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CEE 3030: Civil Engineering Materials

Supplementary Cementitious Materials

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Types of SCMs

Natural (ASTM C 618 Class N) Produced from natural mineral deposits (e.g., volcanic ash, diatomaceous

earth)

May require heat treatment (e.g., metakaolin)

Processed / Manufactured

Silica fume (ASTM C 1240) Slag (ASTM C 989)

Fly Ash (ASTM C 215)

Rice Husk Ash

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Benefits of SCMs

Industrial by-products (waste utilization) Typically cheaper than cement (except for silica

fume and metakaolin)

Environmentally conscious No CO2 emission during processing

Less landfill waste

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Influence of SCMs

Concrete Fresh State

Heat of Hydration

Water demand

Workability

Bleeding

Setting time Concrete Hardened State

Mechanical properties

Durability

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Hydraulic vs. Pozzolanic Reaction (1)

Latent Hydraulic Reactions:

Pozzolanic Reactions:

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Hydraulic vs. Pozzolanic Reaction (2)

Different SCMs React Differently

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Basic Cement Hydration

2C3S + 6H -> C-S-H + 3CH

2C2S + 4H -> C-S-H + CH

Cement Chemistry Notation:

C = CaO; S = SiO2; H = H2O

C-S-H; molar ratios can vary;

strength-giving phase

No cementitious properties (does

not contribute to strength); easily

leached; prone to chemical attack

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SCM Reactions

C3S + H C-S-H + CH

C2S + H C-S-H + CH

FAST

FAST

SCMs + CH + H C-S-HSLOW

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Fly Ash

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Fly Ash

25% limit on cement replacement in Tennessee (15% in GA)

Realistic cement replacement amounts of ~50%

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Class F Fly Ash

Derived from anthracite or bituminous coals from

easternUS.

Pozzolanic reaction ->

Typical composition:

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Class C Fly Ash

Derived from lignite or sub-bituminous coals from

westernUS (particularly Wyoming and Montana).

Pozzolanic and hydraulic reactions ->

Chemical composition:

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Shapes of Fly Ash

Fly ash particles typically exhibit spherical

and irregular shapes.

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Silica Fume

Sili F

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Silica Fume

Highly reactive pozzolan due to high SiO2

contentand extremely small particle size (i.e., large surfacearea).

Approximately 200,000 tons/yr produced in US

Sili F P d t F

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Silica Fume Product Forms

As-produced

(undensified; easily

inhaled)

Densified(agglomerated)

Slurry

Sili F P ti

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Silica Fume Properties

Physical

Particle size ~0.1-0.3 m

Surface area ~15,000-25,000 m2/kg

Generally, black in color

Chemical

85 - 98% SiO2

SiO2 content dependent upon al

Sh f Sili F

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Shape of Silica Fume

Silica fume is almostalways spherical in

shape

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Slag

Slag

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Slag

Also known as ground granulated blast furnace slag.

Slag is the residue from metallurgical processes, either fromproduction of metals from ore or refinement of impure metals.

As of 2005, cost is slightly

lower than portland cement

(was significantly less)

Slag Production

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Slag Production

Slag Pelletization

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Slag Pelletization

May be used as lightweight

aggregate (>4 mm)To be used in concrete, pellets

must be ground

Slag Properties

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Slag Properties

Chemical

35 - 45% CaO

32 - 38% SiO2 8 - 16% Al2O3 5 - 15% MgO

Physical

Particle size < 45m

Surface area ~ 400-600 m2/kg

Angular particle shape

Generally, white to off-white color

Slag at high cement replacement

values may cause concrete to turn

greenish! However, this is not why

we call SCM-cement mixes green

concrete!

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Metakaolin

Metakaolin

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Metakaolin

Calcined (700-900 C) clay

Georgia is major source of kaolin (clay)

Typical cement replacement amounts of

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Metakaolin

Al2Si2O5(OH)4

+ 700-900 C

Al2Si2O7

dehydroxylation,

puckering of layers

Metakaolin

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Metakaolin

Average particle size:

1-2 m

Chemical composition:45-55% SiO240-45% Al2O3

Average surface area:

10,000-25,000 m2/g

Other SCMs

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Other SCMs

Rice Husk Ash

90 million tons of rice husks produced worldwide

each year Particle size ~ 10-20 m

High reactivity (85% SiO2)

Diatomaceous Earth

Volcanic Ash

Proprietary Blends

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Effect of SCMs on Cement and Concrete

Properties

Benefits of SCMs

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Many of the beneficial effects of using SCM are related to the

effect they have on the pore structure by:

These effects refine the pore structure and reduce the permeabiltyof concrete thereby making it more resistant to the penetration of

deleterious agents.

Micro-filler effect

Increased C-S-H

Wall effect

Pore blocking

Benefits of SCMs

Heat of Hydration (1)

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Heat of Hydration (1)

Most SCMs reduce overall heat of hydration and rate

of heat liberation Eliminated need for ASTM Type IV cement

Setting

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Setting Slag and Class C Fly Ash:

setting (15-60 minutes for initial, 30-120 minutes for final)

Class F Fly Ash: setting (more than Class C); dependent upon chemical

composition

Silica Fume:

setting due to

high reactivity

0

50

100

150

200

250

300

350

Control MK235 (8%) MK349 (8%) SF (8%)

Sample

Time(minutes)

Initial S

Final Se

Water Demand

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ate e a d

Fly Ash:

water demand due to ball bearing effect ofspherical particles

For every 10% FA, ~2-3% reduction in water

demand

Silica Fume: at typical replacement amounts,

water demand

Slag: water demand

Effect on Decreasing Water Demand:

FA > Slag > SF

Workability

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y

Silica fume containing concretes tend to besticky and more difficult to finish, leading to

decreased workability or the need for high-range

water reducer.

Slag and fly ash improve workability.

Bleeding

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g

Fly ash: bleeding

Slag: bleeding; depends upon fineness of slag

particles (fine particles decrease bleeding and vice

versa for coarse particles)

Silica fume: bleeding and may eliminate it

altogether, thus making finishing difficult

Rate of Strength Gain (1)

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g ( )

Rate of Strength Gain (2)

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g ( )

0

10

20

30

40

50

6070

80

1 3 7 28 90

Age (days)

Streng

th(MPa)

Control

MK235MK349

SF

SF Redo

Total Strength Gain

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g

Effect of Silica Fume

Using smaller particle sizes than cement, SCMs improve

particle packing, leading to decreased transition zoneporosity and increased overall strength gain.

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Durability: Permeability (1)

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Durability: Permeability (2)

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0

1000

2000

3000

4000

5000

6000

7000

8000

0.40 0.50 0.60

w/cm

Chargepass

ed(Coulombs)

Control

MK235

MK349

SF

MOD