Water-reducing admixtures

Water-reducing admixtures are not adsorbed equally by the various anhydrous and hydrated cement constituents and in studies with calcium lignosulfonate, the approximate maximum adsorption figures shown in Table 1.5 have been obtained [38,39], In addition, adsorption isotherms have been studied at various ages of C3A hydration [36] and it has been shown that it is the initial hydration products (less 

The water-reducing admixtures are the group of products which possess as their primary function the ability to produce concrete of a given workability, as measured by slump or compacting factor, at a lower water-cement ratio than that of a control concrete containing no admixture. 

The other members of the water-reducing admixture group possess some other function which could not be obtained by mix design considerations. 

The various types of water-reducing admixtures possess different but characteristic adsorption isotherms which qualitatively reflect their effect on cement hydration kinetics, as shown in Fig. 1.17. 

Fig. 1.13 The effect of water-reducing admixtures on paste visosity at different addition levels.

Fig. 1.17 Adsorption isotherms for various water-reducing admixtures.

Fig. 1.2 Compressive strength development of concrete containing various types of water-reducing admixture.

In order to understand more fully the effect that water-reducing admixtures have on the plastic properties of fresh concrete, and to gain an insight into the mechanism of action of this category of materials, it is useful to study the effect on the water-cement system. The topic can be considered from the 

Table 1.2 Typical analyses of lignosulfonate-based water-reducing admixtures (after Edmeades)

Fig. 1.18 Reduction in cement paste viscosity by various water-reducing admixtures as a function of amount adsorbed.

Table 1.8 Viscosity reduction of cement pastes by various water-reducing admixtures

The three categories of major ingredients discussed above for the formulation of water-reducing admixtures account for the majority of commercially available products, but there may be limited use of insitol [28], polyacrylamide [29], polyacrylic acids [30] and polyglycerol [31]. 

It can be seen, therefore, that only three chemical materials form the basis of all the water-reducing admixtures, i. e. lignosulfonate, hydroxycarboxylic acid, and hydroxylated polymers. 

In the absence of knowledge of the surface area of cement hydrates available for adsorption at the time of addition, it is difficult to estimate how many layers of water-reducing admixture molecules are adsorbed, but attempts have been made [40] indicating that over 100 layers may be formed with calcium lignosulfonate and salicylic acid at normal levels of addition. However, these calculations were based on specific surface areas of 0.3-1.0 m g-l, whereas other studies [27, 38, 39] have indicated 

In the form of sodium salts all are very soluble and have low freezing points, so that solidification in winter conditions is unlikely. Figure 1.8 shows the types and formulae of materials which have been reported to find application in the formulation of this type of water-reducing admixture. However, the only materials finding widescale application in formulations are the salts of gluconic and heptonic acids. 

Commercial lignosulfonates used in admixture formulations are predominately calcium or sodium based with sugar contents of 1-30%. Typical analyses of two commercially available lignosulfonate water-reducing admixtures are shown in Table 1.2 [12]. 

---