1. CON 123
Cementitious Materials
Session 9
Silica Fume

2. What is Silica Fume?
“Very fine non-crystalline
silica produced in electric
arc furnaces as a
byproduct of the
production of elemental
silicon …”
-- ACI 116R

3. What is Silica Fume?
 Cement grains and silica fume particles at the
same magnification.
 In a 15 percent silica fume replacement of
cement, there are approximately 2,000,000
particles of silica fume for each grain of Portland
cement.
 The longer white bar in the silica fume side is 1
µm long.
 ACI 234R-96, Guide to the Use of Silica Fume in
Concrete.

4. Specification for Silica Fume
ASTM C 1240
Silica Fume—finely
divided residue resulting
from the production of
silicon, ferro-silicon, or
other silicon-containing
alloys that is carried from
the burning surface area
of an electric-arc furnace
by exhaust gases.

5. Silica Production & Silica Fume
 Silica fume is a byproduct
of producing silicon metal
or ferrosilicon alloys in an
electric-arc furnace.
 The raw materials going
into the furnace, the
electric-arc furnace (2000
C), and the smoke
collection system are
shown (next slide).

6. Silica Production & Silica Fume

7. Silica Fume
 The densified material
is on the left and the as-
produced material is on
the right and slurried on
top.
 Silica Fume
 Powdered
 Densified

8. Effect of Silica Fume in Concrete

9. Physical Properties
Particle size (typical) < 1 micron
Bulk Density
(as-produced) 130 – 430 kg/m3
(densified) 480 – 720 kg/m3
Specific Gravity 2.2
Surface Area (BET) m2/kg 13,000 – 30,000

10. Silica Fume in Concrete
 Silica Fume Provides
 Technical Advantages
 Life-cycle Cost Advantages
 Resource Conservation
 Reduced Solid Waste Disposal
 Reduced Greenhouse Gas Production from PC

11. Technical Benefits of Silica Fume
 Benefits
 Pore Refinement
 High Early Age
Strength
 Increased
Stickiness
 Black-Gray Color
 ASR Resistance
 Sulfate Resistance
 Consumes CH

12. Effects on Concrete Properties
 High Strength Development
 Water Demand Increased
 No Bleeding of Concrete
 Reduced Permeability
 High Range Water Reducers required
 Increased Chloride Permeability
 Increased Plastic Shrinkage

13. Silica-Fume Concrete: Typical Strengths
Compressive Strength, MPa
15%
70
10%
60
5%
50
0%
40
30 Control mixture
cement: 390 kg/m3
20
w/c: 0.41
10 air: 5%
0 0 3 7 28 60
Age, days SI

14. Silica-Fume Concrete: Typical Strengths
(SI Version)
 This slide shows the effects of adding increasing
amounts of silica fume to a low water-
cementitious ratio, air-entrained concrete. Note
that:
1. Increased strength will be seen at all ages.
2. There is a diminishing return that governs the
addition of silica fume. Strength will not
continue to increase at the same rate with
increased additions.

15. Silica-Fume Concrete: Typical Strengths
(SI Version)
 This concrete mixture is typical of what has
been used in many parking structures and
bridge decks, where the amount of silica fume
added would be approximately 7.5 percent.
 This mixture would not be optimal for high-
strength applications. See the following slides
for high-strength concrete performance.

16. Advantages of Silica Fume & Pozzolans
 Socket where a sand grain has been pulled away from
cement paste in 1-day old mortar.
 The sand grain was originally at the top of the picture.
 Note the massive CH layer engulfing the sand grain and
by some channel type gaps
 Below: Socket where a sand grain has been pulled
away from cement paste in 28-day old mortar
containing silica fume.
 The sand grain was originally at the top of the picture.

17. Advantages of Silica Fume & Pozzolans
 Enhance
Interfacial Zones
 Reaction
W/Calcium
Hydroxide

18. Corrosion-Resisting Concrete
 Within the general area of improving durability, by far
the greatest use of silica fume has been in concrete
susceptible to corrosion caused by chloride ion attack
of the reinforcing steel.
 This category includes concrete exposed to either
deicing salt or to marine salts.
 The protection mechanism in this case is primarily the
reduction of the permeability of the concrete, which
significantly increases the time that it takes for the
chlorides to reach the level of the reinforcing steel.

19. Corrosion-Resisting Concrete
 Parking structures
 Bridge decks
 Marine structures

20. Silica-Fume Concrete: Typical Values
Silica Fume RCP Compressive Strength
(by mass of cement)
0% > 3,000coulombs = 35 MPa
7-10% < 1,000 coulombs > 50 MPa
> 10% < 500 coulombs > 65 MPa
Don’t fall into strength trap!
SI

21. Silica-Fume Concrete: Typical Values
(SI Version)
 These values are typical for concrete containing about
360-390kg/m3 of cement with a w/cm < 0.40.
 The “strength trap” is the situation in which strength
achieved with silica-fume concrete will greatly exceed
most project specifications.
 This factor may cause concrete producers and
contractors to relax their control over the concrete
production and curing.
 While strength may be adequate for structural
design, rapid chloride permeability values may not be
within specifications.

22. Silica-Fume Concrete:
Corrosion Protection
 3-7% silica fume added by mass of cement
 Mixture may include fly ash or slag
 w/cm < 0.40: use HRWRA
 Total cementitious materials < 415 kg/m3
 Permeability estimated using ASTM C 1202
SI

23. Silica-Fume Concrete:
Corrosion Protection (SI Version)
 This slide summarizes many years of experience using
silica fume for improved durability in bridge decks and
parking structures.
 More and more concrete in this application is now
including either fly ash or slag along with the silica
fume.
 See the examples in Chapter 5 for typical mixture
proportions.
 ASTM C 1202, “Standard Test Method for Electrical
Indication of Concrete’s Ability to Resist Chloride Ion
Penetration.”

24. Concrete with Silica Fume
 3-7% of Mass of  Cautions
Cement  > Plastic Shrinkage
 Used with HRWRA  High Water Demand
 Adjust Admixtures  Stickiness
 Black-Gray Color
 Must be Moist Cured

25. Silica Fume and the Environment
 Recovered/ recycled material
 Replaces Portland cement
 Reduction of CO2
 Reduction of Sox and NOx
 Reduced Material extraction

26. LEED Credits
 Leadership in Energy and Environmental Design
(LEED) is a system developed by the
 United States Green Building Council to rate a
building's environmental performance.
 This system has become the principal method by
which buildings can achieve green
 building certification. The system is based on credits
earned in five major categories.
 Silica Fume can positively impact in two of the credit
categories.