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).
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
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
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.
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.
Hinweis der Redaktion
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.conc696
SI version of previous slide.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.
SI version of previous slide.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.”