152413Naphthalene Sulfonate Superplasticizers

Naphthalene superplasticizers have been in use longer than any of the others, and are available under a greater number of brand names. They are available as both a powder and a brown liquid; in the liquid form they typically have a solids content of about 40%. They are generally available as either calcium salts, or more commonly, sodium salts. (Calcium salts should be used in the case where a potentially alkali-reactive aggregate is used.)

The particular advantages of naphthalene superplasticizers, apart from their being slightly less expensive than the other types, appears to be that they make it easier to control the rheological properties of high-strength concrete because of their slight retarding action.

For a superplasticizer, great concern should be paid on the superplasticizer-cement compatibility, apart from the commonly considering water-reducing ratio. As is well known, the production of modern HPC typically requires a low w/b ratio. This means that high dosages of superplasticizers have to be incorporated to improve workability. In conjunction with the relatively low water contents of modern HPC, typically in the range of 125 to 145 L/m3, this may lead to superplasticizer-cement compatibility problems. In particular, the rapid rate of water consumption, which occurs during the formation of ettringite (C3A • 3CaSO4 • 32H2O), drastically reduces the already small amount of mixing water. This can result in insufficient water available to provide proper workability, particularly when there is no optimum content of calcium sulfate [15]. Recent work by Mokhtarzadeh and French [16] has highlighted the care that must be taken in choosing an appropriate superplasticizer for any particular HPC mix; the choice must be made by considering the superplasticizer effects on both workability and long-term strength.

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