This document discusses various types of admixtures that are added to concrete to modify its properties. It describes 15 types of admixtures classified according to their function, including plasticizers, superplasticizers, retarders, accelerators, air-entraining agents, and pozzolanic materials. Common chemical admixtures are discussed in more detail, along with their effects on properties of fresh and hardened concrete. Mineral admixtures like fly ash, blast furnace slag, rice husk ash, and silica fume are also summarized in terms of their composition and impact on improving concrete quality and durability.

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