Cement - Superplasticizer Compatibility
This page describes the current level of knowledge worldwide about the issues of incompatibility between cements and water reducing chemical admixtures. The definition, classification, and chemical compositions of water reducing chemicals are discussed first, followed by their mechanism of action in concrete. Test methods to evaluate effectiveness of plasticizers are also reviewed. Specific issues of incompatibility are then addressed, namely, loss of workability, alteration of setting behaviour, reduced rates of strength gain, and change in long term behaviour. The factors controlling the compatibility are discussed next in detail, including the role of the chemistry (of cement and admixture), the presence of other mineral and chemical admixtures, and the proportioning of concrete. Finally, a detailed insight into the needs for further research is provided.
Modern concretes almost always possess additives, either in the mineral form or chemical form. Particularly, chemical admixtures such as water reducers and set controllers are invariably used to enhance the properties of fresh and hardened concrete.
A ‘Chemical Admixture’ is any chemical additive to the concrete mixture that enhances the properties of concrete in the fresh or hardened state. This does not include paints or coatings. ACI 116R  defines the term admixture as ‘a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing’.
A number of types of chemical admixtures are used for concrete. The general purpose chemicals include those that reduce the water demand for a given workability (called ‘water reducers’), those entraining air in the concrete for providing resistance to freezing and thawing action (called ‘air entrainers’), and those chemicals that control the setting time and strength gain rate of concrete (called ‘accelerators’ and ‘retarders’). Apart from these chemicals, there are others for special purposes – namely, viscosity modifying agents, shrinkage reducing chemicals, corrosion inhibiting admixtures, and alkali-silica reaction mitigating admixtures.
Apart from the multitude of chemical admixtures, a number of different types and brands of cement are also available in the market today. With the increasing number of types and brands of cement, as well as variants of the water-reducing chemicals, there are issues that arise related to the compatibility between these two ingredients of concrete. Most users apply a trial-and-error approach to these chemicals, often resulting in an unfortunate negative experience and/or low cost-effectiveness, which produce a bias against admixtures in general.
Common problems that arise as a result of incompatibility between
cement and water reducers are: rapid loss of workability, excessive quickening
/ retardation of setting, and low rates of strength gain. Very often,
there even exists incompatibility between a particular chemical and a
certain batch of the same otherwise compatible cement, indicating that
the nature of the problem is complex, and needs further understanding.
Moreover, high performance concretes, which are in wide use today, almost
always incorporate a mineral admixture or filler such as silica fume,
fly ash and limestone powder. This further complicates the physico-chemical
behaviour of the cement-based system since the mineral admixtures play
an important role in the evolution of the hydration reactions and the
availability of free water during the early ages of concrete.
2. WATER REDUCING CHEMICALS
A water reducing chemical, as the name implies, is used to reduce the water content of a concrete mixture while maintaining a constant workability. The resultant effect of the reduced water content is the increased strength and durability of concrete. However, water reducers may also be employed to ‘plasticize’ the concrete, i.e. make concrete flowable. In this case, the water content (or water to cement ratio) is held constant, and the addition of the admixtures makes the concrete flow better, while the compressive strength (which is a function of the water to cement ratio), is not affected. Another use of water reducers is to lower the amount of cement (since water is proportionately reduced) without affecting both strength and workability. This makes the concrete cheaper and environmentally friendly, as less cement is consumed.
Water reducers are classified broadly into two categories: (1) Normal and (2) High range. The normal water reducers are also called ‘plasticizers’, while the high range water reducers are called ‘superplasticizers’. While the normal water reducers can reduce the water demand by 5 – 10%, the high range water reducers can cause a reduction of 15 – 40%.
Water reducing chemicals are generally supplied as liquid formulations, with the active solids content in the range of 30 – 40%. Normal water reducers are typically used at dosages of 0.3 – 0.5% liquid by weight of cement. At higher dosages, there is a danger of excessive retardation, bleeding, and air entrainment. High range water reducers do not have these problems and are capable of being used at higher dosages of 0.7 – 1% (or more) liquid by weight of cement.
Lignosulphonate salts of sodium and calcium, hydroxycarboxylic
acids (citric and gluconic acid) and carbohydrates (corn syrup and dextrin)
are examples of normal water reducers. The subject of interest for this
study, namely superplasticizers are generally chemicals of the type presented
in Table 1. All the superplasticizers are water soluble polymers. As for
other polymers, the behaviour of superplasticizers is also a function
of the structure and the degree of polymerization.
Table 1. Superplasticizing chemicals [from Rixom and Maivaganam, 2003]
Lignosulphonates are generally regarded as ‘1st generation’ superplasticizers, while the sulphonated formaldehyde condensates are called ‘2nd generation’, and the polycarboxylates and polyacrylates are termed as 3rd generation superplasticizers. Currently, the most widely used superplasticizers are the sulphonated formaldehyde condensates. However, the beneficial effects of polycarboxylates are ensuring a gradual shift towards these chemicals.
In terms of costs, polycarboxylic ether and sulphonated melamine formaldehyde are almost equal (taken on an effective solids basis), sulphonated naphthalene formaldehyde is about half the cost of the PCE, while lignosulphonate is the cheapest (about ¼ of PCE). However, in terms of effectiveness to achieve a specific workability of the concrete, the amount of PCE required is much lesser than SNF or lignosulphonate. Thus, the overall cost of a normal plasticized concrete would not be affected based on the choice of the chemical (with the exception of SMF, which are more expensive considering the desired workability of concrete).
2.1 Characteristics of the different chemicals
Used at high dosages, lignosulphonates are capable of producing high range water reduction. However, a major problem with the use of lignosulphonates as superplasticizers is the excessive retardation and air entrainment in concrete. Modified lignosulphonates as superplasticizers lead to concrete with lesser variation in properties.
Sulphonated salts of melamine formaldehyde condensates are good to achieve a high initial slump. However, due to their poor slump retention characteristics, they are unsuitable for long haul applications, and more particularly for ready mixed concrete. On the other hand, these admixtures are excellent for precast concrete, where the concreting time is short. They are also suitable for cold climates. However, SMFs are not widely used in India owing to the poor cost-competitiveness compared to SNF.
Sulphonated salts of naphthalene formaldehyde condensates possess all the necessary characteristics to make them suitable for hot weather concreting. Mainly, these possess good slump retention characteristics, enabling their use in ready mixed concrete where long hauls are common. Slump retention characteristics are also improved by blending SNF with lignosulphonates, which is not possible in the case of SMF. The cost of SNF is also low, making it the most used superplasticizer in India and around the world. However, the maximum incompatibility issues arise with SNF, and these will be discussed at length later in this report.
Polycarboxylates and acrylic copolymers are the most effective of all the chemicals. These can cause a reduction in water content of as much as 40%. Thus, they are highly preferred to make high and ultra high strength concrete, where the w/c may be as low as 0.20. Generally, these chemicals exhibit excellent slump retention characteristics and do not cause any delay in the gain of strength of the concrete. The downside of these admixtures is their high cost. However, as stated earlier, for the same category of concrete (workability) PCEs can work at lower dosages than SNFs and lignosulphonates. Thus, the overall cost of concrete is not affected. Only in the case of special concretes such as self compacting concrete (SCC), the use of PCEs can substantially increase the concrete cost. It must be stated, though, that making good quality SCC without these latest generation superplasticizers is almost impossible. Limited experience with these chemicals indicates that they work well at low water to cement ratios, and exhibit fewer compatibility problems compared to SNF.
2.2 Mechanism of action of water reducers
Figure 1. Mechanism of electrostatic repulsion (top) and steric hindrance (bottom)
Electrostatic repulsion depends on the composition of the solution phase and the adsorbed amount of the SP (greater the adsorption, better the repulsion) [Nakajima and Yamada, 2004]. On the other hand, steric repulsion depends on the length of main chain, length and number of side chains [Sugiyama et al., 2003].
In the case of PCE based admixtures, for fluidity
retention, the main chain should be short, with large numbers of long
side chains [Sugiyama et al., 2003]. Because of the steric repulsion mechanism,
PCEs are generally more effective than the sulphonate based admixtures,
and generally do not experience much problems at low water to cement ratios.
However, they are more sensitive to overdosing, and can lead to problems
like excessive air entrainment and retardation.
Apart from affecting the early age physical properties of concrete,
SPs can also cause some changes in the morphology of hydration products.
Size of portlandite crystals decreases with addition of admixtures [Grabiec,
1999]. Ettringite in the presence of SPs (at high dosage) crystallizes
in small and massive clusters rather than the conventional needle shape
[Hanna et al., 2000; Prince et al., 2002]. In general, SPs improve rheological
properties by yielding smaller hydrate particles and preventing hydration
products from bridging neighbouring cement particles. There is also a
difference in porosity and pore size distribution of superplasticized
concrete compared to normal concrete. Higher numbers of smaller pores
are produced in superplasticized mixtures, which could have an influence
on the degree of shrinkage.
3. CEMENT – SUPERPLASTICIZER INCOMPATIBILITY
3.1 What is incompatibility?
The term incompatibility refers to the adverse effect
on performance when a specific combination of cement and superplasticizer
is used. Common problems include flash setting, delayed setting, rapid
slump loss, improper strength gain, inordinate cracking etc. These issues
in turn affect the hardened properties of concrete, primarily strength
Figure 2. Complexity of the superplasticized cement paste system (based on Aïtcin )
3.2.1 Cement composition and fineness
Cement is composed of four major compounds, namely, C3S (tricalcium silicate), C2S (dicalcium silicate), C3A (tricalcium aluminate), and C4AF (tetracalcium aluminoferrite). In addition, a number of minor oxides, such as alkali oxides (K2O and Na2O), MgO, and SO3 – which is contributed by gypsum, which is added in the final stages of cement manufacture as a set regulator. Because of the final grinding operation, the particles of cement are charged (the surface charge is measured as the Zeta potential). While C3S and C2S possess a negative zeta potential, C3A and C4AF particles are positively charged.
Role of C3A
The C3A content, or more specifically, the C3A to
SO3 ratio has a profound effect on the early age behaviour of cement paste.
In a normal hydration process of OPC, the amount of C3A is such that the
end product of aluminate hydration is monosulphate (AFm). However, in
low C3A cement (sulphate resistant) ettringite remains after the initial
In the early stages of cement hydration, the reactions that dominate are the reaction of C3S with water to produce CSH and calcium hydroxide, and the reaction of C3A with gypsum to produce ettringite (that later converts to monosulphoaluminate in ordinary Portland cement paste). It is during this period that the interaction of the SP with cement occurs. SP molecules with sulphonate functional groups have an affinity for the aluminates, which are positively charged. As a result, they compete with the sulphate released from gypsum for the aluminate reaction sites [Ramachandran, 2002; Jolicoeur and Simard, 1998, Jolicoeur et al., 1994]. When the solubility of the calcium sulphate is low, the SP molecules tend to get adsorbed first on the aluminate compounds, thus preventing the normal setting reaction involving the formation of ettringite. Based on the raw material for calcium sulphate and on the temperatures attained during the final grinding process, the calcium sulphate present in OPC can be a mixture of dihydrate (gypsum), hemihydrate, or anhydrite. In order to prevent the SP molecules from interfering with the aluminate hydration, it is imperative that the SO3 becomes available in solution as early as possible. Thus, the solubility of the calcium sulphate is important. While hemihydrate and synthetic anhydrite possess greater solubility than gypsum, natural anhydrite is very slowly soluble. The presence of natural anhydrite has always been found in systems presenting compatibility problems. It must be also noted that the solubility of sulphates would decrease in the presence of SPs with sulphonate functional groups, thus affecting the normal setting process of the cement [Hanna et al., 2000]. In order to avoid such problems, cements usually contain sufficient amounts of quickly soluble alkali sulphates. However, alkalis themselves affect compatibility in a number of ways, as discussed below.
Alkalis in cement are essential from the point
of view of accelerating C3S hydration. However, excess alkalis could have
adverse effects, one of them being the alkali aggregate reaction. Hence,
there is typically a strict control on the alkali limits. The use of cements
high in alkali causes workability problems in concrete without any admixtures,
but cements low in alkali are known to result in poor rheology of the
concrete in concretes using sulphonate based admixtures [Jiang et al.,
1999]. This is again interconnected with the availability of soluble sulphates,
discussed in the previous section. The problem with low alkali cements
can be overcome by adding an optimum amount of soluble alkalis, primarily
in the metasilicate or sulphate forms [Li et al., 2003]. Jiang et al.
 found that 0.4 – 0.5% soluble alkali content was optimum
to maximize fluidity and reduce fluidity loss of the concrete. Higher
alkali contents promote the solubility of sulphate ions and decreases
the loss of fluidity with SNF [Dodson and Hayden, 1989; Chandra and Bjornstrom,
2002]. However, there are also negative effects – in the presence
of high amount of alkalis when using an alkali sulphate rather than calcium
sulphate, it is difficult for ettringite to crystallize so that rapid
stiffening is experienced [Prince et al., 2002].
3.2.2 Admixture type and dosage
The type and dosage of admixture have major effects
on the cement – admixture compatibility issues. In the previous
section, the affinity of SPs with sulphonate groups for the aluminate
compounds in cement has already been described.
Molecular weight of polymer
In the case of the lignosulphonates [Rixom and Mailvaganam, 1999; Mollah et al., 1995], the presence of low molecular weight ingredients is known to cause excessive air entrainment leading to loss of strength. In addition, the high sugar content of these admixtures could cause unnecessary retardation, especially at high dosages. A further unpredictability might arise depending on whether the chemical is a sodium salt or a calcium salt. Reports from the industry indicate that neither type of chemical is compatible with all cements. In order to be effective, lignosulphonates should be modified – the sugars should be removed by fermentation, and low molecular weight matter should be removed by centrifuging. Lignosulphonate admixtures produce a complex salt with Ca2+, thus decreasing the Ca2+ concentration in the liquid phase, resulting in a delay in the hydration of alite, and causing set retardation.
Time of addition of the SP
Delayed addition is proposed as a means of retaining fluidity in the case of SMF and SNF based admixtures [Uchikawa et al., 1992; Aiad et al., 2002]. The amount of admixture adsorbed reduces when cement hydrates; in other words, adsorption is greater on unhydrated compounds compared to the hydrated phases [Flatt et al., 1998]. As a result, when a delayed addition is done, there is more admixture available in the solution to maintain the fluidity. According to Aiad , the optimum delaying time of the admixture is 10 – 15 min, and this time does not depend on the cement and SP type. Delayed addition has been shown to result in lesser participation of the polymer in the formation of the organo-mineral phase [Flatt and Houst, 2001].
The role of the type of superplasticizer on fresh concrete properties was investigated taking into account following factors: type of superplasticizer (Sulphonated melamine-formaldehyde condensate (SMF), Sulphonated naphthalene-formaldehyde condensate (SNF), Lignosulfonate (LS), and Polycarboxylic Ether (PCE)), superplasticizer dosage, water to cement ratio, and cement properties.
3.2.3 Other issues affecting compatibility
The w/c of concrete seems to affect the performance of SPs. As already stated, SNF admixtures are more prone to slump loss problems at low w/c, as compared to the PCEs. Collepardi  feels that the lower w/c in superplasticized concrete and the resulting lower interparticle separation of the cement particles makes it more sensitive to loss of water by evaporation or reaction with cement during transportation of the concrete. In general, most compatibility problems only exist at low w/c.
Added to the cement-water reducer interactions, the effect of an “inert” component on the workability could be significant (for example, when there is some interaction between a superplasticizer and the fines of the sand being used leading to a high loss of workability during transportation), and early-age cracking increases even when the concrete is of better quality (which can occur when fillers absorb the bleed water, causing higher plastic shrinkage, or when a high paste content causes excessive thermal shrinkage). The fines content in river sand affects the performance of SPs [Papayianni et al., 2005].
The placing of concrete at high ambient temperatures adds a new dimension to the problem of incompatibility. Low temperature has been reported to decrease fluidity. This decrease in workability at low temperature cannot be compensated with SP [Gettu et al., 1997]. On the other hand, high temperatures increase SP adsorption which increases fluidity [Greisser, 2002]. Conversely, temperature increase causes increase in reactivity of C3A which causes higher ettringite contents with fine morphology in the presence of SP [Greisser, 2002 ; Spiratos et al., 2003], thus causing a higher rate of slump loss. The influence of temperature on cement – SP interaction is closely associated with the cement composition [Charles, 2008]. Concrete placement temperatures exceeding 40 oC are routinely encountered in practice, which exacerbates the sensitivity of the cement – superplasticizer combination. In this study, the influence of temperature on the properties of fresh cement pastes containing various cements and superplasticizers was investigated. The variation in the flowability with time of normal Portland cement pastes with different superplasticizers (PCE and SNF) were studied at various temperatures (25, 35 and 45 oC) using Marsh Cone test, and the setting times of the pastes were also measured. Cement having low C3A to SO3 ratio was found to be more sensitive to temperature variations in fluidity retention characteristics than cements having higher C3A to SO3 ratio. Also, cement having higher Equivalent Alkali content was more sensitive to temperature variations.
Cement – superplasticizer interaction in concrete is a complex blend of chemical and physical mechanisms that are interdependent. The complicated nature of the problem prevents the development of simple solutions to address the field related issues of application of superplasticizers.
Studies on cement-water reducer interactions in India have been limited to the workability evaluation of concretes containing these chemicals, in specific regions where rapid slump loss has been observed in concreting operations. There have not been any investigations to understand the physico-chemical nature of this interaction. Thus, the results from these studies are not broad-based, i.e. they apply to a small group of cements and/or chemical admixtures. There is a distinct need for the characterization of Indian cement and admixture properties, in order to understand the nature of their interactions. Moreover, the wide range of cements used, varying transportation durations and climatic conditions necessitate a fundamental study that explains the mechanisms of interaction and helps establish methods for identifying incompatibility in practical situations.
In addition, simple methodologies are required to be able to identify
systems prone to undesirable effects due to such interactions and to further
understand the fundamental nature of admixture behaviour in cement-based
systems. The understanding of these interactions should be both at the
applications scale (for example, studying flowability and retention of
workability in pastes and concretes), as well as at the micromolecular
scale, where some insight can be obtained into the physico-chemical interactions
between cement particles and water reducer molecules.
REFERENCES (alphabetical list)
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