16-01-2014, 12:51 PM
Silica Fume User’s Manual
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INTRODUCTION
The contents of this report reflect the views of the author who is responsible
for the facts and the accuracy of the data presented herein. The contents do
not necessarily reflect the official views or policies of the Federal Highway
Administration. This document does not constitute a standard, specification,
or regulation.
Trade or manufacturers’ names that appear herein are cited only because
they are considered essential to the objectives of the document. The Federal
Highway Administration does not endorse products or manufacturers.
The Silica Fume Association was formed in 1998 to serve as a voice for
producers of silica fume. Please visit the SFA web site (www.silicafume.org)
for information on additional products produced under the FHWA-SFA
cooperative agreement.
SILICA FUME DEFINITION
The American Concrete Institute (ACI) defines silica fume as “very fine non-
crystalline silica produced in electric arc furnaces as a by-product of the production
of elemental silicon or alloys containing silicon” (ACI 116R). It is usually a gray
colored powder, somewhat similar to portland cement or some fly ashes. Figure
1.1 shows a typical silica fume as it appears after being collected from a furnace.
PRODUCTION
Silica fume is a by-product of producing silicon metal or ferrosilicon alloys in
smelters using electric arc furnaces. These metals are used in many industrial
applications to include aluminum and steel production, computer chip fabrica-
tion, and production of silicones, which are widely used in lubricants and
sealants. While these are very valuable materials, the by-product silica fume is of
more importance to the concrete industry.
REACTIONS IN CONCRETE
The benefits seen from adding silica fume are the result of changes to the
microstructure of the concrete. These changes result from two different but
equally important processes. The first of these is the physical aspect of silica
fume and the second is its chemical contribution. Here is a brief description of
both of these aspects: