Cement is the essential component of concrete which, when hydrated, binds the aggregates together to form the hard, strong and monolithic whole that is so useful. Well over 95% of the cement used in concrete throughout the world is Portland cement in its various forms. It is by no means a simple material, and its complexities have an impact on the properties and behaviour of concrete from mixing right through to the end of its life.
It is therefore important to have some understanding of its manufacture, its composition, the processes involved in its hydration and of its final hardened structure if it is to be used effectively.
The crucial components of Portland cement are calcium silicates, which in the manufacturing process are formed by heating a mixture of calcium oxide (CaO) and silicon dioxide (or silica, SiO2) to high temperatures. Both of these occur in the earth’s crust in large quantities, the former in various forms of calcium carbonate (CaCO3), e.g. chalk and limestone, and the latter in a variety of mineral forms in sand, clay or shale.
Cement production is a largescale operation requiring huge quantities of the raw materials, and the production plants are therefore normally sited close to a suitable source of one or both of these, which occasionally even occur in a single source such as marl. The raw materials all contain some other components, and in particular clays contain oxides of aluminium, iron, magnesium, sodium and potassium.
These cannot be avoided; the first two have a significant effect on the manufacture and composition of the resulting cement, and as we will see when discussing durability, some of the others can have significant effects even though they are present only in small quantities.
Manufacturing Process of Portland Cement
The manufacturing process is relatively simple in principle, although the high temperatures and large quantities involved required sophisticated monitoring and control systems to ensure that a uniform high-quality product is obtained. The stages are:
- Initially the limestone or chalk and clay or shale are blended in carefully controlled proportions (normally about 80/20) and interground in ball or roller mills until most or of all the particles are smaller than 90 mm. The composition of the mixture is critical, and it may be necessary to add small quantities of other materials such as ground sand or iron oxide.
- The heart of the manufacturing process consists of heating this mixture (known as the raw meal) to about 1400 – 1500°C. In modern cement plants this takes place in two stages. First the raw meal is fed into the top of a pre-heater tower that includes a pre-calcining vessel (whose use improves the overall energy efficiency of the whole process). As it falls through this it is flash-heated to about 900°C for a few seconds, during which about 90% of the carbonate component decomposes into calcium oxide and carbon dioxide (the calcining reaction). The mixture then passes into a heated rotary kiln that takes the form of an inclined steel cylinder lined with refractory bricks; it can be up to tens of metres long and several metres in diameter (depending on the capacity of the plant) and it is rotated about its longitudinal axis, which is set at a slope of about 3 degrees (Fig. 1).
- The kiln is heated at its lower end to about 1500°C by the combustion of a fuel–air mixture. The most common fuel is powdered coal, but oil and natural gas are also used; waste organic materials such as ground tyres are often added to the main fuels. The pre-heated meal from the pre-calciner is fed into the higher end of the kiln, and it takes between 20 and 30 minutes to reach and pass out of the lower heated end as a granular material called clinker. As the temperature of the feed increases as it moves through the kiln, decarbonation becomes complete at about 1100°C and then, in the so-called burning zone, the oxides start to combine to form a mixture consisting mainly of calcium silicates, calcium aluminates and calcium aluminoferrites. The chemistry involved is fairly complex, with compound formation at 1400–1500°C being greatly helped by the small quantities of alumina and iron oxide that are present (typically 5% and 3% respectively) and that act as a molten flux.
- The clinker emerges from the kiln at about 1200°C and is then cooled to about 60°C before being mixed with a small quantity (3 – 5%) of gypsum (calcium sulphate dihydrate, CaSO4.2H2O), and sometimes a small quantity (up to 5%) of a filler such as limestone powder, and then ground, usually in a ball mill, to give the Portland cement. The grinding process also increases the temperature of the clinker/gypsum mixture so cooling by water sprayed onto the outside of the grinding mill is required. The increased temperature causes some dehydration of the gypsum.
Physical Properties of Portland Cement
Portland cements are fine grey powders. The particles have a relative density of about 3.14, and most have a size of between 2 and 80 µmm. The particle size is, of course, dependent on the clinker grinding process, and it can be and is varied depending on the requirements of the cement.
The particles are too small for their distribution to be measured by sieve analysis, and instead the specific surface area (SSA), the surface area per unit weight, is normally used as an alternative measurement.
This increases as the particle size reduces i.e. a higher value means smaller average particle size. There are a number of ways of measuring this, but unfortunately they all give somewhat different values.
It is therefore necessary to define the method of measurement when specifying, quoting or using a value. The Blaine method, which is the most commonly used, is based on measuring the rate of flow of air under a constant pressure through a small compacted sample of the cement. Values of SSA measured with this method range from about 300 to 500 m2 /kg for most cements in common use.
Chemical Composition of Portland Cement
We have seen that Portland cement consists of a mixture of compounds formed from a number of oxides at the high temperatures in the burning zone of the kiln.
For convenience, a shorthand notation for the principal oxides present is often used: CaO (lime) = C; SiO2 (silica) = S; Al2O3 (alumina) = A; Fe2O3 (iron oxide) = F.
The four main compounds, sometimes called phases, in the cement are:
Tricalcium silicate 3CaO.SiO2 in short C3S
Dicalcium silicate 2CaO.SiO2 in short C2S
Tricalcium aluminate 3CaO.Al2O3 in short C3A
Tetracalcium aluminoferrite 4CaO.Al2O3.Fe2O3 in short C4AF
Strictly, C4AF is not a true compound, but represents the average composition of a solid solution. These compounds start to form at somewhat different temperatures as the clinker heats up when passing down the kiln.
C2S (often known as belite) starts to form at about 700°C, C3S (known as alite) starts to form at about 1300°C, and as the temperature increases to the maximum of about 1450°C most of the belite formed at lower temperatures is transformed into alite.
C3A and C4AF both start to form at about 900°C. Each grain of cement consists of an intimate mixture of these compounds, but it is difficult to determine the amounts of each by direct analysis; instead the oxide proportions are determined, and the compound composition then calculated from these using a set of equations developed by Bogue (1955). These assume:
- all the Fe2O3 is combined as C4AF
- the remaining Al2O3, after deducting that combined in the C4AF, is combined as C3A.
The equations in shorthand form are:
(C3S) = 4.07(C) – 7.60(S) – 6.72(A) – 1.43(F) – 2.85(Š) …….(1)
(C2S) = 2.87(S) – 0.754(C3S) ……..(2)
(C3A) = 2.65(A) – 1.69(F) …….(3)
(C4AF) = 3.04(F) ……..(4)
Where Š = SO3, (C3S), (C2S) etc. are the percentages by weight of the various compounds, and (C), (S) etc. are the percentages by weight of the oxides from the oxide analysis. The value of (C) should be the total from the oxide analysis less the free lime, i.e. that not compounded.
The Bogue equations do not give exact values of the compound composition, mainly because these do not occur in a chemically pure form, but contain some of the minor oxides in solid solution (strictly alite and belite are slightly impure forms of C3S and C2S, respectively).
For this reason, the calculated composition is often called the potential compound composition. However, the values obtained are sufficiently accurate for many purposes, including consideration of the variations in the composition for different types of Portland cement, and their effect on its behaviour. The approximate range of oxide proportions that can be expected in Portland cements is given in the first column of figures in Table 1.
As might be expected from our description of the raw materials and the manufacturing process, CaO and SiO2 are the principal oxides, with the ratio of CaO:SiO2 normally being about 3:1 by weight. The two calcium silicates (C3S and C2S) therefore form the majority of the cement.
However the composition of any one cement will depend on the composition, quality and proportions of the raw materials, and will therefore vary from one cement plant to another and even with time from a single plant. Table 1 illustrates the effects of this on the compound composition by considering four individual cements, A, B, C and D, whose oxide proportions vary slightly (by at most 3%), but which are all well within the overall ranges. The compound compositions calculated with the Bogue formulae show that:
- The principal compounds, C3S and C2S, together amount to 71 – 76% of the cement.
- The relative proportions of each compound vary considerably, by at least two orders of magnitude more than the small variations in the oxide composition. For example, the four ratios of C3S/C2S are 2, 5.9, 0.8 and 1.2, and the C3A content of cement D is 4 to 6 times less than that of the other cements.
As we shall see, such variations have considerable effects on the hydration process and properties of the hardened cement, and therefore careful control of the raw materials and manufacturing processes is vital if cement of uniform quality is to be produced.
Cement A can be considered to have a ‘typical’ or ‘average’ composition for Portland cement (most modern cements have a C3S content in the range 45 – 65% and a C2S content in the range 10 – 30%).
Cements B, C and D are common and useful variations of this, i.e. they have higher early strength, low heat and sulphate-resisting properties respectively. (Note: the compound compositions in Table 1 do not add up to 100% – the remainder comprises the minor compounds, which include the gypsum added to the clinker before grinding.)
Modifications of Portland Cement
When discussing the properties and compositions of cements in sections and we pointed out that these can be altered either by variations in the composition of the raw material or by changes in the manufacturing process.
Now we will discuss ways in which the cement can be altered from ‘average’ or ‘normal’ to obtain properties that are more useful for specific purposes.
Setting, strength gain and heat output: The relative timescales of the dormant, setting and strength-gain periods govern some of the critical operations in concrete practice, for example the transport and placing of the concrete, and the time at which formwork can safely be removed.
One way of modifying these properties is to alter the compound composition by varying the type and relative proportions of the raw materials used in the cement manufacture.
For example, increased proportions of C3S and C3A can reduce the setting time, and if a cement with a higher C3S and lower C2S content is produced, as in cement B in Table 1, this will have a higher rate of strength gain than cement A (but it is important to understand the difference between rapid setting and rapid strength gain – the two do not necessarily go together).
Rapid hardening properties can also be achieved by finer grinding of the cement, which gives an increased surface area exposed to the mix water, and therefore faster hydration reactions.
Since the hydration reactions are exothermic, a consequence of rapid hardening is a higher rate of heat output in the early stages of hydration, which will increase the risk of thermal cracking in large concrete pours from substantial temperature differentials at early ages, i.e. during the first few days after casting.
To reduce the rate of heat of hydration output a ‘low-heat’ cement with a lower C3S and higher C2S content may be used, i.e. as in cement C in Table 1, or by coarser grinding. The disadvantage is a lower rate of gain of strength.
Sulphate resistance: If sulphates from external sources, such as groundwater, come into contact with the HCP, reactions can take place with the hydration products of the calcium aluminate phases, forming calcium sulphoaluminate – etttringite – or, strictly, reforming it, since it was also formed very early in the hydration process.
Crucially the reaction is expansive and can therefore lead to disruption, cracking and loss of strength in the relatively brittle, low-tensile-strength HCP. (Its earlier formation would not have had this effect, as the paste would have still been fluid, or at least plastic.) The solution is a low-C3A-content cement such as cement D in Table 1, which is therefore an example of a sulphate-resisting cement.
White cement: The grey colour of most Portland cements is largely due to ferrite in the C4AF phase, which derives from the ferrite compounds in the clay or shale used in the cement manufacture.
The use of non-ferritecontaining material, such as china clay, results in a near-zero C4AF-content cement, which is almost pure white, and therefore attractive to architects for exposed finishes.
White cement is significantly more expensive than normal Portland cements owing to the increased cost of the raw materials, and the greater care needed during manufacture to avoid discoloration.
It is also possible to modify the properties of concrete by other means, involving the use of admixtures and/or cement replacement materials.
Cement Standards and Nomenclature
The first edition of the UK standard for Ordinary Portland Cement was issued in 1904, since when there have been a further 14 editions with increasingly complex and rigorous requirements. The last of these was in 1996, and a unified European standard, BS-EN 197-1:2000, has now replaced this. This covers five types of cement – CEM I, CEM II, CEM III, CEM IV and CEM V.
The last four these are mixtures or blends of Portland cement with other materials of similar or smaller particle size, and we will leave discussion of these for latter articles.
The cement and variations described in this article are type CEM I. The standard states that at least 95% of this should be ground clinker and gypsum – the remaining maximum 5% can be a ‘minor additional constituent’ (such as limestone powder). There are sub-divisions within the main type that reflect the performance of the cement as altered by composition and/or fineness.
The strength characteristics are determined by measuring the compressive strength of prisms made of a standard mortar with a sand:cement:water ratio of 3:1:0.5 by weight, which has been mixed, cast and stored under defined and carefully controlled conditions.
The cement is then given a number – 32.5, 42.5 or 52.5 – depending on the strength in MPa achieved at 28 days, and a letter, either N or R (N for normal and R for rapid), depending on the strength at either 2 or 7 days.
Limits to initial setting time are also included in the standard. The previous Ordinary Portland Cement from BS 12 roughly corresponds to a CEM I 42.5N, and although it is strictly not now correct to use the term ‘OPC’, it will take a long time before it dies out.
Sulphate-resisting Portland Cement has a separate standard (BS 4027), for which there is no European equivalent; the most significant difference to other Portland cements is the requirement for a C3A content of less than 3.5%.
There is no separate standard for white cement. Many other countries have their own standards. For example, in the USA the American Society for Testing and Materials (ASTM) classifies Portland cement in their specification C-150-94 by type number:
- type I is ordinary Portland cement
- type II is moderate sulphate-resistant or moderate heat cement
- type III is a rapid-hardening cement
- type IV is low heat cement
- type V is sulphate-resistant cement.
It is for most readers, to go into further details about these standards. They can be found in most libraries and on-line when necessary.