Materials Characterization Paper 

In Support of the

Final Rulemaking: 

Identification of Nonhazardous Secondary Materials That Are Solid Waste

Cement Kiln Dust (CKD) 

February 3, 2011 

===================================================================== 

1. Definition of CKD 

Cement kiln dust (CKD) is a fine-grained, solid, highly alkaline material removed from the 

cement kiln exhaust gas by scrubbers (filtration baghouses and/or electrostatic precipitators). 

The composition of CKD varies among plants and over time at a single plant. Much of the 

material comprising CKD is incompletely reacted raw material, including a raw mix at various 

stages of burning, and particles of clinker. 

2. Annual Quantities of CKD Generated and Used 

(1) Sectors that generate CKD: 

• CKD is generated by NAICS industry sector 327310, Cement Manufacturing 

(U.S. Census Bureau 2002). This sector included 247 facilities in 2002 according 

to the U.S. Census of Manufacturers. EPA reports that this sector included 144 

facilities in 2006 (EPA 2006). 

(2) Quantities of CKD generated: 

• An estimated 13 to 17 million short tons of CKD are generated per year (EPA 

2008).1

(3) Trends in generation of CKD: 

• Generation of CKD is directly connected to the production of cement clinker (the 

initial product of a cement kiln before processing). Significant increases in U.S. 

clinker capacity are expected to materialize between 2008 and 2012, anticipating 

an increase in production. The U.S. cement industry has announced plans to 

increase clinker capacity by nearly 25 million metric tons between 2007 and 

2012. The capacity expansion reflects a $5.9 billion investment, which will 

increase capacity 27 percent compared to 2006 U.S. clinker capacity (Cement 

Americas 2008). Assuming that capacity utilization is constant, a CKD 

production increase of 27 percent is also anticipated. 

• The U.S. cement industry has adopted a year 2020 voluntary target of a 60 percent 

reduction (from a 1990 baseline) in the amount of CKD disposed per 

 1

 Because of the high rate of direct reuse, CKD generation rates are not routinely measured, and limited 

data are available. U.S. cement kiln industry personnel estimate that CKD generation (including material returned to 

the kiln) is equivalent to approximately 15 to 20 percent (by weight) of total annual clinker production (EPA 2008). 
Cement Kiln Dust (CKD) 

2

ton of clinker produced (EPA 2007). Through CKD reduction efforts by Portland 

Cement Association (PCA) member companies, this goal was accomplished in 

2004. The PCA is currently developing a new CKD reduction goal (IEEE-IAS 

2008). 

3. Uses of CKD 

(1) Ingredient uses of CKD: 

• Because of the high percentage of raw mix and clinker in CKD, CKD can be 

directly reused in the cement kiln for clinker manufacture (EPA 2008). The 

cement industry recycles more than 75 percent of its CKD, amounting to nearly 

eight million tons each year (EPA 2006). 

• CKD partially substitutes for virgin raw materials in kiln feed. Kiln feed is 

generally comprised of about 80 percent carbonate of lime and about 20 percent 

silica with much lower quantities of alumina and iron. Limestone is the primary 

source of calcium for nearly all cement plants. Shale, clay, and sand are the 

primary materials fed as sources of silica and alumina (EPA 1995). CKD can 

substitute for any or all of these materials, depending on the composition of the 

clinker from which it was generated. 

(2) Non-combustion uses of CKD:

• CKD not returned to the production process can be sold for various types of 

commercial applications, including agricultural soil enhancement, base stabilizing 

for pavements, wastewater treatment, waste remediation, low-strength backfill 

and municipal landfill cover. These applications depend primarily on the chemical 

and physical characteristics of the CKD. The major parameters that determine 

CKD characteristics are the raw feed material, type of kiln operation, dust 

collection systems, and fuel type. Since the properties of CKD can be 

significantly affected by the design, operation, and materials used in a cement 

kiln, the chemical and physical characteristics of CKD must be evaluated on an 

individual plant basis (IEEE-IAS 2008). 

(3) Quantities of CKD landfilled: 

• The majority of CKD is recycled directly back to the cement kiln, thus avoiding 

disposal; however, CKD must periodically be removed from the system due to 

increasing concentrations of various contaminants that may compromise the 

quality of the clinker. The PCA estimates that 6.4 million tons of CKD were 

landfilled in 2003 (EPA 2006). 

(4) Quantities of CKD stockpiled/stored:

• CKD that cannot be recycled back into the cement kiln (because of excessive 

alkali content, as well as other operational factors) is removed from the system 

and often collected onsite in piles or monofills (EPA 1995). Current estimates of 

the stockpiled quantity of CKD are not available. 

• A recent trend at some cement manufacturing facilities is the removal or “mining” 

of CKD placed in landfills or other long-term management units. From the PCA 
Cement Kiln Dust (CKD) 

3

CKD surveys of member companies, it was estimated that the amount of CKD 

removed from onsite landfills has grown from just over 13,400 metric tons in 

1998 to more than 261,000 metric tons in 2006. Because CKD is very similar to 

the raw materials entering the kiln system and may contain partially processed 

feed or final product, it requires less processing prior to use in the kiln and thus, 

less energy. Consequently, CKD is becoming more valuable as manufacturing 

costs rise (IEEE-IAS 2008). 

Exhibit 1: Overview of Generation and Use of CKD in 2004 

Commodity 

Annual 

Quantity 

Generated 

Annual Quantity Used as 

Ingredient Annual 

Quantity 

Landfilled 

Annual 

Quantity in 

Other Uses 

Total 

Quantity 

Stockpiled 

Cement 

Kilns Other 

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

Cement kiln 

dust 

13.23–16.53 

million 0.37 milliona

 0 1.5 million 1.23 million Not Available

Sources: 

U.S. EPA, 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, June 3, 2008. 

Notes: 

a. This figure is based on data in the above referenced report indicating that approximately 333,000 metric tons of 

CKD captured from cement kiln air pollution control devices were used in cement kilns in 2004, which converts 

to approximately 367,000 short tons. This estimate does not reflect the indeterminate, and potentially large, 

quantity of cement kiln dust captured elsewhere in kiln, much of which is also re-routed back into kilns to be used 

as an input in cement production. 

4. Management and Combustion processes for CKD 

(1) Types of units using CKD:

• CKD is returned to cement kilns during the clinker manufacturing process. 

(2) Sourcing of CKD:

• A closed-loop process captures CKD from exhaust gas scrubbers at cement plants 

and returns it directly to the kiln. 

(3) Processing of CKD: 

• CKD captured from kiln exhaust gasses is directly reinjected back into the kiln, 

and does not require processing. CKD mined from stockpiles may require 

processing prior to injection into the cement kiln.2

 

 2

 The sources consulted for this document do not provide detailed information on the actual processing 

steps. 
Cement Kiln Dust (CKD) 

4

(4) State status of CKD use as ingredient: 

• According to state responses to a 2006 survey by the Association of State and 

Territorial Solid Waste Management Officials (ASTSWMO), Florida and Iowa 

have pre-approved the use of CKD in the production of cement as a beneficial 

use, exempting it from the case-by-case approval process for designation of 

beneficial use (ASTSWMO 2007, p.B-9). 

5. CKD Composition and Impacts 

(1) Composition of CKD:

• CKD is comprised of thermally unchanged raw materials, dehydrated clay, 

decarbonated (calcined) limestone, ash from fuel, and newly formed minerals 

corresponding to all stages of processing up through the formation of the clinker. 

An unusual feature of CKD is that, unlike typical by-product materials that are 

substantially different than the product, CKD is essentially cement clinker that 

does not quite meet commercial specifications (EPA 1995). 

• The primary constituents in CKD are silicates, calcium oxide, carbonates 

(expressed as loss of CO2 and H2O on ignition), potassium oxide, sulfates, 

chlorides, various metal oxides, and sodium oxide (EPA 1995). 

• CKD contains insignificant amounts of trace metals and therefore metal 

concentrations are not usually a concern for most applications. A comprehensive 

study by the PCA evaluated the presence of trace metals in CKD from 79 plants 

in the United States and 10 plants in Canada using both conventional and other 

fuels. Each CKD sample was tested for the eight RCRA metals: arsenic, barium, 

cadmium, chromium, lead, mercury, selenium, and silver. The samples were also 

analyzed for antimony, beryllium, thallium and nickel. Results showed that the 

average level of trace metals found in the CKD were significantly below the 

regulation limits (IEEE-IAS 2008). 

• The predominant trace metals in CKD include antimony, barium, lead, 

manganese, strontium, thallium, and zinc, and the minor trace metals include 

beryllium, copper, hexavalent chromium, mercury, nickel, silver, and thallium 

(EPA 1995). The concentration of these metals in CKD varies significantly by 

cement plant depending on the mix of raw feed materials used in clinker 

production and the type of fuel used in the cement kiln (May 1999). 

• Exhibits 2 and 3 detail the composition of cement kiln dust generated by different 

types of kilns and the composition of available alternatives, respectively. We note 

that it would be inappropriate to compare the composition of cement kiln dust to 

any one of the alternative materials individually, given that cement kiln dust 

displaces a combination of these materials in the cement production process. 
Cement Kiln Dust (CKD) 

5

Exhibit 2: Compositions of Cement Kiln Dust 

Constituent 

Cement Kiln Dust, by source 

(Concentration Ranges – weight percent)1

Long Dry Kilns, Dry 

Kilns with Preheaters, 

and/or Calciners Wet Kilns 

Rotary Kiln with 

Cyclone Preheater 

Rotary Kiln with Grate 

Preheater 

Silicon Dioxide 4.3 - 10.1 4.1 - 7.7 7 - 11 2 – 19 

Aluminium Oxide 1.0 - 3.3 1.3 - 3.3 

3 – 62

 0.5 – 82

Titanium Dioxide 0.07 - 0.2 0.08 - 0.2 

Iron Oxide 0.7 - 2.3 0.8 - 2.0 

1 – 32

 0.5 - 42

Manganese (III) Oxide 0.01 - 0.2 0.02 - 0.04 

Calcium Oxide 11.0 - 45.0 15.9 - 38.0 41 - 51 6 - 26 

Magnesium Oxide 0.4 - 2.0 0.4 - 1.9 0.5 - 2 0 - 2 

Sulfur Trioxide 0.1 - 7.7 0.1 - 6.0 0.5 - 4 7 - 41 

Potassium Oxide 0.2 - 9.7 0.2 - 12.1 0.5 - 4 14 - 40 

Chlorine 0.08 - 2.7 4.2 - 6.3 0 - 0.3 0.9 - 4.5 

Sodium Oxide 0.07 - 1.12 0.1 - 4.1 0 - 0.5 0.5 - 3 

Loss on Ignition 

(Carbon Dioxide and 

Water) 

No Data 22 - 25 29 - 38 4 - 24 

pH 6.11 - 12.83 (s.u.) 11.64 - 12.98 (s.u.) No Data No Data 

Notes: 

1. U.S. EPA (1993) 

2. For rotary kilns, the U.S. EPA (1993) source reported combined estimates for aluminum oxide and titanium oxide. It 

also reported combined estimates iron oxide and manganese oxide. 

Exhibit 3: Composition of Alternatives to Cement Kiln Dust (percent by weight)

 Clay Minerals1

Limestone2

Cherty 

Shale3 Sand4 Constituent

Kaolin 

Clay

Palygor-skite 

Clay

Cheto 

Clay

Hectorite 

Clay

Montmorillonite Barasym

Silicon Dioxide 43.9 - 44.2 60.900 60.400 34.700 62.9 - 70.1 49.700 15 - 18 94.6 > 93 

Aluminium Oxide 38.5 - 39.7 10.400 17.600 0.690 16 - 19.6 38.200 3 - 5 1.82 - 

Titanium Dioxide 1.39 - 2.08 0.490 0.240 0.038 0.09 -0.22 0.023 - 0.11 - 

Iron Oxide (II, III) 0.21 - 1.13 3.380 1.500 0.270 0.8 - 3.67 0.034 1 - 1.5 0.7 - 

Manganese Oxide 0.0 - 0.002 0.058 0.099 0.008 0.006 - 

0.009 - - - - 

Calcium Oxide - 1.980 2.820 23.400 1.59 - 1.68 - 38 - 42 0.88 - 

Magnesium Oxide 0.03 10.200 6.460 15.300 3.05 - 3.69 0.014 0.5 - 3.0 0.25 - 

Sulfur 0.0 - 0.02 0.110 - 0.010 0.04 - 0.05 0.100 - 0.05 - 

Potassium Oxide 0.05 - 

0.065 0.800 0.190 0.130 0.078 - 0.53 <0.01 - 0.35 - 

Chlorine - - - - - - - 0.84 - 

Sodium Oxide 0.0 - 0.013 0.058 0.063 1.26 0.27 - 1.53 0.26 - 0.19 - 

Fluorine 0.013 - 

0.02 0.542 0.287 2.600 0.084 - 0.111 0.760 - - - 

Phosphorus Oxide 0.034 - 

0.045 0.800 0.020 0.014 0.026 - 0.049 0.001 - 0.42 - 

Lithium Oxide - - - 2.180 - - 

Notes 

1. Clay Minerals Society (undated) 

2. MineralsZone (2005) 

3. USGS (2002) 

4. Northern Filter Media (2005) 
Cement Kiln Dust (CKD) 

6

. 

(2) Impacts of CKD use:

• In clinker manufacture, CKD partially offsets the need for virgin material feed, 

such as limestone and natural constituents (rock). Thus, returning CKD to the 

cement kiln can reduce the unit consumption of virgin feed stock in the kiln, 

which may then reduce the costs of raw materials and emissions from extraction 

and processing of virgin materials, as CKD requires less processing (and thus less 

energy) for use in cement than virgin raw materials. However, using CKD in 

place of virgin materials may change the emissions from the kiln to the extent 

CKD has a different emissions profile than the alternative virgin materials when 

burned in the kiln. 

• Some of the metal constituents in CKD (e.g., mercury and lead) may be 

volatilized when the material is heated to high temperatures in the cement kiln. 

• The specific lifecycle impacts of CKD use as a raw material in clinker production 

are not evaluated here because of uncertainties in lifecycle scenario development. 

For example, it is difficult to determine the replacement ratio between CKD and 

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

material to be modeled is unclear. In addition, CKD may substitute for a variety 

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

slag, CCPs, foundry sand, etc.); the choice of material often depends on locationspecific 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. 
Cement Kiln Dust (CKD) 

7

References 

Association of State and Territorial Solid Waste Management Officials (ASTSWMO). 2007, 2006 Beneficial Use 

Study Report, published by Association of State and Territorial Solid Waste Management Officials, 

November 2007. 

Clay Minerals Society. (undated), Source Clay Physical/Chemical Data, accessed at: 

http://www.clays.org/SOURCE%20CLAYS/SCdata.html 

Cement Americas, January 1, 2008, Cement Americas Forecast 2008, accessed at: 

http://cementamericas.com/mag/cement_forecast_0101/index.html. 

IEEE-Industry Applications Society (IEEE-IAS). 2008, Beneficial Uses of Cement Kiln Dust, presented at the 2008 

IEEE/PCA 50th Cement Industry Technical Conference., Miami, FL, May 19-22, 2008 

May, Mallory. May 1, 1999, A Study on Trace Metals in Portland Cement Kiln Dust, accessed at: 

http://cementamericas.com/mag/cement_study_trace_metals/. 

MineralsZone, 2005, Limestone Profile, accessed at: 

http://www.mineralszone.com/stones/limestone.html

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

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http://www.epa.gov/epaoswer/other/ckd/cement2.htm. 

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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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