Retarding admixtures

Oil well cements (78) are usually made from Pordand cement clinker and may also be blended cements. The American Petroleum Institute Specification for Materials and Testing for Well Cements API Specification 10) (78) includes requirements for nine classes of oil well cements. They are specially produced for cementing the steel casing of gas and oil wells to the walls of the bore-hole and to seal porous formations (79). Under these high temperature and pressure conditions ordinary Pordand cements would not dow propedy and would set prematurely. Oil well cements are more coarsely ground than normal, and contain special retarding admixtures. 

It is important to ensure that when using corrosion inhibitors with other conventional admixtures, they are added separately, at different times of the mix cycle. Corrosion-inhibiting admixtures which also accelerate the set of concrete may require the combination of a retarding admixture when ambient and mix temperatures exceed 35°C. In like manner, set-retarding corrosion inhibitors may require the addition of an accelerator to offset the retardation of early strength development (e.g. use of sodium nitrite in conjunction with sodium benzoate). 

ASTM recognizes three types of retarding admixtures type B, which simply retards the hydration of Portland cement type D, which not only provides set retardation but also water reduction and type G, which is a retarding supeplasticizer. The materials that are generally used in these admixtures include ... 

In bridge construction, the use of retarding admixtures produces uniformity in the rate of setting and lessens the risk of deflection in partially hardened concrete that may occur in continuously reinforced structures such as spandrel beams. The retarded concrete poured over the supports remains plastic until the final pours are placed at mid-span. 

Fig. 7.12 The extension of revibration time of concrete using various dosages of a retarding admixture (Shutz [28]).

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

When considering the use of retarding admixtures in situations where the temperature exceeds 30°C, the relevance of the ASTM C 494 compliance test data (done under laboratory conditions) to the field conditions the concrete will experience must be considered. The actual temperatures experienced in most hot-weather countries (> 40°C) go beyond the scope of many current recommended practices. For example the ACI guide to hot-weather concreting (ACI 305 R-89) does not meet the needs of, and is not practical... 

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... 

Designated type as given by principal and secondary functions (or the relevant standards), e.g. accelerating, water-reducing or water-reducing/retarding admixture (e.g., ASTM, Type D). 

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. 

Tuthill, L.H., Adams, R.F. and Hemme, J.M. Jr. (1960). Water Reducing Admixtures and Retarding Admixtures, ASTM Special Technical Publication No. 266, Philadelphia, 101-8. 

Retarding admixtures also help to eliminate cold joints and other discontinuities when concrete is placed in layers by enabling adjacent layers to be vibrated into each other. Since setting time governs the optimum time at which concrete can be revibrated, the slower the set, the more effectively... 

HOT-WEATHER CONCRETING (PLASTIC SHRINKAGE IN FLAT-SLABS DUE TO THE USE OF RETARDING ADMIXTURES)... 

Retarding admixtures are water soluble chemicals which delay the setting of cement. 

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