Superplasticizers and Their Nature as Concrete Admixture
ASTM C494 defined as “water reducing high range admixtures”. For simplicity, we will term them as superplasticizers. There have four broad categories like
• Sulfonated naphthalene formaldehyde condensates
• Sulfonated melamine formaldehyde condensates
• Modified lignosulfonates
• Sulfonic-acid esters and carbohydrate esters and other esters.
First two superplasticizers are very common and for simplicity, they can be defined as naphthalene based and melamine based superplasticizers respectively.
Superplasticizers are organic polymer that is soluble in water. A complex process of polymerization is used to synthesize to produce high molecular mass of long molecules. This makes them relatively expensive.
As these are manufactured materials to achieve specific purpose, their behaviors can be modified, in other word, may be optimized to produce customized length of these molecules having minimum number of cross-linking. They are modified to reduce impurities to reduce or nullify unexpected side-effect (obviously harmful effects are considered) even under high dosages.
Efficiency of them improves significantly with large molecular mass; but mass within some limits perform better. Their chemical behavior has also an effect, but any generalize comments on superiority of particular chemical type is not possible.
The difficulties in comparing in chemical nature of superplasticizers in between melamine-based and naphthalene based products is due to
Influence in performance is not by only one property of superplasticizers; moreover cement type, in terms of chemical properties, also play an important role.
Most superplasticizers available in market are in the form of salts mainly of sodium but some calcium salts are produced as well. Calcium salts are less soluble in water. The additional sodium ions produce additional alkalis which can be compared to products of reactions in concrete mix during hydration of cement and also relevant to alkalis-silica reaction.
As they contribute to alkalis in mix, soda content in the admixtures, not only in superplasticizers, should be pre-defined. Some countries like Germany, soda content by mass of cement or cementitious materials is limited to 0.02%.
Sometimes superplasticizers based on napthalne are modified by including co-polymer with some functional group of sulfonic and carboxyl origin. This development offers us stable electrostatic charge over cement particles which prevents flocculation due to surface adsorption of cement particles.
These copolymers become more active when temperature rises which are especially suitable in hot weather concreting.In such extreme weather it is a great challenge to maintain high workability up to 1 hr. after mixing, if required.
While using such admixtures, engineers have to go through manufacture’s information; where such information is not available or inadequate or unreliable, special chemical tests should be conducted. Some physical tests are available which help us to distinguish them from other water-reducing agents.Long molecules wraps cement particles within them rendering high negative charge which exert repelling force between each other. This action results in dispersion and deflocculation of cement particles. Thus an improved workability of concrete is achieved which can be used in two ways
• Producing a high workable concrete which may be primary concern in some special cases
• Producing very high strength concrete.
At a particular water content and water/cement ratio of the mix, influence of dispersion of superplasticizers improves workability of concrete; the typical value is from 3” (75 mm) to 8” (200 mm), maintaining a cohesive mix. The compacting effort required to placed resulting concrete is very minimal, even no compaction is required in some cases, surprisingly with reduced excessive segregation or bleeding.
In this condition, concrete is termed as flowing concrete which is very useful to place very massive sections having reinforcement congestion, in unreachable areas, in road or floor slabs. It also desired in construction conditions where rapid placing is required.
Special care must be taken in designing formwork, as such concrete produce full scale hydrostatic pressure. When this concrete is compacted properly, no problem with bond failure is observed.
The second and most crucial use of superplasticizers is to produce concrete with normal workability having low water content thus reducing water/cement ratio. When water/cement ratios is kept down up to 0.2, the 28-day strength (cylinder specimen) is reached 22000 psi (150 Mpa). In general super plasticizers can decrease water demand in concrete for a particular workability by (25~35) percent which will yield a concrete having (50~75) percent more strength at 24-hrs; an even more increase is sometimes found in such early ages. After 7 hrs., concrete cube strength of 4300 psi (30 Mpa) can be achieved with a practical mixes. Under special curing like high-pressure steam curing even more early strength can be attained.
Requirements of superplasticizers to render flowing property to concrete and high strength property to concrete are provided respectively by ASTM C1017 and ASTM C494 and both requirements are found in BS 5075: Part 3. It is worth mentioning that, commercially available superplasticizers, satisfies both requirements of improving strength and workability as per above standards.
Mechanism of action of superplasticizers:
The fundamental structure of hydration product of cement paste is not altered by superplasticizers; the actual effect is to dispersion of cement particles and subsequent better hydration. This is the explanation of strength increase of concrete with the application of superplasticizers under a constant water/cement ratio. An increase in strength of 10 percent at 24 hrs and 20 percent increment at 28 days are reported, but such behavior cannot be taken as universal in all conditions.
The most important thing is that strength retrogression was not reported with the passage of time.The microstructural behavior of such concrete is not fully established yet; but it is believed that super plasticizers interact with tri-calcium aluminate (known as C3A) to retard hydration process:
The physical impact is to ettringite formation (as crystal) which is almost of cube shape and small in size, unlike needle like shape. Thus cubic shape crystals improve mobility of cement paste. But it is not the main mechanism of improving workability of concrete as Workability is improved when partial hydration is occurred at which already ettringite crystals are formed. The complete behavior of superplasticizers still is not known.
Superplasticizers have no significant effect on surface tension of mixing water; so air entrainment observed under the action of them is low and can be used at a high dosages. Regarding set retarding action, as discussed above, ASTM C494 classified superplasticizers having set-retarding property as Type G.
When naphthalene-based products is used in concrete, they mainly offer retardation action for a particular size range of cement particles (4~30µm). When particle size is smaller than 4 µm, there have abundance of alkalis and SO3 and retardation is not significant.Again larger particles usually subjected to less initial hydration irrespective of presence of superplasticizers.