Admixures

                                 ADMIXTURES

 A material other than water, aggregates, and hydraulic cements used as an ingredient of concrete or mortar and added to the batch immediately before or during mixing.

Reason:

Improve or modify some or several properties of portland cement concrete.

Compensate for some deficiencies.

A. Chemical Admixtures

 Type A: Water-reducing (WR)

 Type B: Set retarding (SR)

 Type C: Set accelerating (SA)

 Type D: WR + SR

 Type E: WR + SA

 Type F: High-range water-reducing (HRWR)

 Type G: HRWR + SR

B. Mineral Admixtures

 Class N: Raw or calcined pozzolans

 Class F: Fly ash produced from burning bituminous coal

 Class C: Fly ash normally produced from burning lignite(subbituminous) coal. (both pozzolanic and cementatious)

1) Admixtures for Durability

Frost action: Air-entraining agents

Sulfate and acidic solutions: Pozzolans, polymer emulsions

Alkali-aggregate expansion: Pozzolans

Thermal Strains: Pozzolans

2) Admixtures for Increasing Strength

Water reducing agents

Pozzolans

Consistency: Flowability, slump

Workability: High cohesiveness and high consistency

(Advantage of fine particle size cohesiveness)

 Chemical Admixtures

Surfactants (Surface-Active Chemicals/ Agents)

Air-entraining surfactants

:        At the air-water interface the polar groups are oriented towards the water phase lowering the surface tension, promoting bubble formation and counteracting the tendency for the dispersed bubbles to coalesce.

       At the solid-water interface where directive forces exist at the cement surface, the polar groups become bound to the solid with the non-polar groups oriented towards the water, making the cement surface hydrophilic so that air can displace water and remain attached to the solid particles as bubbles.

Water-Reducing surfactants:

       When water is added to cement, a well-dispersed system is not achieved, because:

The water has high surface tension.

 Cement particles tend to cluster together or form flocs.

   When a surfactant with a hydrophilic chain is added to the cementwater system, the polar chain is adsorbed alongside the cement particle, and thus lowering the surface tension of the water, and making the cement surface hydrophilic.

Mineral Admixtures

Definition: Mineral Admixtures are insoluble siliceous materials, used at

relatively large amounts (15-20% by weight of cement).

 Fine particle size, siliceous material that can slowly react with CH at

normal temperatures, to form cementitious products.

           CH+ S --Normal Temp.(Aq)------ C_S_H

Low heat of hydration

 Transform large pores to fine pores

 Historically, mineral admixtures are volcanic ashes.

 Significance: Durability to thermal cracking, chemical attack, sulfate

attack, workability.

By-Product Mineral Admixtures

 Fly Ash (FA) 1-40micro meter Particle Size; Surface Area=0.5 m2/g

 Blast Furnace Slag (BFS) --- 1-40 micro meter; SA=0.5 m2/g

 Condensed Silica Fume (SF) -- 0.1 micro meter ; SA=20 m2/g

 Rice Husk Ash (RHA) -- 10-20 micro meter; SA=60 m2/g

Internal bleeding is reduced ---Reduced Micro cracking

Effect of Pozzolans:

 It will reduce the available space for formation of large crystals

 Pozzolans will convert CH into C-S-H

 Consistency of concrete is generally measured by the slump test (ASTM C143). This test is performed by measuring the slump (subsidence), in inches, of concrete after removal of the truncated cone mould in which the freshly mixed concrete was placed.