First, let’s look at two different approaches to producing cements that have different characteristics, then in the next section we can practise visualising them using our mind’s eye image.
The requirements of cement differ depending how concrete (or mortar) made from it is to be used. For example, pre-cast products usually need a cement that gives high early strength so that the concrete can be de-moulded and the moulds re-used. Large pours of concrete need a cement with low heat of hydration to minimise thermal cracking. Where sulfate is present in groundwater, the concrete needs to be resistant to deterioration through sulfate attack.
These different demands need different cements with which to make the concrete.
The characteristics of Portland cement can be altered in two principal ways. The
first is to modify the cement clinker to produce the desired characteristics and the second is to mix Portland cement with other materials such as slag or fly ash.
The first approach is exemplified in the American ASTM C 150 standard
specification, which describes five different types of Portland cements, neglecting a further three air-entrained sub-types. ASTM C 150, particularly Table 1, is an elegant and concise summary of how to control the properties of Portland cement by varying its composition.
The second approach can be seen in the European EN-197 standard specification, which describes only one Portland cement, CEM I, and a long list of other
cements containing mixtures of Portland cement and other materials.
Of course, blended cements are available in the USA, and some concrete in Europe is still made with CEM I only, although the proportion has been decreasing for some years. In the UK, CEM I is still available in bulk, but in bagged form it is scarce, except for white cement. Typically, a bag of cement at a builder’s merchant is CEM II/A-LL, a Portland-limestone cement, or CEM II/B-V, a Portland-fly ash cement.
11.1.1 Altering the properties of cement: modify the Portland cement
High early strength:
Suppose you wanted to make a high early strength Portland cement. You would want to produce a clinker that had a high alite content and a low belite content, since alite is more reactive than belite. You might also want to increase the alkali content, as this will increase the alkalinity of the cement pore fluid and accelerate hydration; of course, this might lead to problems with alkali-silica reaction if reactive aggregates are present in the concrete, but here we are just talking about principles.
You would also grind the cement finer as smaller particles will react more quickly.
You might also produce a mineralised cement; alite in mineralised cement is more reactive than alite in normal Portland cement.
Of the above approaches, the two most commonly used are finer grinding of the cement and increasing the alite content.
In ASTM C150-7, there are no particular limits on C3S and C2S contents for Type III cement but the limits for fineness applicable to other cements are removed, and minimum one-day compressive strengths are specified together with higher minimum compressive strengths at 3 days than for other cements.
In EN 197, the specification for high early strength CEM I is similarly empirical – the cement has to meet the required strengths at 2 days and 28 days for that strength class.
Low heat of hydration:
If you wanted to make a cement that had a low heat of hydration, you would want to limit the proportions of the most reactive clinker minerals, the alite and aluminate. The cement would then have a substantial belite content.
To illustrate this, the ASTM C 150-7 standard specification prescribes limits for the clinker phases in cements of low or moderate heat of hydration:
For a Type II cement (moderate sulfate resistance or moderate heat of hydration) the specified maximum C3A content is 8% and the sum (C3S+4.75C3A) must be 100 or less. (Note the weighting applied to the C3A; this is because it is highly reactive, generating heat quickly, and because limiting the C3A content reduces the potential for sulfate attack).
For a Type IV cement (low heat of hydration), the maximum C3S content is limited to 35% and there should be at least 40% C2S. Also, the
maximum permitted C3A content is 7%.
(NB: phase proportions here are calculated using the prescribed Bogue-type formula; these limits do not apply in C 150-07 if optional heat of hydration limits are to be applied).
Sulfate-resisting:
To make a sulfate-resisting Portland cement, the main consideration would be to minimise the C3A content by making a clinker with a low Alumina Ratio; normally, this would be achieved by adding an iron-rich component to the raw feed.
Instead of producing C3A, most of the aluminium will be then present in the ferrite phase, which will have a higher ratio of iron to aluminium compared with the ferrite in normal Portland cements. The objective in minimising the aluminate content is obviously to limit the potential for the formation of ettringite in sulfate-rich environments.
In ASTM C 150-7, the cement C3A content is limited to a maximum of 5%;
Annex A1 provides a modified calculation for the ferrite phase content if the cement AR is less than 0.64, on the basis that such cements will not contain any C3A.
11.1.2 Altering the properties of cement: combine Portland cement with other materials
We’ve just seen that one way of varying cement properties is to adjust the composition of the Portland cement clinker. Another way is to blend Portland cement with other cementitious materials, just as the Romans did when they blended lime mixed with volcanic glass or ground brick and tile to produce pozzolanic cements.
In the terminology of the European cement standard EN-197, mixtures of Portland cement with other cementitous mineral additions are called “Portland composite cements” if they are produced at the cement plant by intergrinding or blending1. If mixed by blending at the concrete batching plant, they are
“combination cements”. In American usage, mixtures of Portland cement with other cementitious materials are widely known as “blended cements” whether blended or interground.
Here, we will use “composite cements” to mean blended cements with at least two cementitious components, regardless of how or where they were blended.
Taking the three headings as before for variants of Portland cements:
High early strength:
To make a high early strength cement, the simplest approach to achieve the highest early strengths would be not to blend anything with Portland cement;
most materials you might add, such as slag or fly ash, react more slowly than Portland cement. However, microsilica and metakaolin are highly reactive and may improve early strengths. Sprayed concrete mixes may contain metakaolin or microsilica. Mixes of Portland cement and CAC can give early strength due to flash setting.
Composite cements may be designated an “R” suffix under EN-197, indicating high early strength, but in this context, “high early strength” is a relative
concept. Composite cements with the “R” suffix are almost always in the 32,5 or 42,5 strength classes and only have to achieve 10 MPa or 20 MParespectively at 2 days. For the highest strength class, 52,5, a cement has to achieve 30 MPa at 2 days; 52,5 R cements are almost all CEM I (ie: pure Portland cements) ground to increased fineness and often with raised alite contents.
1 “Intergrinding” means that the additional material (eg: slag or fly ash) is added to the clinker in the cement mill. “Blending” means that the materials are mixed in some other way, usually using specialised dry-blending equipment.
Low heat of hydration:
Combining other cementitious materials with Portland cement is an ideal way to produce a cement with low heat of hydration. Composite cements containing Portland cement with additions such as fly ash or slag will have a lower heat of hydration because of the slower rate of reaction of slag and fly ash.
In EN-197 terminology, an example of a low heat cement would be a cement containing a high proportion of blastfurnace slag: a cement labelled “CEM III B 32.5N-LH” would contain 66%-80% slag and 20%-34% clinker. The “LH” suffix indicates a low heat cement. In EN-197, “low heat” is defined as a cement with a characteristic heat of hydration of not more than 270 J/g.
Sulfate-resisting:
Composite cements made with Portland cement and either fly ash or slag can give at least equivalent sulfate resistance characteristics to that obtained when using sulfate-resisting Portland cement.
Suitably-designed mixes containing a Portland fly ash cement with, for example, 70% Portland cement and 30% fly ash should have good sulfate-resisting
qualities. Alternatively, a Portland blastfurnace cement could be used with a high slag content, for example 30% Portland cement and 70% slag.
To give an example of a sulfate-resisting composite cement in terms of the EN 197-1 standard is not possible, since EN 197-1 does not currently cover sulfate-resisting cements.
However, a cement labelled: “CEM II/B-V 32,5R” would have a fly ash content of 26% – 35% and such cements are sold as having equivalent sulfate resistance to that of sulfate-resisting Portland cement, if the cement has been shown to meet additional criteria relating specifically to sulfate resistance. These additional criteria are not specified in any European standard but in individual national standards and the cement is labelled with an additional suffix, eg: “+SR”.
However, these bureaucratic niceties are not what we are focussing on here; the main point is that composite cements containing slag or fly ash can offer
equivalent, or even better, performance for sulfate-resistance compared with sulfate-resisting Portland cement.
Other benefits revisited:
The headings above don’t really do justice to the positive characteristics of composite cements, so, by way of revision, here are some more (see Chapter 7 for more detail):
Depending on composition, in addition to reduced heat of hydration and better sulfate resistance, benefits of composite cements also include: possibly increased
later strengths; lower permeability and better durability; reduced efflorescence;
reduced shrinkage and creep; reduction in risk of ASR; reduced CO2 emission per unit volume of cement; reduced consumption of natural resources. In the
interests of balance, we should also say that composite cements are more prone to carbonation and might want more care with curing.