Admixtures and their types

1. ADVANCED CONCRETE
TECHNOLOGY
PREPARED BY CHANDU
¾ B.E.CIVIL ENVIRONMENTAL

2. ADMIXTURES
 Admixture is defined as a material, other than cement, water and aggregates, that is used as an
ingredient of concrete.
 Admixture is used to modify the properties of ordinary concrete so as to make it more suitable for any
situation.
 Admixtures are addition to a concrete mix that can help control the set time and other aspects of fresh
concrete.
 Admixtures are chemicals, added to concrete, mortar or grout at the time of mixing, to modify
properties either in the wet state or after mix has hardened.
 In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at
the time of batching/mixing.
 As per the report of the ACI committee 212, admixtures have been classified into 15 groups according
to type of materials constituting the admixtures.

3. Types of admixtures
 Plasticizers
 Superplasticizers
 Retarders and retarding plasticizers
 Accelerators and accelerating plasticizers
 Air-entraining admixtures
 Pozzolanic or mineral admixtures
 Damp-proofing and waterproofing admixtures
 Gas forming admixtures
 Air detraining admixtures
 Alkali-aggregate expansion inhibiting admixtures
 Workability admixtures
 Grouting admixtures
 Corrosion inhibiting admixtures
 Bonding admixtures
 Fungicidal, germicidal, insecticidal admixtures
 Colouring admixtures

4. Function of admixtures
 Retard or accelerate initial setting
 Increase slump and workability
 Reduce or prevent shrinkage
 Modify the rate of capacity for bleeding
 Reduce segregation
 Decrease weight of concrete
 Improve durability
 Decrease the rate of heat of hydration
 Reduce permeability
 To make porous concrete
 To make colouring concrete
 Increase bond of concrete to steel reinforcement

5. Chemical admixtures and their types
 Chemical admixtures are water soluble chemicals that are added in relatively small
amounts to concrete in order to change certain properties.
 Some of the most commonly used chemical admixtures are given below:
 1.Plasticizer admixtures
 2.Superplasticizer admixtures
 3.Accelerator admixtures
 4.Retarder admixtures
 5.Gas forming admixtures
 6.Air entraining admixtures
 7.Workability admixtures

6. Plasticizers
 These are the chemicals that improve the workability of paste.
 This allows a lower w/c to be used for a given workability, resulting in higher
quality concrete.
 The action of plasticizers is mainly to fluidify the mix and improve the workability of
concrete, mortar or grout.
 It may be noted that all plasticizer are to some extent set retarders, depending
upon the base of plasticizers, concentration and dosage used.
 Use of plasticizers permit the reduction of water upto 15% without reducing
workability.

7. Superplasticizers
 Superplasticizers constitute a relatively new category and improved version of
plasticizers.
 Use of superplasticizers permit the reduction of water upto 30% without affecting
workability.
 Superplasticizers can produce:
 1. At the same w/c ratio much more workable concrete than the plain ones.
 2. For the same workability, it permits the use of less w/c/ ratio.
 3. As a consequence of increased strength with lower w/c ratio, it also permits the
reduction in cement content.
 The superplasticizers also produce a homogeneous, cohesive, concrete generally
without any tendency for segregation and bleeding.

8. Accelerator admixtures
 These are chemicals that increase the rate of early cement hydration so that the
cement sets faster.
 The most common reason for using accelerators is speed up the pace of
construction by reducing the time required for the concrete to have some
minimum load-bearing capacity.
 A very cheap and effective accelerator is calcium chloride.
 The accelerating materials added to plasticizers or superplasticizers are
trietheolamine chlorides, calcium nitrates, and flousillicates.

9. Retarder admixtures
 These are the opposite of accelerators; they slow the early hydration.
 They are used primarily to extend the period of workability of the fresh concrete.
 Unlike accelerators, the use of retarders does not tend to harm the long-term
properties of the concrete.
 Sugars (including sucrose, table sugar) is a very cheap and effective retarder.
 Retarding plasticizers are used to give workability retention to the concrete, delay
the setting time and increase initial workability.

10. Gas forming admixtures
 This includes aluminium powder, zinc, magnesium powder, and hydrogen peroxide.
 The addition of gas forming admixtures varies from 0.005 to 0.02% by weight of
cement.
 Most commonly used gas forming admixture is aluminium powder and it results in
formation of hydrogen gas bubbles.
 Gas-forming admixtures help maintain concrete's initial volume, counteracting
settlement and bleeding, by generating or liberating bubbles in the mix.

11. Air entraining admixtures
 These are chemicals that make the concrete less susceptible to damage caused by
freezing of the water in the pores.
 They promote the formation of very small air bubbles in the concrete during
mixing, resulting in a system of approximately spherical voids that are evenly
distributed throughout the hardened cement paste.
 Their use is standard practice in any environment that may experience freezing
weather, and they are probably the most commonly used chemical admixture.

12. Workability admixtures
 A concrete is said to be workable if it is easily transported, placed, compacted and
finished without any segregation.
 General admixtures used for workability of concrete are bentonite clay, finely
divided silica, hydrated lime and talc.
 Reduces rate and amount of bleeding and increases leap strength of concrete.
 Reduce the heat of evolution, increase the durability.
 Accelerate the rate of strength development at early stages.
 Decrease the permeability of concrete.
 Reduce or prevent the settlement or create slight expansion.

13. Pozzolanic or mineral admixtures
 Mineral admixtures are added in concrete to improve the quality of concrete.
 Effect of mineral admixtures on the properties of fresh concrete is very important
as these properties may effect the durability and mechanical properties of
concrete.
 Good pozzolans will not unduly increase water requirement or drying shrinkage.
 Mineral admixtures Possess certain characteristics through which they influence the
properties of concrete differently.
 Fly ash(FA), silica fume(SF), ground granulated blast furnace slag(GGBS), and rice
husk ash(RHA).

14. Pozzolanic materials
 Pozzolanic materials are siliceous and aluminium materials, which in themselves
possess little or no cementitious properties.
 Pozzolan + calcium hydroxide + water = C – S – H(gel).
 This reaction is called pozzolan reaction.
 They are divided into 2 groups
 Natural pozzolans Artificial pozzolans
 1.clay and shales 1.Fly ash
 2.opalinc cherts 2.Blast furnace slag
 3.diatomaceous earth 3.Rice husk ash
 4.volcanic tuffs and pumiciles 4.Silica fume
 5.Metakaoline
 6.Surkhi

15. Properties of mineral admixtures
 Lower the heat of hydration and thermal shrinkage
 Increase the water tightness
 Reduce the alkali aggregate reaction
 Improve resistance to attack by sulphate soils and sea water
 Improve extensibility
 Lower susceptibility to dissolution and leaching
 Improve workability
 Lower costs

16. Fly ash
 Fly ash is finely divided residue resulting from the combustion of powdered coal
and transported by the flue gases and collected by electrostatic precipitator.
 Fly ash is the most widely used pozzolanic material all over the world.
 The use of fly ash also contributes to the environmental pollution control.
 Used in the construction of many high-rise buildings and industrial structures.
 Class F: Produced by burning anthracite, usually has less than 5% CaO. Class F ash
normally has pozzolanic properties only.
 Class C: produced by burning lignite. Class C fly ash may have CaO content in
excess of 10%. In addition to pozzolanic properties it also possess cementitious
properties.

17. Ground granulated blast furnace slag
 GGBS is a nonmetallic product consisting essentially of silicates and aluminates of calcium
and other bases.
 The molten slag is rapidly chilled by quenching in water to form a glassy sand like
granulated material.
 The granulated material when further ground to less than 45 microns will have specific
surface of about 400 to 600 m2/kg.
 The replacement of cement with GGBS will reduce the unit water content necessary to
obtain the same slump.
 Effect of GGBS on hardened concrete:
 1. Reduce heat of hydration
 2. Reinforcement of pore structures
 3. Reduced permeability to the external agencies
 4. Increased resistance to chemical attack.

18. Rice husk ash
 Rice husk ash Is obtained by burning rice husk in a controlled manner without
causing environmental pollution.
 When properly burnt it has high SiO2 content and can be used as a concrete
admixture.
 Rice husk ash exhibits high pozzolanic characteristics and contributes to high
strength and high impermeability of concrete.
 RHA essentially consist of amorphous silica, 5% carbon, and 2% K2o.
 The specific surface of RHA is between 40-100 m2/g.

19. Silica fume
 Silica fume, also referred to as microsilica or condensed silica fume, is another material
that is used as an artificial pozzolanic admixture.
 It is a product resulting from reduction of high purity quartz with coal in an electric arc
furnace in the manufacture of silicon.
 Contains at least 85% SiO2 content with mean particle size between 0.1 and 0.2 micron.
 Minimum specific surface area is 15000 m2/kg. Particle shape is spherical.
 Silica fume has become one of the necessary ingredients for making high strength and
high performance concrete.
 Silica fume was also used for one of the flyovers at Mumbai, where for the first time in
India 75Mpa concrete was used(1999).
 AVAILABLE FORMS:
 1. Undensified forms with bulk density of 200-300 kg/m3.
 2. Densified forms with bulk density of 500-600 kg/m3.
 3. Micro-pellestied forms with bulk density of 600-800 kg/m3.
 4. Slurry forms with density 1400 kg/m3.

20. Compressive strength of different types of silica fumes
Dig:

21. THANK YOU