Admixtures are chemicals that are added to concrete during mixing and significantly change its fresh, early age or hardened state to economic or physical advantage. They are usually defined as being added at rates of less than 5% by weight of the cement, but the typical range for most types is only 0.3 – 1.5%. They are normally supplied as aqueous solutions of the chemical for convenience of dispensing and dispersion through the concrete during mixing.
Admixtures in Concrete
Their popularity and use have increased considerably in recent years; estimates for the UK are that about 12% of all concrete produced in 1975 contained an admixture, and that this increased to 50% by 1991 and is now well over 75%. In some places, notably parts of Europe, North America, Australia and Japan, the proportion is even higher.
Action and Classification of Admixtures
An extremely large number of commercial products are available, which work by one or more of the following mechanisms:
- interference with the hydration reactions to accelerate or retard the rate of hydration of one or more of the cement phases
- physical absorption onto the surface of cement particles causing increased particle dispersion
- altering the surface tension of the mix water causing air entrainment
- increasing the viscosity of the mix water resulting in an increased plastic viscosity or cohesion of the fresh concrete
- incorporating chemicals into the hardened cement paste to enhance particular properties such as increased protection to embedded steel or water repellence.
These result in admixtures usually being classified or grouped according to their mode of action rather than by their chemical constituents. For example the European standard (BS EN 934) includes requirements for:
- water-reducing/plasticising admixtures
- high-range water-reducing/superplasticising admixtures
- set and hardening accelerating admixtures
- set retarding admixtures
- air-entraining admixtures
- water-resisting admixtures
- water-retaining admixtures
- set-retarding/water-reducing/plasticising admixtures
- set-retarding/high-range water-reducing/ superplasticising admixtures
- set-accelerating/water-reducing/plasticising admixtures.
Clearly the last three are admixtures with a combination of actions. We shall consider the five distinct types which together make up more than 80% of the total quantities used in concrete – plasticisers, superplasticisers, accelerators, retarders and air-entraining agents – and briefly mention others.
Plasticisers, also called workability aids, increase the fluidity or workability of a cement paste or concrete. They are long-chain polymers, the main types being based on either lignosulphonates, which are derived in the processing of wood for paper pulp, or polyycarboxylate ether.
They are relatively inexpensive but lignosulphonates in particular can contain significant levels of impurities depending on the amount of processing. Their plasticising action is due to the surfaceactive nature of the component polymer molecules, which are adsorbed on to the surface of the cement grains.
In their normal state the surfaces of cement particles carry a mixture of positive and negative residual charges (a property of all surfaces), which means that when mixed with water the particles coalesce into flocs, thus trapping a considerable amount of the mix water and leaving less available to provide fluidity.
In solution the plasticiser molecules have negative ionic groups that form an overall negative charge of the order of a few millivolts on the cement particles after they are absorbed onto the cement particle surface. The particles therefore now repel each other and become more dispersed, thus releasing the trapped water and increasing the fluidity, as illustrated in Fig. 1.
The particles also become surrounded by a sheath of oriented water molecules, which prevent close approach of the cement grains, a phenomenon known as steric hindrance or steric repulsion.
The overall effect is one of greater lubrication and hence increased fluidity of the paste or concrete. If a constant consistence or fluidity is required then the water content can now be reduced, thus leading to a lower water:cement ratio and increased strength; this is why plasticisers are often known as water-reducers. BS EN 934 requires that the water reduction for constant consistence should be greater than 5%. Values are normally between 5 and 12%. The use of plasticisers has been increasingly widespread since their first appearance in the 1930s.
Significant, and sometimes undesirable, secondary effects with some plasticisers are that they act as retarders, delaying the set and decreasing the early strength gain, and/or that they entrain air in the form of small bubbles.
Depending on the amount of processing in manufacture they may also contain impurities that have other undesirable side-effects at increasing doses, and therefore the magnitude of the primary effects that can be satisfactorily achieved with plasticisers is relatively modest, though nevertheless useful and cost effective.
As the name implies superplasticisers are more powerful than plasticisers and they are used to achieve increases in fluidity and workability of a much greater magnitude than those obtainable with plasticisers. They are also known as high-range water-reducers.
They were first marketed in the 1960s, since when they have been continually developed and increasingly widely used. They have higher molecular weights and are manufactured to higher standards of purity than plasticisers, and can therefore be used to achieve substantially greater primary effects without significant undesirable side-effects. They are a crucial ingredient of many of the special or so-called ‘high-performance’ concretes. BS EN 934 requires that the water reduction for constant consistence should be greater than 12%.
Values vary between 12 and about 30%, depending on the types and efficiency of the constituent chemicals. Currently three main chemical types are used (Dransfield 2003):
- Sulphonated melamine formaldehyde condensates (SMFs), normally the sodium salt.
- Sulphonated naphthalene formaldehyde condensates (SNFs), again normally the sodium salt. 3
- Polycarboxylate ethers (PCLs). These have been the most recently developed, and are sometimes referred to as ‘new generation’ superplasticisers.
These basic chemicals can be used alone or blended with each other or lignosulphonates to give products with a wide range of properties and effects. A particular feature is that polycarboxylates in particular can be chemically modified or tailored to meet specific requirements, and much development work has been carried out to this end by admixture suppliers in recent years.
This has undoubtedly led to improvements in construction practice, but a consequence is that the websites of the major suppliers contain a confusing plethora of available products, often with semi-scientific sounding names.
The mode of action of superplasticisers is similar to that of plasticizers, i.e. they cause a combination of mutual repulsion and steric hindrance between the cement particles. Opinions differ about the relative magnitude and importance of these two effects with different superplasticisers, but a consensus (Collepardi, 1998; Edmeades and Hewlett, 1998) is that:
- with SMFs and SNFs, electrostatic repulsion is the dominant mechanism
- with PCLs, steric hindrance is equally if not more important. This is due to a high density of polymer side-chains on the polymer backbone, which protrude from the cement particle surface (Fig .2).
This leads to greater efficiency, i.e. similar increases in fluidity require lower admixture dosages. The term ‘comb polymer’ has been used to describe this molecular structure. Some typical fluidity effects of admixtures of different types, measured by spread tests on a mortar, are shown in Fig .3.
The limited range and effectiveness of a lignosulphonate-based plasticiser and the greater efficiency of a PCL superplasticiser (in this case a polyacrylate) compared to an SNF-based material are apparent. Some of the more important features of the behaviour of superplasticisers, which directly effect their use in concrete, can be summarised as follows.
- The behaviour of any particular combination of superplasticiser and binder will depend on several factors other than the admixture type, including the binder constituents, the cement composition, the cement fineness and the water:binder ratio (Aitcin et al., 1994).
- Substantially increased performance can be obtained if the superplasticiser is added a short time (1 – 2 minutes) after the first contact of the mix water with the cement. It appears that if the superplasticiser is added at the same time as the mix water, a significant amount is incorporated into the rapid C3A/gypsum reaction, hence reducing that available for workability increase. This effect has been clearly demonstrated for lignosulphonate, SMF and SNF based admixtures, but has been reported as being less significant for at least some PCLs, which are therefore more tolerant of mixing procedures.
- The superplasticising action occurs for only a limited time, which may be less than that required if, for example, the concrete has to be transported by road from mixing plant to site. Methods of overcoming this include:
- blending a retarder with the superplasticiser
- addition of the superplasticiser on site just before discharge from the mixer truck
- repeated additions of small extra doses of the admixture.
The losses with some PCLs have been shown to be lower than with other types, at least over the critical first hour after mixing.
- For any particular binder/superplasticiser combination there is a ‘saturation point’ or optimum dosage beyond which no further increases in fluidity occur (Fig. 4). At dosages higher than this, not only is there no increase in fluidity, but detrimental effects such as segregation, excessive retardation or entrapment of air during mixing – which is suddenly released – can occur.
An accelerator is used to increase the rate of hardening of the cement paste, thus enhancing the early strength, particularly in the period of 24 – 48 hours after placing, perhaps thereby allowing early removal of formwork, or reducing the curing time for concrete placed in cold weather. They may also reduce the setting time.
Calcium chloride (CaCl2) was historically very popular as it is readily available and very effective. Figure 5a shows that it accelerates both the initial and final set, and Fig. 5b shows that a 2% addition by weight of cement can result in very significant early strength increases.
This effect diminishes with time, and the long-term strength is similar to that of nonaccelerated concrete. The calcium chloride becomes involved in the hydration reactions involving C3A, gypsum and C4AF, but the acceleration is caused by its acting as a catalyst in the C3S and C2S reactions (Edmeades and Hewlett, 1998).
There is also some modification to the structure of the C-S-H produced. Of great significance is the increased vulnerability of embedded steel to corrosion owing to the presence of the chloride ions.
This has led to the use of calcium chloride being prohibited in reinforced and pre-stressed concrete, and to the development of a number of alternative chloride–free accelerators, most commonly based on either calcium formate, sodium aluminate or triethanolamine.
However, as with plasticisers and superplasticisers the magnitude of the effects of these depends on the binder constituents and composition and cannot be predicted with certainty, and so should be established by testing.
Retarders delay the setting time of a mix, and examples of their use include:
- counteracting the accelerating effect of hot weather, particularly if the concrete has to be transported over a long distance
- controlling the set in large pours, where concreting may take several hours, to achieve concurrent setting of all the concrete, hence avoiding cold joints and discontinuities, and achieving uniform strength development.
The retardations resulting from varying doses of three different retarding chemicals are shown in Fig. 6. Sucrose and citric acid are very effective retarders, but it is difficult to control their effects, and lignosulphonates, often with a significant sugar content, are preferred.
The retarding action of normal plasticisers such as some lignosulphonates and carboxylic acids has already been mentioned; most commercial retarders are based on these compounds, and therefore have some plasticising action as well.
The mode of action of retarders involves modification of the formation of the early hydration products, including the portlandite crystals. As with other admixtures, temperature, mix proportions, fineness and composition of the cement and time of addition of the admixture all affect the degree of retardation, and it is therefore difficult to generalise.
Air-entraining agents (AEAs) are organic materials which, when added to the mix water, entrain a controlled quantity of air in the form of microscopic bubbles in the cement paste component of the concrete. The bubble diameters are generally in the range 0.02 – 1 mm, with an average distance between them of about 0.2 mm. They are sufficiently stable to be unchanged during the placing, compaction, setting and hardening of the concrete.
Entrained air should not be confused with entrapped air, which is normally present as the result of incomplete compaction of the concrete, and usually occurs in the form of larger irregular cavities. AEAs are powerful surfactants, which change the surface tension of the mix water and act at the air–water interface within the cement paste. Their molecules have a hydrocarbon chain or backbone terminated by a hydrophilic polar group, typically of a carboxylic or sulphonic acid.
This becomes orientated into the aqueous phase, with the hydrocarbon backbone pointing inwards towards the air, thus forming stable, negatively charged bubbles that become uniformly dispersed (Fig. 7).
Only a limited number of materials are suitable, including vinsol resins extracted from pinewood and synthetic alkylsulphonates and alkylsulphates. The major reason for entraining air is to provide freeze–thaw resistance to the concrete.
Moist concrete contains free water in entrapped and capillary voids, which expands on freezing, setting up disruptive internal bursting stresses. Successive freeze–thaw cycles, say, over a winter, may lead to progressive deterioration.
Entrained air voids, uniformly dispersed throughout the HCP, provide a reservoir for the water to expand into when it freezes, thus reducing the disruptive stresses. Entrained-air volumes of only about 4–7% by volume of the concrete are required to provide effective protection, but the bubble diameter and spacing are important factors.
Air entrainment has two important secondary effects:
- There is a general increase in the consistence of the mix, with the bubbles seeming to act like small ball-bearings. The bubbles’ size means that they can compensate for the lack of fine material in a coarse sand, which would otherwise produce a concrete with poor cohesion.
- The increase in porosity results in a drop in strength, by a factor of about 6% for each 1% of air. This must therefore be taken into account in mix design, but the improvement in workability means that the loss can at least be partly offset by reducing the water content and hence the water:cement ratio.
AEAs have little influence on the hydration reactions, at least at normal dosages, and therefore have no effect on the resulting concrete properties other than those resulting from the physical presence of the voids, as described above.
Other Types of Admixture
Other admixtures include pumping aids, water-resisting of waterproofing admixtures, anti-bacterial agents, bonding agents, viscosity agents or thickeners, anti-washout admixtures for underwater concrete, shrinkage-reducing admixtures, foaming agents, corrosion inhibitors, wash-water systems and pigments for producing coloured concrete. Admixtures collectively contribute to the great versatility of concrete and its suitability for an ever-increasing range of applications.
Thanks for reading about “admixtures in concrete”.