Lignosulfonate-based admixtures

Tabie 1.30 Effect of lignosulfonate-based admixtures on basic creep of concrete... 

The lignosulfonate-based admixtures have been used more widely than other water redueers. They are capable of redueing water requirements and retarding the setting times of concrete. They influenee the dispersion and the hydration rate of the individual cement compounds, and, thus, the cement itself. Techniques such as XRD, DTA, DSC, TG, DTG, and conduction calorimetry have been used extensively to follow the hydration of cement and cement compounds containing different t5q)es and amounts of lignosulfonates (LS). 

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

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 effect produced by the incorporation of a lignosulfonate-based water-reducing agent is shown in Table 7.6. The results were obtained from a series of mixes over an 8-month period by a ready-mix plant used in the production of concrete piles. Since the standard deviation of this particular plant was 5.0 MPa for mixes produced without the use of admixtures, it is evident that the use of the admixture resulted in reduced variability. These results indicate that in high workability mixes with cement contents in the median range, the admixture may cause an increase in the standard deviation. Thus in redesigning the mix to have a lower cement content in this class of concrete, adequate consideration should be given to this difference in standard deviation. Increased uniformity can be attained in this instance... 

When fatty-acid or wax emulsion type damp-proofers are used in conjunction with lignosulfonate or hydroxycarboxylic-based admixtures, heavy air entrainment results with attendant strength reduction. Both these effects decrease waterproofing characteristics. At higher workabilities, ligno sulfonates counteract the reduced bleeding effect obtained with waxes and increase bleeding rates [83]. 

The efficiency of water-reducing admixtures, and their relative usefulness are dependent on the aggregate-cement ratio. Hydroxylated polymer and hydroxycarboxylic acid types are more effective than lignosulfonate-based materials at higher cement contents (lower aggregate-cement ratios), whilst the lignosulfonate... 

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. 

An extensive work was carried out by Uchikawa l on the conduction calorimetry of superplasticizers, such as SNF (NS in the figure), lignosulfonate (LS), a co-polymer of acrylic acid with acrylic ester (PC), and a three dimensional polycondensate product of aromatic aminosulfonic acid with trimethyl phenol (AS) (Fig. 11). The first peak in the calorimetry corresponds to the heat of dissolution of alite, the heat of formation of the AFt phase, and the calcium hydroxide formation from free lime. The second peak corresponds to the heat of hydration of alite. The admixtures were found to accelerate the formation ofthe ettringite phase. At w/cratios ofO.3 and 0.5 and a later addition of the admixture, the appearance of the second peak was significantly delayed and the peaks were of lower intensity. Most retardation occurred with polycarboxylic acid and amninosulfonic acid-based admixtures (Fig. 11). DSC was used to determine the amount of lime formed at different times. The DSC results show that the addition of admixtures at different w/c ratios generally decreases the amounts of lime in the presence of superplasticizers (Fig. 12). 

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 ... 

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]. 

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 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 ... 

Many concrete producers use a combination of superplasticizer and a conventional water-reducing, set-modifying or air-entraining admixture to achieve the desired performance. The superplasticizer provides the major portion of the required water reduction and the conventional admixture is added to achieve one or more of the following objectives (1) further water reduction (2) admixture economy (3) the desired air content (4) increased workability and (5) extension of set and workability. Coimnercial conventional admixture formulations used for this purpose are usually based on sodium lignosulfonates, hydroxycarboxylic acids or processed carbohydrates. Such combinations, besides reducing the dosage of the... 

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