Admixture|Types|Function in engineering

                     Admixtures

An admixture is defined as a material other than cement, water, aggregates and fiber reinforcement used as an ingredient of a cementitious mixture to modify the workability of treshly mixed, setting

or hardened properties. It is added to the batch before or during its mixing

Types of Admixture

Admixtures are primarily classified, based on their function, into five major classes, water reducing, retarding, accelerating air-entraining, and super plasticizing. In addition, a specialty class contains

various types of other minor admixtures also.

1.Accelerators

Accelerating admixtures are chemicals that increasing the rate of hydration thus, decrease the setting time and increase the hardening rate of concrete. They are usually used in applications where there is

short curing period, early formwork removal is required, urgent repairs or cold weather concreting applications. Accelerators mainly consist of nitrite, nitrate, thiocyanate, tri-ethanol-amine, calcium chloride, etc. Among these, Calcium chloride (<2%) is mostly used.

2.Retarders

Retarding admixtures postpone the preliminary rate of hydration of cement and lengthen the setting time of cement paste.This admixture is useful for high temperature and where the concrete should be transmitted to a long distance. It can also be utilized for grouting oil wells. Retarding admixtures mainly consist of calcium sulfate, gypsum, sugars, carbohydrates, salts from acids, soluble zinc, SOluble borates, etc. but gypsum (2 to 3%) is mostly used.

3.Air-Entraining Admixtures

Air-entraining admixtures are chemicals that mainly enhance the durability of concrete, its freeze and thaw resistance, improves the workability and the resistance to segregation and bleeding

However, air-entrainers can reduce the strength of econcrete. In general, as the amount of air increases by 1%, the strength decreases by about 5%. The mode of action of this type of admixtures is at it forms uniformly distributed tiny air bubbles inside concrete. Air entrainers mainly consist of natural wood resins, animal fats, vegetable fats, natural oil, alkali salts, polyethylene oxide polymers,

Cutralized vinsol resins, sulfonated compounds, or others.

4. Water Reducing Admixtures

Water reducing admixtures are chemicals that reduce the amount of required water in a concrete mixture. In fact, this type of admixtures acts based on either the mechanism of electrostatic repulsion

or that of steric stabilization, depending on its composition. Water reducers have various benefits such as:

Enhancing the strength for a fixed slump.

Increasing the workability for a fixed water content.

Reducing the cement content for a fixed strength.

Reducing cost.

Water reducing admixtures consist of lignosulfonate, alkyl aryl sulfonates, resin sulfonate, hydro- carboxylic acids, etc.

5.Super Plasticizing Admixtures

Super plasticizing admixtures also called high range water reducers are chemicals that reduce the amount of required water for a fixed slump more than normal or mid-range water reducers do. In general, the water reduction capacity for most super-plasticizers ranges from 12 to 40%.

6.Other Special Admixtures

Following are the various special types of admixtures used in construction field

Coloring agent

Bleeding agent

Grouting agent

Gas forming agent

Corrosion inhibiting agents

Fly ash and blast furnace slag

Functions of Admixture

Following are various functions of admixtures:

Increase workability without increasing water content or to decrease the water content at the same workability.

●Retard or accelerate both initial and final setting times.

●Prevent shrinkage and cracks.

●Reduce segregation of concrete, mortars and grouts for instance.

●Increase the slump or slump-tlow without increasing the water content.

●Retard or reduce heat generation during early hardening

●Accelerate the rate ot strength development.

●Increase the strength ot concrete or mortar

●Increase resistance to freezing and thawing

●Control the expansion caused by the reaction of alkalis with certain aggregate constituent.

●Increase the bond of concrete to steel reinforcement.

●Increase the bond between old and new concrete.

●Improve impact resistance and abrasion resistance.

●Produce colored concrete or mortar.