Superplasticizers effect

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 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 ... 

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],... 

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 ... 

The effect that superplasticizers have on the setting times of concrete depends on a number of factors including the type of superplasticizer, cement composition, and particularly whether there is a simple addition of the admixture to the concrete or if a reduction in water-cement ratio is made. In general it can be stated that ... 

In general, superplasticizers will not have an adverse effect on strength, or strength development of concrete when added to produce highly workable concrete without reducing the water content. In fact several studies have indicated that an increase in strength may occur, and in some cases this can... 

Compatibility with other admixtures due to the inherent surfactant properties exhibited by most of the materials in classes A, B and C, their use with other admixtures such as water reducers should be evaluated in terms of the side effects on the plastic and hardened concrete, viz. severe set retardation and excessive air entrainment. The molecules of materials with fewer surfactant properties compete with the superplasticizer for adsorption sites on the cement particle. This will result in a higher dosage of the superplasticizer. 

It has sometimes been argued that concrete is seldom damaged by frost action if low water-cement ratio mixes are used. However, the paramount effect of air entrainment in improving freeze-thaw resistance was clearly demonstrated in a study of the freeze-thaw resistance of both air-entrained and non-air-entrained superplasticized concrete with low water-cement... 

When high dosages of superplasticizer are used to effect low W/C ratio and high workability. 

Despite wide publication of the adverse effects that retempering causes, job site addition of extra water to compensate for slump loss is still a common practice, particularly in hot-weather conditions. Under these conditions, the use of both conventional and superplasticizing admixtures helps to minimize the amount of water required for re-tempering so that the loss of strength is minimized [46, 47], This is shown in Table 7.14. 

One of the significant limitations of the use of first- and second-generation superplasticizers in ready-mixed flowing concrete is the rapid decrease in the initially achieved high workability and this constitutes one of the chief constraints to their wider acceptance. Therefore, a number of major producers of admixtures have sponsored active research to improve the workability retention characteristics of their superplasticizers. Some recent developments [48] have shown promise, among these are materials based on acrylate polymers (AP). The AP-based materials are reported to be more effective than SNF- or SMF-based surplasticizers in terms of water reduction, slump increase and slump retention. Figure 7.19 shows the remarkable improvement in the retention of workability produced by the AP type superplasticizer compared to an SNF type. 

The important features of this type of admixture is that such benefits are realized by addition of the admixture with the gauging water (like conventional admixtures and very unlike the SNF and SMF materials) thus eliminating some of the problems associated with the first- and second-generation superplasticizers. Table 7.15 [48] presents the effect of the time of... 

Table 7.15 Effect of mode of addition of CAE, SMF and SNF superplasticizers on the slump of Portland cement concrete mixes (Collepardi)...

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