Superplasticizers

Superplasticizers (SP) reveal analogous effect as plasticizers it means that they improve the workability of cement mixture at constant w/c ratio or give the possibility to reduce w/c at constant workability. Therefore the strength can be improved as early as in 1984 Paillere [371] reported the concrete class of 60 MPa. The melamine resins act more effectively than the naphthalene-formaldehyde condensates [361]. The concrete subjected to thermal treatment or autoclaving shows much better strength, with respect to the reference, with superplasticizers addition. The difference between plasticizers and superplasticizers is only one the latter can be used in higher concentrations without causing excessive retardation or air entrainment. 

The most commonly used superplasticizers can be classified to the four groups 

Beside from the polyaiylsulphonate polymers—SMF and SNF, the polycarboxylate (e.g. polyaciylates) polymers PA belong nowadays to the most commonly used superplasticizers (Fig. 6.90) [372], All these polymers dissociate in water, in the case of the first anions with the sulphonate groups -SO3 and the second—carboxylate -C(X) groups are formed. 

However, to the latter group (other substances) polymers of not firlly recognized properties, which properties are given only by producers, are included. They are, among other, the multi-ring srrlphonates, alkylarylsrrlphonates adds, sulphonated polystyrene, sulphonated creosote— formaldehyde condensates and marty others [373], 

Superplasticizer addition should be much higher, in the range from 1 to 3 % by mass of cement than that of plasticizer. The fluidizing effect is increasing, from certain threshold content, proportionally to superplasticizer addition (Fig. 6.91) [361]. 

The mode of action of superplasticizers involves surface adsorption, dispersion of the particles in the aqueous phase assisted by 

At any particular time, the viscosity is lower for the paste containing the admixtures. It has been reported that 1% superplasticizer at a w/c ratio of 0.3 shows the same fluidity as cement paste at a w/c ratio of 0.4 without the admixture. 

In concrete, slump can be increased substantially by the addition of superplasticizers. For example, a slump of 260 mm from an initial slump value of 50 mm is obtained by an addition of 0.6% SMF the same value is realized with an addition of 0.4% SNF. The time when the superplasticizer is added to concrete affects the slump value. By adding the admixture with the mixing water, the slump is increased, but even higher values result if the admixture is added a few minutes after the concrete is mixed with water. 

Higher than normal workability of concrete containing a superplasticizer is maintained for about 30-60 minutes, and then there is a rapid decrease in slump, termed slump loss. To control or extend the workability, the superplasticizer should be added at the point of discharge of concrete. Some admixtures are added to superplasticizers to control slump loss. The acrylate-based superplasticizers are claimed to possess good slump retention qualities. 

The rate of hydration of cement and cement compounds is influenced by superplasticizers. The reported data on the effect of superplasticizers on the hydration of C3A and C3A + gypsum systems are contradietory because of the variations in the materials and conditions of hydration. There is a general consensus that SNF and SMF retard the hydration of 

Another use of such polymer types as concrete additives has been described (14). The polymer can be used to retard the curing process and to incorporate specific elements that enhance the overall performance of the concrete. For example, the pol5rmers may be used as admixtures to chelate specific elements. This chelation ability may be usable in order to ensure the uptake and distribution of a desired element throughout the concrete mixture. 

The performance enhancement may be as a result of the curing process, the strength or other characteristics of the concrete or the ability of the concrete to adhere to incorporated reinforcement materials (14). 

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Superplasticiy Superplasticizers Super Radiometal Supersaturation Superseed Supersensitization... 

Naphthalenesulfonic Acid—Formaldehyde Condensates. The sodium salts of the condensation products of naphthalenesulfonic acid with formaldehyde constitute an important class of compounds which are mainly used in the area of concrete additives (32,33), agricultural formulations, mbber formulations, and synthetic tanning agents. They are also used in photographic materials (34). Hampshire Chemical Co. and Henkel of America, Inc., are the largest suppHers of naphthalene sulfonate in concrete additives (superplasticizer) and reportedly hold 75—80% of this market. It was estimated that naphthalene sulfonate demand from U.S. producers would reach approximately... 

Water-Reducing Agents and Superplasticizers. Common admixtures that improve fluidity of concrete mixes are often used in high strength concrete. These admixtures make possible the incorporation of siUca fume while maintaining necessary workabiUty. Three principal types of... 

In academia too, workers have advanced our knowledge of how water-reducing agents and superplasticizers function and have made our earlier theories look simplistic and pedantic. 

A general update of references and developments in all chapters including a major new chapter on superplasticizers , or high-range water-reducing agents. 

Uchikawa, H. and Hanehara, S. (1997). Proceedings of the Fifth Canmet/ACI International Conference on Superplasticizers, Rome, Italy. 

Anon. (1976). Superplasticizing Admixtures in Concrete, Cement and Concrete Association, 28. 

The superplasticizers are a special category of water-reducing agents in that they are formulated from materials that allow much greater water reductions, or alternatively extreme workability of concrete in which they are incorporated. This is achieved without undesirable side effects such as excessive air entrainment or set retardation. 

The three major types of raw materials used in superplasticizers, SNF, SMF, and polyacrylates are shown in Fig. 2.1, which also illustrates the three different types of polyacrylates. Minor amounts of other materials are often added such as triethanolamine (to counteract retardation), tributyl phosphate (to cut down excessive air entrainment) and hydroxycarboxylic acid salts or lignosulfonates (to increase retardation). In addition proprietary superplasticizers can be blends of two of the main ingredients. 

In order to understand the way in which superplasticizers affect the properties of concrete, studies have often been made on cement pastes in view of the convenience of such investigations. 

Superplasticizers reduce both the yield value and plastic viscosity [24] of cement pastes. At higher dosages (for example 0.8% for SNF) [5, 6], the yield value approaches zero and the system becomes essentially Newtonian. 

The cement composition affects the rheological behavior of the system cement pastes having low C3S/ C2S and C3A/C4AF ratios have a higher viscosity when the superplasticizer addition is delayed [27],... 

There is a relationship between the amount of superplasticizer adsorbed on to the cement and the apparent viscosity [28, 29], The second reference indicates that the relationship is linear, while the relationship in the first reference is linear over at least part of the curve (Fig. 2.5). 

The zeta potential is the difference in potential between that of the total dispersed system and that of the layer at the interface of the dispersed particles (in this case cement) and the dispersing medium (water). Many studies have been made of the effect of superplasticizers on the zeta potential of the cement-water system from which the following conclusions can be drawn ... 

Cement has a positive zeta potential which is diminished and eventually becomes negative on the addition of a superplasticizer [31]. 

For the sulfonate-based polymer types of superplasticizer, the zeta potential increases as the molecular weight increases, up to a maximum, as shown in Fig. 2.6 [34]. 

Earlier work [37] by one of the authors indicated that superplasticizers of the SNF and SMF type were less strongly adsorbed onto the hydrating cement than normal water-reducing agents and this was used to explain why there was less retardation by the superplasticizers. This... 

SMF and SNF superplasticizers are adsorbed rapidly onto hydrating cement but this net effect is made up of very rapid adsorption by C3A and slower adsorption by the silicate phases, as shown in Fig. 2.9. [39],... 

There is no doubt that superplasticizers affect the manner and rate in which the individual components in cement react with water and with each other. 

The chemistry is very complicated, even in the absence of superplasticizers, but the following is a summary of what has been established by the many workers in this field. 

The interaction of superplasticizers with Portland cement is the most complicated situation of all because of reactions between the various components of the cement and the competition, for example between the superplasticizer and gypsum for reaction with C3A. However, in general ... 

The C3S phase is not as strongly retarded as for the individual material because the C3A strongly adsorbs a large proportion of the superplasticizer preferentially. 

Superplasticizers operate by adsorption onto the initial hydrates of C3A, C2S and C3S. In the case of C3A there is evidence that this is more than just a physical effect, and that an organo-mineral compound may be formed. 

Different types of superplasticizers alter the air content of fresh concrete in varying degrees and the effect is also dependent on the way the superplasticizer is used. In general, the following observations are noted ... 

When the superplasticizer is used to produce highly workable concrete, more air will be entrained than a control concrete. This is particularly true of SNF and polyacrylate types (where air-detraining agents are sometimes intentionally added into the formulations) and less so with the SMF type. 

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