Set retarders/set retarding admixtures for concrete

1st part

Set retarders or set retarding admixtures are used in concrete mixes in order to delay the hydration procedure of the cement and thus its setting.

Their action is most desirable in the case of high temperatures. High temperature can reduce the setting time of concrete considerably. Similarly their use is indispensable in case of unforeseen or scheduled delays in casting.

There are different versions of set retarding admixtures if their water reducing capabilities are used as a criterion:

* Simple set retarding admixtures with practically no influence on workability

* Set retarding water reducers which can increase workability for par W/C ratio

* Set retarding superplasticizers which can considerably increase concrete workability for par W/C ratio.

Main uses of set retarding admixtures

1. They help concrete maintain its workability.

2. They are used to delay setting. Thus they can help eliminate cold joints and/or provide more time for finishing procedures.

3. They can be used as water reducers to increase final compressive strength or

4. They can be used as plasticizers/superplasticizers to increase workability.

Chemical base of set retarding admixtures

- Salts of lignosulfate acids

- Salts of hydroxycarboxylic acids

- Polysaccharides and their derivatives

- Hydroxylated polymers

 Set retarding admixtures: way of action

The active ingredients of set retarding admixtures stick on cement particles with an adsorption mechanism.

In this way they isolate cement from water and prevent hydration procedures.

For how much time? This will depend on the thickness of the formed film.

Obviously the biggest dosage of set retarding admixture the thicker the film.

After a time span which can be from several minutes to few hours, the film will disappear and the hydration procedure will go on.

It is critical not to overdose, especially in low temperatures. There are cases where hydration is inhibited and the concrete will never set.

DOSAGES

They vary considerably with the type of set retarding admixture and the temperature. The higher the temperature the less the retardation.

The most usual dosages range between 0.3% - 0.8% by cement weight for set retarding plasticizers and from 0.7% to 1.3-1.5% for set retarding superplasticizers.

End of 1st part

Chris Strogilis

Civ. Eng. – MBA – DipM