Set-retarder

Examples of retarders are shown in Table 10-11. They are added to prevent cement from setting too rapidly. These additives are also referred to as set retarders. Cements with retarders to prevent rapid setting may be used at the high-temperature and high-pressure environments of deep wells. Common retarders are lignosulfonate and certain carbohydrate derivatives, such as welan gum, xanthan gum, cellulose, and polyanionic cellulose. 

L. E. Brothers. Low temperature set retarded well cement compositions and methods. Patent US 5472051, 1995. 

I. Livsey and R. Shaunak. Cement setting retarding agents. Patent GB 2182031, 1987. 

K. A. Rodrigues. Cement set retarding additives, compositions and methods. Patent US 5341881, 1994. 

Setting retarder Extending workability time Sugar, treacle, fruit syrup, blood, egg white, gluten... 

The calcium ion concentration of the solution phase is slightly increased in the early stages of hydration, but subsequently the concentration approaches that of a system containing no admixture. It has been found that the greater the period of set retardation, the larger is the difference in calcium concentration, and it takes longer to reach the same level as a... 

The superplasticizers are a special category of water-reducing agents in that they are formulated from materials that allow much greater water reductions, or alternatively extreme workability of concrete in which they are incorporated. This is achieved without undesirable side effects such as excessive air entrainment or set retardation. 

Compatibility with other admixtures due to the inherent surfactant properties exhibited by most of the materials in classes A, B and C, their use with other admixtures such as water reducers should be evaluated in terms of the side effects on the plastic and hardened concrete, viz. severe set retardation and excessive air entrainment. The molecules of materials with fewer surfactant properties compete with the superplasticizer for adsorption sites on the cement particle. This will result in a higher dosage of the superplasticizer. 

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 ... 

These materials may be used singly or in combination with other organic compounds. Type B admixtures have now been replaced by the bifunctional type D water-reducing set retarders. 

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

With some cement-superplasticizer combinations, various problems have been reported such as low fluidification effect, rapid slump loss, severe segregation, extended set retardation and loss of entrained air. These are briefly reviewed below. 

The silica fume is said to decrease the formation of ettringite by reducing the concentration of calcium (Ca" +) and hydroxyl (OH ) ions involved in the formation of ettringite [146]. The expansion of lime-based admixtures is not affected by water-reducing admixtures to the same extent as observed for CSA-based admixtures. However, set retardation may result due to the increased amounts of calcium hydroxide produced in the presence of the WRA. 

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. 

The presence of heavy metals is often observed to significantly interfere with proper setting of Portland cement. The incorporation of fly ash into Portland cement helps to partially compensate for this set-retardation, and a subsequent supercritical C02 treatment of such modified cement should perform even better. 

---