1、 Retarder definition

Retarder is an admixture that can prolong the setting time of concrete. The main types of retarders are: hydroxycarboxylic acid and its salts, such as tartaric acid, citric acid, gluconic acid and its salts and salicylic acid; Sugary carbohydrates, such as molasses, glucose, sucrose, etc; Inorganic salts, such as borate, phosphate, zinc salt, etc; Sodium lignosulfonate, etc.

2、 Common retarder

(1) Molasses retarder

Molasses retarder is made from the leftover of sugar making through lime treatment. Its main components are calcium hexose, calcium sucrose, etc. Generally, the mixing amount is 0.1% ~ 0.3% (powder) and 0.2% ~ 0.5% (water agent) of the cement quality. The setting time of concrete can be extended by 2 ~ 4H. For each increase of 0.1% (water agent), the setting time will be extended by about 1H. When the mixing amount is greater than 1%, the concrete will be loose and not hard for a long time;

When the content is 4%, the 28d strength is only 1 / 10 of that of no content.

(2) Hydroxycarboxylic acid and its salts retarder

Generally, the content of this kind of retarder is 0.03% ~ 0.10% of the quality of cement, and the setting time of concrete can be extended by 4 ~ 10h. This kind of retarder will increase the bleeding rate of concrete, especially in concrete with low cement dosage or high water cement ratio. If used with air entraining agent, it can be improved.

(3) Lignosulfonate retarder

Generally, the content of this kind of retarder is 0.2% ~ 0.3% of the quality of cement, and the setting time of concrete can be extended by 2 ~ 3H.

3、 Action mechanism of Retarder

The action mechanism of various retarders is different. Generally speaking, organic retarders are mostly surfactants, which have strong activity on the surface of cement particles and new phase of hydration products. They are adsorbed on the surface of solid particles, delaying the hydration of cement and the formation of slurry structure. Inorganic retarders often form a layer of insoluble film on the surface of cement particles, which acts as a barrier to the hydration of cement particles and hinders the normal hydration of cement. These effects will lead to the retarding of cement concrete. The adaptability of retarder to cement varieties is very obvious. The retarding effect of different cement varieties is different, and even the opposite effect will appear. Therefore, trial mixing must be carried out before use to test the effect. Generally, the dosage of retarder is small, and the dosage should be strictly controlled during use. Excessive addition will not only cause non coagulation for a long time, but also accelerate coagulation sometimes. See table 4-22 for the dosage of various retarders. Retarder is mainly used for concrete in high temperature season, mass concrete, pumping and slip form concrete construction and long-distance transportation of commercial concrete. The retarder should not be used for concrete constructed under the daily minimum temperature of 5 ℃, nor for concrete with early strength requirements and steam curing concrete.