Supplementary Cementing Materials
Kimberly Kurtis
Cement
Hydration
School of Civil and Environmental Engineering
Georgia Institute of Technology
Atlanta, Georgia
Dr. Kimberly Kurtis
School of Civil Engineering
Georgia Institute of Technology
Atlanta, Georgia
1
PCA is a good resource on SCMs
PCA CD038, 2002
1
What are Supplementary Cementing
Materials?
AKA “mineral admixtures” or “supplementary
cementitious materials” or “SCMs” (for cool cats)
SCMs are a class of mineral-based materials
possess pozzolanic reactivity and/or latent
hydraulic reactivity
Pozzolan – a siliceous or alumino-siliceous material that, in finely
divided form and in the presence of moisture, chemically reacts at
ordinary temperatures with calcium hydroxide released by the
hydration of Portland cement to form compounds possessing
cementing properties
A hydraulic cement reacts chemically with water to form
compounds (hydrates) that have cementing properties – e.g.
Portland cement
What are Supplementary Cementing
Materials?
Reactive materials which are used as partial replacement
(generally by weight) for portland cement
water-to-cement ratio w/c = mwater/mcement
water-to-cementitious materials ratio w/cm = mwater/(mcement + mSCMs)
A component in “Blended cements”, where portland
cement and SCMs are combined together as 1 product;
These are currently more common in Europe and South
America than in the US.
Materials which can improve strength, workability, and
durability
Generally are less expensive than cement
2
SCMs are GREEN
Reduce energy consumption and CO2 emissions when used as partial
replacement for cement
Productive use of industrial waste which may be landfilled
Can improve concrete durability
Contractors can derive benefits from having projects LEED
(Leadership in Energy and Environmental Design) certified through the
use of SCMs and other sustainable construction practices
There are also some financial benefits to green construction
New Home for Georgia Tech’s
Business School is Green
ATLANTA (September 19, 2003) -- Georgia Tech will
save money and faculty and students will breathe easier in
Georgia Tech’s new Management building - only the
second building in Georgia to be certified as a Leadership
in Energy and Environmental Design (LEED) Silver
green building, a U.S. Green Building Council rating
system. This cornerstone project of Technology Square
Georgia Tech’s new multipurpose complex in the heart of
Midtown Atlanta, achieved the prestigious LEED Silver
certification, the second level of the four-level rating
$CMs are GREEN
Financial incentives are now available to contractors and owners to
reduce CO2 emissions during concrete construction.
One such carbon dioxide offset program, Cool Climate Concrete (C3),
offers $0.50 per metric ton CO2 offset.
When a 4000-psi concrete which would ordinarily be produced with 600
lbs. cement/yd. is produced instead with 35% cement weight, this
amounts to a savings of 200 lbs cement/yd, assuming cement is 95%
clinker.
Assuming that 1 metric ton (2200 lbs.) of CO2 results from the production
of 1 ton portland cement clinker, this substitution results in reduction of
200 lbs. (0.091 metric tons) of CO2 or a savings of 5 cents for each yard
placed.
For more details on carbon offsets in concrete, see
http://www.coolclimateconcrete.com/index.html
3
Types of SCMs
Natural (ASTM C 618 Class N)
• Produced from natural mineral deposits
(e.g., volcanic ash or pumicite,
diatomaceous earth, opaline cherts and
shales)
• May require heat treatment (e.g.,
metakaolin or calcined clay, calcined
shale, rice hull ash, calcined shale)
http://www.gemsmuri.gatech.edu/diatom02.JPG
Processed / Manufactured
• Silica fume (ASTM C 1240)
• Fly Ash (ASTM C 215)
• Slag (ASTM C 989)
http://www.uwm.edu/Dept/CBU/images/about/cem_7.jpg
Origins - Pozzolans
Greeks used volcanic ash with
calcined lime (CaO) as a cement.
Early Romans near town of
Pozzuoli used volcanic soils to
make these hydraulic mortars.
Now the term “pozzolan” is used to
describe a siliceous material that
will react with lime in the presence
of water to produce C-S-H.
4
Pozzolans
Most pozzolons are composed primarily of amorphous silica or
silicates with high surface area
Many are industrial by-products, including fly ash or silica fume
S i
O
OH
Na o r K
SCM Chemical Composition
5
Hydraulic vs. Pozzolanic Reaction (1)
• Pozzolanic Reactions:
Chemical reaction with calcium hydroxide (lime) and
water that leads to the formation of cementitious
products, like C-S-H.
• Latent Hydraulic Reactions:
Chemical reaction with water that leads to setting and
hardening of the material.
Hydraulic vs. Pozzolanic Reaction (2)
Different SCMs React Differently
6
Hydration of Calcium Silicates in
Cement
2C3S + 6H C3S2H3 + 3CH
2C2S + 4H C3S2H3 + CH
C-S-H; molar ratios can vary;
strength-giving phase
No cementitious properties
(does not contribute to
strength); easily leached; prone
to chemical attack
Reminder on cement chemistry notation:
C = CaO; S = SiO2; H = H2O
Pozzolanic Reaction
C3S + H
C2S + H
FAST
FAST
C-S-H + CH
C-S-H + CH
Pozzolan + CH + H
SLOW
C-S-H
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How can SCMs improve concrete properties?
Many of the beneficial effects of using SCM are related to the effect
they have on the pore structure by:
Micro-filler effect – increased
packing of cementitious particles
Increased C-S-H – replacing porous
CH with C-S-H
Wall effect – densifying the ITZ
(interfacial transition zone) at the
cement-aggregate interface
Pore blocking – which occurs
because of a combination of these
factors
Figure credit: M&M text
These effects refine the pore structure and reduce the permeability of
concrete thereby making it more resistant to the penetration of deleterious
agents.
Fly Ash
• The most widely used SCM
• Has been used for > 50 years in concrete
• ASTM C 215 definition:
The finely divided residue that results
from the combustion of coal and that is
transported by flue gases from the
combustion zone to the particle removal
system.
• Also called “pulverized fuel ash” or “PFA”
• Inorganic, noncombustible by-product of
coal burning power plants
• Approximately ½ the cost (or less) of
cement
• Can be used at up to ~50% replacement
for cement
• Slower reacting than most SCMs
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Fly Ash
2 Classes of Fly Ash: C and F
Range in Chemical Composition of Fly Ash
Class F
MgO
CaO
Fe2O3
Fe2O3
Class C
MgO
CaO
Al2O3
SiO2
Al2O3
ASTM C 618 Classification
SiO2
Class F
Normally from bituminous and
anthracite coal
SiO2 + Al2O3 + Fe2O3 ≥
70%
Class C
Normally from sub-bituminous and
lignite coal
SiO2 + Al2O3 + Fe2O3 ≥
50%
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2 Classes of Fly Ash: C and F
Class F Fly Ash
Derived from anthracite or
bituminous coals from
eastern US
Pozzolanic reaction slower rate of reaction than
Class C fly ash
Typical composition:
<10% CaO, >50% SiO2
Class C Fly Ash:
Derived from lignite or subbituminous coals from western
US (particularly Wyoming and
Montana)
Pozzolanic and hydraulic
reactions typically faster rate
of reaction than Class F fly ash
Chemical composition:
>20% CaO, 30-50% SiO2
Fly ash of choice in the Atlanta
area
• Be careful of carbon content in air entrained concrete
Physical Characteristics of Fly Ash
Mainly solid spheres with some cenospeheres
(hollow) or plerospheres (hollow spheres
containing smaller spheres)
Particle size ~ 5-20 µm
Surface area ~ 300-500 m2/kg
Density ~ 540-860 kg/m3
Specific gravity ~ 2.2-2.4
Color ranges from off-white to light gray
http://www.stiash.com/assets/ashes3.jpg
http://www.uwm.edu/Dept/CBU/images/about/cem_7.jpg
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Silica Fume
Highly reactive pozzolan due to high SiO2 content and extremely small
particle size (i.e., large surface area)
ASTM C 1240 definition:
A very fine pozzolanic material, composed mostly of amorphous silica
produced by electric arc furnaces as a byproduct of the production of
elemental silicon or ferrosilicon alloys.
Also called “microsilica” or “condensed silica fume”
Typical cement replacement values of 5-15%
Approximately 200,000 tons/yr produced in US
About 4x the cost of cement
Silica Fume
By-product of silicon and ferrosilicon alloy production
http://www.silicafume.org/general.html
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Silica Fume Product Forms
As-produced (undensified;
easily inhaled)
Densified (agglomerated)
Slurry
Silica Fume Properties
Chemical
85-98% SiO2
SiO2 content dependent upon alloy
http://www.norchem.com/tem.gif
Physical
Very small, spherical particles
Particle size ~0.1-0.3 µm
Surface area ~15,000-25,000 m2/kg
Density ~ 130-430 kg/m3
Specific gravity ~ 2.2
Generally dark gray in color
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Slag
Also known as ground granulated blast
furnace slag (GGBFS) or slag cement
Possesses good latent hydraulic cementing
properties
Slag is the residue from metallurgical
processes, either from production of metals
from ore or refinement of impure metals.
The form of slag used in concrete comes from
the production of iron from ore.
Has been used in concrete for > 100 years
About 70-80% the cost of cement
Typical cement replacement values 20-70%.
Slag
http://www.steel.org/learning/howmade/blast_furnace.htm
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Slag Properties
Chemical
35-45% CaO
32-38% SiO2
8-16% Al2O3
5-15% MgO
Slag at high cement replacement
Physical
values may cause concrete to
Particle size < 45µm
turn greenish! However, this is
Surface area ~ 400-600 m2/kg
not why we call SCM-cement
Density ~ 1050-1375 kg/m3
mixes “green” concrete!
Specific gravity ~ 2.9
Angular particle shape
Generally, white to off-white color
Other SCMs
Metakaolin
Calcined (700-900° C) clay
Georgia is major source of kaolin (clay)
Typical cement replacement amounts of 5-15% (similar to silica fume)
High reactivity (particle size ~ 1-2µm)
55% SiO2, 35-45% Al2O3
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Other SCMs: Metakaolin
250
4,5
Cement 2
Cement 2 & 8% MK 235
Cement 2 & 8% MK 349
200
3
150
2,5
2
100
1,5
4,5
1
Cement 4
50
• Metakaolin seems to
accelerate and intensify
cement hydration,
particularly around the C3A
peak.
250
Cement 4 & 8% MK 235
Cement 4 & 8% MK 349
200
0
0
6
12
18
24
30
36
42
Age (h)
• Early exothermicity
particularly dependent on
alkali content.
Rate of heat evolution (mW/g)
3,5
0
48
54
60
66
72
3
150
2,5
2
100
1,5
1
Cumulative Rate evolution (J/g)
4
0,5
50
0,5
0
0
0
6
12
18
24
30
36
Age (h)
42
48
54
60
66
72
Effect of SCMs on Heat of Hydration
(a)0.8
Control
8% MK235
15% MK235
8% MK349
15% MK349
8% SF
15% SF
0.7
Heat evolution (J/g)
Rate of heat evolution (mW/g)
3,5
Cumulative Rate evolution (J/g)
4
0.6
0.5
0.4
0.3
0.2
0.1
0
0
6
12
18
24
Age (h)
15
Other SCMs
Rice Hull Ash
90 million tons of rice husks produced worldwide each year
Particle size ~ 10-20 µm
High reactivity (85% SiO2)
Waste Glass
Crushed, recycled glass; must be finely ground
Variable composition
Waste Fiberglass
Processed waste glass fibers
White color
Particle size ~ 3-8 µm
High reactivity calcium aluminosilicate (50-55% SiO2; 20-25%
CaO;15-20% Al2O3)
Influence of SCMs
Concrete Fresh State
Water demand
Workability
Bleeding
Heat of hydration
Setting time
Concrete Hardened State
Mechanical properties
Durability
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Effect of SCMs on Water Demand
Fly Ash
↓ water demand due to “ball bearing” effect of spherical
particles
For every 10% FA, ~2-3% reduction in water demand
Silica Fume
↕ water demand (↓ ~2% replacement, ↑ >5% replacement)
Slag
↓ water demand
Effect on water demand:
FA < Slag < SF
Effect of SCMs on Workability
Silica fume containing concretes tend to be “sticky” and
more difficult to finish, leading to decreased workability or
the need for high-range water reducer.
Slag and fly ash improve workability.
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Effect of SCMs on Bleeding
Fly ash: ↓ bleeding
Slag: ↕ bleeding; depends
upon fineness of slag
particles (fine particles
decrease bleeding, coarse
particles have less of an
effect)
Silica fume: ↓ bleeding and
may eliminate it altogether,
thus making finishing difficult
Effect of SCMs on Heat of Hydration
Most SCMs reduce overall heat of hydration and rate of
heat liberation
Good for mass concrete or large sections and for
concreting in hot weather
Eliminated demand for ASTM Type IV cement
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Effect of SCMs on Setting Time
Slag and Class C Fly Ash:
Lengthen setting time (by 15-60 minutes for initial
set , 30-120 minutes for final set)
Class F Fly Ash:
Lengthen setting time (more than Class C);
dependent upon chemical composition
Silica Fume:
↓ setting due to high reactivity
Summary of Effects on Fresh Concrete
Fly Ash
Class F
Class C
Slag
Silica
Fume
Natural Pozzolans
Calcined
Shale
Calcined
Clay
Metakaolin
Water Demand
Workability
Bleeding
Setting Time
Air Content
(dosage rate)
Heat of Hydration
Image source: PCA
19
Effect of SCMs on Rate of Strength Gain
Effect of SCMs on Rate of Flexural Strength Gain
(a)
• w/cm=0.40
• Metakaolin or
silica fume used at
8% wt. cement
11
10
9
Control
MK235
MK349
SF
Strength (MPa)
8
7
6
5
550-600
psi
4
3
300-400
psi
2
1
0
1
3
28
90
Age (days)
20
Effect of SCMs on Total Strength Gain
Using smaller particle sizes than cement, pozzolan improve “particle
packing,” leading to decreased transition zone porosity and increased
overall strength gain.
Effect of Silica Fume
Effect of SCMs on Durability
Pozzolanic materials improve concrete durability
due to:
Refined pore structure (due particle packing
and later formation of C-S-H)
Improved transition zone properties
Reduction in soluble hydration products, like
CH
Effect on Permeability
Reduced Permeability
Effect on Alkali-Silica Reaction
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Effect of SCMs on Permeability
Effect on Chloride Permeability
SCM
Fly Ash, slag & most pozzolans
Silica fume, metakaolin
(highly reactive pozzolans)
Little effect at early age (28 days)
Reduction becomes more significant with age
Substantial reduction at later age
Significant reduction at early age (28 days)
Smaller decreases with age
Image source: PCA
Summary of Effects on Hardened Concrete
Fly Ash
Class F
Class
C
Slag
Silica
Fume
Natural Pozzolans
Calcined
Shale
Calcined
Clay
Metakaolin
Early Age Strength
Gain
Long Term
Strength Gain
Permeability
Chloride Ingress
ASR
Sulfate Resistance
Freezing &
Thawing
Image source: PCA
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Ternary and Quaternary Blends
Use of cement + 2 or 3 SCMs
Overcomes some issues with workability and strength
development
Example:
Instead of using 15% silica fume by weight replacement for
cement, use 5% silica fume and 20% Class F fly ash to
get the same workability and early strength (or better) as
the original concrete but with better durability.
Ternary and Quaternary Blends
Example: combination of silica fume and slag to control
alkali-silica reaction
Expansion at 14 days (%)
0.4
0.3
3% Silica
Fume
0.2
0% Silica Fume
7% SF
0.1
CSA Limit
Expansion limit
10% SF
0.0
0
10
20
30
40
50
Slag (%)
Thomas & Siebert, 1996
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