Admixtures

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Admixtures

admixture (n.) any material other thanwater, aggregates, hydraulic cement andfiber reinforcement, used as an ingredientof concrete or mortar, and added to thebatch immediately before or during mixing.

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Types of Admixtures

1. Air-entraining admixturesadded to improve freeze-thaw durability

2. Chemical admixtureswater-soluble compounds used to improve theproperties of the fresh concrete

3. Mineral admixturesfinely divided solids usually added to improvethe strength of the hardened concrete

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Air Entrainment Goals

tiny air bubbles, uniform in size

uniform dispersion in cement paste

air content = 9% of mortar volume

one billion bubbles per cubic yard

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Air Entrainment

hydrophilic group(s)

hydrophobic component

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Air Entrainment

Repulsion between negative surface charges prevents coalescence

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Air Entrainment

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Air Entrainment

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Chemical Admixtures

set acceleratorsset retarders

stabilizerswater reducers

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Set Accelerators

Typically used in cold weather to reduce settingand curing times; also used to speed removal offormwork

Salts (calcium chloride, sodium chloride) arecheap set accelerators but can corrode rebarand reduce resistance to sulfate attack

Nitrates and nitrites are less effective and moreexpensive but are also non-corrosive

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Set Retarders

Typically used in hot weather to slow setting andcuring so concrete can be finished; also used inmass pours to eliminate cold joints

Sugars, starches, and cellulose derivatives areabsorbed onto the surface of cement particles todelay hydration of the calcium silicates

As little as 0.05% by mass will delay setting forfour hours; 1% prevents setting completely

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Stabilizers

Lets leftover concrete be returned to plant andreused the next day

Forms protective barrier around cement grains toprevent hydration of both calcium silicates andcalcium aluminates for up to 72 hours

Activator dissolves the protective barrier and letssetting proceed in a normal manner with no harmto the concrete

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Water Reducers

low-range (5-10%)

mid-range (10-15%)

high-range (15-30%)“superplasticizers”

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Water Reducers

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Superplasticizers

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Slump Loss

withsuper

without super

Slu

mp

-in

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Mineral Admixtures

Natural Pozzalans

volcanic ash

diatomaceous earth

burnt clays

Artificial Pozzalans

fly ash

silica fume

blast furnace slag

burnt rice hulls

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Pozzalans

pozzalan (n.) siliceous materials which bythemselves have little or no cementitiousvalue but will, in finely divided form and inthe presence of moisture, chemically reactwith calcium hydroxide (CH) at ordinarytemperatures to form compounds (CSH)that have cementing properties.

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Hydration Chemistry

CS H CSH CH heat

calcium silicate

water

calcium silicate hydrate gel

calcium hydroxide crystals

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True Pozzalans

silicateshigh specific surface

usually sphericalusually glassy

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Uses of Pozzalans

Save money by replacing expensive cement

Lower heat of hydration due to slow strength gain

Increase sulfate resistance if low in alumina

Improve workability due to spherical shape

Increase strength by converting CH to CSH

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Effects on Strength

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Fly Ash

Tiny spheres of glassy silica and alumina that are electrostatically precipitated from exhaust

gases given off by coal-fired power plants

Anthracite = Class F fly ashLignite = Class C fly ash

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Blast Furnaces

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Blast Furnace Slag

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Silica Fume

Tiny spheres of glassy silica electrostatically precipitated from exhaust gases given off by

electric arc furnaces used to make silicon

Carbon-free = white silica fumeCarbonaceous = black silica fume

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Silica Fume

0

4000

8000

12000

16000

0 10 20 30 40 50 60Age (days)

Stre

ngth

(ps

i)

705 lb/yd3 cement102 lb/yd3 silica fume

705 lb/yd3 cement