WATER REDUCERS AND RETARDERS

Most water reducers and retarders are based on similar formulations, and they are treated together in this chapter. These admixtures can be divided into four categories, viz., normal water reducers, water-reduc-ing-retarding admixtures, water-reducing-accelerating admixtures, and retarders. 

A normal water reducer lowers the water requirement to attain a given slump. Thus, for the same slump and a constant cement content use of lower w/c ratios, results in general improvement in strength, permeability, and durability. Alternatively, the desired slump is achieved without a change in w/c ratio by lowering the cement content. A water-reducing admixture may also be used to increase the slump to facilitate placements. According to ASTM, water reducers should be able to lower the water requirements by 5% of the control. The water reduction depends on the 

The water-reducing-retarding admixtures are used for avoiding cold joints and facilitating large pours. They are used particularly in hot weathering operations. These admixtures should increase the initial set time by at least one hour, with a maximum by 3 A hours. The compressive strength should be at least 110% of the control at 3, 7, and 28 days. 

The water-reducing-accelerating admixtures may be used in winter concreting as they permit early form removal and enable concrete to be available for service earlier. The addition of these admixtures should result in an initial set one hour earlier, with a maximum at 3 A hours earlier. The compressive strength requirement is 125% of control mix at 3 days and 110% at 28 days. 

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The replacement of Portland cement by fly ash class F (ASTM C 618) has been found to reduce the rate of slump loss in a prolonged mixed concrete, and the extent of the reduction is greater with increased cement replacement (Fig. 7.37). Fly ash also was found to be beneficial in reducing slump loss in concretes with conventional water-reducing and retarding admixtures [95], The effect of fly ash on reducing slump loss can be attributed to chemical and physical factors. It was found that the surface of fly ash particles may be partly covered with a vapor-deposited alkali sulfate that is readily soluble [103, 104], Thus the early hydration process of Portland cement is effected because sulfate ions have a retarding effect on the formation of the aluminates. Indeed, fly ash was found to be a more effective retarder than an... 

Composition. The materials that are used for water reducers and retarders are generally composed of the following compounds ... 

Water-reducing and retarding admixtures are required to extend the setting time from 1 to 3.25 hours with respect to the reference concrete. Water-reducing and accelerating admixtures accelerate the setting time between 1 and 3.5 hours. 

Table 7.10 Physical properties of concrete with and without the delayed addition of water-reducing set-retarding admixture (Dodson)...

Mielenz, R.C. (1960). Symposium on the Effects of Water-Reducing and Set Retarding Admixtures on the Properties of Concrete, ASTM SP-266, 161-82. 

Other types of admixtures used with calcium aluminate cements include water reducers and superplasticizers (Section 11.4), which also act as retarders, and thickening agents, such as carboxymethylcellulose. Complex formulations may be used for special purposes for example, a ready-mix mortar for high-performance road repair might contain 55% Ciment Fondu and 45% sand, with glass fibre, aluminium powder, Li2C03, sodium gluconate and methyl ethyl cellulose (M93). 

Admixtures are substances that are added during the mixing process in small quantities related to the mass of cement, in order to improve the properties of fresh or hardened concrete [7,8]. The most utilised admixtures are water reducers and superplasticizers that may be added to improve the workability of concrete or reduce the amount of mix water accelerators that are used to increase the rate of development of early strength of concrete set-retarders that reduce the setting time of concrete air-entraining agents that increase the freeze-thaw resistance of concrete (Section 3.1). Recently, corrosion inhibitors have been developed in order to increase the corrosion protection of embedded steel these will be treated in Chapter 13. 

What is the role of admixture on concrete properties State the difference between water reducing, self-retarding and set accelerating admixture. 

Uses Emulsifier, dispersanL opacifier, stabilizer, thickener for topical skin prods., lotions and creams, powds., conditioners, rinses, foundations, cleansers, moisturizers, makeup, and sunscreens stabilizer, vise, builder for emulsions Features Decreases water evap. and retards freezing reduces greasiness low HLB... 

Dehydration or Chemical Theory. In the dehydration or chemical theory, catalytic dehydration of ceUulose occurs. The decomposition path of ceUulose is altered so that flammable tars and gases are reduced and the amount of char is increased ie, upon combustion, ceUulose produces mainly carbon and water, rather than carbon dioxide and water. Because of catalytic dehydration, most fire-resistant cottons decompose at lower temperatures than do untreated cottons, eg, flame-resistant cottons decompose at 275—325°C compared with about 375°C for untreated cotton. Phosphoric acid and sulfuric acid [8014-95-7] are good examples of dehydrating agents that can act as efficient flame retardants (15—17). 

Another group of compounds called oxygen scavengers retard oxidation by reducing the available molecular oxygen. Products in this group are water soluble and include erythorbic acid [89-65-6] C HgO, and its salt sodium erythorbate [6381-77-7] C HgO Na, ascorbyl pahnitate [137-66-6] 22 38 7 ascorbic acid [50-81-7] C HgO, glucose oxidase [9001-37-0] and sulfites (23). 

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