Additions are defined as ‘finely divided materials used in concrete in order to improve certain properties or to achieve special properties’ (BS EN 206, 2000). Somewhat confusingly, there are a number of alternative names favoured in different countries and at different times: cement replacement materials, fillers, mineral additives, mineral admixtures, supplementary cementing materials, cement substitutes, cement extenders, latent hydraulic materials or, simply, cementitious materials.
They are nearly always inorganic materials with a particle size similar to or smaller than that of the Portland cement, and they are normally used to replace some of the cement in the concrete mix (or sometimes supplement it) for property and/or cost and/or environmental benefits.
Several types of materials are in common use, some of which are by-products from other industrial processes, hence their potential for economic advantages and environmental and sustainability benefits.
However the principal reason for their use is that they can give a variety of useful enhancements of or modifications to the properties of concrete. They can be supplied either as separate materials that are added to the concrete at mixing, or as preblended mixtures with the Portland cement.
The former case allows choice of the rate of addition, but means that an extra material must be handled at the batching plant; a pre-blended mixture overcomes the handling problem but means that the rate of addition is fixed. Pre-blended mixtures have the alternative names of extended cements, Portland composite cements or blended Portland cements.
Generally, only one material is used in conjunction with the Portland cement, but there are an increasing number of examples of the combined use of two or even three materials for particular applications.
The incorporation of additions leads to a rethink about the definition of cement content and water:cement ratio. Logically these should still mean what they say, with cement being the Portland cement component.
Since, as we will see, additions contribute to the hydration reactions, the Portland cement and additions together are generally known as the binder, and hence we can refer to the binder content and water:binder ratio when discussing mix proportions. (To add to the confusion, the alternative terms powder, powder content and water:powder ratio are sometimes used when additions that make little contribution to the hydration reactions are incorporated.) BS-EN 206 recognises two broad divisions of additions:
- Type 1: nearly inert additions
- Type 2: pozzolanic or latent hydraulic additions.
This reflects the extent to which the additions are chemically active during the hydration process and therefore the extent to which they contribute to or modify the structure and properties of the hardened paste.
It will be useful at this stage to explain what is meant by pozzolanic behaviour before going on to consider some of the most commonly used additions.
Type 2 additions exhibit pozzolanic behaviour to a greater or lesser extent. A pozzolanic material is one that contains active silica (SiO2, or S in shorthand form) and is not cementitious in itself but will, in a finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form cementitious compounds.
The key to the pozzolanic behaviour is the structure of the silica; this must be in a glassy or amorphous form with a disordered structure, which is formed by rapid cooling from a molten state.
Many of the inter-molecular bonds in the structure are then not at their preferred low-energy orientation and so can readily be broken and link with the oxygen component of the calcium hydroxide. A uniform crystalline structure that is formed in slower cooling, such as is found in silica sand, is not chemically active.
Naturally-occurring pozzolanic materials were used in early concretes, but when a pozzolanic material is used in conjunction with a Portland cement, the calcium hydroxide (CH in shorthand) that takes part in the pozzolanic reaction is the portlandite produced from hydration of the cement. Further quantities of calcium silicate hydrate are produced:
S + CH + H → C-S-H
The reaction is clearly secondary to the hydration of the Portland cement, which has led to the name ‘latent hydraulic material’ in the list of alternatives above.
The C-S-H produced is very similar to that from the primary cement hydration (the molar ratios of C/S and H/S may differ slightly with different pozzolanic materials) and therefore make their own contribution to the strength and other properties of the hardened cement paste and concrete.
The most commonly used type 1 addition is ground limestone, normally known as limestone powder. Addition of up to 5% of this to Portland cement is permitted in many countries without declaration.
For example, BS EN 197-1:2000 allows this amount to be included in a CEM I cement as a ‘minor additional constituent’. Higher additions are also used in some types of concrete, most notably self-compacting concrete in which high powder contents are required for stability and fluidity of the fresh concrete.
The main enhancement of properties is physical – the fine powder particles can improve the consistence and cohesiveness of the fresh paste or concrete.
However, although there is no pozzolanic reaction, there is some enhancement to the rate of strength gain due to the ‘filler effect’ of improved particle packing and the powder particles acting as nucleation sites for the cement hydration products, and there is some reaction between the calcium carbonate in the limestone with the aluminate phases in the cement.
The main Type 2 additions in use worldwide are:
- fly ash, also known as pulverised fuel ash (pfa) – the ash from pulverised coal used to fire power stations, collected from the exhaust gases before discharge to the atmosphere; not all ashes have a suitable composition and particle size range for use in concrete
- ground granulated blast furnace slag (ggbs) – slag from the ‘scum’ formed in iron smelting in a blast furnace, which is rapidly cooled in water and ground to a similar fineness to Portland cement
- condensed silica fume (csf), often called microsilica – extremely fine particles of silica condensed from the waste gases given off in the production of silicon metal
- calcined clay or shale – a clay or shale heated, rapidly cooled and ground
- rice husk ash – ash from the controlled burning of rice husks after the rice grains have been separated
- natural pozzolans – some volcanic ashes and diatomaceous earth.
We will now discuss the first four of the materials in the above list in more detail, using metakaolin (also known as HRM – high reactivity metakaolin) as an example of a calcined clay.
All these four are somewhat different in their composition and mode of action, and therefore in their uses in concrete. Rice husk ash has similarities with microsilica, and natural pozzolans are not extensively used.
Chemical Composition and Physical Properties
Typical chemical compositions and physical properties of these four materials are given in Table .1, together with typical equivalent properties of Portland cement for comparison. Two types of fly ash are included, high- and low-lime, which result from burning different types of coal.
High-lime fly ash is not available in many countries, and the low-lime form is most commonly available. It is normally safe to assume that when fly ash is referred to in textbooks, papers etc. it is the low-lime version unless specifically stated otherwise. The following features can be deduced from the table:
- All the materials contain substantially greater quantities of silica than does Portland cement, but crucially, most of this is in the active amorphous or glassy form required for the pozzolanic action.
- Microsilica is almost entirely active silica.
- The alumina in the fly ash, ggbs and metakaolin are also in an active form, and becomes involved in the pozzolanic reactions, forming complex products. The metakaolin comprises nearly all active silica and alumina.
- Two of the materials, high-lime fly ash and ggbs, also contain significant quantities of CaO. This also takes part in the hydration reactions, and therefore neither material is a true pozzolan, and both are to a certain extent self-cementing. The reactions are very slow in the neat material, but they are much quicker in the presence of the cement hydration, which seems to act as a form of catalyst for the production of C-S-H.
- The above considerations lead to maximum effective Portland cement replacement levels of about 90% for high-lime fly ash and ggbs, 40% for low-lime fly ash and metakaolin and 25% for csf. At higher levels than these, there is insufficient Portland cement to produce the required quantities of calcium hydroxide for the secondary reactions to be completed. However, high-volume fly ash (HVFA) concrete, with up to 70% fly ash and low water:binder ratios, has been of increasing interest in recent years.
- Fly ash and ggbs have particle sizes similar to those of Portland cement, whereas the metakaolin particles are on average nearly ten times smaller and the microsilica particles 100 times smaller (although the ggbs and metakaolin are both ground specifically for use in concrete, and so their fineness can be varied). The consequences of the associated differences in surface area are:
- the rate of reaction of the metakaolin is higher than that of fly ash and ggbs, and that of microsilica highest of all (but remember that all are still secondary to that of the Portland cement)
- both metakaolin and microsilica result in a loss of fluidity of the cement paste and concrete if no other changes are made to the mix, with again the effect of csf being greater that that of metakaolin. To maintain fluidity, either the water content must be increased, or a plasticiser or superplasticiser added. The latter is the preferred option, since other properties such as strength are not compromised. With a sufficient dosage of superplasticiser to disperse the fine particles, a combination of excellent consistence with good cohesion and low bleed can be obtained.
- The spherical shape of the fly-ash particles leads to an increase in fluidity if no other changes are made to the mix. Some increase is also obtained with ggbs.
- All the materials have lower relative densities than Portland cement, and therefore substitution of the cement on a weight-for-weight basis will result in a greater volume of paste.
We should also note that variability of fly ash due to changes in the coal supply and power station demands can be a significant problem. Some processing of the ash is therefore often carried out to ensure a more uniform, high-quality material for use in concrete.
This includes screening to remove large particles, and the removal of particles of unburnt carbon, which are very porous and can reduce the consistence of the fresh concrete. All the above considerations have led to an everincreasing use of the various additions in all types of concrete in the last few decades.
Supply and Specification
Additions can be supplied as separate materials or pre-blended with Portland cement. For many years blends with ggbs have been known as Portland Blast Furnace cements and blends with fly ash Portland Pozzolanic cements.
There is an array of relevant standards throughout the world, covering the materials individually but also as blends. It is worthwhile briefly considering the designations for the latter in the current European standard BS EN 197-1:2000. This includes five main types of cement, with CEM I being Portland cement containing at least 95% ground clinker and gypsum with up to 5% minor additional constituents. The other four types are:
- CEM II Portland composite cement: Portland cement with up to 35% of another single constituent, which can be ggbs, microsilica, a natural or calcined pozzolan, fly ash, burnt shale or limestone powder, or with up 35% of a mixture of these additions
- CEM III Blast furnace cement: Portland cement with 35–95% ggbs
- CEM IV Pozzolanic cement: Portland cement with 11–35% of any combination of microsilica, natural or calcined pozzolan or fly ash
- CEM V Composite cement: Portland cement with 35–80% of a mixture of blast furnace slag with natural or calcined pozzolan or fly ash.
Within each main type there are a number of subtypes for the different types and quantities of additions, which results in 27 products within the whole family of cements. This may seem unnecessarily complex, but the standard covers all of the cements produced throughout Europe, and in any country or region only a few of the 27 will be available.
Each of the products has its own unique letter-code designation which, together with the strength-class designations, leads to a lengthy overall designation for any one cement. You should consult the standard itself for a complete list and full details if and when you need these.