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Admixture of Cement and Concrete- Types of Admixtures - Mechanism of Admixtures in Concrete


Admixtures 


The admixtures are ingredients that are added to concrete other than cement, water, and aggregate to obtain a specific property that is added immediately before or after mixing. A properly designed concrete mix should be workable, finishable, strong, water tight, durable and wear resistant. To some extend these properties can be obtained without the help of admixtures. 




Reasons For Using Admixtures in Concrete


The major reasons for using admixtures in concrete are mentioned:

  • For the reduction for cost of concrete construction
  • Admixtures give effective properties in concrete, that a normal mix
  • To maintain the quality throughout mixing, transporting, placing and curing, where the conditions are worse.
  • To overcome emergencies during concreting works.
But it has to be kept in mind that " No admixture of any type or amount is a substitute for good concreting practice".

Factors Affecting Admixture in Concrete


The effectiveness of admixture when used in a given concrete mix, depends upon certain factors. They are:
  • The property of Cementing Materials used - The type, brand  and amount
  • The water Content
  • The shape of the Aggregate
  • The grading and proportion of aggregates
  • The mixing time
  • The slump value of the mix
  • The temperature of the concrete

Types of Admixtures


There are mainly three major types of admixtures used in concrete mix design. Each category serving different responsibilities and action in concrete. The major category and subdivisions of admixtures are given in the below flowchart.

Fig. Classification of Admixture

Chemical Admixtures


Chemical Admixtures are admixtures that are soluble in water. They are added in relatively small amounts to bring some change in certain properties of the concrete.  The manufacture of chemical admixture is a large profitable industry, profitable than the manufacture of portland cement. The chemical admixture can be classified into following types:
  1. Accelerators
  2. Retarders
  3. Air Entraining Agents
  4.  Plasticizers/Water Reducers/
  5. Superplasticizer
  6. Damp Proofing and waterproofing admixtures
  7. Water retaining Admixtures
  8. Multiple admixtures
  9. Special Admixtures


Mineral Admixtures


The mineral admixture are also called as "supplementary cementacious materials". These unlike chemical admixture, make use of recycled products, waste materials to increase the concrete performance. These are called pozzolanos, due to their pozzolanic activity when added to concrete mix. 

The properties are:

  • The mineral admixtures increases the strength 
  • It reduce permeability 
  • The hydration heat is reduced thus increasing durability.
  •  Any deficiency in aggregate grading is corrected.
  • Increase Workability
  • Improve extensibility
  • Lower cost
  • Less suceptible to dissolution and leaching activity

Properties of Mineral Admixtures


The pozzolanic materials or mineral admixtures are :
  • Siliceous or siliceous aluminious materials
  • They have no cementacious value
  • The pozzolanic activity is carried out in moisture. This involves reaction with calcium hydroxide and gives products possessing cementaciuos property.

Portland Pozzolan Chemical Reaction 



C3S / C2S + H2O —›C-S-H + Ca (OH)2 

 Ca (OH)2 + Reactive SiO2 —› Pozzolanic C-S-H

Classification of Mineral Admixtures



The mineral admixtures can be classified into two broad categories:

  1. Natural Pozzolans
  2. Artificial Pozzolans
Natural Pozzolans : Clay and shales, Volcanic Tuffs, Pumicies, Diatomacceous Earth, Opalinc Cherts
Artificial Pozzolans : Fly ash, Blast Furnce Slag, Rice Husk Ash, Metakaoline, Surkhi, Silica Fume

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