Materials Characterization Paper 

In Support of the 

Proposed Rulemaking: 

Identification of Nonhazardous Secondary Materials That Are Solid Waste 

Silica Fume (used as ingredient in clinker manufacture) 

March 18, 2010 

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1. Definition of Silica Fume 

Silica fume is a fine, dust-like material generated during silicon metal and ferrosilicon and 

related ferroalloys (e.g., stainless steel) production. Specifically, it is produced by the reduction 

of high purity quartz with coal or coke and wood chips in an electric arc furnace during silicon 

metal or ferrosilicon alloys production. The glassy, spherical particles are extremely small, 

measuring less than one micrometer (μm) in diameter, with an average diameter of about 0.1 μm. 

Silica fume particles are composed primarily of silicon dioxide (usually more than 85 percent). 

2. Annual Quantities Generated and Used 

(1) Sectors that generate silica fume: 

• Silica fume is generated by NAICS sector 331112, Electrometallurgical 

Ferroalloy Product Manufacturing. This includes facilities that manufacture 

stainless steel and cast iron. 

(2) Quantities and prices of silica fume generated: 

• The Silica Fume Association estimates that silica fume production in the United 

States in 2004 was between 110,000 and 132,000 short tons (EPA 2008). 

• In 2006, one major silica fume supplier reported the bulk price of silica fume, 

freight on board (FOB) (i.e., not including shipping costs), at $205/ton (Robl and 

Groppo 2007). It is unclear if this is representative of the price of silica fume 

from other suppliers. 

(3) Trends in generation of silica fume: 

• Growing U.S. customer needs for alloy and stainless steel is expected to continue 

to have a strong positive influence on ferroalloy demand (USGS 2006). As a 

result, production of ferroalloys and of silica fume as a byproduct are likely to 

remain constant or increase in the next several years. 
Silica Fume 

 

3. Uses of Material/Commodity 

(1) Ingredient uses of silica fume: 

• Silica fume can be used in clinker manufacture as a partial replacement for silicacontaining raw materials (e.g., clay, shale, sand, and limestone). Because of the 

high price of this material, however, and the relative abundance of other 

secondary materials and virgin materials, use of silica fume as a raw material for 

clinker is less common, though it still occurs (Guerra 2008). In 2004, 20,000 tons 

of silica fume were used in cement kilns (EPA 2008). 

• Cement kilns are represented by NAICS industry sector 327310, Cement 

Manufacturing, which included 247 facilities in 2002 (U.S. Census Bureau 2002). 

(2) Non-combustion uses of silica fume:

• Silica fume can be also be used at cement plants as a supplementary cementitious 

material (i.e., after the production of clinker in the kiln) either by premixing silica 

fume with finished portland cement to produce a blended cement (during the 

cement production process) or by adding the silica fume to portland cement 

concrete as a mineral admixture. In addition, silica fume is frequently used in 

consumer products such as toothpaste (Guerra 2008). Use of silica fume in noncombustion uses in 2004 totaled approximately 70,000 tons (EPA 2008).

(3) Quantities of silica fume landfilled: 

• The Silica Fume Association estimates that about 28,000 short tons of silica fume 

were landfilled in 2004 (EPA 2008). Given the commodity value of silica fume, it 

is not clear why any is landfilled; possible reasons include contamination of the 

silica fume or lack of market access due to location of the furnaces. 

(4) Quantities of silica fume stockpiled/stored:

• Data regarding stockpiling or storage of silica fume are not readily available, but 

demand for the material and the relatively modest quantities produced suggest 

that little, if any, silica fume is collected for long-term storage. 

Exhibit 1: Overview of Generation and Use of Silica Fume in 2004 

Annual Quantity Used as 

Ingredient Annual 

Quantity 

Generated 

Cement 

Kilns Other 

Annual 

Quantity 

Landfilled 

Annual 

Quantity in 

Other Uses 

Total 

Quantity 

Stockpiled 

Commodity 

---------------------------------- Short Tons -----------------------------

Silica fume 

0.11-0.13 

million 0.02 million 0 0.03 million 0.07 million 

Undetermined; 

not likely to be 

significant

Sources: 

Unless otherwise noted, data is from U.S. EPA, April 28, 2008, Study on Increasing the Usage of Recovered 

Mineral Components in Federally Funded Projects Involving Procurement of Cement or Concrete to Address the 

Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users. 

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Silica Fume 

 

4. Management and Combustion processes 

(1) Types of combustion units using silica fume:

• Silica fume is used in cement kilns during the clinker manufacture process. 

(2) Sourcing of silica fume:

• Silica fume is marketed by a variety of admixture suppliers in the U.S. who obtain 

the material from the generators and prepare it for resale. There are five domestic 

silica fume producers listed in the Silica Fume Association, all of whom are 

located in the Southeastern United States. The product is typically sold packaged 

in 25 pound bags and in bulk (Robl and Groppo 2007). 

(3) Processing of silica fume: 

• Prior to use as a supplementary cementitious material, silica fume may be 

densified by tumbling the material in a silo, which causes surface charges to build 

up. The charges draw the particles together to form weak agglomerates. Because 

of the increased bulk density, the material is more economical for truck transport 

(Silica Fume Association 2005). 

(4) State status of silica fume use as ingredient: 

• At this stage, we have not identified any states that have specifically granted 

beneficial use designation to the use of silica fume in clinker manufacture, or that 

prohibit use of silica fume in clinker, but we have not performed an exhaustive 

investigation of state activities and regulations. 

5. Commodity Composition and Impacts 

(1) Composition of silica fume:

• Silica fume consists of 85 percent amorphous (non-crystalline) silicon dioxide 

(SiO2). There may be additional materials in silica fume depending on the metal 

being produced in the smelter from which the silica fume was collected (Silica 

Fume Association 2005). 

(2) Impacts of silica fume use as ingredient:

• If used in clinker manufacture, silica fume partially offsets natural sources of 

silica, such as clay, shale, sand, and limestone. Thus, using silica fume in the 

cement kiln would avoid the energy use and environmental releases associated 

with the extraction and processing of these virgin materials. 

• The specific lifecycle impacts of silica fume use as a raw material in clinker 

production are not evaluated here because it appears silica fume has limited use in 

this application, and because of uncertainties in lifecycle scenario development. 

For example, it is difficult to determine the replacement ratio between silica fume 

and other raw feed materials in clinker production. Thus, the correct quantity of 

material to be modeled is unclear. In addition, silica fume may substitute for a 

variety of virgin raw materials, as well as other secondary materials (e.g., blast 

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Silica Fume 

 

furnace slag, CCPs, foundry sand, cement kiln dust, etc.); the choice of material 

often depends on location-specific factors such as the proximity of material 

sources to the cement kiln and relative availability of different materials. Avoided 

upstream impacts depend heavily on the specific material being displaced in the 

lifecycle scenario. 

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Silica Fume 

 

References 

Guerra, Erika, 2008, Holcim Support Group Ltd., Corporate Industrial Ecology, personal communication, August 

26, 2008. 

Robl, Thomas and John Groppo, University of Kentucky, 2007, Advanced Multi-Product Coal Utilization ByProduct Processing Plant, accessed at: 

http://www.netl.doe.gov/technologies/coalpower/cctc/ccpi/pubs/KY/UKRF%20Apr%202007%20FinalRep.

pdf. 

Silica Fume Association, 2005, Silica Fume User’s Manual, accessed at: http://www.silicafume.org/pdf/silicafumeusers-manual.pdf. 

United States Census Bureau, 2002 Economic Census, Industry Snapshots for Cement Manufacturing, accessed on 

August 18, 2008 at: 

http://quarterhorse.dsd.census.gov/TheDataWeb_HotReport/servlet/HotReportEngineServlet?emailname=b

h@boc&filename=mfg1.hrml&20071204152004.Var.NAICS2002=327310&forward=20071204152004.V

ar.NAICS2002. 

United States Environmental Protection Agency (EPA). June 3, 2008, Study on Increasing the Usage of Recovered 

Mineral Components in Federally Funded Projects Involving Procurement of Cement or Concrete to 

Address the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users. 

United States Geological Survey (USGS). 2006, Minerals Yearbook: Ferroalloys, accessed on August 18, 2008 at: 

http://minerals.usgs.gov/minerals/pubs/commodity/ferroalloys/myb1-2006-feall.pdf. 

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