13 - Concrete Admixtures - Memphis

Admixtures

CIVL 3137 1

CIVL 3137 2

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.

CIVL 3137 3

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

CIVL 3137 5

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

CIVL 3137 7

Air Entrainment

In Roman times, ox blood was used to improvedurability (though I’m not sure they knew why).

Rediscovery in modern times due to accidentalobservation that freeze-thaw resistant sectionsof some roads had been made with cement thathad been contaminated with beef tallow addedas a grinding aid

CIVL 3137 8

Air Entrainment

hydrophilic group(s)

hydrophobic component

CIVL 3137 9

Air Entrainment

Repulsion between negative surface charges prevents coalescence

CIVL 3137 10

Air Entrainment

CIVL 3137 11

Air Entrainment

CIVL 3137 12

Chemical Admixtures

set acceleratorsset retarders

water reducersstabilizers

Chemical Admixtures

• Type A – water reducing• Type B – retarding• Type C – accelerating• Type D – water reducing and retarding• Type E – water reducing and accelerating• Type F – high-range water reducing (HRWR)• Type G – HRWR and retarding• Type S – specific performance admixturesCIVL 3137 14

CIVL 3137 15

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

CIVL 3137 16

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

CIVL 3137 17

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

CIVL 3137 18

Water Reducers

low-range (5-10% reduction)

mid-range (10-15% reduction)

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

CIVL 3137 19

How Water Reducers Work

CIVL 3137 20

Using Superplasticizers

CIVL 3137 21

Drawback: Slump Loss

withsuper

without super

Slum

p -i

n

Supplementary Cementitious Materials

(Mineral Admixtures)

CIVL 3137 22

CIVL 3137 23

Mineral Admixtures

Natural Pozzalans

volcanic ash

diatomaceous earth

rice husk ash

calcined clays

Artificial Pozzalans

fly ash

silica fume

blast furnace slag

(metakaolin)

CIVL 3137 25

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.

CIVL 3137 26

Hydration Chemistry

CS H CSH CH heat

calcium silicate

water

calcium silicate hydrate gel

calcium hydroxide crystals

CIVL 3137 27

True Pozzalans

silicates

high specific surface

usually glassy

usually spherical

(high in SiO2)

(fine powder)

(no crystals)

Natural Pozzolans

CIVL 3137 28

volcanic ash

rice husk ash

diatomaceous earth

calcined clay (metakaolin)

Fly Ash

CIVL 3137 29

Silica Fume

CIVL 3137 30

CIVL 3137 31

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

CIVL 3137 32

Effects on Strength

CIVL 3137 35

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

CIVL 3137 36

Blast Furnace Slag

Formed when iron ore, coke and a flux (either limestone or dolomite) are melted together in

a blast furnace to produce molten iron

Air cooled slagPelletized slag

Granulated slag

CIVL 3137 37

Blast Furnaces

Air Cooled Slag

CIVL 3137 38

Blast furnace slag is allowed to slowly coolin air then processed through a screeningand crushing plant into many sizes for useprimarily as a construction aggregate.

Common uses are in ready-mix concrete,precast concrete, hot mix asphalt, septicdrain fields and pipe trench backfill.

http://www.nationalslag.org/blast-furnace-slag

Air-Cooled Slag

CIVL 3137 39

Granulated Slag

CIVL 3137 40

Granulated slag is rapidly cooled by largequantities of water to produce a sand-likegranule that is primarily ground into acement known as Ground GranulatedBlast Furnace Slag (GGBS), or Type Sslag cement. It is also mixed with Portlandcement clinker to make a blended Type 1Scement.

http://www.nationalslag.org/blast-furnace-slag

CIVL 3137 41

Granulated Blast Furnace Slag

GGBFS

CIVL 3137 42

Pelletized Slag

CIVL 3137 43

Pelletized slag is quickly cooled usingwater or steam to produce a lightweightaggregate. Due to its reduced weight, it isused to make lightweight concrete and aslightweight fill over marginal soils.

http://www.nationalslag.org/blast-furnace-slag

CIVL 3137 44

Pelletized Slag

CIVL 3137 45

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

CIVL 3137 46

RAW MATERIAL HANDLINGAND TRANSPORT

CHARGE FEED

ELECTRODES

RECOVEREDENERGY

SILICAFUME

1

TAPPING ANDREFINING

3

SOLIDIFICATION4

CRUSHING ANDSIZING OF PRODUCT

5

OFF‐GAS2

ELECTRIC ARCFURNACE

SILICON

C Si02

CIVL 3137 47

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

CIVL 3137 48