Lignosulfonates admixture

Lignosulfonate admixtures can be used to produce concrete of a required workability and strength characteristic at lower cement contents than the comparative plain concrete with no adverse effect on the durability of the concrete or total structure. The only exception to this rule would be in conditions where high-sulfate ground waters may be involved when the minimum cement contents of relevant codes of practice should be observed. 

Lignosulfonate admixtures containing triethanolamine should not be used in situations sensitive to increased volume deformations. 

A partial explanation of the water reduction by lignosulfonate admixture is its ability to entrain air. It is reported that lignosulfonate promotes higher water reduction than hydroxycarboxylic acid-based admixtures. 

It is now 20 years and 13 years since the first and second editions respectively of Chemical Admixtures for Concrete were published. A first glance at the international admixture business could lead to the impression that not a lot had changed in 20 years certainly not enough to justify the complete revision of the second edition of the book. While it is true that products based on lignosulfonates, sodium glucoheptonate and com symp are still provided to the market in thousands of tonnes every year, there have been several significant changes ... 

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)...

In the formulation of admixtures from lignosulfonate (Table 1.1), the following comments are relevant ... 

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. 

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... 

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... 

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 ... 

The addition of a water-reducing admixture to a cement suspension can be shown to disperse the agglomerates of cement particles into smaller particles [33,38, 47] and can be seen clearly in photomicrographs as shown in Fig. 1.21. Maximum dispersion occurs at a level of 0.3-0.5% by weight of calcium lignosulfonate [33, 34] which would indicate the presence at the surface of about 0.2-0.4% calcium lignosulfonate. The separation of particles results in an increase in the surface area of the system by 30-40% [33, 38], which may explain the more rapid rate of cement hydration after the initial retardation period. 

As far as the final hydration products of ordinary Portland cement are concerned, there is an indication from isothermal calorimetry [57] that there is very little difference in the presence or absence of a calcium lignosulfonate water-reducing admixture. In this work, the heat evolved per unit of water incorporated into the hydrate has been determined for two cements, with the results shown in Fig. 1.25. It can be seen that the relationship between the amount of heat evolved and the amount of water combined with the cement is maintained whether the admixture is present or not. This work also indicated that the retardation in the early stages is compensated for at later times by an acceleration. 

The increase in workability obtained is, or course, a function of the dosage of admixture used and this is illustrated in Fig. 1.27 for lignosulfonates and the hydroxycarboxylic acid material. It will be appreciated that considerable retardation would be obtained at the higher dosage levels. 

These limited results give some indication that it is possible to achieve high-workability material without a consequential loss in cohesion by the use of water-reducing admixtures of the lignosulfonate. 

It can be concluded that water-reducing admixtures of the lignosulfonate and hydroxycarboxylic acid types will not alter the relationship between the compressive strength and the tensile and flexural strengths. 

For concrete used in dam construction, the results shown in Table 1.23 [99] have been obtained. From these results, it will be seen that a reduction in the water-cement ratios was obtained and in the large majority of cases (80% of the specimens) an improvement in freeze-thaw resistance was obtained. In fact, the average resistance of admixture-containing concrete was 39% greater than the control specimens. The ability of the lignosulfonates to... 

The recorded data on lignosulfonate water-reducing agents indicate that, as far as freeze-thaw durability is concerned, because of the low water-cement ratios possible, an enhancement to the durability will invariably be obtained. When the admixtures are used to effect a reduction in the cement content, there are strong indications that a considerable enhancement of durability is obtained, presumably due to a reduction in the cement matrix which is the part of the concrete susceptible to frost damage. The higher aggregate content would therefore allow easier dissipation of stresses. 

It can be concluded from the assessment of the data in this section that inclusion into a concrete mix of a water-reducing admixture of the lignosulfonate, hydroxycarboxylic acid and air-entraining type should not lead to any deterioration in the durability of that concrete to freeze-thaw cycling. Indeed there are strong indications that, when used either as a means of reducing the water-cement ratio or, alternatively, of reducing the cement content, more durable concrete may result. 

Similar data have been obtained for other lignosulfonate-based water-reducing admixtures where cement and workability were kept constant with a reduction in the water-cement ratio and the reinforcement bond was measured by ASTM C234 91a, with the result shown in Table 1.29 [107]. 

The data presented in this section illustrate that, with the exception of those accelerating water-reducing admixtures containing calcium chloride, there is an abundance of evidence to support the conclusion that water-reducing admixtures of lignosulfonate chemical form certainly will not accelerate any kind of corrosion with reinforcement and, when used to reduce the water-cement ratio, will form a more permeable and durable protective cover for the reinforcement. In view of the chemical nature of the other types of materials such as the hydroxycarboxylic acids and hydroxylated polymers, it seems most likely that these materials too would have no deleterious effect in this respect. 

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