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The admixture which we use in the concrete for delaying its setting time then such type of admixtures are called as retarders.
Gypsum is a retarding agent which we add in the cement during its manufacturing process.
Gypsum is work as a starting agent as well as it restricts soundness in the cement.
Apart from gypsum of various admixtures already listed in some previous post.
The chemical name of gypsum is calcium sulphate.
Thank you.
It is defined by percentage of weights or volume only .not related to time . Mostly around 0.8 to 1 % of weight if memory ok .
Any admixtures used in concrete, it is as per the recommendations of the manufacturer. Refer to the manufacturer's specification for the quantity of admixture to be used per cubic meter of concrete to delay the setting time.
Many admixtures are available in the market to increase the setting time of concrete called Retarders.
Quantity of retarders is depended upon the recommendations of manufacturer per m^3.
Retarders are extending the hydration process of cement particles does increase the initial setting time of concrete. Following are some commonly used retarders:
Calcium sulphate and gypsum. Acids and salts also act as retarders Commonly sugar is the most economical retarder used in concrete.sugar of 0.05 to 0.02% of total concrete is mixed. (ASTM C - 494- TYPE D & G ). Modern retarders are lignosulfonate acids, carboxylic acid, calcium acetate.
Concreting in some situations becomes quite complex where workability of concrete has to be maintained for longer durations and their retarders as an admixture can be quite beneficial. Retarders are useful in extending hydration of cement particles which in turn increases the initial setting time of concrete, therefore, helps in preventing the formation of cold joints, increased efficiency of cement grout while grouting oil wells at 200°C, transporting concrete from RMC to construction site locations.
The most used retarders are calcium sulphate a.k.a gypsum which is inter-grounded along with clinker, materials such as starches, cellulose, acids and salts of acids show similar reading effect. Common sugar being most economical retarder is often used in concrete whose dosage lies between (0.05-0.20)% of the total mass of concrete. In case of American codes ASTM C-494 (Type D & Type G) show retarding effect.
Modern retarders are bifold beneficial in terms retarding effect as well as water reducers which are mostly lignin-sulphonic acids, hydroxylated carboxylic acids, calcium acetate and also enhance 28 days compressive strength by (10-20)%. Chemical admixture being quite unpredictable in properties for the different dosage of admixtures.
As of if excess of sugar is added which can lead to the delayed setting of concrete by 72 hours which in turn is quite detrimental for concrete. Chemical admixtures are such materials must be technically controlled very efficiently or they can lead to vulnerable effects.
