2. Set-Retarding
⦁ This type of chemical admixtures decreases the initial rate of
reaction between cement and water and thereby retards the
setting of concrete.
⦁ It functions by coating the surface of C3S (Tri calcium silicate)
components,thus,delaying this reaction with the water
.
⦁ Reaction products are slow to form as such the setting and
hardening of concrete are delayed reducing early compressive
strengths.
⦁ Retardation in setting time up to 8-10 hours is possible by
suitable use of retarders.
3. Set-Retarding
⦁ The main ingredients of retarders are as follows:
⦁ Lignosulphonic acids and their salts.e.g.Na,Ca or NH4,
⦁ Hydro-carboxylic acids and their salts.
⦁ Carbohydrates including sugar
.
⦁ Inorganic salts based on flourates, phosphates, oxides, borax and
magnesium salts.
4. Set-Retarding: Advantages
⦁ Improves workability, cohesion and extends setting time,
provides protection against delays and stoppages and facilitates
keeping workable concrete for extended period .
⦁ In the large construction, good workability of the concrete
throughout the placing period and prevention of cold joints is
ensured by adding retarders in the concrete.
⦁ Extended setting time minimise risks of long distance delivery
in hot weather
, improves pumpability of concrete by extended
setting period and improved workability of concrete.
5. Set-Retarding: Advantages
⦁ Reduces bleeding and segregation where poor sand grading
are unavoidable.
⦁ Reduces adverse environmental effects of various nature on
concrete and embedded steel by considerable reduction in
permeability.
6. Effect of use of Retarder
⦁ Retarding admixture forms a film around the cement
grain that prevent or delays the reaction with water
.After
some times,this film breaks and normal hydration takes
place.
7. Accelerating Admixtures
⦁ These admixtures when added to concrete, mortar or grout
increases the rate of hydration of hydraulic cement, shortens
the time of set, accelerates the hardening or development of
strength of concrete/mortar
.
⦁ These admixtures function by interaction with C3S (Tri-
calcium silicate) component of the cement thus increasing the
reaction between cement and water
8. Accelerating Admixtures
⦁ Many substances are known to act as accelerators for
concrete. They include Alkali Hydroxides, Silicates, Fluoro-
Silicates, Organic Compounds, Calcium Formates, Calcium
Nitrates, Calcium Thio Sulphates, Aluminium Chlorides,
Potassium Carbonates, Sodium Chlorides & Calcium
Chlorides.
9. Accelerating Admixtures: Advantages
⦁ Shortens the setting time of cement and therefore increases
the rate of gain of strength.
⦁ Enables earlier release from precast moulds thus speeding
production.
⦁ Reduces segregation and increase density and compressive
strength.
⦁ Cures concrete faster and therefore uniform curing in winter
and summer can be achieved.
⦁ Reduces water requirements, bleeding, shrinkage and time
required for initial set.
11. Mineral Admixtures
⦁ Mineral admixtures are finely divided siliceous materials which
are added to concrete in relatively large amounts, generally in
the range 20 to 70 percent by mass of the total cementitious
material.
13. Ecological Advantage
⦁ Power plants using coal as fuel and metallurgical furnaces
producing cast iron, silicon metal, and ferrosilicon alloys are
the major sources of byproducts that are being produced at
the rate of millions of tonnes every year in many countries.
⦁ Dumping of these by-products into landfills and streams
amounts to a waste of the material and causes serious
environmental pollution.
⦁ Disposal as concrete aggregate or for roadbase construction is
a low-value use which does not utilize the pozzolanic and
cementitious potential of those materials.
14. Ecological Advantage
⦁ With proper quality control, large amounts of many industrial
by-products can be incorporated into concrete, either in the
form of blended portland cement or as mineral admixtures.
⦁ Whenever a pozzolanic and/or cementitious byproduct can be
used as a partial replacement for portland cement in concrete,
it represents significant energy and cost savings.
15. Advantage
⦁ It has been amply demonstrated that the best pozzolans in
optimum proportions mixed with Portland cement improves
many qualities of concrete,such as:
⦁ Lower the heat of hydration and thermal shrinkage;
⦁ Increase the watertightness;
⦁ (Reduce the alkali-aggregate reaction;
⦁ Improve resistance to attack by sulphate soils and sea water;
⦁ Improve workability;
⦁ Lower costs.
16. Pozzolanic Reaction
⦁ Pozzolana + Calcium Hydroxide + Water C – S – H (Gel)
⦁ The mechanism by which pozzolanic reactionexercises a
beneficial effect on the properties of concrete is the same
irrespective of whether a pozzolanic material is added to
concrete in the form of a mineral admixture or as a
component of blended portland cement.
⦁ Pozzolanic (e.g.,low-calcium fly ash),
⦁ Cementitious (e.g.,granulated iron blast-furnace slag),
⦁ Both cementitious and pozzolanic (e.g.,highcalcium fly ash)
17. Classification
⦁ Natural materials: have been processed for the sole purpose of
producing a pozzolan. Processing usually involves crushing,
grinding, and size separation; in some cases it may also involve
thermal activation.
⦁ By-product materials: are not the primary products of the
industry producing them. Industrial byproducts may or may
not require any processing.
18. Natural Pozzolans
⦁ Volcanic glasses: Santorini Earth of Greece, Bacoli Pozzolan of
Italy
, and Shirasu Pozzolan of Japan are examples of pozzolanic
materials which derive their lime-reactivity characteristic
mainly from the unaltered aluminosilicate glass.
⦁ Volcanic tuffs: Pozzolans of Segni-Latium (Italy), and trass of
Rheinland and Bavaria (Germany), represent typical volcanic
tuffs.
⦁ Calcined clays or shales: clay and shales will not show
appreciable reactivity with lime unless the crystal structures of
the clay minerals present are destroyed by heat treatment
19. Natural Pozzolans
⦁ Diatomaceous earth: This group of pozzolans is characterized
by materials of organic origin.
⦁ Diatomite is a hydrated amorphous silica which is composed
of skeletal shells from the cell walls of many varieties of
microscopic aquatic algae. The largest known deposit is in
California.
