I) forms, partly from silica derived from the sand. As more sand reacts, calcium hydroxide from the lime and from cement
hydration is gradually used up by continued formation of C-S-H(I). With continued autoclaving, 1.1 nm tobermorite starts to crystallize from the C-S-
H(I); the total proportion of C-S-H(I) declines and that of 1.1 nm tobermorite gradually increases. C-S-H(I) is therefore mainly an intermediate compound.
The final hydration products are then principally:
1.1nm tobermorite Possibly some residual C-S-H(I) Hydrogarnet
Unreacted sand is likely to remain in the final product. There may also be some residual
calcium hydroxide if insufficient silica has reacted and some residual anhydrite and/or hydroxyl-ellestadite if anhdrite was present in the mix.
SEM image of polished section showing a detail - a cell wall - of a block made with cement, lime and sand mix. Some residual unreacted sand particles remain (examples arrowed), often with rims of hydration product showing the size of the original particle. Most of the matrix is composed of tobermorite. Black areas at top left and bottom right are epoxy resin used in preparing the polished section filling air voids (air cells).
The objective is to react sufficient silica from the sand to form tobermorite from the available lime supplied by the lime and cement. This will depend on a range of factors, including the inherent reactivities of the materials, their fineness (especially the sand), and the temperature and pressure. If the autoclaving time is too short, the tobermorite content will not be maximised and some unreacted calcium hydroxide will remain and block strengths will be then less than optimum. If the autoclaving time is too long, other hydration products may form which may also be detrimental to strength and an unnecessary energy cost will be incurred.
There are different forms of tobermorite: 1.1 nm tobermorite and 1.4 nm tobermorite. Also, there are different types of 1.1 nm tobermorite and these behave differently when heated. Their crystal structure is that of layered sheets, with water molecules between the layers - on heating, the inter-layer water is lost; as a result, some 1.1 nm tobermorites shrink (a process known as lattice shrinkage) but some don’t.
1.4 nm tobermorite (C5S6H9) - forms at room temperature and is found as a natural mineral. It decomposes at 55 °C to 1.1 nm tobermorite, and so is not found in AAC.
Calcium silicate hydrate compositions in AAC
1.1 nm tobermorite (C5S6H5) is usually the main hydration product in AAC where cement, lime and sand are used
C-S-H(I) - more crystalline than C-S-H in dense concrete, typically 0.8<Ca/Si<1.0. Xonotlite (C6S6H) - forms with longer autoclaving times, or higher temperatures
'Normal'tobermorite shows lattice shrinkage, while non-shrinking tobermorite is called 'anomalous' tobermorite. Tobermorite in AAC made with cement, lime and sand is usually normal tobermorite. Tobermorite in autoclaved aerated concrete made with cement, lime and PFA is usually anomalous tobermorite. Aluminium and alkali together in solution (such as will be present in mixes of cement, lime and PFA) tend to produce anomalous tobermorite, with some aluminium and alkali taken up into the tobermorite crystal structure. The differences between the different forms of autoclaved calcium silicate hydrates are not well-defined; in an AAC block, intimately-mixed hydrates of different compositions and crystallinity are likely to occur.
Other hydrothermally-formed minerals
Gyrolite (C2S3H2) - not normally found in AAC Jennite (C9S6H11) occurs as a natural mineral; not found in AAC C-S-H(II) - Ca/Si≈ 2.0. Does not occur in AAC C2SH (α-C2S hydrate) can occur in autoclaved products but is undesirable Hydroxyl-ellestadite (C10S3.3SO3.H2O) - may be found in AAC; also occurs at the
cooler end of cement kilns
Environmental benefits of Autoclaved Aerated Concrete
The use of autoclaved aerated concrete has a range of environmental benefits:
Insulation: most obviously, the insulation properties of aircrete will reduce the heating costs of buildings constructed with autoclaved aerated concrete, with consequent fuel savings over the lifetime of the building.
Materials: lime is one of the principal mix components and requires less energy to produce than Portland cement, which is fired at higher temperatures. Sand requires only milling before use, not heating, and PFA is a by-product from electricity generation. NB: lime may require less energy to manufacture compared with Portland cement but more CO2 is produced per tonne (cement approx. 800-900 kg CO2/tonne compared to lime at 1000 kg CO2 per tonne).
Carbonation: less obviously, the cellular structure of aircrete gives it a very high surface area. Over time, much of the material is likely to carbonate, largely
offsetting the carbon dioxide produced in the manufacture of the lime and cement due to the calcining of limestone.
Cement analysisTraditionally, cement analysis was carried out using wet-chemical techniques. Now, the days of flasks bubbling away over bunsen burners in the laboratory of a cement works are largely gone, replaced by X-ray analysis equipment of various types.
At a cement works (=plant, factory, production facility) raw materials, clinker and cement are analysed using X-ray fluorescence (XRF) and, often, X-ray diffraction (XRD). These techniques are used routinely, day in, day out and are the principal means of controlling composition of raw materials, the raw feed, clinker and cement, in other words XRF provides rapid compositional data for controlling almost all stages of production.
Another technique used either routinely, or as required is microscopy. Cement microscopy has a wide range of applications in examining raw materials, coal, clinker and cement. Optical microscopy is used routinely on some cement works as a technique of kiln control.
The basics of X-ray fluorescence and X-ray diffraction are described below. Cement microscopy has its own page.
X-ray fluorescence (XRF)
The basic principle of XRF analysis is simple. (The physics of what happens is complex, but we won't concern ourselves too much with the details.)
If we zap a sample of, say, cement with a beam of X-rays, the X-ray beam will cause other X-rays to be generated within the cement. Some of these X-rays escape from the cement and are collected by a suitably-positioned X-ray detector. Many of these collected X-rays have energies which are characteristic of the atomic number of the atom in which they were generated.
In other words, by measuring the energies of the X-rays, we can tell what elements they came from.
If we measure these energies under carefully-controlled conditions, we can count the X-rays from each element over a period of time, say one minute, and from this we can calculate the proportions of each element in the cement sample.
Looking at this in just slightly more detail, the beam of X-rays zapping the target cement specimen excite the atoms within the target by removing an electron from an orbital 'shell' around the atom. This electron is replaced by another from a different shell; the transfer of electrons from one shell to another results in a loss of energy specific to those shells and that element. The lost energy is radiated from the atom in
the form of an X-ray. This process, in which one X-ray excites another X-ray, is called 'X-ray fluorescence'.
The specimen of cement or other material may be in the form of a glass bead or a pellet of pressed powder.
Beads are made by heating the specimen together with a flux, typically lithium tetraborate, at about 1100 C to form a glass. This approach has the advantage that the specimen is then a homogeneous material, allowing more accurate X-ray analysis.
Pressed pellets are made by grinding the specimen finely and compressing the resulting powder to form a pellet. This pellet is then analysed directly. Preparing pressed pellets is quicker and easier than preparing glass beads, but the specimen is then a heterogeneous material. This makes the calculation of the processes of X-ray fluorescence and absorption within the specimen more complicated and there may be some loss of accuracy, although it should be minimal.
XRF is at the heart of the control of the production process in any modern cement works and is central to the ability of the cement maker to produce a consistent product.
Cement microscopyCement microscopy is a very powerful technique, used for examining clinker, cement, raw materials, raw feed and coal. Every stage of the cement manufacturing process can be improved through the use of a microscope.
Most cement microscopy is done using a petrographic microscope. Usually the specimen is a polished section of cement clinker examined using reflected light, although it may be a powder mount or a thin section examined using transmitted light.
A scanning electron microscope (SEM) may also be used. The combination of SEM with X-ray microanalysis (=EDX, EDAX) is very powerful as it enables the analysis of individual crystals or particles.
Details of the history of a clinker can be seen - raw material fineness and homogeneity, clinker composition, temperature profile in the kiln, for example. From this information, the likely performance of the cement can be predicted or the cause of production problems identified such as poor combinability, or low grindability.
Some cement manufacturers use microscopy as a technique for kiln control, with clinker samples being examined continuously. Other manufacturers use it occasionally on an 'as required' basis while some manufacturers never use it at all. (As cement microscopists ourselves, if we may let a little bias creep in here, we would say these cement producers who don't use microscopy are really missing out on making better cement at lower cost.)
Important characteristics the microscopist examines are:
Overall nodule microstructure - dense, porous, dense micronodules interconnected by tenuous 'bridges'. This gives a broad relative indication of burning conditions.
Alite crystal size - coarse alite may indicate a slow heating rate, excessive burning or coarse silica in the raw feed; silicate reactivity may be lower than it could be with improved burning conditions.
Belite crystal size - larger belite crystals suggest longer time in the burning zone.
Aluminate and ferrite crystal size - coarse flux phases suggest slow cooling; finer, intergrown, flux phases indicate faster cooling. Belite colour also indicates the cooling rate; fast-cooled crystals are clear while slower cooling allows impurities to crystallize out along lattice planes imparting a yellow colour.
Large clusters of belite or free lime - these may indicate coarse particles in the raw feed.
Overall distribution of silicates - ideally, belite crystals will be dispersed evenly throughout each clinker nodule, either as individual crystals or in small clusters. If large clusters of belite are present, burnability is likely to be less good and the clinker will be harder to grind.
Other clinker mineral characteristics can indicate very slow cooling, reducing conditions, an excess of alkali over sulfate in the clinker and other adverse conditions.
Coming soon! We are preparing a book, an introduction to scanning electron microscopy of cement and concrete. Click here for more details.
Our own dedicated cement microscopy website has more information on both optical and scanning electron microscopy.
The International Cement Microscopy Association (ICMA) holds an annual meeting, usually in the USA. The proceedings of the meetings are a valuable source of reference.
SEM (Scanning Electron Microscopy) of cement and
concrete: an introductory e-book
SEM / EDX is an important technique in the study of cement and concrete. This e-book, currently in preparation, will describe the application of scanning electron microscopy (SEM) and X-ray microanalysis (EDX) to cement and concrete, including:
basics of SEM, EDX/WDX and image analysis choosing equipment: what is necessary, what is desirable and what are just luxuries? specimen preparation: fracture surfaces and polished sections examination of clinker and cement: how to do it and what information can be
obtained examination of concrete: how to do it and what information can be obtained,
including: estimation of water-cement ratios; measurement of mix proportions; cement type determination; quantitative X-ray microanalysis and hydrate characterisation; identification of deleterious processes
The author, Nick Winter, worked for a major cement manufacturer for eight years, where he worked mainly on scannning electron microscopy. For the last twenty years, he has been a director of WHD Microanalysis Consultants Ltd., a small independent company specialising in SEM and optical microscopy of cementitious materials. Nick is no stranger to running an SEM on a small budget, and this book tells you how to make the most of your money in purchasing equipment and how to make the most of the equipment once you have got it.
The book will be of interest to anyone applying SEM to cementitious materials or setting up an electron microscope laboratory. The price has yet to be determined.
If you would like us to e-mail you when the book is available (with no commitment) please complete the short form below.
Deleterious processes in concreteThe main categories of deleterious processes in concrete, by which the concrete deteriorates as the result of chemical attack, are outlined below. Each of these is a huge subject in its own right, the subject of many papers and conferences. Here we can only summarize the main characteristics and suggest sources of more detailed information.
Of course, concrete can also suffer from physical damage, for example through impact, abrasion or frost action.
It should be stressed that concrete which has been designed appropriately for the conditions in which it is used, and produced in accordance with the design, should have a very long service life. That said, sometimes problems do arise. Often, these can be traced back to poor design or workmanship.
Some of the most common causes of concrete deterioration are:
Cracking and spalling due to corrosion of steel reinforcement Leaching Sulfate attack Alkali-silica reaction Carbonation
There are many other forms of chemical attack which can damage concrete, including oils or fats, acid, and salt solutions.
Strictly speaking, carbonation is not detrimental to concrete; in fact the compressive strength of carbonated concrete is higher than that of uncarbonated concrete. However, since carbonation can be closely associated with other causes of concrete deterioration, and involves major chemical changes in the cement paste within the affected concrete, it will be considered here too.
Usually, concrete which is affected by one of these processes will also show signs of others. To take an extreme example, consider a concrete cured at high temperature - this could be a steam-cured concrete or concrete from a large pour where the heat of hydration of the cement has raised the temperature above about 70 C.
In such a concrete, a particular form of sulfate attack known as delayed ettringite formation (DEF) could occur. DEF is often associated with alkali-silica reaction, if susceptible aggregate is also present. Both of these processes induce cracking in the concrete; this may allow water to percolate through the cracks, resulting in leaching. In addition, the cracks will allow carbon dioxide to penetrate deep into the concrete as gas or dissolved in rain water, initiating carbonation at depth within the concrete. If steel reinforcement is present in the affected area, corrosion is likely to occur. This concrete would then be showing signs of all five deleterious processes discussed here.
Alkali-silica reaction in concreteAlkali-silica reaction (ASR) can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition.
ASR is the most common form of alkali-aggregate reaction (AAR) in concrete; the other, much less common, form is alkali-carbonate reaction (ACR). ASR and ACR are therefore both subsets of AAR.
ASR is caused by a reaction between the hydroxyl ions in the alkaline cement pore solution in the concrete and reactive forms of silica in the aggregate (eg: chert, quartzite, opal, strained quartz crystals).
A gel is produced, which increases in volume by taking up water and so exerts an expansive pressure, resulting in failure of the concrete. In unrestrained concrete (that
is, without any reinforcement), ASR causes characteristic 'map cracking' or 'Isle of Man cracking'.
Gel may be present in cracks and within aggregate particles. The best technique for the identification of ASR is the examination of concrete in thin section, using a petrographic microscope. Alternatively, polished sections of concrete can be examined by scanning electron microscopy (SEM); this has the advantage that the gel can be analysed using X-ray microanalysis in order to confirm the identification beyond any doubt.
Figure 1 Concrete thin-section, viewed with a petrographic microscope, showing a chert aggregate particle (at the right of the image) from which alkali-silica gel has extruded into adjacent cracks.
Figure 2 Polished section of concrete, viewed with a scanning electron microscope, showing a chert aggregate particle with extensive internal cracks due to ASR. The cracks extend from the aggregate into the nearby concrete (arrowed).
Figure 3 Detail of the chert particle in the previous image and adjacent cement paste, showing alkali-silica gel extruded into cracks within the concrete. Ettringite is also present within some cracks.
The conditions required for ASR to occur are:
A sufficiently high alkali content of the cement (or alkali fromother sources) A reactive aggregate, such as chert Water - ASR will not occur if there is no available water in the concrete, since
alkali-silica gel formation requires water
The use of pozzolans in the concrete mix as a partial cement replacement can reduce the likelihood of ASR occurring as they reduce the alkalinity of the pore fluid.
With some aggregates, expansion due to ASR increases in proportion with the amount of reactive aggregate in the concrete. Other aggregates show what is called a “pessimum” effect; if the proportion of reactive aggregate in test mixes is varied, while other factors are kept constant, maximum concrete expansion occurs at a particular aggregate content. Higher or lower proportions of reactive aggregate will give a lower expansion.
The process of ASR is believed to be in many respects similar to the pozzolanic reaction, such as occurs normally in concrete containing pulverised fuel ash (PFA), for example. However, there is an important difference. In the pozzolanic reaction small pozzolanic
particles are reacting in a Ca-rich environment, while ASR occurs in mature concrete and involves larger particles of aggregate.
The pozzolanic reaction mechanism is believed to be a process in which silicate anions are detached from the reactive aggregate by hydroxyl ions in the pore fluid. Sodium and potassium ions are the ions most readily-available to balance the silicate anions and an alkali-silicate gel is formed. This can take up (imbibe) water and is mobile. The alkali-silicate gel is unstable in the presence of calcium, and calcium silicate hydrate (C-S-H) is formed.
In the pozzolanic reaction where a pozzolan is used as a partial cement replacement, the particles are small. As there is much calcium available in young concrete, the alkali-silicate gel forms in a thin layer around the pozzolanic particle and quickly converts to C-S-H. No expansion results.
In the case of alkali-silica reaction, the reaction usually occurs much later, possibly years after the concrete was placed. Large aggregate particles (large, that is, compared with cement-sized pozzolan) generate a significant volume of gel which then takes up water and expands within the hardened, mature concrete.
Because the concrete is mature, calcium availability is limited as most of the calcium is bound up in stable solid phases. The rate of supply of calcium is therefore insufficient to convert the gel quickly to C-S-H. Expansion of the gel as water is taken up, may result in damage to the surrounding concrete. Over time, the gel slowly does take up calcium; eventually the composition of the alkali-silica gel may become very similar to that of the calcium silicate hydrate in the cement paste (see Figure 4). By then, though, the damage to the concrete may have already been done.
Figure 4 Same image as in Figure 3, with X-ray spectra superimposed showing how alkali-silica gel composition changes with time to become more like the
surrounding calcium silicate hydrates. In a) the gel spectrum shows large peaks due to silicon and potassium (the alkali) and only a very weak peak due
to calcium. In b) the calcium peak has become much stronger and the potassium peak much weaker. In c) the potassium peak has disappeared
entirely and the gel has approximately the same composition as the normal calcium silicate hydrate comprising the bulk of the cement paste. Clearly, the gel is older with increasing distance from the aggregate particle in which it originated - the 'oldest' gel has had more time in which to take up calcium from the surrounding paste, and has now become calcium silicate hydrate.
Sulfate attack in concrete and mortar
Sulfate attack can be 'external' or 'internal'.
External: due to penetration of sulfates in solution, in groundwater for example, into the concrete from outside.
Internal: due to a soluble source being incorporated into the concrete at the time of mixing, gypsum in the aggregate, for example.
External sulfate attack
This is the more common type and typically occurs where water containing dissolved sulfate penetrates the concrete. A fairly well-defined reaction front can often be seen in polished sections; ahead of the front the concrete is normal, or near normal. Behind the reaction front, the composition and microstructure of the concrete will have changed. These changes may vary in type or severity but commonly include:
Extensive cracking Expansion Loss of bond between the cement paste and aggregate Alteration of paste composition, with monosulfate phase converting to ettringite
and, in later stages, gypsum formation. The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement paste
The effect of these changes is an overall loss of concrete strength.
The above effects are typical of attack by solutions of sodium sulfate or potassium sulfate. Solutions containing magnesium sulfate are generally more aggressive, for the same concentration. This is because magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates mainly as gypsum.
Other sources of sulfate which can cause sulfate attack include:
Seawater Oxidation of sulfide minerals in clay adjacent to the concrete - this can produce
sulfuric acid which reacts with the concrete Bacterial action in sewers - anaerobic bacterial produce sulfur dioxide which
dissolves in water and then oxidizes to form sulfuric acid In masonry, sulfates present in bricks and can be gradually released over a long
period of time, causing sulfate attack of mortar, especially where sulfates are concentrated due to moisture movement
Figure 1 Scanning electron microscope image of sulfate attack in concrete. Ettringite (arrowed) has replaced some of the calcium silicate hydrate in the cement paste; the darker areas of paste have been partly decalcified. As a consequence of these alterations, the paste will be weakened. Although much of the cement paste here remains apparently unaltered (eg: top right), if widespread within the concrete (which in this instance it was) sulfate attack can significantly weaken the concrete.
Internal sulfate attack
Occurs where a source of sulfate is incorporated into the concrete when mixed. Examples include the use of sulfate-rich aggregate, excess of added gypsum in the cement or contamination. Proper screening and testing procedures should generally avoid internal sulfate attack.
Delayed ettringite formation
Delayed ettringite formation (DEF) is a special case of internal sulfate attack.
Delayed ettringite formation has been a significant problem in many countries. It occurs in concrete which has been cured at elevated temperatures, for example, where steam curing has been used. It was originally identified in steam-cured concrete railway sleepers (railroad ties). It can also occur in large concrete pours where the heat of
hydration has resulted in high temperatures within the concrete.
DEF causes expansion of the concrete due to ettringite formation within the paste and can cause serious damage to concrete structures. DEF is not usually due to excess sulfate in the cement, or from sources other than the cement in the concrete. Although excess sulfate in the cement would be likely to increase expansion due to DEF, it can occur at normal levels of cement sulfate.
A key point in understanding DEF is that ettringite is destroyed by heating above about 70 C.
A definition of delayed ettringite formation
DEF occurs if the ettringite which normally forms during hydration is decomposed, then subsequently re-forms in the hardened concrete.
Damage to the concrete occurs when the ettringite crystals exert an expansive force within the concrete as they grow.
In normal concrete, the total amount of ettringite which forms is evidently limited by the sulfate contributed by the cement initially. It follows that the quantity of ettringite which forms is relatively small. Ettringite crystals form widely-dispersed throughout the paste. If expansion causes cracking, ettringite may subsequently form in the cracks but this does not mean the ettringite in the cracks caused the cracks initially.
DEF causes a characteristic form of damage to the concrete. While the paste expands, the aggregate does not. Cracks form around these non-expanding 'islands' within the paste - the bigger the aggregate, the bigger the gap.
Figure 2 Diagram showing how paste expansion produces a small gap around small aggregate particles and a bigger gap around larger particles.
Figure 3 Scanning electron microscope image of limestone aggregate particle. The cement paste has expanded and a gap has formed between between the aggregate and the cement paste. This is characteristic of damage to concrete due to DEF. The aggregate is no longer contributing to concrete strength, since it is effectively detached from the cement paste. Often, these gaps become filled with ettringite.
Conditions necessary for DEF to occur are:
High temperature (above 65-70 degrees C approx.), usually during curing but not necessarily
Water: intermittent or permanent saturation aftercuring Commonly associated with alkali-silica reaction (ASR)
In laboratory tests, limestone coarse aggregate has been found to reduce expansion.
DEF usually occurs in concrete which has either been steam cured, or which reached a high temperature during curing as a result of the exothermic reaction of cement hydration.
As the curing temperature of concrete increases, ettringite normally persists up to about 70 C. Above this temperature it decomposes. In mature concrete, monosulfate is usually the main sulfate-containing hydrate phase and this persists up to about 100 C. DEF could occur in concrete which was heated externally, eg: from fire.
An ettringite molecule contains 32 molecules of water; ettringite formation therefore requires wet conditions.
DEF and ASR appear to be closely linked; in one study (Diamond and Ong, 1994) a mortar made using limestone aggregate was cured at 95 C. Subsequent ettringite formation within the paste was scarce and expansion was minimal. However, if aggregate susceptible to ASR was used instead of limestone, ettringite formation and expansion were both much greater. This, and other studies, suggests that ASR is, or can be, a precursor for DEF expansion.
The effect of cement composition on DEF is not well understood. Some factors correlate strongly but the causes are not clear. In laboratory tests, DEF expansion was shown to correlate positively with cement-related factors, including:
high sulfate high alkali high MgO cement fineness high C3A high C3S
DEF is still by no means fully understood. For further reading on this subject, try:
Lawrence C D 'Laboratory Studies of Concrete Expansion Arising from Delayed Ettringite Formation,' (1993) published by the British Cement Association.
Lawrence C D (1995) Cement and concrete research, Vol 25, p903.Diamond and Ong (1994) in 'Cement Technology' (Ceramic Transactions Vol.
40, p79). American Ceramic Society.Kelham S, Cement and Concrete Composites, Vol. 18, p171.
Variability of Portland cementPortland cement variability can occasionally cause problems.
"Are all nominally similar cements the same?"
"Does cement from the same source (same cement works, same kiln) always behave consistently?"
The answer to both questions is 'not necessarily.'
That said, let us first look at the other side of the coin.
Consider a modern cement works - it probably produces 1000 - 4000 tons of cement a day.
Raw materials are dug from the ground and are intrinsically variable - limestone from one end of the quarry might well contain more clay or silica than limestone from the other end for example.
Fuels are variable - coal varies in ash and sulfur contents and these days, a works may also burn ancillary fuels such as solvents or car tyres.
Variation in raw materials and fuels are likely to result in changes in kiln conditions and consequently, to some extent at least, to changes in the cement. Of course, some changes may be for the better, but unless they are maintained consistently, the result will be variability in the product.
Given all these sources of potential variability, another way of asking the question might be:
'How does the cement manufacturer achieve his normally high degree of consistency in the product?'
The short answer is that modern cement is normally more consistent and of a higher quality than ever before. This is due to more rigorous testing of the final product, by online analysis of raw materials and clinker and by improved monitoring of kilns, mills etc.. Of course, all this has been made possible by the advances in computer technology in recent years.
For example, until fairly recently, chemical analysis would be done by conventional wet chemical analysis. A sample of clinker or raw material would arrive at the laboratory and over the following few hours a chemist would have crushed it, dissolved it, heated it, titrated it and do whatever else was required by the analytical method. For each element.
In a modern works this is automated; a robot arm grabs the sample, crushes it, presses it into a pellet and analyses it by X-ray fluorescence and reports the result in less time than the chemist could determine one single element.
Consequently, if the composition of the raw materials, or the clinker, starts to deviate from the target, corrective action can be taken quickly, not some hours later or maybe the next day as was once the case. The result is that cement is now much less variable.
Despite all this progress, cement does still vary and, despite constant improvements, it probably always will. Below, we will consider some of the causes of this variability.
Cement-induced concrete strength variability
The most important property of Portland cement that is likely to vary, is the strength of concrete produced from it. Other important properties are setting times and workability (which may also impact on strength if more water is added to the mix) and colour.
Why might cement be different this week from the cement obtained from the same source last week? Cement made on a Monday is unlikely to be exactly the same as cement made at the same place on Thursday, but the chances are good that any differences will be insignificant. Sometimes, though, differences are greater.
Cement-induced concrete strength variability can be due to both physical and chemical causes. Some of the more important causes are discussed below, although there are many others.
The main physical cause of strength variability - changes in cement particle size
In any reaction, a finer material is likely to react more quickly than a coarser material and cement is no exception. Differences in cement particle size, expressed as fineness or surface area, will affect strength development. If a cement has a different particle size, there will evidently be no point in looking at the cement composition for the cause if a problem has arisen with the cement mill.
The fineness to which the cement is ground will evidently affect the rate at which concrete strengths increase after mixing. Grinding the cement more finely will result in a more rapid increase in strength. Fineness is often expressed in terms of total particle surface area, eg: 400 square metres per kilogram. However, of as much, if not more, importance as fineness is the particle size distribution of the cement; relying simply on surface area measurements can be misleading.
Some minerals, gypsum for example, can sometimes grind preferentially producing cement with a high surface area, but very fine gypsum and relatively coarse clinker particles. Strength gain won't be as fast as expected; indeed, there could be false-setting problems.
Grinding the cement more finely is likely to be the immediate response of a cement producer if he sees early strengths starting to decrease.
Chemical causes of strength variability
These include primarily:
alite content and reactivity alkali and sulfate content
Alite content and reactivity
Alite is the most reactive cement mineral that contributes significantly to concrete strength, so a higher alite content should give better early strengths ('early' in this context means up to about 7 days). Normally this is so, but not necessarily, as much depends on burning conditions in the kiln.
Generally, more alite is good from the viewpoint of early strengths. However, it is possible that lighter burning of a particular clinker could result in cement giving a higher early strength due to the formation of more reactive alite, even if there is a little less of it. Not all alite is created equal!
Large alite crystals are generally less-reactive than smaller ones. Kiln conditions leading to large alite crystals are therefore likely to produce clinker that will result in lower early strengths in the cement. Such kiln conditions are likely to be a slow rate of heating of the clinker in the kiln or prolonged burning at high temperature. Slow cooling of the clinker may also cause the alite to be less-reactive.
Alkali and sulfate: 'optimum sulfate content'
For a particular cement, there will be what is called an 'optimum sulfate content,' or 'optimum gypsum content.' Sulfate in cement, both the clinker sulfate and added gypsum, retards the hydration of the aluminate phase. If there is insufficient sulfate, a flash set may occur; conversely, too much sulfate can cause false-setting. The solubility of the sulfate, and therefore its availability to produce retardation or to cause a false set (also known as 'plaster set') depends on the starting materials used and the temperature in the cement mill.
A balance is therefore required between the ability of the main clinker minerals, particularly the aluminate phase, to react with sulfates in the early stages after mixing and the ability of the cement to supply the sulfate. The optimum sulfate content will be affected by many factors, including aluminate content, aluminate crystal size, aluminate reactivity, solubilities of the different sources of sulfate, sulfate particle sizes, milling temperatures and whether admixtures are used.
If this were not complicated enough, the amount of sulfate necessary to optimize one property, strength for example, may not be the same as that required to optimize other properties such as drying shrinkage. Additionally, concrete and mortar may have different optimum sulfate contents.
It is thought that sulfates affect strength development by influencing the microstructure of the calcium silicate hydrates, although the details of this are unclear.
More straightforward is the concept that sulfates affect workability. Concrete is normally made to a given 'slump' and not to a fixed water/cement ratio. If the sulfate content deviates from the optimum, the workability will be reduced and more water will probably be added to the mix. This will result in lower strengths at all ages.
Effect of alkalis on strength
Much of the clinker sulfate is present as alkali sulfate, usually in the form of aphthitalite (Na2SO4), arcanite (K2SO4) or calcium langbeinite (2CaO.K2O.SO3) , or a mixture of some or all of these.
Sodium and potassium sulfates can affect strength development by increasing the alkalinity of the pore fluid; when a sulfate ion reacts with aluminate phase to make ettringite and is removed from solution, the loss of the sulfate ion is balanced by the creation of two hydroxyl ions. This increased alkalinity accelerates alite hydration, giving higher early strengths.
We have here touched on some of the ways in which cement variability can affect concrete strength development but it is a complex subject and by no means fully understood.