What is Chemical Admixture Function, types and Uses in concrete

What is admixture?

The admixture is essentially for in the four ingredients in concrete i.e., the coarse aggregates, fine aggregates cement, and water. However, in order to improve certain properties of concrete both in plastic and hardened stage, it is necessary to add the fifth ingredient in the concrete mix. The fifth ingredient can be a mineral or chemical admixture.

The admixtures ranging from the addition of minerals, chemicals, and waste materials have been used to modify the properties of concrete in plastic and hardened stages. Admixtures are generally added in a relatively small quantity to change the properties of the mix. 

Functions of chemical admixtures

Some of the important functions for which the admixtures could be used are given below following the points.

1. It increases the impermeability.
2. It produces lightweight concrete (i.e., to reduce the weight of concrete per mo).
3. It reduces the chances of bleeding.
4. It reduces the chances of segregation.
5. It increases the strength of concrete.
6. It produces cellular concrete.
7. It produces colored concretes.
8. It imparts anti-corrosive properties.
9. It reduces the heat of hydration.
10. It produces concretes for acidic and alkaline environments.

Chemical Admixtures

When chemicals are added in a concrete mix to improve the performance characteristics are called chemical admixtures. Depending upon the quantity to be improved the chemical admixtures can be of following types : 

1. Accelerators
2. Retarders
3. Water-reducing admixtures
4. Grouting admixtures
5. Air-entraining admixtures
6. Waterproofing and permeability reducing admixture
7. Plasticizers

1. Accelerators

Accelerators are the chemical admixtures added to speed up the initial set of concrete. These are added to concrete they are more sufficient to increase the rate of hydration of cement and Shorten the setting time. The addition of accelerator helps in attaining early strength which is required for many purposes.

• Early removal of formwork.
• Reducing the specified period of curing.
• Emergency repair work.
• Placing the concretes at low temperatures.
• Early placement of structure in service

Commonly used accelerators in concrete are following given below  

(i) Calcium Chloride (CaCl2) - It is the most widely used accelerator. It is available in the flakes. Calcium chloride can generally be used in amounts up to 2% by mass of cement 7861 (Part-II) recommends a maximum of 1.5% of CaCl2 for plain and reinforced concrete in cold weather conditions. But recent studies have shown that CaCl2 is harmful to pre reinforced concrete. 

(ii) Accelerators containing fluorosilicates & triethylamine - They are capable of red the period (for which concrete remains in plastic stage) to less than 10 minutes. These are compared expensive. 

Other examples of chemical accelerators are NaCl, Na2SO4, NaOH, etc. 

2. Retarders

Chemical admixtures are generally used to delay or prolong the setting of the cement paste in concrete. Retarders slow down the chemical process of hydration so that concrete remains place and workable for a longer time than normal concretes. The addition of a retarder helps in : 

• Hot weather concreting.
• Transportation of ready mix concrete so that concrete can be placed and compacted in the plastic stage.
• Holding back the hydration process and leaving more water for workability.

Commonly used retarders are given below following the points.

1. Calcium sulphate (Gypsum) - It is generally added during the manufacture of cement to retard the setting time. Gypsum, if added beyond a specified limit (3 to 4% by weight of clinkers) makes the cement unsound. 

2. Sugar - At normal temperatures, 0.2% addition of sugar can extend the final setting time to about 72 hours or more. But when it is increased beyond 0.2% (say 0-25%), rapid setting and remarkable reduction in 28 days compressive strength take place. 

3. Skimmed Milk - Due to sugar content present in skimmed milk, it can give retarding en By addition of 0.1% of Sodium hexametaphosphate, the initial and final setting times of 12 and 13 hours can be obtained respectively. 

Other examples of retarders are - Ammonium chloride, Calcium tartrate, Calcium acetate, etc.

3. Water reducing admixtures

Water reducing admixtures are used to given below following the points. 

• Improve the quality of concrete.
• Obtain specified strength at a lower cement content.
• Increase the slump of a given concrete mix without increasing the water content.

The specific effects of water-reducing and retarders vary with the type of cement, water-cement ratio, mixing temperature, etc. Therefore, it is generally recommended that they should be tested with available site materials to determine their ability to produce desirable properties. 

The chemicals generally used as water-reducing admixtures belong to the hydroxylated carboxylic acids and their salts. Other examples of water reducing chemical admixtures are carbohydrates and organic acids.

4. Grouting  admixture

Some retarders are especially useful in cement grout slurries, especially where are given below.

• The grouting is prolonged.
• The grout has to reach a considerable distance.
• Hot water is encountered underground.

Cement grouts containing pozzolanic materials are often used in given below following the points.

1. Cementing oil wells.
2. Preventing rapid loss of water from cement paste.

Other examples of grouting admixtures are - Gels, Clays, pre-gelatinized starch, and methylcellulose.

5. Air-entraining admixtures

These chemical admixtures are useful to incorporate a controlled amount of air in the form of very small disconnected bubbles in the concrete during mixing, without significantly altering the setting or the rate of hardening characteristics of the mix. 

The entrapped air bubbles (approximately 0.01 to 0-25 mm diameter by reducing internal friction) acts as flexible ball bearing rollers and modify the workability of concrete mix in the plastic stage. The main functions of air-entrained concrete are given below following the points.

• To improve workability.
• For easier placing.
• To decrease the bleeding and water gain. 4
• To increase the resistance of concrete for frost action.
• For increased durability.
• To decrease the chances of bleeding.
• To make cellular concrete and lightweight concrete.

The entrapped air bubbles reduce the capillary forces in concrete. Within the normal range of If content, the maximum reduction in compressive strength and flexural strength hardly exceeds 15% and 10% respectively. Air-entrained concrete is considered to be more plastic and workable than non-air-entrained concrete. 

Chemical compounds used for air-entrainment are given below following the points.

• Natural wood resins e.g., Vinsol Resin.
• Animal and vegetable fats and oils e.g., olive oil.
• Various sulphonated compounds.

Air entraining admixtures made from neutralized vinsol resin is mostly used for mass concrete and concrete used in highway pavements. 

6. Waterproofing and permeability reducing admixture

Water under pressure and in contact with one surface can be forced through channels between the two surfaces. The passage of water in this way gives the measure of the permeability of concrete The chemical admixtures required to reduce the permeability of concrete are known as permeability reducer admixtures. The water can also enter concrete by the action of capillary forces. The materials added in concrete to reduce the flow through capillary channels are known as damp proofers. 

Air entraining agents can be used as permeability reducers and damp proofers because the resistance of concrete to the penetration can be improved by air bubbles induced. Even water-repelling agents like soda and potash soaps are sometimes added to improve the resistance of concrete to the penetration of moisture. The chemicals used as waterproofers are given below following the points.

• Asphaltic products
• Thick viscous liquids
• Thin films
• Sodium silicate compounds forming a gel.

7. Plasticizers

Plasticizers are the chemical compounds that impart very high workability with a large decrease in water content (at least 20%) for a given workability. There are several types of admixtures available but the most popular and most often used is the plasticizer. The plasticizer is a chemical admixture and this publication covers in detail its classification, interaction, useful effects on concrete properties and precautions to be exercised in its usage. 

Plasticizers are extensively used in concrete mixes for the past four decades in the world over. However, in India, this usage has considerably increased in the last five years. Plasticizers or any other type of admixtures are not substitutes for badly designed concrete mixes or bad construction practices. They basically facilitate and modify good mixes and good construction practices to achieve certain specific requirements that can be easily achieved. Chemical compounds used as plasticizers are given below following the points.

• Anionic surfactants e.g., Lignosulphonates.
• Non-ionic surfactants e.g., Hydroxylated carboxylic acids.

Plasticizers are generally added in the amount of 0.1% to 4% by weight of cement. A 02 concentration of chemical admixtures by weight can bring the following advantages given below.

• It increases in slump value from 25 mm to 100 mm.
• It requires 15% - 20% less quantity of cement,
• It increases the compressive strength by 30% with a 20% reduction in water-cement ratio.

The main purpose of adding admixtures are the following given below.

(A) In fresh concrete

1. Increases workability and/or permeability without increasing the water-cement ratio.
2. Improves cohesiveness and thereby reducing segregation or bleeding.
3. Improves to some extent set retardation.
4. It provides high workability for column-beam junctions and thin walls with a high percentage of steel.

(B) In hardened concrete

1. Increases strength by reducing the water-cement ratio, maintaining the same workability.
2. Reduces permeability and improves durability by reducing the water-cement ratio.
3. Reduces the heat of hydration and drying shrinkage by reducing cement content.

It is necessary to do trial mixes using plasticizers with the same concrete material as proposed to be used at the site. All plasticizers should be tested adequately for their desired performance in full scale before being used in any large construction work.

Also read - What is Mineral Admixture Function, types and Uses in concrete