3030-7-scms
TRANSCRIPT
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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