Water-reducing admixtures

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 way in which the four types of water-reducing admixtures discussed so far affect the strength gain characteristics of concrete containing them is shown in Fig. 1.2. The four concrete mixes have been designed to have approximately the same 28-day compressive strength, i.e. the admixture-containing mixes would contain approximately 10% less cement than the control mixes. 

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

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. 

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]. 

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

The accelerating water-reducing admixtures are simple blends of either calcium chloride, nitrate, thiocyanate or formate with a lignosulfonate or a hydroxycarboxylic acid salt. In some cases it may not possible to obtain a completely sediment-free solution and agitation of store tanks may be necessary. Typically, a mixture of approximately 33% calcium chloride and 4% calcium lignosulfonate by weight in water would be used. 

Air-entraining water-reducing admixtures containing lignosulfonates can be based on impure lignosulfonate raw materials, as stated earlier, where only 2-3% additional air is required. However, this air may not be of the amount, type, and stability required, therefore additions of surfactants are made. Several different types can be used but in the majority of cases they are based on alkyl-aryl sulfonates (e. g. sodium dodecyl benzene sulfonate) or fatty-acid soaps (e.g. the sodium salt of tail-oil fatty acids). Additions of these types will allow incorporation of sufficient stable air of the correct bubble size to meet durability requirements under freeze-thaw conditions. 

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. 

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]. 

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... 

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

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 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. 

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... 

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

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

The nature of the bond between the molecules of the water-reducing admixture and the surface of the cement constituent hydrates has been described as ionic group outwards in many references [33, 42,], mainly based on work [33, 43] showing migration of cement particles under the influence of an electric current when lignosulfonate molecules are adsorbed on the surface. Similar results have been reported for hydroxycarboxylic acids [44], Other relevant data are summarized below ... 

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