Types of Admixtures for Concrete

Admixtures

 

                   Admixtures are ingredients other than aggregates, water, fibers, and hydraulic cement that are added to the concrete batch immediately before or during mixing. A proper use of admixtures offers some particular  beneficial effects to concrete, improved quality, acceleration or retardation of setting time, increases  frost and sulfate resistance, control of strength development, and improved workability.
 

Calcium chloride (CaCl_2)

                 Calcium chloride is generally used in cold weather to accelerate the setting time and produces an early finish of the concrete. CaCl_2 can effect the characteristics of concrete causing corrosion of unprotected reinforcement, in a decrease the resistance to freeze,  increased internal stresses, an increase in the attack of sulfates, temperature rise, and an increase in the amount of drying shrinkage between 10 - 50 %.

Set Retarding Agents

                These agents are primarily used for delaying the setting time of concrete. Set retards are useful, but are not the solution to poorly designed mixes. These retarders must be uniformly dispersed throughout the whole mix [mortar] . Failure to do so will result in the admixture remaining in voids and causing some areas to remain plastic. This leads to plastic shrinkage.

Water Reducing Agents

                A certain amount of the water is added to concrete to make the concrete place able and to ignite the hydration reaction of Portland cement. Over 50% of the water have no useful effect and have direct results in durability, the strength of concrete and drying shrinkage. These water reducing agents added to the concrete reduce the water demands of the mix, increase the strength, to reduce the cost, maintain the workability, reduce segregation, reduce honeycombing, reduce cracking, reduce bleeding,  and permeability, increase bond strength of concrete to steel, and reduce drying shrinkage. An average of 12 to 15 % reduction in the total water content will accomplish this goal.

Accelerating Agents

                These Accelerating agents are used to shorten the setting time and increase the early strength of the concrete. These agents are mainly used for concrete needing to be used immediately in some special cases like the marine constructions etc.

Air Entraining Agents

These Air entraining agents are used primarily to increase the resistance of freezing and thawing. They also produce

•     lower water demands for a particular slump,

•     high  resistance to deicing chemicals,

•     to improve workability,

•     reduce segregation and bleeding,

•     increase durability, and improve pump ability and

•     reduce the amount of fine aggregate needs in the mixture.

Even though air entrainments have no effect on drying shrinkage, they create a slight deficit in the concrete strength.