Admixtures are added to concrete to alter its properties and improve workability, strength, and curing time. The main types of admixtures are accelerators, retarders, plasticizers, and super plasticizers. Accelerators speed up curing and are used when early strength is needed, while retarders slow curing and are used for long transportation. Plasticizers and super plasticizers reduce the water content needed for a given workability, thereby increasing strength. Caution must be taken with dosages and some admixtures are not suitable for certain concrete types.

2.  WHY WE USE ADMIXTURE?
 ADMIXTURES ARE USED IN CONCRETE TO ALTER ITS PROPERTIES IN VARIOUS WAYS.
 SOME COMMON USES INCLUDE IMPROVING WORKABILITY, INCREASING OR DECREASING
CURING TIME, AND INCREASING CONCRETE STRENGTH.
 TYPES OF ADMIXTURE
 ACCELERATORS
 RETARDERS
 PLASTICIZERS (WATER REDUCING AGENTS)
 SUPER PLASTICIZERS (HIGH RANGE WATER REDUCERS)

3.  ACCELERATORS
 ACCELERATORS ARE USED WHERE RAPID SETTING AND HIGH EARLY STRENGTH REQUIRED.
 WE ALSO USE IT WHERE RAPID REMOVAL OF FORM WORK REQUIRED.
 HOW IT WORKS?
 ACCELERATORS GENERALLY SPEED UP THE CHEMICAL REACTION OF THE CEMENT AND
WATER SO IT ACCELERATE THE RATE OF SETTING OR GAIN EARLY STRENGTH OF CONCRETE.
 CaCl2 CAN BE USED AS ACCELERATORS BUT AN AMOUNT UP TO 2% BY MASS OF CONCRETE.
 IS 9103:1999(CODE BOOK FOR CONCRETE ADMIXTURE) RECOMMENDS USE UP TO 1.5% OF
CaCl2 FOR RCC WORK IN COLD WEATHER.
 ACCELERATOR INCREASE THE HYDRATION TEMPERATURE OF ABOUT 110C.
 HEAT OF HYDRATION MORE IN ACCELERATORS.

4.  CAUTION
 ACCELERATORS ARE CHLORIDE BASED SO IT ALSO INCREASE CORROSION.
 SO WE SHOULD NOT USE THEM IN WATER RETAINING STRCTURES,PRESTRESSED CONCRETE
AND STEAM CURED CONCRETE.
 IF WE USE ACCELERATOR MORE THAN ITS REQUIRED AMOUNT IT CAN CAUSE INSTANT
SETTING AND LOSS OF CEMENT MIX.
 ACCELERATORS ARE MORE EFFECTIVE IN COLD WEATHER.

5.  RETARDERS
 RETARDERS ARE USED WHERE CONCRETE HAS TO BE TRANSPORTED FOR
LONG TIME.
 TO PREVENT COLD JOINTS DUE TO DURATION OF PLACING.
 HOW IT WORKS?
 RETARDERS GENERALLY SLOW THE CHEMICAL REACTION OF THE CEMENT AND WATER SO IT
DELAY THE RATE OF SETTING AND GAIN LESS EARLY STRENGTH OF CONCRETE.
 MOST COMMON RETARDERS ARE GYPSUM(CaSo42H2O),ZINK,SUGAR(0.5-1%).
 HEAT OF HYDRATION LESS IN RETARDERS.
 CAUTION
 IF WE MIXED MORE THAN RECOMMENDED VALUE RETARDATION
CAN LAST FOR DAYS.

6.  PLASTICIZERS (WATER REDUCING AGENTS)
 PLASTICIZERS ARE USED TO INCREASE PLASTICITY IN THE FRESH CONCRETE FOR IMPROVING WORKABILITY.
 THESE ARE MAINLY USED TO ACHIEVE HIGHER STRENGTH BY REDUCING THE WATER CEMENT RATIO.
 HOW IT WORKS?
 PLASTICIZERS CREATE REPULSIVE FORCE TO INCREASE WORKABILITY.( THE FORCE BY WHICH BODIES REPEL
ONE ANOTHER)
 WHEN PLASTICIZERS ADDED IN CEMENT THEY GET ABSORBED BY THE CEMENT
PARTICLES.
 IF O.1 TO 0.4% OF PLASTICIZERS ADDED BY WEIGHT IN CEMENT CONCRETE
IT WILL REDUCE 5-15% OF WATER REQUIRMENT.
 CARBOXYLIC ACID,LIGNOSULPHONIC ACID USED AS PLASTICIZERS.

7.  SUPER PLASTICIZERS (HIGH RANGE WATER REDUCERS)
 SAME AS PLASTICIZERS BUT NOW A DAYS IT USES MORE.
 FINER THE CEMENT HIGHER THE SUPER PLASTICIZERS DOSE.
 IF O.1 TO 0.4% OF SUPER PLASTICIZERS ADDED BY WEIGHT IN CEMENT CONCRETE IT WILL
REDUCE 30-50% OF WATER REQUIRMENT.
 C3A HAVE MAJOR INFLUENCE ON THE PROPERTIES OF
SUPER PLASTICIZERS