A Review of Literature Related to the Use 

of Spray Dryer Absorber Material 

Production, Characterization, Utilization Applications, 

Barriers, and Recommendations

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A Review of Literature Related to 

the Use of Spray Dryer Absorber 

Material 

Production, Characterization, Utilization 

Applications, Barriers, and Recommendations 

1014915 

Technical Report, September 2007 

Cosponsor 

University of North Dakota 

Energy & Environmental Research Center 

Coal Ash Resources Research Consortium 

15 North 23rd Street, Stop 9018 

Grand Forks, ND 58202-9018 

Project Manager 

L.V. Heebink 
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iii 

CITATIONS 

This report was prepared by 

University of North Dakota 

Energy & Environmental Research Center 

Coal Ash Resources Research Consortium 

15 North 23rd Street, Stop 9018 

Grand Forks, ND 58202-9018 

Principal Investigators 

L.V. Heebink 

T. Buckley 

D. Pflughoeft-Hassett 

D. Hassett 

This report describes research sponsored by the Electric Power Research Institute (EPRI) and the 

University of North Dakota (UND) Energy & Environmental Research Center (EERC) Coal Ash 

Resources Research Consortium (CARRC). 

The report is a corporate document that should be cited in the literature in the following manner: 

A Review of Literature Related to the Use of Spray Dryer Absorber Material: Production, 

Characterization, Utilization Applications, Barriers, and Recommendations. EPRI, Palo Alto, 

CA and UND EERC CARRC, Grand Forks, ND: 2007. 1014915. 

v 

REPORT SUMMARY 

This report is a compilation of an extensive literature review focused on spray dryer absorber 

(SDA) systems and their by-products. It includes background information on the current SDA 

production and use statistics, characterization data on SDA material reported in the literature, an 

overview of the current commercial and potential uses of SDA material reported in the literature, 

identification of barriers to the increased use of SDA material, and the authors’ conclusions and 

recommendations based on the literature reviewed. This report is intended to assist utility ash 

managers, ash marketers, regulators, and researchers in understanding the issues surrounding 

SDA material utilization so that they can move from disposal to utilization. 

Background 

SDA systems are a semi-dry SO2 control technology that currently make up about 12% of the 

flue gas desulfurization (FGD) systems at U.S. power plants. SDA systems have typically been 

employed on small-to-moderate size units that burn low-sulfur coal in the western United States, 

where water supply is limited. The material produced by SDA systems primarily consists of 

calcium sulfite commingled with varying amounts of fly ash and unspent lime. Because most 

units in the United States do not pre-collect fly ash, the SDA material often contains a high 

percentage of it. SDA material presents a unique challenge for use applications, and as a result 

most coal-fired power plants currently dispose of SDA material. The use of SDA systems is 

expected to grow along with wet FGD systems over the next 10 years as a result of increased 

requirements for SO2 control, heightening the need for increased utilization options. 

Objectives 

• To summarize current information in the literature relative to SDA material production, 

characteristics, and uses 

• To rank the potential of different uses for SDA material 

• To provide recommendations for maintaining/increasing the use of SDA material in the 

future 

Approach 

The authors performed a detailed survey of the U.S. and European literature on the use of SDA 

material in a variety of applications. They summarized information on SDA material 

characteristics, production, and use, with particular emphasis on the benefits and disadvantages 

of the higher fly ash content of the SDA material in the United States. Based on the review, the 

authors developed a prioritized list of potential applications to advance the use of SDA material 

along with a set of recommendations for further research. 
vi 

Results 

According to American Coal Ash Association (ACAA) statistics, in 2006 U.S. utilities produced 

about 1.5 million short tons of SDA material and used only about 0.14 tons, or a little over 9%. 

Mining applications represented by far the largest type of application, accounting for 85% of the 

use. Other uses included concrete and concrete products, flowable fill, roadways, waste and soil 

stabilization, and agriculture. Key attributes of SDA material that must be considered in 

developing applications are the material’s inherent variability, its high sulfur content, and its 

potential for expansion. 

Based on the literature review, the authors ranked the SDA material for use in various 

applications: 

• High potential: cementitious products (such as masonry, flowable fill, and synthetic 

aggregate) and mining applications 

• Moderate potential: cement replacement in concrete, engineering applications, agriculture, 

soil stabilization, and wet FGD sorbent 

• Low potential: binder material, cement manufacture, hazardous waste fixation, marine 

applications, mineral wool, sulfuric acid production, and wallboard 

Barriers inhibiting the use of the material were identified and include the following: 

• Inconsistent terminology used to define the material 

• Lack of understanding of the material 

• Limited data on environmental and health effects 

• Inconsistent guidelines on beneficial use 

• Economics 

EPRI Perspective 

Maintaining and increasing the utilization rate of coal combustion products (CCPs) is a primary 

strategic goal of EPRI’s CCP Use research program. SDA materials currently have a low 

utilization rate, and their volume is expected to significantly increase over the next several years. 

Understanding SDA material characteristics and developing increased management options are 

key components of the overall CCP use research strategy. 

Keywords 

Flue gas desulfurization products 

Spray dryer absorber material 

Coal combustion products 

Utilization 
vii 

ABSTRACT 

Coal-fired power plants account for the majority of sulfur dioxide (SO2) emissions in the United 

States. Legislative actions in the United States and elsewhere have been responsible for most 

industrial SO2 controls, resulting in the installation of flue gas desulfurization (FGD) systems. In 

the United States, approximately 85% of FGD systems are wet, 12% are spray dryer absorber 

(SDA) systems, and 3% are dry injection systems. This report is a compilation of an extensive 

literature review on SDA systems and their byproducts. 

Most SDA systems in the United States collect fly ash (40–75%) along with SDA material, 

producing spherical, glassy fly ash particles coated by and intermixed with fine crystals of 

calcium/sulfur reaction products. Depending on their source, SDA materials can vary widely in 

their physical, chemical, and mineralogical properties, and the successful utilization of SDA 

material is highly dependent on these properties. Characterization data cited in the literature are 

included in this report. 

In the literature reviewed, a number of current commercial and potential uses of SDA material 

were identified, including agriculture, binders, cement manufacture, cement replacement in 

concrete, civil engineering, flowable fill, fixating agent for waste, marine applications, masonry, 

mineral wool, mining applications, soil stabilization, sulfuric acid production, synthetic 

aggregate, wallboard, and wet FGD sorbent. Many of the commercial uses are being successfully 

implemented in Europe but are slower to enter the marketplace in the United States. Applications 

currently commercial in the United States include agriculture, concrete, concrete products, 

flowable fill, mining applications, soil stabilization, structural fills and embankments, and 

synthetic aggregate. Potential uses in the research and development stage were rated as high-, 

moderate-, or low-potential commercial applications. High-potential applications for the U.S. 

market are estimated to be those that take advantage of the presence of the fly ash component of 

the SDA material, can tolerate relatively high sulfur content, and either have limited 

susceptibility to expansion or reduce expansion potential in the production process. These 

applications fall into two categories: cementitious products and mining applications. 
viii 

Barriers inhibiting the use of the material were identified and include the following: 

• Inconsistent terminology used to define the material 

• Lack of understanding of the material 

• Limited data on environmental and health effects 

• Inconsistent guidelines on beneficial ash use 

• Economics 

Based on the information obtained in the research, the authors recommend the following to 

maintain existing commercial markets and develop new ones for SDA materials produced in the 

United States: 

• Work within existing organizations such as ASTM International and the American Coal Ash 

Association to develop and put into use appropriate terminology and definitions for SDA 

materials. 

• Develop an understanding of the impact of compositional variability on the performance 

characteristics of SDA materials. 

• Develop an understanding of the oxidation profiles of SDA materials, and evaluate the 

impacts of oxidation on product performance. 

• Educate potential users, regulatory representatives, and other stakeholders about SDA 

materials. 

• Address quality, compositional, environmental, and performance criteria in research, 

development, and demonstration efforts. 
ix 

NOMENCLATURE 

ACAA American Coal Ash Association 

AASHTO American Association of State Highway and Transportation 

 Officials 

AFBC atmospheric fluidized-bed combustion 

Ca6Al2(SO4)3(OH)12 · 26H2O ettringite 

CaCl calcium chloride 

CAIR Clean Air Interstate Rule 

CaO calcium oxide, lime 

Ca(OH)2 calcium hydroxide, hydrated lime, portlandite, unreacted 

 lime 

CARRC®

 Coal Ash Resources Research Consortium®

Ca3Si(CO3)(SO4)(OH)6 · 12H2O thaumasite 

CaSO3 calcium sulfite 

CaSO3 · 1⁄2 H2O hannebachite, calcium sulfite hemihydrate 

CaSO4 calcium sulfate, anhydrite 

CaSO4 · 2 H2O gypsum, calcium sulfate dihydrate 

CCP coal combustion product 

CDF controlled density fill 

CHP combined heat and power 

CLSM controlled low-strength material 

DM Deutsche Mark 

DOE U.S. Department of Energy 

DSI dry sorbent injection 

ECOBA European Coal Combustion Products Association 

EERC Energy & Environmental Research Center 

EPA U.S. Environmental Protection Agency 

EPRI Electric Power Research Institute 

ERKOM “Waste Management of Residues from Coal-Fired Power 

 Plants and Waste Incineration Plants” (known as ERKOM 

 from the German title) 

ESP electrostatic precipitator 

EU European Union 

FBC fluidized-bed combustion 

FGD flue gas desulfurization 

FSI furnace sorbent injection 

FSS fixated scrubber solids 

HCl hydrogen chloride 
x 

LIMB limestone injection multistage burner 

LOI loss-on-ignition 

MnDOT Minnesota Department of Transportation 

MW megawatt 

NOx nitrogen oxides 

NR not reported 

OSU Ohio State University 

OVwG Oberverwaltungsgericht: Administrative Court of Appeals 

RCRA Resource Conservation and Recovery Act 

SCS Stearns, Conrad and Schmidt 

SDA spray dryer absorber 

SEM scanning electron microscopy 

SO2 sulfur dioxide 

SO3 sulfur trioxide 

TCLP toxicity characteristic leaching procedure 

TGA thermogravimetric analysis 

UND University of North Dakota 

VAW Vereinigte Aluminium-Werke AG, Lünen 
xi 

CONTENTS 

1 INTRODUCTION ....................................................................................................................1-1

Background ...........................................................................................................................1-3 

Development and Status of FGD Systems in the United States ......................................1-3 

FGD Systems...............................................................................................................1-4 

Wet Calcium-Based FGD Systems .........................................................................1-4 

Dry and Semidry FGD Systems ..............................................................................1-4 

Projected Future SDA Installations in the United States ................................................1-10 

Current SDA Material Production and Use Rates ...............................................................1-12 

Projected Future SDA Production in the United States ..................................................1-15 

2 RESULTS AND DISCUSSION OF THE LITERATURE REVIEW..........................................2-1

Definitions of SDA Material ...................................................................................................2-1 

Characterization of SDA Material..........................................................................................2-2 

Physical Properties...........................................................................................................2-3 

Qualitative Physical Properties ....................................................................................2-3 

Particle Size.................................................................................................................2-3 

Specific Surface Area ..................................................................................................2-5 

Bulk Density.................................................................................................................2-5 

Specific Gravity............................................................................................................2-7 

Optimum Moisture Content and Maximum Density .....................................................2-8 

Unconfined Compressive Strength ..............................................................................2-9 

Permeability ...............................................................................................................2-10 

Mineralogical and Chemical Composition ......................................................................2-11 

Chemical Analyses ....................................................................................................2-13 

pH ..............................................................................................................................2-17 
xii 

Current and Potential Uses of SDA Material.......................................................................2-19 

Agriculture ......................................................................................................................2-19 

Binders ...........................................................................................................................2-23 

Cement Manufacture......................................................................................................2-23 

Cement Replacement in Concrete .................................................................................2-24 

Civil Engineering.............................................................................................................2-25 

Flowable Fill....................................................................................................................2-28 

Fixating Agent for Waste ................................................................................................2-29 

Marine Applications ........................................................................................................2-30 

Masonry..........................................................................................................................2-30 

Mineral Wool...................................................................................................................2-31 

Mining Applications.........................................................................................................2-31 

Soil Stabilization .............................................................................................................2-32 

Sulfuric Acid Production .................................................................................................2-33 

Synthetic Aggregate .......................................................................................................2-33 

Wallboard .......................................................................................................................2-35 

Wet FGD Sorbent...........................................................................................................2-36 

3 SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS.................................................3-1

SDA Production and Material Characteristics .......................................................................3-1 

Utilization of SDA Material.....................................................................................................3-2 

High-Potential Applications...............................................................................................3-4 

Cementitious Products.................................................................................................3-4 

Masonry ..................................................................................................................3-4 

Flowable Fill ............................................................................................................3-5 

Synthetic Aggregate................................................................................................3-5 

Mining Applications......................................................................................................3-5 

Moderate-Potential Applications.......................................................................................3-6 

Cement Replacement in Concrete...............................................................................3-6 

Engineering Applications .............................................................................................3-6 

Agriculture....................................................................................................................3-7 

Soil Stabilization...........................................................................................................3-7 

Wet FGD Sorbent ........................................................................................................3-8 
xiii 

Low-Potential Applications ...............................................................................................3-8 

The following applications are expected to have low utilization potential. 

Predictions for utilization rates have not been made. ..................................................3-8 

Binder ..........................................................................................................................3-8 

Cement Manufacture ...................................................................................................3-8 

Fixating Agent for Hazardous Waste Sludge...............................................................3-8 

Marine Applications .....................................................................................................3-9 

Mineral Wool................................................................................................................3-9 

Sulfuric Acid Production...............................................................................................3-9 

Wallboard.....................................................................................................................3-9 

Barriers to SDA Material Utilization.......................................................................................3-9 

Conclusions.........................................................................................................................3-11 

Recommendations ..............................................................................................................3-11 

4 REFERENCES .......................................................................................................................4-1

A BULK CHEMICAL COMPOSITION – REPORTED AS OXIDES – OF SDA 

MATERIAL REPORTED IN LITERATURE REVIEWED.......................................................... A-1

B TOTAL ELEMENTAL COMPOSITION OF SDA MATERIAL REPORTED IN 

LITERATURE REVIEWED ....................................................................................................... B-1

xv 

LIST OF FIGURES 

Figure 1-1 Terms Used to Refer to SDA Material in the Literature ...........................................1-2

Figure 1-2 Diagram of the SDA Process Showing the Fly Ash Precollection Option and 

the Solids Recycle [2].........................................................................................................1-6

Figure 1-3 ACAA Definitions of Dry FGD Material [19] ...........................................................1-12

Figure 1-4 Dry Scrubber FGD Material Use in the United States In 2006. Of 1,488,951 

Short Tons Produced, 136,639 Short Tons of Dry Scrubber FGD Material (9.18%) 

Was Beneficially Used [22] ..............................................................................................1-13

Figure 1-5 Spray Dry Absorption Product Use in Europe (EU 15) in 2004. Of 466,278 

Short Tons Produced, 373,684 Short Tons (80.1%) Was Beneficially Used [23] ............1-14

Figure 2-1 ASTM International Definitions [28] .........................................................................2-1

xvii 

LIST OF TABLES 

Table 1-1 Typical SDA Operation Material Distribution [8] .......................................................1-6

Table 1-2 SDA Units in the United Statesa................................................................................1-8

Table 1-3 Dry FGD Plants in Europe [16, 17] .........................................................................1-10

Table 1-4 U.S. SDA Market Projections [18]...........................................................................1-10

Table 1-5 Details of U.S. SDA Market Projections [18]...........................................................1-11

Table 2-1 Particle-Size Distribution and Mean Mass/Particle Size of SDA Material 

Reported in Literature Reviewed, μm.................................................................................2-4

Table 2-2 Specific Surface Area of SDA Material Reported in Literature Reviewed, m2/g........2-5

Table 2-3 Bulk Density of SDA Material Reported in Literature Reviewed ...............................2-6

Table 2-4 Bulk Density of SDA Material Specified as Aerated or Poured or Loose; 

Packed or Compacted; and Tapped or Settled Reported in the Literature Reviewed........2-6

Table 2-5 Specific Gravity of SDA Material Reported in Literature Reviewed ..........................2-7

Table 2-6 Optimum Moisture Content and Maximum Dry Density of SDA Material 

Reported in Literature Reviewed........................................................................................2-8

Table 2-7 Unconfined Compressive Strength of SDA Material Reported in Literature 

Reviewed, psi...................................................................................................................2-10

Table 2-8 Permeability Coefficient of SDA Material Reported in Literature Reviewed, 

cm/sec..............................................................................................................................2-11

Table 2-9 Typical Composition of SDA Material Containing Fly Ash as Noted in the 

Reviewed Literature, wt% ................................................................................................2-12

Table 2-10 Typical Range of Main Components of SDA Material with Precollection of 

Fly Ash, wt% ....................................................................................................................2-13

Table 2-11 Composition (wt%) of SDA Material Overall and as a Function of SO2

Separation Efficiency. Adapted from Kolar [17] ...............................................................2-14

Table 2-12 Summary of Bulk Chemical Composition, Reported as Oxides, of SDA 

Material Containing Fly Ash Reported in Literature Reviewed, wt%................................2-15

Table 2-13 Summary of Bulk Chemical Composition, Reported as Oxides, of SDA 

Material with Precollection of Fly Ash Reported in Literature Reviewed, wt% .................2-16

Table 2-14 Summary of Total Elemental Composition of SDA Material Containing Fly 

Ash Reported in Literature Reviewed, ppm .....................................................................2-17

Table 2-15 Summary of Total Elemental Composition of SDA Material with Precollection 

of Fly Ash Reported in Literature Reviewed, ppm............................................................2-20

Table 2-16 pH Values of SDA Material Reported in Literature Reviewed...............................2-20
xviii 

Table 3-1 Commercial Applications for SDA Material in the United States and Europe...........3-2

Table A-1 Bulk Chemical Composition of SDA Materials Containing Fly Ash Reported in 

Literature Reviewed .......................................................................................................... A-2

Table A-2 Bulk Chemical Composition of SDA Materials with Precollection of Fly Ash 

Reported in Literature Reviewed....................................................................................... A-5

Table B-1 Total Elemental Composition of SDA Materials Containing Fly Ash Reported 

in Literature Reviewed ...................................................................................................... B-3

Table B-2 Total Elemental Composition of SDA Materials with Precollection of Fly Ash 

Reported in Literature Reviewed....................................................................................... B-7
1-1 

1

INTRODUCTION 

The use of flue gas desulfurization (FGD) technologies to reduce sulfur dioxide (SO2) emissions 

from flue gases at coal-fired power plants gained prominence in the 1980s in the United States. 

FGD systems are currently used on approximately 22% of U.S. coal-fired power plants. A 

variety of FGD system types are used globally, and the types of FGD systems used in the United 

States are summarized in a 2003 Energy & Environmental Research Center (EERC) report [1]. 

The FGD material that is the focus of this report is termed spray dry absorber material or SDA 

material. The system is referred to as a spray dry absorber (SDA). An SDA system captures SO2

from the flue gas by use of slaked lime slurry, which is sprayed into the flue gas, dried by the 

heat of the flue gas, and collected in a particulate control device. SDA systems may follow a 

particulate control device that collects the fly ash or the fly ash may intermingle with the lime 

slurry and be collected in combination with the SDA material. Recycle of the combined solid 

may be used to improve sorbent utilization. Alkaline fly ash such as that generated from 

subbituminous coals and some lignite coals will sorb SO2 gases. As noted by Redinger, the use of 

fly ash precollection is common in Europe but is not common in the United States. SDA systems 

are considered efficient and reliable and have a lower capital cost than wet FGD systems [2]. 

Operating costs for SDA systems are higher than wet FGD systems but the water usage is lower. 

The resulting product currently has a low utilization rate in the United States. 

The product produced from an SDA system is a dry FGD material commonly referred to as SDA 

material or dry FGD material. Other terms are also used to refer to this material, and a number of 

these were identified in a previous literature search on the topic of sulfite-rich FGD materials [3]. 

These terms are noted in Figure 1-1. This report will use the term “SDA material” throughout, 

except where the generic term “dry FGD material” was used in the review literature. In the 

United States, SDA material typically contains fly ash, so the authors have elected to note 

precollection of fly ash or the level of fly ash content when known. 
Introduction 

1-2 

• Spray/spray-dry/dryer/drier absorber/atomization/absorption (SDA, SAV) sludge, ash, material, 

product, byproduct, by-product, end-product, waste, or residue 

• Semidry absorber/atomization/absorption (SDA) sludge, ash, material, product, byproduct, byproduct, end-product, waste, or residue 

• Spray absorption process (SAP) sludge, ash, material, product, byproduct, by-product, end-product, 

waste, or residue 

• Spray dryer (SPD) residue 

• Spray dryer by-product (SDB) 

• Calcium spray dryer/drier ash, material, product, byproduct, by-product, end-product, waste, or 

residue 

• Lime spray dry/dryer/drier (LSD) ash, material, product, byproduct, by-product, end-product, waste, or 

residue 

• Advanced SO2 control (ASC) ash, material, product, byproduct, by-product, end-product, waste, or 

residue 

• Sulfite-rich flue has desulfurization sludge, ash, material, product, byproduct, by-product, endproduct, waste, or residue 

• Sulfite sludge 

• Scrubber residue or sludge 

• Dry flue gas desulfurization sludge, ash, material, product, byproduct, by-product, end-product, 

waste, or residue 

• Dry scrubber sludge, ash, material, product, byproduct, by-product, end-product, waste, or residue 

• Nonoxidized flue gas desulfurization sludge, ash, material, product, byproduct, by-product, endproduct waste, or residue 

• Flue gas desulfurization sludge, ash material, product, byproduct, by-product, end-product, waste, or 

residue (with no reference to the FGD process) 

• Spent slurry 

Figure 1-1 

Terms Used to Refer to SDA Material in the Literature 
Introduction 

1-3 

Coal-fired power plants are currently evaluating options to comply with U.S. regulations that 

will require reductions of emissions of air toxics and acid gases. Responses are expected to result 

in an increase in SO2 emission controls and a subsequent increase in the volumes of FGD 

products produced in the United States. SDA systems, already being used by coal-fired power 

plants primarily in the western United States, will be one option that power plants may install, 

especially where water resources are limited. While all types of FGD material production are 

likely to increase, the potential increase in production volumes of SDA materials is a subject that 

raises the issue of materials management because of the current low utilization rate in the United 

States. With goals of 50% utilization of coal combustion products (CCPs) set by the U.S. 

Department of Energy (DOE) and the American Coal Ash Association (ACAA) to be achieved 

by 2010, additional high-volume production of one or more materials with limited potential for 

utilization in the current market threatens to offset the great strides to increase CCP utilization in 

the United States. The first step in determining the best options for utilization of SDA material is 

to identify current commercial applications and any potential applications that have been 

investigated or demonstrated. 

The goal of the project was to assess the current state of the knowledge regarding the 

characterization and utilization of SDA material and make recommendations on how to improve 

the use of the material. The project focused primarily on products produced from SDA systems; 

however, the report does contain limited information on other FGD processes and products. 

This report provides background information on how SDA systems function, descriptions of the 

variability of SDA materials, and information on the current production and use statistics. This 

background information provides a framework from which to evaluate the information on 

utilization applications summarized from the literature. 

Background 

Development and Status of FGD Systems in the United States 

Currently, coal-fired power plants account for the majority of SO2 emissions in the United States. 

Health concerns, including breathing difficulty, respiratory illness, and aggravation of existing 

cardiovascular disease, are associated with exposure to high ambient concentrations of SO2. The 

emission of SO2 from coal-fired power plants can also lead to acid deposition in the environment. 

In an effort to address health and environmental concerns related to SO2 in ambient air, 

legislation has been enacted to regulate most industrial SO2

 emissions. In the United States, 

major regulations include the Clean Air Act Amendments of 1970, 1977, and 1990. The 1990 

amendment required a permanent 10-million-ton reduction (to almost half the 1980 level) in SO2

emissions between 1980 and 2010. On March 10, 2005, the U.S. Environmental Protection 

Agency (EPA) issued the Clean Air Interstate Rule (CAIR), which will permanently cap 

emissions of SO2 and nitrogen oxides (NOx) in the eastern United States. SO2 emission 

regulations have also been proposed in many other industrialized nations. Most members of the 

European Economic Community are regulated, and Canadian laws are similar to those in the 

United States. 
Introduction 

1-4 

Coal-fired power plants generally employ one of two strategies to control SO2 emissions: 1) burn 

compliance fuels or 2) install FGD systems. Compliance fuels can be obtained by burning lowsulfur coal (coals with sulfur content below 2% by weight), blending low- and high-sulfur coals, 

and washing coal. Most modern power plants, particularly plants built after 1978, are required to 

have an FGD system. A variety of FGD systems are in use and others are in various stages of 

development. Commercialized FGD processes include wet, semidry, and completely dry 

processes. Regardless of type, FGD processes typically use a calcium- or sodium-based alkaline 

sorbent. The sorbent is injected in the flue gas in a spray chamber/vessel or directly into the duct. 

The SO2 is adsorbed, neutralized, and/or oxidized by the alkaline sorbent into a solid compound, 

either calcium or sodium sulfite or sulfate. The solid is removed from the flue gas stream using 

downstream equipment. 

FGD Systems 

For a typical coal-fired power plant, FGD systems will remove ~90% or more of the SO2

 in the 

flue gas. According to EPA, approximately 85% of the FGD systems installed in the United 

States are wet, 12% are SDA systems, and 3% are dry injection systems [4]. The following is a 

brief description of wet and dry FGD systems. 

Wet Calcium-Based FGD Systems 

In a wet FGD system, flue gas is ducted to a spray chamber/vessel (absorber) where an aqueous 

solution of sorbent is injected into the flue gas. The most popular type of sorbent used is 

limestone, but lime can also be used. A portion of the water in the solution is evaporated and the 

waste gas stream becomes saturated with water vapor. SO2 dissolves into the solution droplets 

where it reacts with the alkaline particulates. The resulting wet FGD material falls to the bottom 

of the spray chamber/vessel, where it is collected [4]. 

Wet FGD systems are the most popular technology to control SO2 emissions because they use a 

widely available and inexpensive sorbent (limestone), can produce FGD gypsum (a usable 

product) when using forced oxidation, are reliable, and can achieve efficiency up to 99% [1]. 

Several types of wet FGD processes are available and include limestone-forced oxidation, 

limestone with natural or inhibited oxidation, lime with or without buffers, lime dual alkali, 

magnesium-promoted lime, a seawater process, a sodium-scrubbing process, and ammonia 

scrubbing [1, 5]. 

Dry and Semidry FGD Systems 

There are four types of dry/semidry FGD systems: 1) SDA (semidry) systems; 2) duct sorbent 

injection (DSI) (dry); 3) furnace sorbent injection (FSI) or limestone injection multistage burner 

(LIMB) (dry); and 4) fluidized-bed combustion (FBC) (dry). From 1980 to 1992, 7200 

megawatts (MW) of dry FGD systems were installed at electric utilities in the United States and 

have mainly been applied to units burning low-sulfur coals. Of the 43 electric utility dry FGD 

system installations in the United States, the majority lie west of the Mississippi River [5, 6]. The 

technology is also used in western Europe. 
Introduction 

1-5 

Dry FGD systems like DSI, FSI, and LIMB inject powdered sorbent directly into the furnace, 

economizer, or downstream ductwork. The resulting dry product is removed using particulate 

control equipment such as an electrostatic precipitator (ESP) or baghouse/fabric filter. The flue 

gas is generally cooled prior to entering the particulate control device. Water can be injected 

upstream of the absorber to enhance SO2

 removal [4]. Atmospheric fluidized beds use a sorbent 

such as limestone or dolomite to capture sulfur released by the combustion of coal. Jets of air 

suspend the mixture of sorbent and burning coal during combustion, converting the mixture into 

a suspension of red-hot particles that flow like a fluid [7]. 

The SDA, or semidry, process consists of four operations: sorbent preparation, the spray dryer 

absorber (also referred to as a chamber or vessel), particulate collection, and product 

management. An alkaline sorbent is delivered to the power plant in covered railroad cars, trucks, 

or river barges and then stored in a suitable container to which is it usually pneumatically 

conveyed. The most popular sorbent is lime (calcium oxide, CaO precalcined) or calcium 

hydroxide (hydrated lime, Ca(OH)2), but a select number of systems use sodium carbonate. The 

sorbent is mixed with water, then classified to prevent any large grit particles from going to the 

spray dryer absorber, where they can cause orifice plugging in the spray nozzles or rotary 

atomizer. The resulting aqueous slurry is sprayed into the hot flue gas in the spray dryer absorber 

in a cloud of fine droplets. The residence time is sufficient to allow the SO2 and other acid gases 

such as sulfur trioxide (SO3) and hydrogen chloride (HCl) to react simultaneously with the 

sorbent and for the water to evaporate. A diagram of the SDA process is shown in Figure 1-2. In 

this figure, a fly ash precollection ESP is shown as an option. In Europe, fly ash precollection is 

common for SDA systems, allowing the fly ash stream to be utilized without being mixed with 

the SDA material. In the United States, it is not common for the fly ash to be precollected, so the 

fly ash and SDA sorbent combine and pass into the particulate control device as shown in the 

figure. It is also typical that a portion of the SDA material is recycled back into the SDA, also 

indicated in the figure. 

The resulting material is a dry powder product, which is a calcium sulfite (CaSO3 · 1⁄2 H2O, or 

hannebachite)rich material, referred to as SDA material in this report. A small portion of the dry 

product is collected at the bottom of the absorber, but the bulk of the material is collected in 

either an ESP or a fabric filter/baghouse. The SDA material may contain up to 75% fly ash by 

mass, depending on the location of the SDA installation. The distribution of materials collected 

from the bottom of the absorber and from the particulate control device (ESP or fabric 

filter/baghouse) in a typical SDA operation is given in Table 1-1 [8]. Since there is a certain 

amount of unreacted lime in the SDA material, most SDA systems recycle part of their products 

(SDA material and fly ash) back into the feed slurry to increase lime utilization and take 

advantage of the inherent alkalinity of some fly ashes [9, 10, 11]. 
Introduction 

1-6 

B&W Proprietary & Confidential Dynegy / S&L 1/10/06 6 2006 Western Fuels Symposium 6

H2O H2O

CaO

SDA

Recycle 

Solids

Disposal / 

Utilization

BH / ESP

SDA Process

ESP

Fly ash

Pre-Collection

Figure 1-2 

Diagram of the SDA Process Showing the Fly Ash Precollection Option and the Solids 

Recycle [2] 

Table 1-1 

Typical SDA Operation Material Distribution [8] 

Material Bottom of Drying 

Chamber (%) 

Particulate Control 

Device (%) 

Fly Ash 70 72 

Calcium Sulfite 13 15 

Calcium Sulfate 7 9 

Unreacted Lime 5 1 

Water 5 3 
Introduction 

1-7 

The advantages of SDA systems over wet scrubbing include: 

1. Less costly construction materials typically made of mild steel, thus lower capital costs. 

2. Dry products that do not require the use of expensive handling equipment or a wastewater 

stream. 

3. Fewer unit operations requiring less space, making SDA a good choice for retrofit. 

4. Flexibility of the feed system, allowing immediate feed control of sorbent to follow boiler 

load. 

5. High reliability. 

6. Less sensitive and simpler process chemistry [5, 9, 12, 13]. 

7. Removal of SO3

 from flue gas. 

Perhaps the greatest disadvantage of SDA systems is the higher cost of lime sorbents used in 

relation to the limestone used for wet scrubbing [9, 12, 13]. In addition, SDA systems produce a 

product that is difficult to sell and is often disposed of. SDA material is typically disposed of in a 

manner similar to fly ash. 

SDA systems are the second most popular FGD technology. SDA systems are used mostly for 

relatively small-to-medium-capacity boilers (40–500 MW) that burn low- to medium-sulfur 

coals. Currently, there are 26 SDA units in operation on coal-fired power plants in the United 

States. These units are shown in Table 1-2. SDA units in the United States were previously 

reported by Beidleman and Hilbert [14] and Soud [15]; however, multiple current sources were 

used to develop this updated table.
1-8 

Table 1-2 SDA Units in the United Statesa

Nameplate 

Capacity 

(MW) Utility Plant and Unit with FGD 

System 

City State 

By Plant By FGD Unit 

In-Service 

Date 

Design Coal 

Sulfur (wt%) Sorbent 

Antelope 

Valley 

Unit 1 

Beulah ND 900 450 1984 1.20 Lime 

Antelope 

Valley 

Unit 2 

Beulah ND 900 450 1986 1.20 Lime 

Basic Electric Power Cooperative 

Laramie River 

Station Unit 3 Wheatland WY 1650 550 1982 0.50 Lime/Alkaline Fly Ash 

Duke Energy East Bend Unit 2 Rabbit Hash KY 669 669 1981 5.20 Lime East Kentucky Power Cooperative, Inc. 

H.L. Spurlock 

Unit 2 Maysville KY 1118 525 1981 3.60 Lime 

Grand River Dam Authority Coal-Fired Complex Unit 2 Chouteau OK 1010 520 1986 1.50 Lime/Alkaline Fly Ash Great River Energy Stanton Station Stanton ND 188 188 1982 0.70 Lime Kansas City Power & Light Hawthorn Station Kansas City MO 594 594 2001 0.33 Lime/Alkaline Fly Ash Marquette Board of Light and Power Shiras 3 Marquette MI 40 40 1983 0.50 Limestone Otter Tail Power Company Coyote Station Beulah ND 400 400 1981 0.80 Lime/Alkaline Fly Ash PacifiCorp Wyodak Power Plant Gillette WY 365 365 1986 0.80 Lime/Alkaline Fly Ash Platte River Power Authority 

Rawhide 

Energy 

Station 

Wellington CO 294 294 1984 0.30 Lime/Alkaline Fly Ash 

Sierra Pacific Power CompanySC Valmy Unit 2 Valmy NV 521 267 1985 0.50 Lime 
Table 1-2 (continued) SDA Units in the United Statesa 

Nameplate 

Capacity 

(MW) Utility Plant and Unit with FGD 

System 

City State 

By Plant By FGD Unit 

In-Service 

Date 

Design Coal 

Sulfur (wt%) Sorbent 

South Carolina Electric & Gas Company Cope Station Cope SC 430 430 1996 1.90 Lime Tri-State Generation & Transmission Association 

Craig 

Unit 3 Craig CO 1274 446 1984 0.70 Lime 

Sunflower Electric Power Corporation 

Holcomb 

Unit 1 Holcomb KS 360 360 1983 1.00 Lime/Alkaline Fly Ash 

Springerville 

Unit 1 Springerville AZ 1560 380 1985 0.70 Lime/Alkaline Fly Ash 

Springerville 

Unit 2 Springerville AZ 1560 380 1985 0.70 Lime/Alkaline Fly Ash Tucson Electric Power Company 

Springerville 

Unit 3 Springerville AZ 1560 400 2006 0.70 Lime/Alkaline Fly Ash 

Cherokee 

Unit 3 Denver CO 715 151 — 0.40 Lime/Alkaline Fly Ash 

Cherokee 

Unit 4 Denver CO 715 351 2003 0.40 Lime/Alkaline Fly Ash 

Hayden 

Unit 1 Hayden CO 446 184 1998 0.40 Lime/Alkaline Fly Ash 

Hayden 

Unit 2 Hayden CO 446 262 1999 0.40 Lime/Alkaline Fly Ash 

Xcel Energy 

Sherco Unit 

3 Becker MN 2400 900 1987 0.90 Lime/Alkaline Fly Ash 

aMultiple current sources were used to develop this update table. bLignite. cSubbituminous. dBituminous 
Introduction 

1-10 

In nine European countries, 49 dry FGD plants were reported [16]. The plants are categorized by 

country in Table 1-3. 

Table 1-3 

Dry FGD Plants in Europe [16, 17] 

Country Sites with Dry 

FGD Plants 

Dry FGD 

Production 

(approx.) 

(short tons/year) 

Year 

Reported

% of FGD Units 

Using SDA 

Process 

Austria 4 50,000 1995 41.7 in 1993 

Denmark 2 83,200 1997 37.3 in 1993 

Czech Republic 8 140,000 1998 NRa

Finland 4 32,000 1999 NR 

Germany 20 378,000 1996 7.0 in 1993 

Italy 1 10,000 2000 NR 

Poland 5 500,000 – 600,000 2000 NR 

Spain 1 5,000 1999 NR 

Sweden 4 10,000 1999 100 

a

Not reported. 

Projected Future SDA Installations in the United States 

The Babcock & Wilcox Company tracks projected future SDA installations in the United States 

(see Table 1-4) [18]. Details indicating new capacity versus retrofit installations, generating unit 

size, coal type, geographic location, and reagent system are provided in Table 1-5. The numbers 

of projected SDA installations shown in Tables 1-4 and 1-5 are planned projects that have been 

announced, but many factors could change these planned installations. Additionally, installations 

of circulating dry scrubber-type systems, producing a product expected to be similar to SDA 

material, are projected to increase. 

Table 1-4 

U.S. SDA Market Projections [18] 

Time Horizon 

1 – 5 Years (2007 – 2012) 5 – 10 Years (2013 – 2017) 

Projected SDA 

Installations # Gen. Units Gen. Capacity (MW) # Gen. Units Gen. Capacity (MW) 

New Capacity 14 7,050 3 1,850 

Retrofits 28 11,350 16 7,400 
Introduction 

1-11 

Table 1-5 

Details of U.S. SDA Market Projections [18] 

1 – 5 Years 

(2007 – 2012) 

5 – 10 Years 

(2013 – 2017) Time Horizon 

New Capacity Retrofits New Capacity Retrofits

Generating Unit Size (MW) 

>800 2 3 1 

500 to 800 7 7 3 6 

250 to 500 1 8 8 

100 to 250 2 7 1 

<100 2 3 

Coal Type (sulfur content, %) 

Lignite 1 

Subbituminous 13 18 3 16 

Bituminous – western 3 

Bituminous – eastern 7 

Geographic Location by EPA Region 

1 (ME, NH, VT, MA, RI, CT) 2 

2 (NY, NJ) 3 

3 (PA, WV, VA, MD, DE) 2 

4 (KY, TN, NC, SC, GA, AL, MS, FL) 2 

5 (OH, IN, IL, MI, WI) 1 10 14 

6 (AR, LA, OK, TX, NM) 3 2 1 

7 (IA, MO, KS, NE) 4 2 1 

8 (CO, UT, WY, MT, ND, SD) 4 7 

9 (CA, NV, AZ) 2 

10 (AK, WA, OR, ID) 

Reagent Stream 

Fly ash recycle 11 26 3 16 

Lime only 3 2 
Introduction 

1-12 

Current SDA Material Production and Use Rates 

ACAA reports the yearly production and use of CCPs in the United States, including statistics on 

FGD products from wet and dry systems. SDA material is reported with all other products from 

dry FGD systems, with the exception of FBC systems, in an FGD dry scrubber material category. 

The ACAA definitions of dry FGD material and FGD dry scrubber material are included in 

Figure 1-3 [19]. 

• Dry FGD material – the product that is produced from dry FGD systems and consists primarily of 

calcium sulfite, fly ash, portlandite (Ca(OH)2), and/or calcite. Lime-based sorbent system dry FGD 

material main constituents are calcium sulfite and dry fly ash, along with minor quantities of calcium 

sulfate. Sodium-based sorbent system main constituents are sodium sulfite and dry fly ash along with 

minor quantities of sodium sulfate. Dry FGD material is being used in construction, engineering, and 

agricultural applications; however, most of the material is stored in landfills. 

• FGD material dry scrubbers – the dry powdered material from dry scrubbers that is collected in a 

baghouse along with fly ash and consists of a mixture of sulfites, sulfates, and fly ash. 

Figure 1-3 

ACAA Definitions of Dry FGD Material [19] 

The production of dry scrubber FGD material has been reported as a separate category in the 

annual ACAA CCP Production and Use Survey since the 2002 statistics were released in 2003 

[20]. Since then, the reported production has varied from a low of 935,394 short tons in 2002 to a 

high of 1,829,830 short tons in 2004 [20, 21]. ACAA reported that 1,488,951 short tons of dry 

FGD material was produced in the United States in 2006. Of that, 136,639 short tons (or 9.18%) 

was beneficially used, which is the lowest percentage reported by ACAA’s survey [22]. 

Figure 1-4 illustrates the major markets for dry scrubber FGD material in the United States as 

reported by respondents to the 2006 annual ACAA CCP Production and Use Survey. 

The use application categories reported for dry scrubber FGD material in the ACAA annual 

survey since 2002 statistics have included concrete/concrete products/grout, cement/raw feed for 

clinker, flowable fill, structural fills/embankments, road base/subbase/pavement, soil 

modification/stabilization, mineral filler in asphalt, mining applications, waste 

stabilization/solidification, agriculture, aggregate, and miscellaneous/other. However, the 

categories vary annually because of variations in respondents to the annual ACAA survey and 

added SDA units or unit closures (e.g., Xcel Energy’s Riverside Unit 7 went out of service in 

2004). ACAA typically notes a utility response rate of 54%–60% for its annual Production and 

Use Survey. 
Introduction 

1-13 

Figure 1-4 

Dry Scrubber FGD Material Use in the United States In 2006. Of 1,488,951 Short Tons 

Produced, 136,639 Short Tons of Dry Scrubber FGD Material (9.18%) Was Beneficially 

Used [22] 

SDA material production and use figures are incorporated into the overall reporting category of 

dry FGD material by ACAA, and quantities of SDA material produced and used have never been 

reported separately by ACAA. Using the 2004 and 2005 production figures for dry FGD 

material, and an estimate of sodium-based dry FGD material production based on sodium-based 

sorbent use predictions for 2007, and taking into consideration the ACAA response rate, the 

authors estimated SDA annual production of approximately 3.6 million tons. Using the data in 

Table 1-2 and production figures provided by several industrial power plants, it was estimated 

that 350–400 tons of SDA material is produced for each MW of capacity. Using the 

approximation of 350–400 tons of SDA material produced per MW unit size multiplied by 

9556 MW of existing SDA units indicated that ~3.3–3.8 million tons of SDA material is likely 

produced annually in the United States. The authors estimate is significantly higher than the 

ACAA production figures indicate; however, the ACAA statistics are expected to reflect the use 

profits for dry FGD materials even though not all utilities are responding. 

The European Coal Combustion Products Association (ECOBA) reports the production and use 

of CCPs in Europe (EU 15). EU 15 refers to the fifteen countries that formed the European 

Union until the end of April 2004. These countries include the United Kingdom, France, 

Belgium, Luxembourg, the Netherlands, Germany, Italy, the Irish Republic, Denmark, Greece, 

Portugal, Spain, Sweden, Finland, and Austria. Production of SDA material in 2004 was 

463,000 short tons, down from 540,000 in 2003 [23]. Of this, 41% of SDA material produced in 

2004 was beneficially used in nonmining applications, which are broken down into general 
Introduction 

1-14 

engineering fill, flowable fill, plant nutrition, and other uses. An additional 39% of the SDA 

material produced was used for mine reclamation and restoration purposes, for a total of 80% 

SDA product utilization. Therefore, 20% was disposed of [23]. The beneficial use and mine 

reclamation and restoration applications are compiled in Figure 1-5. 

Figure 1-5 

Spray Dry Absorption Product Use in Europe (EU 15) in 2004. Of 466,278 Short Tons 

Produced, 373,684 Short Tons (80.1%) Was Beneficially Used [23] 

ECOBA lists several specific current uses for SDA material in Europe [24, 25, 26]: 

• As a component of mining mortar for stabilizing underground cavities 

• As an addition in the production of sand–lime bricks 

• In the production of cement clinker in a special clinker production method (Müller-Kühne 

Process) 

• As a sorbent in a wet FGD process in power plants 

• As a sulfur fertilizer in agriculture 
Introduction 

1-15 

In May 2004, ten Accession Countries joined the existing EU 15, resulting in an EU comprising 

25 member states. The Accession Countries were Cyprus, the Czech Republic, Estonia, Hungary, 

Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia. SDA material is produced in anthracite 

and lignite coal-fired plants in Poland. From anthracite, 93,304 thousand short tons of SDA 

material was produced, with 98% utilized in mining and beneficial landfill/land reclamation in 

2000. In addition, 1,134,240 thousand short tons of fly ash + SDA material was produced, with a 

100% utilization rate. Applications included building materials, bricks and ceramics, roads, and 

mining. From lignite, 424,389 thousand short tons of fly ash + SDA material was produced, with 

100% utilization in beneficial landfill/land reclamation [27]. 

Projected Future SDA Production in the United States 

Based on the current production figures, estimated production rate, and MW of SDA scrubbed 

units in the United States, it is estimated that up to 7 million tons of additional SDA material 

could be produced from SDA units installed in the next 5 years, making the total annual 

production as high as 10 million tons, nearly 300% of the current total, by 2012. With an 

estimated potential additional 9250 MW of SDA scrubbing added between 2012 and 2017, it is 

estimated that up to an additional 3.5 million tons of SDA material could be generated annually 

in that time period. If all the projected SDA units are installed as noted in Tables 1-4 and 1-5, 

and no existing units are taken off line, the total SDA material production could reach 

13–14 million tons annually in 10 years. This estimate reflects an increase of 400%–500%. 

2-1 

2

RESULTS AND DISCUSSION OF THE LITERATURE 

REVIEW 

Technical reports, journal articles, patents, conference proceedings, book chapters, and news 

articles were assembled and reviewed to prepare this report. A multitiered search approach was 

used to obtain literature including sources that were the most readily available and those that 

required a more in-depth search. The search resulted in a significant number of documents 

containing references to dry FGD materials including SDA materials. A tiered approach was also 

used for the review of the literature obtained. Researchers used knowledge about these materials 

to choose the documents for review. The literature reviewed included information on the 

characteristics of SDA material and commercial and potential utilization applications for SDA 

material. An effort is under way to add all of the gathered references to the EERC FIRST 

SEARCH online database located at www.undeerc.org/carrc/firstsearch/. 

Definitions of SDA Material 

ASTM International [28] and ACAA [19] have developed and published terminology and 

definitions related to CCPs including FGD materials. These terms and definitions do not 

currently include SDA system or material definitions; however, the definition of dry FGD 

material provided by both organizations is inclusive of SDA material that contains fly ash. The 

ACAA definition of dry FGD material (see Figure 1-3) includes materials that would be 

produced in dry FGD systems utilizing sodium-based sorbents, but the ASTM International 

definition (see Figure 2-1) includes only calcium-based materials. SDA systems utilize lime 

slurry, so these materials are calcium-based materials. Strictly applying the ASTM International 

and ACAA definitions for dry FGD material (which includes fly ash as a component of the 

material), material from an SDA system using fly ash precollection would not be defined as a dry 

FGD material; however, the term SDA material is used here to refer to materials that do and do 

not contain a fly ash component to be consistent with the literature reviewed. 

• Dry FGD material – the product that is produced from dry FGD systems and consists primarily of 

calcium sulfite, fly ash, portlandite (Ca(OH)2), and/or calcite. ASTM equivalent terms are dry FGD ash 

and lime spray drier ash. 

• FGD material dry scrubbers – the dry powdered material from dry scrubbers that is collected in a 

baghouse along with fly ash and consists of a mixture of sulfites, sulfates, and fly ash. 

Figure 2-1 

ASTM International Definitions [28] 
Results and Discussion of the Literature Review 

2-2 

Characterization of SDA Material 

SDA materials can vary widely in their physical, chemical, and mineralogical properties, 

depending on their source. The following factors affect both the quantities and characteristics of 

SDA material: 

• Composition of the coal feedstock (ash content, sulfur content, heating value) 

• Combustion conditions 

• Sorbent type 

• SO2

 uptake efficiency (Ca/S ratio) 

• Fly ash collection location and efficiency 

• Composition and mineralogy of the fly ash 

• Recirculation rate 

• Load level 

• Stoichiometric (sorbent) ratio 

Regardless of the type of process used to scrub the flue gas, all FGD products include spent 

sorbent as sulfites or sulfates plus unreacted sorbent. The quantity of the sorbent used is usually 

proportional to the sulfur content of the coal burned but is also a result of the percent of SOx

recovery desired and system operating parameters [29]. The calcium sulfite content is dependent 

on the SO2 removal efficiency [17]. 

Because fly ash commonly makes up a large proportion of the SDA material (40%–75%), the 

overall physical properties and morphology of most SDA materials are similar to fly ash; 

therefore, handling properties are similar to fly ash. Most U.S. systems collect the fly ash and 

SDA material together in an ESP or baghouse/fabric filter, producing spherical glassy fly ash 

particles coated by and intermixed with fine crystals of calcium/sulfur reaction products [10, 12]. 

The fly ash particles provide reaction sites for the removal of sulfur and, consequently, become 

coated with reaction products. The result is a dry, free-flowing powder with particles smaller and 

finer than fly ash [10]. However, if the SDA material is collected as a separate product stream, 

then it will appear as a fine, dusty powder with an off-white color. The major difference between 

SDA material and conventional fly ash can be attributed to the higher calcium and sulfur content 

of SDA material. SDA materials are finer and more caustic, have a higher heat of hydration, and 

produce more alkaline leachate when compared to conventional fly ash [13]. 

Dry FGD materials from plants that used different types of boilers and burned different types of 

coal were collected, and it was found that the products had a moderate to wide range of 

variability in their chemical, physical, and mineralogical properties [30]. The amount of unused 

sorbent (portlandite, Ca(OH)2) varied the most, by a factor of 14. This variation depended on 

whether or not the plant recycled its products. This example shows that the physical and 

chemical properties of dry FGD materials can be highly variable from plant to plant and that an 

analysis of the material should be conducted prior to beneficial use, particularly when the 

material has the potential to expand or will contact water. 
Results and Discussion of the Literature Review 

2-3 

The following is an analysis of the physical properties, chemical characteristics, and mineralogy 

of SDA material, based on the literature reviewed. It is important to note that the properties and 

characteristics of the SDA materials are affected by varying amounts of fly ash present, which 

was not always reported. 

Physical Properties 

The physical properties of SDA material are important because they affect storage, handling, 

disposal, and engineering uses. Uniformity of the material and the ability to pack tightly are 

especially important for some applications. 

SDA materials are generally free-flowing to average-flowing, but higher moisture content can 

affect flowability and storage problems. Both qualitative and quantitative physical properties can 

affect transfer and transport. The quantitative physical properties that can affect transfer and 

transport include particle size, bulk density, specific gravity, and temperature. Disposal is 

affected by optimum moisture and maximum density, unconfined compressive strength, and 

permeability [12]. 

Qualitative Physical Properties 

Qualitative physical properties of SDA material include abrasiveness, hygroscopicity, tackiness, 

corrosivity, and tendency to aerate. The abrasive nature of SDA material has been demonstrated 

through reported wear on handling equipment. SDA material has also been found to have 

hygroscopic tendencies due to potential hydration reactions of the lime, calcium sulfate, and 

calcium sulfite fractions of the material. Evidence of its tacky nature, if it does absorb water, has 

been demonstrated by solids buildup at the elbows of pneumatic equipment. If wetted, it can 

stick and build up on mechanical conveyors. When wet, a moderate corrosivity exists. In 

addition, it has the tendency to aerate and retain air. This makes it easy to convey pneumatically 

but limits the speed at which it can be conveyed [12, 13]. 

Particle Size 

The particle-size distribution of a material is characterized by the proportion of particle sizes 

within a series of specific size intervals. Particle-size distribution is important because many 

engineering parameters are related to the variation of particle size of a material. The fineness of 

CCPs is also an important characteristic because high-surface-area CCPs are generally more 

chemically activated which, in turn, may be of importance in evaluating utilization options [31]. 

The exposed surface area of a given volume will be greater for the smaller CCP particles. 

The particle size of SDA material is very fine and tends to be smaller than conventional fly ash. 

Most SDA materials can be classified as in the silt size range (1/256–1/16 mm, or 3.9–62.5 μm). 

SDA material particle-size distributions are relatively uniform [13]. Particle sizes depend on both 

combustion and collection system designs [32]. 
Results and Discussion of the Literature Review 

2-4 

The particle-size distribution and/or mean mass/particle size of SDA materials have been 

reported in numerous publications. A compilation is shown in Table 2-1. The reported particlesize distribution ranged from 1 to 400 μm, with 1–70 μm more typical. The reported mean 

mass/particle size range is 1–45 μm. 

Table 2-1 

Particle-Size Distribution and Mean Mass/Particle Size of SDA Material Reported in 

Literature Reviewed, μm 

Particle-Size Distribution Mean Mass/Particle Size Source (s) 

NR 16–45 [2, 30] 

NR 13.5 [33] 

20–40 NR [1] 

5–60a 10–15a [16] 

2–74 NR [29] 

1–70 NR [31,34b] 

NR 4–30 [17] 

NR 2–30 [10] 

1–400 7.4–18.0c [35b] 

NR 20–28 [12b] 

18–60 24–29 [13] 

NR 20–25c [36] 

a

 Precollection of fly ash; 3%–10% fly ash content. 

b

 Literature review reported within source. 

c

 Reported as mean diameter. 

Conventional fly ash mean mass/particle size has been reported as 35–55 μm [10], 25–40 μm 

[36], and a mean of 45 μm at one plant [12, 13]. 

It was reported that 78%–80% of the SDA material was in the fraction finer than 40 μm, while 

57%–78% of conventional fly ash was in this finer fraction [12, 13]. Similarly, four SDA 

material samples tested had greater than 80% of particles by weight finer than 25 μm, indicating 

very fine particles [37, 38]. Kolar reported that 85%–100% of the SDA material was less than 60 

μm, where 60 μm was the maximum particle size observed with precollection of fly ash [17]. It 

was reported that the SDA material from Basin Electric Power Cooperative pilot plant was a 

very fine-grain, powdery material, similar in particle size to the fly ash normally produced at a 

coal-fired power plant [39]. 
Results and Discussion of the Literature Review 

2-5 

Early work performed for a University of Tennessee thesis was summarized in another report 

[40]. The SDA samples tested contained a much smaller percentage of particles less that 2.5 μm 

in diameter than conventional fly ash. The particle-size distribution analysis also showed that the 

highest percentage of particles in the SDA material was found to be in the range of 4–10 μm in 

diameter. The thesis was not available to researchers for review; therefore, the data could not be 

reviewed in complete context. 

Specific Surface Area 

The specific surface area of FGD samples tested by the Ohio State University (OSU) 

corresponded with particle size, indicating mostly nonporous materials [37, 38]. Specific surface 

area ranges are a function of the fly ash content in the SDA material [17]. The specific surface 

area of SDA materials reported in the reviewed literature (0.2–16 m2

/g) is shown in Table 2-2. 

Table 2-2 

Specific Surface Area of SDA Material Reported in Literature Reviewed, m2

/g 

Specific Surface Area Source (s) 

1.64 – 7.47 [2, 30] 

9.49 ± 3.82 [37] 

~9.4 [38] 

1.6 – 7.5 [34a] 

0.2 – 3.39 [35a] 

1.5 – 16 [17] 

a

 Literature review reported within source. 

Bulk Density 

Density is defined as the mass (or weight) per unit volume of a material. The reviewed literature 

had several means of reporting bulk density as compiled in Tables 2-3 and 2-4. Included with 

bulk density ranges are aerated, poured, packed, and tapped bulk density ranges. Bulk density 

measurements are used for different purposes, depending on the intended use of the material. 

Overall, the range of reported bulk density measurements was 400–1760 kg/m3

 (25–110 lb/ft3). 

The fly ash in dry FGD materials has similar particle size, particle density, and morphology to 

those of conventional fly ashes, but dry FGD materials have lower bulk densities [30]. The 

difference in bulk density is due to variations in the chemical and mineralogical characteristics of 

the reacted and unreacted sorbent. The range in bulk density has been attributed to the fly ash 

content [17]. Contrary to Dawson et al. [30], Klimek et al. indicated that the bulk density of SDA 

material is higher than the bulk density of conventional fly ash because SDA material has a 

larger fine fraction (78%–80% finer than 40 μm for SDA material vs. 57%–78% for fly ash) [13]. 
Results and Discussion of the Literature Review 

2-6 

The bulk density of fly ash has been reported as 1000 kg/m3 (62 lb/ft3) [41, 42]. The loose, 

tapped, and compacted bulk densities were similar for fly ash and SDA material containing fly 

ash ranging from 30–90 lb/ft3 (480–1440 kg/m3) overall for the conventional fly ash [36]. 

Table 2-3 

Bulk Density of SDA Material Reported in Literature Reviewed 

Bulk Density Source(s) 

630 kg/m3 (39 lb/ft3) [38] 

780–1250 kg/m3 (49–78 lb/ft3) [32] 

960–1440 kg/m3 (60–90 lb/ft3)a [11] 

700 kg/m3 (44 lb/ft3)b [16] 

400–1100 kg/m3 (25–69 lb/ft3) [17] 

600 kg/m3 (37 lb/ft3)b, c [41, 42] 

a

Reported as density. 

b

Precollection of fly ash; up to 20% fly ash content. 

c

Small portion of fly ash. 

Table 2-4 

Bulk Density of SDA Material Specified as Aerated or Poured or Loose; Packed or 

Compacted; and Tapped or Settled Reported in the Literature Reviewed 

Aerated or Poured or 

Loose Bulk Density 

Packed or Compacted 

Bulk Density 

Tapped or Settled 

Bulk Density Source(s)

480–960 kg/m3 (30–60 lb/ft3) NR 784–1250 kg/m3 (49–78 lb/ft3) [2] 

580–960 kg/m3 (36–60 lb/ft3) NR 720–1250 kg/m3 (45–78 lb/ft3) [34a] 

585–962 kg/m3 (37–60 lb/ft3) 784–1250 kg/m3 (49–78 lb/ft3) 780–1250 kg/m3 (49–78 lb/ft3) [30] 

580–790 kg/m3 (37–50 lb/ft3) 780–1020 kg/m3 (49–64 lb/ft3)

b

 NR [12, 13] 

550–680 kg/m3 (34–42 lb/ft3) 710–760 kg/m3 (44–47 lb/ft3) NR [13a] 

480–1040 kg/m3 (30–65 lb/ft3) 730–1680 kg/m3 (45.5–105 lb/ft3) 640–1040 kg/m3 (40–65 lb/ft3) [35a] 

980–1460 kg/m3 (61–91 lb/ft3) NR NR [31] 

480–640 kg/m3 (30–40 lb/ft3) 1280–1760 kg/m3 (80–110 lb/ft3) 720–1040 kg/m3 (45–65 lb/ft3) [36] 

a

 Literature review reported within source. 

b

 Refers to packed (tapped) bulk density. 
Results and Discussion of the Literature Review 

2-7 

Specific Gravity 

Specific gravity is defined as the ratio of weight in air of a given volume of solids at a stated 

temperature to the weight in air of an equal volume of distilled water at the same temperature 

(usually 20°C). Specific gravity is often used as a method of comparison for engineering 

materials. This differs from bulk density because only the solid fraction of the material is used; 

the void fraction is not considered. 

Specific gravities reported in the reviewed literature as shown in Table 2-5 range from 2.088 to 

2.84. A specific gravity range of 1.5 to 3.1 was reported for fly ash in a previous literature review 

[31]. 

Table 2-5 

Specific Gravity of SDA Material Reported in Literature Reviewed 

Specific Gravity Source(s) 

2.29 – 2.80 [2, 30] 

2.088 – 2.560 [33] 

2.3 – 2.8 [34a

] 

2.29 – 2.80b [12, 13] 

2.48 – 2.84 [31, 39] 

2.50 – 2.71c [41] 

a

Literature review reported within source. 

b

Used ASTM International D854 (Standard 

Test Methods for Specific Gravity of Soil 

Solids by Water Pycnometer). 

c

Precollection of fly ash; up to 20% fly ash content. 

Early work performed for a University of Tennessee thesis as summarized in another report 

indicated that average specific gravities of lignite-fired calcium-based SDA material increased as 

a function of unreacted sorbent content [40]. The specific gravity of compacted fly ash was 2.52 

and for compacted SDA material was 3.14–3.71. The thesis was not available to researchers for 

review; therefore, the inconsistency of the data from other available data could not be evaluated. 
Results and Discussion of the Literature Review 

2-8 

Optimum Moisture Content and Maximum Density 

The moisture content of a CCP is a measure of the amount of water present in the voids in the 

CCP and is expressed as a weight percentage of total dry weight. The natural moisture content is 

a function of the deposition environment of the CCP and must be determined experimentally for 

each individual CCP. The natural moisture content of a CCP must be known to calculate the 

quantity of water that must be added or removed to bring the CCP to its optimum moisture 

content for compaction. 

The optimum moisture content of a CCP is related to the maximum density obtained by 

compaction in the laboratory. The values of moisture content versus dry density are plotted to 

form a compaction curve. As indicated by the curve, density is dependent on moisture content. 

The highest point on the compaction curve corresponds to the maximum dry density and 

optimum moisture content [31]. Calculations to determine optimum moisture content do not 

generally allow for the water that will be consumed by the formation of secondary hydrated 

phases such as ettringite in the use application. Many of these phases contain up to 50% moisture 

or even higher. The rate of water addition used by engineers in these calculations can lead to 

incomplete formation of cementitious phases and later expansion caused by delayed ettringite 

formation. As shown in Table 2-6, the optimum moisture content range reported in the reviewed 

literature was 10%–63% and the maximum dry density reported in the reviewed literature ranged 

from 790 to 1860 kg/m3

 (49–116 lb/ft3). 

Table 2-6 

Optimum Moisture Content and Maximum Dry Density of SDA Material Reported in 

Literature Reviewed 

Optimum Moisture Content Maximum Dry Density Source(s) 

40% – 68%a 833 – 1056 kg/m3 (52 – 66 lb/ft3)

b [38] 

28% – 63% 790 – 1300 kg/m3 (49 – 81 lb/ft3) [43] 

16% – 60% 880 – 1630 kg/m3 (55 – 102 lb/ft3) [34c

] 

16% – 38% 1140 – 1670 kg/m3 (71 – 104 lb/ft3) [13, 30] 

10% – 54% 977 – 1860 kg/m3 (61 – 116 lb/ft3) [35c

] 

18% – 54% 977 – 1630 kg/m3 (61 – 102 lb/ft3) [12, 13c

] 

18% – 54% NR [31] 

19% 1610 kg/m3 (101 lb/ft3) [44] 

18% – 54% 980 – 1460 kg/m3 (61 – 91 lb/ft3) [40] 

30% – 32%d 1240 – 1350 kg/m3 (77 – 84 lb/ft3)

d [41] 

a Weight of water to weight of solids. 

b Optimum density instead of maximum density. 

c

 Literature review reported within source. 

d

 Precollection of fly ash; up to 20% fly ash content. 
Results and Discussion of the Literature Review 

2-9 

Observations in the literature reviewed include the following: 

• Results indicated that the optimum moisture content of the materials increased and the 

maximum dry density of the compacted material decreased with an increase in the unreacted 

lime content of the material [40]. 

• SDA materials had optimum moisture contents higher than the corresponding fly ashes and 

increased with increasing sulfur content. In contrast, density values at optimum moisture 

decreased as the sulfur content increased [10]. 

• Data show that the maximum density is not particularly sensitive to slight variations in the 

water content in the vicinity of the optimum. Most FGD products can be considered 

lightweight materials with compacted densities lower than those of a typical natural soil. 

Lightweight fills impose smaller loads on the natural soils upon which they are placed, 

resulting in less settlement in the underlying soils and less likelihood that the soils will fail 

[38]. 

• Optimum moisture contents of 18%–54%, with an average of 28%, were reported in a 

previous literature review. Within that same report, the project samples yielded optimum 

moisture contents of 16%–38%, with an average of 28.5%. These moisture contents represent 

the amount of moisture added to the dry material even though common practices base the 

optimum value on moisture content determinations performed after compaction. Moisture 

content determinations performed during this study used a drying temperature of 110°C, 

causing hydrated water to be included in the determination. It requires a temperature of over 

205°C to dehydrate calcium sulfate completely [13]. 

The natural moisture content of SDA material has been reported at 1%–5% [31] and with a range 

of <0.1%–13.2% in previously reviewed literature [35], while the residual moisture of SDA 

material has been reported to be about 2% [45]. The values reported for optimum moisture 

content for compaction indicate that a substantial amount of moisture should be added to SDA 

materials in order to obtain maximum density [12]. 

Unconfined Compressive Strength 

Unconfined compressive strength is usually determined using ASTM International D2166, 

Unconfined Compressive Strength of Cohesive Soil, or ASTM International D1633, 

Compressive Strength of Molded Soil–Cement Cylinders. The two procedures are similar except 

that ASTM International D6133 assumes there is no deformation of the sample during 

compression and uses its original dimension to calculate unit compressive strength. Results 

reported in the reviewed literature are summarized in Table 2-7. 

Typically, the samples continued to gain strength between 28 and 56 days of curing. It was 

hypothesized that the most probable reason for the wide range of strengths is the difference in 

chemical composition [13]. 
Results and Discussion of the Literature Review 

2-10 

It was noted that for low-sulfur SDA materials, a false set may occur as a result of the high 

calcium and low sulfur content, resulting in disruptive expansion reactions [10]. Strengths were 

found to increase with curing time, with values being higher after 28 days than after 10 days 

[38]. The lowest unconfined compressive strength values closely resemble low-strength soil–

cement mixtures, and the highest values represent low-strength concrete [40]. 

Table 2-7 

Unconfined Compressive Strength of SDA Material Reported in Literature Reviewed, psi 

1-Day 7- Day 21- Day 28- Day 56- Day Maximum Source 

NR NR NR 51 – 88 NR NR [38] 

60a NR NR 2700 NR NR [46] 

NR 280 – 4690 NR 20 – 790 140 – 1650 NR [34] 

NR 78 – 1780 NR NR NR NR [10] 

NR NR NR NR NR 12 – 3000 [13b] 

NR 41 – 536 81 – 2250c

 50 – 1411 72 – 1775 NR [40b

] 

a

Used ASTM International C109 (Standard Test Method for Compressive Strength of Hydraulic 

Cement Mortars (Using 2-in. or [50-mm] Cube Specimens). b

Literature review reported within source. 

c

Separate sample set. 

Scanning electron microscopy (SEM) confirmed that the presence of unreacted CaO or Ca(OH)2

increases the firmness of the SDA materials. The available moisture dissolves some of the 

calcium compounds, which then recombine in a pozzolanic reaction with aluminosilicates 

contained in the fly ash, forming high-specific-volume compounds [40]. 

Permeability 

Permeability is defined as the rate of flow through a material. Permeability coefficients, typically 

reported in cm/sec, describe flow through a unit area under a unit hydraulic gradient. Hydraulic 

gradient correlates the forces causing water to flow and the forces resisting flow. A material is 

considered permeable if it has interconnected pores, cracks, or other passageways through which 

water or gas can flow. The range of permeability coefficients, or hydraulic conductivities, of fly 

ash compacted to its maximum dry density is from 10-7 to 10-4 cm/sec for bituminous fly ashes, 

3 × 10-6 to 1 × 10-5 cm/sec for subbituminous ashes, and 1 × 10-7 to 9 × 10-6 cm/sec for lignite 

ashes [31]. Permeability coefficients for SDA materials from the reviewed literature compiled in 

Table 2-8 ranged from less than 9 × 10-10 to 6.5 × 10-3 cm/sec. 
Results and Discussion of the Literature Review 

2-11 

Mineralogical and Chemical Composition 

Successful engineering applications depend heavily on the mineralogical properties of SDA 

materials. SDA material has been described as a combination of spherical glassy fly ash particles 

coated by and intermixed with fine crystals of calcium/sulfur reaction products [13]. Three SDA 

materials (with up to 20% fly ash) produced in Europe were described as dominated by fine, 

easily crushed spheres composed of finely aggregated crystals of calcium sulfite hemihydrate 

[41]. They hypothesized that the spheres were probably formed from the atomized slurry 

droplets, suggesting rapid water evaporation as the slurry meets the hot SO2-laden gases. There 

was also evidence of calcite, gypsum, clay mineral, and quartz, usually coated with a fine layer 

of sulfite. An SDA material tested consisted of 46% amorphous glassy material, 26.6% 

hannebachite, 17% mullite, 5.5% hydrated lime, and 3% quartz [47]. 

Table 2-8 

Permeability Coefficient of SDA Material Reported in Literature Reviewed, cm/sec 

Permeability Coefficient Source(s) 

10-9– 10-6 [34a

] 

9 × 10-10– 9.7 × 10-5 [34] 

3.1 × 10-9 – 2.7 × 10-7 [30] 

10-7– 6.5 × 10-3 [35] 

3.1 × 10-9– 6.8 × 10-7 [13] 

<9 × 10-10– 9.7 × 10-5 [13a

] 

10-7– 10-6 [11, 31, 40] 

a

 Literature review reported within source. 

The chemical composition of SDA material depends on the sorbent used for desulfurization, the 

proportion of fly ash collected with the FGD product, coal sulfur content, SO2

 removal, and other 

factors. Dry FGD materials contain higher concentrations of calcium and sulfur and lower 

concentrations of silicon, aluminum, and iron than fly ash. The principal reaction product of dry 

FGD is hannebachite (calcium sulfite hemihydrate): 

CaO (lime) + SO2 (g) + 1⁄2H2O = CaSO3 · 1⁄2H2O Eq. 2-1

Under more oxidizing conditions, gypsum (calcium sulfate dihydrate) may also form: 

CaO (lime) + SO2 (g) + 1⁄2O2 + 2H2O = CaSO4 · 2H20 Eq. 2-2
Results and Discussion of the Literature Review 

2-12 

Sulfite-to-sulfate ratios range from 2:1 to 3:1 [10]. Unreacted sorbent remains as portlandite 

(Ca(OH)2) in the residuals. At ambient temperature and moist conditions, calcium sulfite (CaSO3) 

will slowly oxidize to calcium sulfate (CaSO4), and calcium hydroxide will be converted to 

CaCO3 by CO2 in the air. SDA material from lime-based systems has similar chemical 

composition to stabilized wet FGD material [10]. 

The major constituents of Basin Electric Power Cooperative’s pilot SDA material were found to 

contain 14.6% CaSO3 and 8.9% CaSO4 [39]. The resulting sulfite/sulfate ratio was 1.64. 

Unreacted lime concentrations were 1.6%, and CaCO3 was 2.1%. Fly ash comprised 71.5% of 

the material, while the reaction products and unreacted sorbent were the remaining 28.5%. 

The weight ratio of fly ash to sorbent-derived material can vary between 0.7 and 8, depending on 

the composition of the coal [45] and the proportion of the fly ash that is removed before the 

scrubber. Without precollection, the fly ash proportion will be 70%–85% by weight, producing a 

spherical grain. When fly ash precollection is utilized, a filter separation efficiency of >80% will 

produce an SDA material with <30% fly ash, and very good filters (>99%) will produce contents 

as low as 1%–4% [17]. If fly ash is recovered before the SDA system, then the product consists 

mainly of irregular clusters of sulfite crystals, forming agglomerates up to 50 μm across [17, 45]. 

Examples of the composition of SDA material containing fly ash are shown in Table 2-9. 

Table 2-9 

Typical Composition of SDA Material Containing Fly Ash as Noted in the Reviewed 

Literature, wt% 

Material From LowSulfur Coal 

From LowSulfur Coal From HighSulfur Coal Five 

Samples European Coal 

Fly Ash 75 75 40 12–29 20–85 

Calcium Sulfite 

(CaSO3 · 1⁄2H2O) 16 13 38 28–44 9–47 

Calcium Sulfate 

(CaSO4)

a 6 6 15 6 1.7–17 

Unreacted Lime 

(Ca(OH)2) 2 4 5 10–29 1–15 

CaCO3 NRc NR NR 15–33 4.5–13.7 

CaCl 

(Calcium Chloride) NR NR NR NR 0.8–6.3 

Moisture (Free Water) 1 2 2 NR NR 

Source(s) [36] [10] [10] [38, 48] [49] 

a

 Mixed hydrates. 

b

 Includes some CaCO3. 
Results and Discussion of the Literature Review 

2-13 

As shown in Tables 2-10 and 2-11, precollection of fly ash does change the composition of the 

final SDA material. The impact of SO2 collection efficiency on SDA material composition is 

shown in Table 2-11. 

Chemical Analyses 

Bulk chemical compositional data of SDA materials as reported in the reviewed literature are 

compiled in Appendix A. Conventionally, major/minor components of CCPs are reported as 

oxides. A typical report may include a weight percent value for SiO2, Al2O3, Fe2O3, CaO, SO3, 

MgO, Na2O, K2O, P2O5, TiO2, BaO, MnO2, SrO, moisture content, and loss-on-ignition (LOI). 

These data are summarized in Table 2-12 for SDA material containing fly ash and in Table 2-13 

for SDA material with precollection of fly ash. 

Table 2-10 

Typical Range of Main Components of SDA Material with Precollection of Fly Ash, wt% 

Main Components Typical Rangea

 Range 

Fly Ash/Lime Inerts 3 – 10 <8 

Calcium Sulfite (CaSO3) 55 – 70 17 – 68 

Calcium Sulfate (CaSO4) 5 – 15 3.5 – 29 

Unreacted Lime (Ca[OH]2) 2 – 10 0.5 – 15 

CaCO3 5 – 15 5 – 13 

CaCl2 • n H2O 1 – 4 0.8 – 9.5b

Moisture (Free H2O) 1 – 3 NR 

Source [16] [49] 

a

 0.7%–2% sulfur coal and precollection of fly ash with efficient ESP. 

b

 Reported as CaCl. 
Results and Discussion of the Literature Review 

2-14 

Table 2-11 

Composition (wt%) of SDA Material Overall and as a Function of SO2 Separation Efficiency. 

Adapted from Kolar [17] 

SO2 Separation Efficiencya

Main Components Range with and 

Without Fly Ash 90% 85% 80% 75% 70% 

Fly Ash/Lime Inerts 0–85 15.4 17.0 19.5 21.0 22.8 

Calcium Sulfite 

(CaSO3) 15–75 33.6 35.8 39.8 40.5 41.5 

Calcium Sulfate 

(CaSO4) 2–30 5.0 5.3 5.9 6.0 6.15 

Unreacted Lime 

(Ca[OH]2) 0–25 38.1 33.4 25.3 22.4 18.8 

CaCO3 1–30 NR NR NR NR NR 

CaCl2 · 4 H2O 1–15b 4.9 5.5 6.5 7.1 7.75 

Moisture (Free H2O) 1–4 3.0 3.0 3.0 3.0 3.0 

a

Prerequisites: SO2=1600 mg/m3; fly ash=1000 mg/m3; and HCl=160 mg/m3 (all standard temperature 

and pressure, dry). 

b

Reported as CaCl2 · n H2O. 
Results and Discussion of the Literature Review 

2-15 

Table 2-12 

Summary of Bulk Chemical Composition, Reported as Oxides, of SDA Material Containing 

Fly Ash Reported in Literature Reviewed, wt% 

Parameter Range Sources 

SiO2 6 – 46 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

Al2O3 4 – 44 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

Fe2O3 1 – 44 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

CaO 0.2 – 52 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

SO3 0 – 32 [2, 14, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

MgO 0.1 – 14 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

Na2O 0.1 – 46 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

K2O 0.1 – 6.37 [2, 14, 30, 31, 33, 34, 35, 40, 44, 49, 50, 51, 52] 

P2O5 0.03 – 1.2 [2, 14, 31, 33, 35, 40, 44, 50, 51, 52] 

TiO2 0.2 – 1.19 [2, 14, 30, 33, 35, 40, 44, 49, 50, 51, 52] 

BaO 0.39 – 0.85 [44, 50] 

MnO2 0 – 0.12 [2a, 14, 44, 50] 

SrO 0.11 – 0.46 [2, 44, 50] 

Moisture <0.1 – 13.2 [31, 33, 35, 40, 44, 50, 51, 52] 

LOI or C 0.19 – 20.5 [14, 31, 35, 44, 50] 

Unaccounted 1.7 – 6.2 [40, 51, 52] 

a

 Mn3O4

When reviewing compositional data on CCPs, it is important to understand that reporting of 

major/minor components as oxides is merely a reporting convention and is not necessarily 

indicative of the actual chemical forms in the ash. One important example of this is found in the 

reported calcium oxide concentration. In fly ash or bottom ash, the calcium is usually present as 

a component of the glassy phase with other elements and is not present as primarily CaO as 

reported. In FGD materials, calcium is likely associated with sulfur and is present as calcium 

sulfite or calcium sulfate. When calcium oxide is present as lime in any CCPs and is exposed to 

water by sluicing or storage in a pond situation, that lime will be converted to calcium hydroxide 

or hydrated lime. However, the bulk compositional data are useful in evaluating CCPs for 

various use applications because of the voluminous comparative historical data, empirical 

evaluations, and comparison with other tests and standards. 
Results and Discussion of the Literature Review 

2-16 

Additional chemical parameters were reported as a weight percent of the evaluated SDA material 

in the reviewed literature. These included CaSO3, CaSO3 · 1⁄2H2O, CaSO4, CaSO4 · 2H2O, 

Table 2-13 

Summary of Bulk Chemical Composition, Reported as Oxides, of SDA Material with 

Precollection of Fly Ash Reported in Literature Reviewed, wt% 

Parameter Range Source(s) 

SiO2 1.4 – 11.1 [41a, 45, 49] 

Al2O3 0.8 – 5.4 [41a, 45, 49] 

Fe2O3 0.4 – 4.4 [41a, 45, 49] 

CaO 32.9 – 60 [41a, 45, 49] 

SO3 2 – 30 [41a, 49] 

MgO 0.4 – 1.9 [41a, 45, 49] 

Na2O <0.1 – 0.3 [41a, 49] 

K2O <0.2 – 0.6 [41a, 49] 

TiO2 0 – 0.2 [45, 49] 

LOI or C 1.3 – 2.1 [45] 

a

 May contain up to 20% fly ash content. 

CaCO3, CaCl2, SO4, organic carbon, total sulfur, free lime, hydroxide, Ca(OH)2, Cl, Ca as 

available CaO, CO2, SO2, SO3

-2, SO4-2, CO3-2, Cl-

, loss at 500°C, and loss at 900°C. Loss at 500°C 

determines free moisture and moisture of hydration; at 900°C, other compounds such as 

carbonates decompose to oxides. 

The total elemental composition of SDA materials as reported in the reviewed literature are 

compiled in Appendix B. The highly variable data for SDA material containing fly ash are 

summarized in Table 2-14. 

The trace metal content of SDA material with precollection of fly ash is, as a rule, lower than 

that of fly ash and comparable to that of soil [16]. The limited data available in the reviewed 

literature for SDA material with precollection of fly ash are summarized in Table 2-15. The 

narrow range of concentrations for each element is within the range for SDA material containing 

fly ash. 
Results and Discussion of the Literature Review 

2-17 

pH 

Kost et al. suggest that the pH of dry FGD products depends primarily on the sorbent used and 

secondarily on the FGD technology [37]. They concluded that the high pH values of most FGD 

samples were due to the presence of oxides and hydroxides of Ca and Mg. When these products 

are exposed to water and CO2, they will convert to carbonates by carbonation reactions and the 

pH will decrease. The pH values reported in the reviewed literature, shown in Table 2-16, ranged 

from 9 to 13. 

Table 2-14 

Summary of Total Elemental Composition of SDA Material Containing Fly Ash Reported in 

Literature Reviewed, ppm 

Element Range Source(s) 

Aluminum (Al) 10,000 – 230,000 [12, 13, 30, 31, 35, 37, 46, 53, 54] 

Antimony (Sb) 0.8 – 29 [12, 13, 30, 31, 35, 55, 56] 

Arsenic (As) 0.4 – 1200 [12, 13, 17, 30, 31, 35, 37, 44, 46, 47, 54, 55, 56] 

Barium (Ba) 0.76 – 12,000 [12, 13, 30, 31, 35, 44, 46, 54, 55, 56] 

Beryllium (Be) 0.7 – 63 [12, 13, 30, 31, 35, 37, 46, 54, 55, 56] 

Boron (B) <10 – 1460 [12, 13, 30, 31, 35, 37, 44, 46, 47, 50, 54, 55, 56] 

Bromide (Br) 0.3 – 21 [31, 35] 

Cadmium (Cd) 0.01 – 70 [12, 13, 17, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56] 

Calcium (Ca) 7100 – 401,000 [12, 13, 30, 35, 37, 46, 53, 54] 

Cesium (Cs) 1 – 22 [35] 

Chloride (Cl) <0.1 – 10,200 [35] 

Chromium (Cr) 3 – 1000 [12, 13, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56] 

Cobalt (Co) <0.5 – 172 [12, 13, 17, 30, 31, 35, 37, 46, 54, 55, 56] 

Copper (Cu) 3 – 655 [12, 13, 30, 31, 35, 37, 44, 46, 47, 49, 55] 

Fluoride (F) 0.4 – 1000 [31, 35] 

Iodine (I) 0.1 – 0.6 [35] 

Iron (Fe) 6300 – 367,000 [12, 13, 30, 31, 35, 37, 44, 46, 53, 54, 55, 56] 

Lead (Pb) <0.3 – 800 [12, 13, 17, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56] 

Lithium (Li) 15.1 – 530 [12, 13, 30, 35, 37, 46, 54] 

Magnesium (Mg) 3000 – 151,300 [12, 13, 30, 31, 35, 37, 46, 53, 54] 
Results and Discussion of the Literature Review 

2-18 

Table 2-14 (continued) 

Summary of Total Elemental Composition of SDA Material Containing Fly Ash Reported in 

Literature Reviewed, ppm 

Element Range Source(s) 

Manganese (Mn) 24.5 – 1432 [12, 13, 30, 31, 35, 37, 46, 47, 54, 55, 56] 

Mercury (Hg) <0.001 – 10 [12, 13, 17, 30, 31, 35, 44, 46, 49, 55, 56] 

Molybdenum (Mo) <0.018 – 514 [12, 13, 30, 31, 35, 37, 46, 47, 54, 55, 56] 

Nickel (Ni) 1.4 – 460 [12, 13, 17, 30, 31, 35, 37, 44, 46, 47, 49, 54, 55, 56] 

Phosphorus (P) 21 – 2200 [37, 46, 54] 

Potassium (K) 1600 – 9300 [12, 13, 30, 35, 37, 46, 54] 

Rubidium (Rb) 48 – 530 [35] 

Selenium (Se) <0.4 – 760 [12, 13, 17, 30, 31, 35, 37, 44, 46, 47, 50, 54, 55, 56] 

Silicon (Si) 22,000 – 157,200 [12, 13, 30, 35, 37, 46, 54] 

Silver (Ag) <0.024 – 8 [12, 13, 30, 31, 35, 44, 46, 54, 55, 56] 

Sodium (Na) 710 – 240,000 [12, 13, 30, 35, 46] 

Strontium (Sr) 30 – 13,000 [12, 13, 30, 31, 35, 37, 46, 54] 

Sulfur (S) 3000 – 170,000 [37, 53, 54] 

Thallium (Tl) 0.1 – 42 [12, 13, 17, 30, 31, 35, 46, 55, 56] 

Tin (Sn) 0.01 – 962 [12, 13, 30, 31, 35] 

Titanium (Ti) 1050 – 6700 [12, 13, 30, 35, 46] 

Uranium (U) 0.8 – 140 [12, 13, 30, 35] 

Vanadium (V) 0.4 – 950 [12, 13, 30, 31, 35, 37, 46, 44, 54, 55, 56] 

Zinc (Zn) <6 – 9000 [12, 13, 30, 31, 35, 37, 44, 46, 49, 54, 55, 56] 
Results and Discussion of the Literature Review 

2-19 

Current and Potential Uses of SDA Material 

When SDA material was first produced, there were no obvious uses for it because of the novelty 

of the process, a lack of applications for the main component calcium sulfite, the undesirable 

calcium chloride content, a wide range in chemical composition, and the low volume per location 

[17]. The literature reviewed contained numerous references to current commercial and potential 

uses of SDA material. Many of the uses have commercial potential but are still in the research 

and development phase. The following is a summary of current commercial and potential uses of 

SDA material noted in literature reviewed. It should be noted that this summary is not exhaustive 

and only represents uses reported in the reviewed literature. 

Agriculture 

It is widely know that gypsum (CaSO4

 · H2O) has been used to improve soil conditions for 

centuries. Gypsum has been shown to improve water retention characteristics, increase 

infiltration rates, reduce soil crusting, and mitigate salinity and sodicity in alkaline soils or arid 

and semiarid regions [58]. Studies on the use of FGD gypsum compare it to natural gypsum in 

terms of the benefits when applied to agriculture soil. Although FGD gypsum is widely used in 

agriculture, other FGD products, including SDA material, contain very little mineral gypsum; 

therefore, their suitability for agriculture applications is different and should not be compared to 

gypsum. ACAA reported 168,190 short tons of FGD gypsum and 846 short tons of dry FGD 

material were used in agriculture applications in 2006, down drastically from 19,259 short tons 

in 2005, possibly because of variations in respondents to the ACAA survey [22, 59]. ECOBA 

reported 36,376 short tons of SDA material used for plant nutrition in 2004 [23]. The low 

solubility of calcium sulfite (CaSO3 · 1⁄2H2O) in SDA material makes it a poor source of calcium 

and sulfur for agriculture use. The sulfites may be harmful to plants, by producing hydrogen 

sulfide gas under anaerobic conditions, unless placed far in advance of planting [60]. However, 

the substitution of alkaline sulfite-rich dry FGD products for conventional liming materials in 

agriculture is a potential use for these products, as noted in the research studies summarized 

below. It is important to note that there are instances in the literature of agricultural use of SDA 

material in Europe, but the authors focused primarily on literature from the United States. 
Results and Discussion of the Literature Review 

2-20 

Table 2-15 

Summary of Total Elemental Composition of SDA Material with Precollection of Fly Ash 

Reported in Literature Reviewed, ppm 

Element Range Source(s) 

Arsenic (As) 4.8 – 11 [41a] 

Barium (Ba) 270 – 4000 [41a] 

Boron (B) 100 – 150 [41a] 

Cadmium (Cd) 0.9 – 6.9 [41a, 49] 

Chromium (Cr) 34 – 60 [41a, 49] 

Copper (Cu) 43 – 80 [41a, 49] 

Lead (Pb) 28 – 110 [41a, 49] 

Manganese (Mn) 170 – 420 [41a]

Mercury (Hg) <0.3 [41a, 49] 

Molybdenum (Mo) 2.7 – 5.4 [41a]

Nickel (Ni) 32 – 80 [41a, 49] 

Selenium (Se) 3.9 – 6.5 [41a]

Vanadium (V) 60 – 70 [41a]

Zinc (Zn) 94 – 380 [41a, 49] 

a

 May contain up to 20% fly ash content. 

Table 2-16 

pH Values of SDA Material Reported in Literature Reviewed 

pH Source(s) 

9.7 – 12.8 [2] 

11.8 – 12.5 [37, 57] 

9.3 [47] 

9 – 13 [34a

, 41b] 

12.4 [53] 

11.7 – 12.4 [39] 

a

Literature review reported within source. 

b

Precollection of fly ash; up to 20% fly ash content. 
Results and Discussion of the Literature Review 

2-21 

Calcium sulfite phytotoxicity is a concern when applying SDA material to agriculture soils, as 

evident below in the literature cited. Strategies for reducing toxicity are also examined. 

Decreased maize growth was reported in an acid soil when levels of an FGD product containing 

CaSO3 exceeded 2 g/kg in a soil initially at pH 4.2 [61]. Toxicity from volatile sulfur compounds 

and increased aluminum toxicity were suspected, but the relative contribution of each could not 

be quantified. The SO3

-2 ion is thermodynamically unstable in the presence of oxygen. It as been 

reported that rapid CaSO3 oxidation to CaSO4 is expected in the oxidizing atmosphere of most 

agronomic soils, and additional reactions that generate SO2 may occur with CaSO3 addition to 

low-pH media [62]. The oxidation of sulfite to sulfate at near-neutral to alkaline pH values is 

extremely slow at atmospheric pressure. The fact that oxidation is relatively rapid in soils leads 

to the conclusion that other mechanisms may play a crucial role. It is likely that in an agricultural 

soil, fungi, bacteria, and microfauna may aid in the ultimate oxidation of sulfite to sulfate 

through metabolic processes. Sulfite and SO2

 both are phytotoxic, but toxicity effects may be 

temporary, subsiding when most SO3

-2 has been oxidized to SO4-2 [58]. Research by others 

indicated that raising the soil pH or allowing oxidation of sulfite to sulfate to occur are two 

effective strategies for reducing calcium sulfite phytotoxicity [63]. The toxic effects resulting 

from calcium sulfite use in soils were related to low soil pH and can be considerably diminished 

by applying an acidity-neutralizing amendment such as hydrated lime. In addition, the oxidation 

of sulfite to sulfate in water and soil systems can occur in approximately 3–5 weeks. Others also 

demonstrated that both sulfate and sulfite FGD products can be used to reduce toxicity in acidic 

subsoil layers. Although short-term phytotoxicity was observed in the sulfite-rich sample, the 

addition of small amounts achieved the goal of mitigating soil aluminum toxicity and increasing 

plant growth. This suggests that application of the sulfite-rich sample or similar materials to soils 

in autumn may improve crop growth the following growing season [58]. The lower toxicity of 

sulfite in alkaline environments is likely due to the reduced concentrations of bisulfite ions, 

which are pH-dependent and maximum at below pH 5. Bisulfite combining with atmospheric 

oxygen is the mechanism used in much of the forced oxidation found in the production of 

gypsum from sulfite scrubber material. 

The impact SDA material has on the environment and plant growth was studied by the 

University of Georgia and by Argonne National Laboratory. The University of Georgia study 

showed the incorporation of SDA material into native soils had no impact on germination rates 

of corn, soybeans, and cotton [47]. Concerning the effect on the elemental composition of plant 

tissues, arsenic, selenium, molybdenum, boron and, to some extent, calcium increased within all 

crops grown in Year 1. Tissue concentrations of manganese and sodium decreased, and nickel, 

lead, iron, and copper levels were not affected. Leachate salinity and soil salinity rose 

immediately (from 0.2 to 2.9–3.3 dS/m) after the application and remained elevated over the two 

years of the study. The leachate pH was not affected by the application of SDA material, but the 

application did produce a stable increase in soil pH (from 5.5 to 8.1). Arsenic, selenium, and 

boron tended to accumulate in the plants. Boron emerged in the leachate, indicating possible 

impacts to groundwater quality. Plant growth experiments using soil treated with SDA material 

were conducted at Argonne National Laboratory. Three different soil types and soybean and corn 

crops were tested, using a combination of SDA material with soil at 0%–4% by weight. For both 

corn and soybeans, growth decreased as the amount of SDA material increased. Soybeans grown 

in soil with 4% SDA material exhibited stunted growth when compared to the control crop. For 

corn, a similar reduction in growth was noticed, although not to the same degree as for soybeans. 

Boron concentration in the leaves of the corn and soybeans grown in soil treated with SDA 
Results and Discussion of the Literature Review 

2-22 

material were found to be up to 20 times greater than those found in the leaves of the control 

grown in soil containing no SDA material [64]. 

The use of SDA material as a liming agent has been studied. The British Coal Corporation 

analyzed the use of a European SDA material as a liming agent [41]. Pure limestone has a liming 

or neutralizing value (calcium oxide content equivalent) of 56, and the SDA material tested had a 

neutralizing value of 25. The study concluded that if an appropriate amount of the SDA material 

were added to achieve adequate liming, the recommended upper limits for trace element 

concentrations in soils after sewage sludge application would not be exceeded. The use of CCPs 

(including SDA material) as compared to CaCO3 as a liming agent in strongly acidic soils was 

studied, and it was concluded that CCPs functioned similarly to CaCO3 and that they had very 

few adverse effects on soil enzyme activities compared with those of CaCO3 [53]. In order of 

effectiveness for increasing soil pH were FBC ash > LIMB ash > SDA material > fly ash. 

Because CCPs contain CaO and MgO, the study concluded that it seems likely that the ability of 

CCPs to act as liming agents may be related to their content of these oxides. For example, the 

high initial alkalinity associated with the application of CCPs to soil was reduced by conversion 

of CaO to CaCO3 in soils exposed to atmospheric CO2. The results of this study also provide 

information on the use of soil enzyme activities as indicators of soil quality. They clearly 

demonstrate that caution is required in such use because the relationships between soil qualities 

are complex and measurements of the changes in enzyme activities without reference to proper 

controls can be misleading. 

Several commercial uses of SDA material in fertilizer applications were identified in Europe; 

however, the use of SDA material as a fertilizer has not reached the commercial stage in the 

United States. 

Many soils throughout northern Europe are experiencing a growing sulfur deficiency, and 

therefore, fertilizer companies are increasing the amount of sulfur in commercial fertilizers. The 

application of SDA material as fertilizer for sulfur deficiency is now permitted in Germany, 

Denmark, and Austria, as described below [16]. 

SDA material with low fly ash content has been used as a sulfur fertilizer since 1994 in 

Denmark, and it has been used as an admixture to and spread with liquid manure. The 

Lanskontoret for planteavl (Agricultural Research Institute) has carried out tests that indicate that 

SDA material, collected without fly ash, is suitable for use on farm soils, and the Danish 

Ministry of Environmental Protection has given permission for this application, with certain 

conditions. In commercial applications, the residues are mixed with dolomitic lime to prevent 

dusting and to allow application of the two materials simultaneously. It is necessary to apply the 

two materials together because the fertilizing potential of the SDA material is too small to make 

separate application viable. Dolomitic lime contains magnesium and acts slowly and 

continuously. The SDA material can also be mixed with liquid manure, agricultural lime, or 

wastewater sludge [65]. 
Results and Discussion of the Literature Review 

2-23 

A three-year VGB Group research project under the “Waste Management of Residues from 

Coal-Fired Power Plants and Waste Incineration Plants” (known as ERKOM) research program 

yielded results that gained approval of SDA material as a fertilizer in Germany in 1999 by the 

Scientific Advisory Board of the Federal Ministry of Agriculture [66, 67]. The minimum sulfur 

content of a fertilizer of this type is to be 10%, which is achieved by SDA material when the 

major amount of fly ash is precollected before the SO2 removal. The German Ministry of Food, 

Agriculture, and Forestry has specified SDA material as a sulfur–calcium fertilizer in legislation. 

The SDA material was also licensed in Austria based on the research results in Germany. The 

total cost of the VGB Group research project of approximately 0.5 million Deutsche Mark (DM) 

has produced annual savings for the operators in the region of 2.8 million DM [66]. 

Binders 

A novel binder for interior plasters is described in U.S. Patent 5,522,928 using a calcium sulfite 

component, water, and a finely divided residual material component comprising primarily 

calcium sulfite. SDA material is called out specifically as the calcium sulfite component [68]. 

Kolar summarizes a multiphase binder described in German Patent DE 3 82 16 57 C2 [17]. SDA 

material is mixed with fly ash, and precalcination, oxidation, and calcination are carried out. A 

salable binder is produced by adding additives and grinding. The use of the product as a flooring 

binder is considered promising, and other potential applications include use in insulating 

building materials and raw materials for double floor plates where it may be substituted for 

cement products. Binder applications have not reached commercialization in the United States. 

Cement Manufacture 

SDA material has been used commercially to manufacture cement in Germany. A German coalfired power plant treats SDA material using a fluidized-bed process. Pelletized anhydrite is 

produced that can be used as a substitute for natural anhydrite in industrial processes such as 

cement manufacture [69]. Two anhydrite production processes using SDA material are the FläktDorr-Oliver Process and the Vereinigte Aluminium-Werke AG, Lünen (VAW) Process [17]. 

Another process used dry FGD material to manufacture cement, once utilized in the MüllerKühne Process at WSZ Wolfener S.u.Z. in Bitterfeld in Germany [16]. 

U.S. Patent 4,470,850 claims that a dry FGD material (likely SDA material) can be used in the 

place of fly ash and gypsum as a solidification regulator in the production of fly ash cement [70]. 

The essential reaction product in the dry FGD material that functions as the solidification 

regulator is the calcium sulfite hemihydrate. Although this application appears to be technically 

feasible, no dry FGD material has been reported as used for cement manufacture in the United 

States since 2003 [71]. 
Results and Discussion of the Literature Review 

2-24 

Cement Replacement in Concrete 

Research has been conducted on the use of SDA material as a concrete admixture. Below is a 

summary of the research studies found in the literature reviewed, which consisted primarily of 

European sources. It is not known if the limited amount of literature pertaining to commercial or 

demonstrated use of SDA material as a concrete admixture is due to technical constraints or a 

lack of market development. ACAA reported 9660 short tons of dry scrubber FGD material used 

in concrete, concrete products, and grout in 2006 [22]. 

Research carried out at KEMA, Netherlands, has investigated the use of SDA material as a 

partial replacement for sand and cement in concrete [45, 72]. The SDA materials tested had no 

precollection of fly ash and contained a mixture of about 70% fly ash and 30% sorbent reaction 

products. For initial comparative tests, 20% of the portland cement in the concrete was 

substituted by SDA material. Except for a slight retardation in setting times, concretes in which 

cement was partly substituted by SDA material showed strength and durability performances 

comparable to or superior to reference concretes. No destructive ettringite (Ca6

Al2(SO4)3(OH)12 · 

26 H2O) reactions were observed in tests carried out with an exposure time of two years. The 

presence of chloride was noted in some SDA materials, restricting their application in reinforced 

concretes [45]. The study concluded that the compositions could be applied successfully to 

concrete products, but that further verification under real conditions was required. Long-term 

tests were still being carried out when the report was published. 

A demonstration project was carried out by the Technical University of Denmark to assess the 

use of SDA material as a cement and concrete admixture [73]. Three precast, reinforced concrete 

front elements containing 20 and 30 wt% SDA material were manufactured. Additives, such as 

superplasticizers and an air-entraining agent, were added to some specimens. After a period of 

1 year, corrosion tests indicated a low probability of corrosion for most of the specimens. The 

results suggest that when SDA material is used as an admixture in mortar to replace part or all of 

the usual fly ash, increased compressive strength is achieved. The grain size of the SDA material 

has some influence on strength. Progressive substitution of cement by SDA material will result 

in gradually decreasing strength. During this project, it was found that the mineral thaumasite 

(Ca3Si(CO3)(SO4)(OH)6 · 12H2O) formed under certain conditions. Because thaumasite greatly 

reduces the strength of cement and concrete, the project was abandoned. 

A review of studies in Europe by Kolar indicated that nonreinforced concrete might be a more 

suitable application for SDA materials than reinforced concrete [17]. Chloride poses a risk of 

corrosion of steel reinforcement. Concrete pipes produced containing SDA material in Austria 

had swelling attributed to the formation of ettringite. Prevention of this was not successful. 

Additionally, the product was not frost-resistant. It was concluded that these factors limited the 

use of SDA material-containing concretes in building to solid or hollow bricks for interior walls 

or backing. Dry storage of the bricks would also be necessary. 
Results and Discussion of the Literature Review 

2-25 

The use of dry FGD material as a raw material for cellular or gas concrete was studied in 

Denmark and Poland [16]. Cellular concrete is made of cement and/or white lime and ground 

quartz sand and steam-hardened at 180°–200°C (356°–392°F), resulting in a porous structure. 

There are over 20 factories for the production of cellular concrete in Poland, many of which use 

fly ash as a raw material. Testing has concluded that the addition of approximately 20% dry FGD 

material is positive and that it can be used for cellular concrete manufacturing. However, the 

market incentive is low, and further development work is required. Earlier work in Denmark 

used up to 30% SDA material, resulting in problems including chlorine corrosion in the 

autoclaves and thaumasite formation [17]. 

Two sources of SDA material and one source of limestone FSI product were tested for partial 

cement replacement in concrete. The products were substituted for 30 wt% Type I cement. Three 

product concrete mixtures, one fly ash concrete mixture, and one control concrete mixture were 

prepared. One SDA sample met the ASTM International C618 criteria except for SO3

 content, 

which was higher than the criterion. The other SDA material and the limestone FSI product did 

not meet ASTM International C618 criteria for silica, alumina, iron oxides, SO3, and LOI. 

However, for all samples, their fineness and pozzolanic activity indices met ASTM International 

C618 criteria. In addition, the autoclave expansion test results were within the ASTM 

International C618 limits. The concrete mixtures made with SDA material achieved a higher 

compressive strength than that of the control mixture at all testing ages (3, 7, 28, and 

90 days). The SDA mixtures had longer setting times than the LIMB mixtures and controls [34]. 

In one project, portland cement concrete incorporated with 10% SDA material was used on a test 

section of road pavement [46]. Prior to the field placement, laboratory specimens were made 

using 5% and 10% SDA material. Laboratory and field results indicated that compressive 

strengths increased with increased percent of SDA material for all test ages (7, 14, 21, and 

28 days). A comparison of the laboratory and field concrete mixes shows a similar 28-day 

compressive strength. 

Civil Engineering 

Dry FGD material, including SDA material, mixed with fly ash has been used commercially in 

civil engineering in Europe and has been a topic of study in research projects. Civil engineering 

applications including landfill construction, embankments, structural fill, and road base have 

been documented in the literature. ECOBA reported 61,729 short tons of SDA material used for 

general engineering fill in 2004 [23]. ACAA reported 249 short tons of dry scrubber FGD 

material used in the road base/subbase/pavement category but no use in the structural 

fill/embankment category in 2006 [22]. 

In Nordic countries, there are six blending stations at coal-fired power plants blending dry FGD 

material and fly ash, sometimes with the addition of cement. These blends are referred to in the 

literature generically as stabilisate product.1

 An example is Cefill (or Cefyll), produced by the 

Swedish company Cementa AB, which was developed in the 1980s and has been producing 

Cefill since at coal-fired combined heat and power (CHP) power plants in Västerås, Sweden. 

 

1

 This is similar to the process used in the United States whereby wet FGD material is mixed with fly ash to produce 

fixated scrubber solids (FSS) or Pozzotec. 
Results and Discussion of the Literature Review 

2-26 

Three qualities of Cefill are typically produced with 30%–70% dry FGD material, 30%–70% fly 

ash, and 0%–8% cement. Compressive strengths vary from 5 to 30 MPa (725–4350 psi) and 

water permeabilities vary from 10-10 to 10-12 cm/sec. A similar stabilisate product called REALIT 

is produced at coal-fired power plants in Dürnrohr, Austria [16]. 

A few commercial examples of the use of Cefill and REALIT as a capping material are provided 

by Bengtsson [16]. Cefill has frequently been used as a bottom- and top-capping material for 

hazardous waste such as metal ore mine residues and as a vertical seal for preventing horizontal 

groundwater flow contamination. Many other projects have been carried out in Sweden based on 

the tight and flexible nature of the cured Cefill material. REALIT was used to seal off a sludge 

deposit. 

The construction of road dams, filling of foundation cavities and cable trenches, and sealing of 

waste disposal sites using Cefill are applications that have been abandoned in Denmark but are 

being intensively pursued in Sweden and Finland [17]. 

In Europe, stabilisate products have been used extensively as a base course material in 

applications such as storage areas, coal yards, road banks, parking lots, and noise protection 

walls. An example is a 2-km-long road built at a coal-fired CHP station in Finland using a 1-mthick layer of a 50% dry FGD material and 50% fly ash without cement addition covered with 

0.5–1.5-m fly ash and a 0.24-m layer of crushed stone [16]. This test road has been tested 

annually for important engineering parameters. Results have been very satisfactory, showing 

high frost resistance. REALIT was used in the base liner of a squeezed sewage sludge disposal 

site in Austria [74]. 

A common European application of stabilisate products is for landscaping and land reclamation 

[16]. Examples include fill for reclamation of destroyed land and old/closed quarries, fill in an 

area behind a new quay in a harbor, and reclamation of lignite pit mines. It is common practice to 

recultivate the top surface of reclaimed land with a layer of soil for planting trees and grass. 

In addition, utilization of a stabilisate product is under consideration in the Czech Republic as 

embankment material for roads and railways. 

The fine-grain Dutch SDA material is suitable for use in the stabilized layer of road construction, 

whereas the artificially prepared grains (lumps) did not meet Dutch standard technical 

requirements [45]. However, research in Denmark has shown that problems may occur with 

swelling during application as a foundation material [65]. The expansion of several types of FGD 

materials was measured, and it was concluded that compacts containing SDA material, alone and 

in combination with fly ash, expanded by less than 0.2% after a year of exposure, with no 

evidence of structural deterioration after that time [41]. 

The use of SDA material in a truck ramp for vehicles to unload trash [57]. Difficulties in 

achieving uniform conditions during construction were experienced; however, no problems with 

performance and no evidence of failure were reported. 
Results and Discussion of the Literature Review 

2-27 

Dawson et al. evaluated literature on the potential for SDA material to be used as structural fill 

and concluded that all but the nonreactive SDA material has potential to be used as a structural 

fill [10]. For reactive SDA material, compressive strengths in excess of 2000 psi can be 

achieved, whereas low-reactive SDA material strengths greater than 100 psi can be expected. 

Swell properties of the material and the strength that can be achieved when compacted are 

factors in the application of FGD materials for engineering purposes. Swelling primarily 

occurred in FGD samples containing free lime and occurred in two episodes [38]. The first 

episode occurred almost immediately upon water addition and is attributed to hydration reactions 

such as the hydration of lime (CaO) to portlandite (Ca[OH2]) and of anhydrite (CaSO4) to 

hemihydrate (CaSO4 · 1⁄2 H2O). The second swelling episode usually began after about 10 days 

and is thought to be a result of the formation of secondary minerals such as ettringite. The 

strengths achieved varied when FGD samples were compacted at optimum water content. 

The use of SDA material for cured compacted products suitable for use as landfill materials, 

embankments, roadbase compositions, and similar applications is described in U.S. Patent 

4,354,876 [8]. It is specifically stated that the material used is obtained from a lime-based dryscrubbing FGD operation containing fly ash. The material is mixed with water and compacted 

under sufficient load to achieve at least 70% of the laboratory dry density. The resulting cured 

compacted products have compressive strengths of at least ~25 psi and permeabilities of less 

than 1 × 10-5 cm/sec. The process of the invention is designed to take full advantage of the 

unique self-bonding capabilities of the material. The SDA material may need to be conditioned 

for this use. Cementitious additives may be required for unreactive to moderately reactive SDA 

materials. 

A test road used SDA material in the pavement layers, the pavement, the subbase, and the 

embankment [46]. SDA material proved useful; especially as replacement for common earthen 

borrow material2

 in embankments. The granular and select granular borrow material met or 

exceeded Minnesota Department of Transportation (MnDOT) specifications. Environmental 

testing showed the placement had no adverse impact on groundwater quality; however, barium in 

the SDA material–common borrow mixture slightly exceeded Minnesota drinking water limits. 

Although the placement was successful from an engineering and environmental standpoint, it did 

not offer economic advantages because additional costs were incurred for mixing and 

transporting the SDA material. The SDA material might have an economical advantage if lime or 

lime kiln dust was required for borrow stabilization. 

In Kansas, SDA material (containing about 80% fly ash) has been mixed with dry economizer 

ash, moistened with water, and used to create a lining layer (~1.25 m thick) for a landfill site. 

The conditioned SDA material was spread in layers about 0.5–0.75 m thick and compacted to 

give a layer with a permeability of <10-6 cm/sec. Wet boiler slag was brought separately to the 

landfill and encapsulated in the conditioned SDA material [65]. 

 

2

 Earthen borrow is sand, gravel, or other material used for grading. 
Results and Discussion of the Literature Review 

2-28 

Flowable Fill 

Flowable fill is a material that flows like a liquid, is self-leveling, requires no compaction or 

vibration to achieve maximum density, and hardens to a predetermined strength [28]. Controlled 

low-strength material (CLSM), controlled density fill (CDF), and infill are names used to 

describe flowable fill that comprises a blend of cement, fly ash, sand, and water. ACAA and 

ECOBA have reported commercial uses for dry scrubber FGD material (9843 short tons in 2006) 

and SDA material (45,195 short tons in 2004), respectively, in flowable fill applications [22, 23]. 

Research conducted to date indicates that SDA material can be an effective component in the 

production of flowable fill. 

The potential for SDA material to be used in flowable fill as a replacement for conventional fly 

ash was examined. The design mixes consisted of varying amounts of SDA material, cement, 

lime, admixtures, and water. The mixes were tested in the laboratory for flowability, unity 

weight, moisture content, unconfined compressive strength, erodiblity, set time, penetration, and 

long-term strength characteristics. Tests were conducted for up to 90 days of curing. The study 

concluded that flowable fill containing SDA material can be an economical alternative to 

conventional materials. SDA flowable fill mixtures tested gained good strength (400 psi obtained 

in 1–2 days for standard flowable fill and 400 psi obtained in 1/3–6 hours for quick-set flowable 

fill) and had excellent placeability. The authors suggested, especially for high-cementitiouscontent flowable fill, that long-term strength tests be conducted to estimate the potential for later 

excavation. Furthermore, chemical reaction and mechanisms that accelerate initial set time need 

to be studied. Long-term strength tests for more than 1 year are needed, and full-scale field tests 

would be valuable. Resilient modulus, stress–strain behavior, freeze–thaw, swell potential, and 

corrosivity characteristics also need to be studied [75]. 

Others mixed SDA material with an additional fly ash and stabilized the mixture by adding about 

3 wt% lime kiln dust [76]. The stabilized product was either used as a flowable fill or left in a 

storage yard for several years. Samples extracted from these sites were analyzed using x-ray 

diffraction and scanning electron microscopy. The analytical results show the formation of 

thaumasite, ettringite, and an intermediate phase with varying chemical composition of calcium, 

aluminum, silicon, and sulfur. Ettringite and thaumasite are not present in CCPs but are 

secondary minerals formed from the reaction of the CCPs with water. Most of the thaumasite 

formed in the system was growing directly from the gypsum matrix. Thaumasite was also 

growing in the void space. Thaumasite grown in the system occur as short, stubby crystals. 

Ettringite crystals, on the other hand, grow in isolated pockets when the conditions of a saturated 

lime environment are available. 
Results and Discussion of the Literature Review 

2-29 

Fixating Agent for Waste 

SDA material, and other alkaline-rich FGD materials have the potential to solidify or fixate 

wastes. Alkaline FGD materials may be useful to stabilize metals in acidic hazardous wastes that 

have reduced solubilities at higher pH, such as cadmium, iron, manganese, zinc, copper, and 

cobalt [34]. Waste sludge stabilization is similar to soil and road base stabilization; however, 

since the sludges generally have a high moisture content and low solids content, more FGD 

material needs to be used to fixate waste. Research studies that have evaluated the use of SDA 

material as a fixating agent for waste are summarized here. 

The great fineness of SDA material, especially with low fly ash content, yields a high water 

retention capacity, which could be used for the thickening of sewage sludge and river or harbor 

silt. Some applications have been successful; however, negative experiences have occurred in 

Denmark because calcium hydroxide appears to cause early hardening [17]. 

In one study, different waste materials (cadmium and chromium plating precipitation sludges, 

waste oil digestion sludge, and a sedimentation slurry from an aluminum can reclamation center) 

were mixed with SDA material [77]. In all mix designs, a ridged structural material evolved as a 

result of the expansive and pozzolanic reactions occurring from the wetted SDA material. 

Structural and physical characteristics and leaching were evaluated. The cadmium plating waste–

SDA material mix exhibited a 7-day unconfined compressive strength of up to 5.221 MPa (759 

psi) and the chromium plating sludge–SDA material mix reached 7.587 MPa (1103 psi). The 

SDA material mixed with water exhibited a 7-day unconfined compressive strength up to 5.035 

MPa (762 psi). The unconfined compressive strength for weak concrete is 10.346 MPa (1504 

psi). The raw and waste materials fixated with SDA material were leached with acetic acid and 

with deionized water. The SDA material did not leach above Resource Conservation and 

Recovery Act (RCRA) regulations. The cadmium waste and aluminum can-processing waste 

heavy metal leaching characteristics were reduced from hazardous to nonhazardous by fixation 

with SDA material, as was chromium plating waste at mass ratios of 1:1, 2:1, and 25:1 (SDA 

material to dry chromium waste). The oil sludge was not affected by SDA material fixation with 

regard to inhibiting leaching. 

A study on the leachability of SDA material from Argonne National Laboratory concluded that if 

proper fixation techniques can be developed, SDA material has the potential to be used as an 

impounding agent in the codisposal of chemical wastes containing lead, cadmium, and other 

elements whose leachability decreases with increasing pH [64]. 

A laboratory study conducted at the Western Research Institute evaluated the ability of FGD 

materials to stabilize the organic and inorganic constituents of hazardous wastes. Two sources of 

atmospheric fluidized-bed combustion (AFBC) material and two sources of SDA material were 

used in this study, and four types of hazardous waste streams were obtained including separator 

sludge, mixed metal oxide–hydroxide waste, metal-plating sludge, and creosote-contaminated 

soil. Each product was mixed with each hazardous waste, allowed to equilibrate, and then 

leached using the toxicity characteristic leaching procedure (TCLP). It was found that chromium 

was leached from both SDA material-stabilized mixtures. However, the products tested can be 

used to stabilize the cadmium found in the metal oxide–hydroxide hazardous waste. Mineralogy 

tests were performed on a number of products and hazardous waste mixtures. Quartz and 
Results and Discussion of the Literature Review 

2-30 

ettringite were the most dominant mineral phases; others included gypsum, portlandite, and 

calcite [34]. 

This application has not reached commercialization in the United States. 

Marine Applications 

It has been suggested that SDA material could be used in marine applications such as artificial 

reefs and offshore sea defenses, although ACAA does not report this as a separate use 

application. The literature reviewed, and summarized here, showed positive results when reefs 

and blocks were prepared using SDA material and placed in a seawater environment. All studies 

showed that artificial reefs and blocks could provide a favorable habitat for marine life without 

adverse impacts to the environment. Tests carried out in the United Kingdom by the Coal 

Research Establishment show that blocks containing SDA material, exposed to both tidal and 

totally submerged conditions, showed no evidence of surface friability or cracking [41]. Block 

weights generally increased with time to levels compatible with water adsorption values found in 

laboratory tests. The strength of blocks containing a proportion of portland cement and fly ash 

increased compared with those containing only SDA material, but all specimens behaved 

adequately. Similarly, the Marine Sciences Research Center conducted an investigation on the 

preparation and evaluation of blocks prepared from a variety of CCPs and placed in a seawater 

environment. Testing indicated that properly designed reef blocks possessed adequate structural 

integrity for marine environments, exhibited no adverse environmental effects, and functioned 

acceptably as habitat for marine life [10, 78]. Dump blocks from SDA material and fly ash 

mixtures bonded with 7.5% portland cement were produced. It was indicated that the blocks 

could be used for artificial reef construction. The compressive strength was 5 Nm/m2

, and very 

low leaching was observed [45]. 

Masonry 

SDA material has shown promise for use as a raw material to manufacture masonry products; 

however, only one reference to the commercial manufacture of masonry was found in the 

literature. Based on results by KEMA and on further testing in Germany, considerable amounts 

of dry FGD material were used in German and Dutch sand–lime brick manufacturing [16]. 

However, the manufacturing of sand–lime bricks seems to have ceased (as of the writing of that 

report). Detailed information of the application is limited, both for competitive reasons and 

because of the risk of unfavorable publicity regarding the use of “waste products” in a highquality application. ACAA reported 9660 short tons of dry scrubber FGD material used in 

concrete, concrete products, and grout in 2006 [22]. 

International patent WO 82/00819 and U.S. Patent 4,377,414 describe a method to use SDA 

material in the production of shaped cementitious products such as pellets, bricks, tiles, and 

blocks [79, 80]. The method uses a closely controlled compaction process. First, an SDA 

material containing fly ash is uniformly contacted with a critical amount of water and then 

immediately compacted at a critical compaction ratio to provide a manageable green body in 

which the fly ash particles are positioned with respect to one another so that the interstitial spaces 

are sufficient to accommodate the volumetric changes in the cementitious materials without any 
Results and Discussion of the Literature Review 

2-31 

deleterious expansion of the product. The product is then cured in as little as two days at 120°–

180°F (49°–82°C). 

Evaluation of the use of SDA material in sand–lime bricks has been carried out in the 

Netherlands by KEMA [45, 81]. Sand–lime bricks are made of fine white lime and high-quartz 

sand molded, pressed, and steam-cured in autoclaves at 175°–214°C (347°–417°F). About 3000 

sand–lime bricks were manufactured and tested. The test bricks were manufactured with 

mixtures of SDA material and fly ash replacing about 20% of the sand in the conventional 

bricks. The quality of the bricks containing SDA material was satisfactory in comparison to 

conventional bricks, with higher compressive strength and splitting-tensile strength. The results 

were good with respect to compressive strength and splitting-tensile strength, porosity, and 

absorption coefficient with capillary action of water. No efflorescence was found when only 

moderate amounts of sodium sulfate (<0.4 wt%) or potassium sulfate (<2.2 wt%) were present. 

The carbon content of the fly ash must be limited to <7 wt%. The oxidation rate of calcium 

sulfite to calcium sulfate is extremely low, particularly in air. SDA materials have also been used 

in the manufacture of sand–lime bricks in Germany [69]. It has been noted that laboratory tests 

showed that the best results were obtained by replacing 50% of the sand with SDA material 

containing 70% fly ash [17]. For the pressing of solid bricks, it is necessary to reduce the SDA 

material proportion to 20%. 

Mineral Wool 

SDA material can be used to manufacture mineral wool used in insulation and ceiling tiles, as 

described in the research study conducted by the EERC [82]. SDA material with a high level of 

fly ash was used to prepare mineral wool in a pilot-scale cupola furnace. The SDA material was 

formed into balls, melted at 2600°–3000°F (1430°–1650°C) in the cupola, and fiberized by 

means of a compressed airstream. The mineral wool product was easily fabricated and appeared 

to be comparable in fiber diameter to that of commercial mineral wool. There were no reported 

commercial uses of this application in the United States, and ACAA does not report this as a 

separate use application. 

Mining Applications 

One large-volume use for alkaline, dry FGD material is in reclamation of acidic minespoils. Dry 

FGD material could be used to neutralize the spoil acidity and reestablish the vegetative cover to 

stabilize soils and reduce erosion [83]. It has been indicated that SDA material could potentially 

be used in a grout mix design to fill mine voids [34]. ECOBA reported 181,881 short tons of 

SDA material used for mine reclamation and restoration in 2004 [23]. Mining applications are 

the largest reported use of dry scrubber FGD material in the United States. ACAA reported 

115,696 short tons of dry scrubber FGD material used in mining applications in 2006 [22]. 
Results and Discussion of the Literature Review 

2-32 

Brendel et al. summarized the findings from an EPRI study entitled Advanced SO2 Control ByProduct Utilization: Laboratory Evaluation, EPRI CS-60443 [34]. In this study, grout mixes 

were prepared with cement and FGD materials (including SDA material) at various proportions 

with sufficient water to achieve a flow in the range of 5–35 seconds. Three mix designs were 

created with SDA material including 3:1, 1:1, and 1:2 (cement to SDA material) ratios. Data 

show increased strength gain from 7 days [2.3–16.1 MPa (334–2335 psi)] to 28 days [4.5– 

26.1 MPa (652–3785 psi)]. Data show that more cement does not always improve strength. 

SDA material has been used as a mine fill in the United States and Europe. Instances cited in 

literature are described below. 

A former limestone room and pillar mine in Sugar Creek, Missouri, is being stabilized using up 

to 700 tons of SDA material per day. The SDA material is mixed with water to create a slurry 

that is injected into the mine through 10-inch-diameter cased boreholes drilled through 160 feet 

or more of overburden. Because this particular SDA material contains ammonia, ventilation 

during mine stabilization was provided by constructing steel reinforced shotcrete walls between 

selected pillars to control airflow. As of September 2002, over 71,000 tons of the SDA 

material/water slurry had been injected into the mine [84]. 

In North Dakota, SDA material (containing about 75% fly ash) has been mixed with wet bottom 

ash to a water content of 20%–25% and used to backfill an old lignite strip mine. The mine was 

lined with clay (0.5–1.5 m thick) and capped with a clay layer (~1.5 m thick), overburden, and 

top soil [65]. 

In Germany and Poland, SDA material has been used successfully in the mining industry as filler 

in packing and backfill operations and as an additive in mining mortars [16, 69]. SDA material 

has been used in the backfilling of gravel pits without bottom sealing in Austria. Mining mortar 

from SDA material and fly ash was used in 1990 in Germany for the construction of 

underground retaining walls, for backfilling and consolidation, and for special uses. The use of 

SDA material for filling abandoned tunnels in lime, ore, and coal mines has been officially 

sanctioned and approved by German court decisions (OVwG (Oberverwaltungsgericht: 

Administrative Court of Appeals) Saarlouis, File 1-W 125/89; 1 F 17/89) [17]. 

Soil Stabilization 

SDA material has physical and chemical characteristics similar to those of a lime–fly ash mix 

and, thus, has the potential to be used in soil stabilization. The ability for the SDA material to 

form ettringite without swelling is key to its engineering performance. It also needs to be durable 

enough to withstand potential damage due to freezing and thawing and wetting and drying 

action. ACAA reported 299 short tons of dry scrubber FGD material used in soil modification 

and stabilization in 2006 [22]. Soil stabilization with SDA material has been documented in only 

a few laboratory and field demonstration projects. 
Results and Discussion of the Literature Review 

2-33 

OSU performed laboratory experiments to characterize the engineering properties of silty clay 

stabilized with SDA material and FSI products. Tests were conducted to evaluate compressive 

strength, permeability, swelling potential, compressibility, and leachate composition. These tests 

were performed on compacted samples of SDA material–soil mixtures at the optimum moisture 

content. The FGD materials tested substantially improved the strength and stiffness of soil, 

which was dependent on the mix ratio. All stabilized soil mixtures developed strength of at least 

0.69 MPa (100 psi) by 28 days. Generally, the permeability of the stabilized soil decreased with 

time as a result of the chemical reaction occurring within the mixture. Every sample showed an 

increase in volume over time due to ettringite formation. Consolidated test results showed low 

compressibility of stabilized soils. The concentration of heavy metals in the leachate was 

considerably lower compared to EPA drinking water standards [34]. 

ICF Northwest Inc. performed tests on a variety of FGD materials, including SDA material, 

mixed with A-7 clay, with and without additional lime. Soil had a liquid limit of 28.5 and a 

plastic limit of 24. The mixtures consisted of a) 90% soil, 7% FGD material, and 3% lime; 

b) 90% soil and 10% FGD material; and c) 90% soil, 7% fly ash, and 3% lime. The 28-day 

compressive strength of SDA material mixed with lime was 4.34 MPa (630 psi) and without lime 

was 2.83 MPa (410 psi) [34]. 

Sulfuric Acid Production 

The Müller-Kühne process is based on Müller’s tests to produce sulfuric acid from anhydrite and 

Kühne’s idea to produce cement at the same time by adding carbon (coke), clay, and sand. As 

early as 1985, investigations were started to substitute natural raw materials with products such 

as SDA material [17]. Dry FGD material was processed into sulfuric acid in the Müller-Kühne 

Process at WSZ Wolfener S.u.Z. in Bitterfeld in Germany until recently [16]. No information 

was found to indicate why this process is not currently used. 

No instances of sulfuric acid production research in the United States were noted in the reviewed 

literature, and ACAA does not report this as a separate use application. 

Synthetic Aggregate 

The production of synthetic aggregate using SDA material has been demonstrated in several 

countries on a commercial level. However, other commercial attempts have not been as 

successful or were only demonstrated on a laboratory scale. Production methods tend to use 

mixtures of fly ash and SDA material, either pelletized or briquetted. Most synthetic aggregates 

are lighter than natural aggregates, and are suitable for the manufacture of lightweight precast 

products such as roofing tiles, masonry blocks, or as a concrete and asphalt paving material. 

Although synthetic aggregate was not reported as an application for SDA material by ACAA in 

2006 [22] or ECOBA in 2004 [23], numerous instances of commercial use and research projects 

were noted in the reviewed literature. 
Results and Discussion of the Literature Review 

2-34 

In the United States, a manufacturing plant is being operated at the Birchwood Power Facility by 

Universal Aggregates, LLC. The plant is turning CCPs, including SDA material, into 

1,667,000 tons of lightweight aggregate annually [85]. The process used by Universal 

Aggregates, LLC, is described in U.S. Patent 6,054,074 [86]. The method mixes a sulfurcontaining CCP, recycle fines containing calcium hydroxide, an aluminum-containing material, 

and water to produce an agglomerated product. This is then combined with curing fines that 

contain calcium oxide and is cured. The cured material is screened to separate dry fines, which 

are recycled, and the aggregate, which is used as a product. CCPs used in the method can include 

wet FGD material with fly ash, dry FGD material such as SDA material, wet lime kiln dust with 

dry lime kiln dust, or FBC material with fly ash. Specifications are provided for the various 

materials. Further investigation of the end product is being conducted by the Research and 

Development Department of CONSOL Energy (CONSOL R&D) with Universal Aggregates, 

LLC, under a DOE cooperative agreement. The objective is to conduct a systematic study of the 

durability of manufactured aggregates using a variety of CCPs, including SDA material, with 

different chemical and physical properties. The manufactured aggregates and aggregate product 

materials were tested under different freeze–thaw, wet–dry, and long-term natural weathering 

conditions [33]. 

Sherburne EnviRock initiated production of manufactured, lightweight aggregate intended for 

use in the concrete masonry block market in 2001. A year later EcoBlend, a concrete mineral 

admixture also intended for use in concrete masonry, was added to production. SDA material 

containing fly ash is used as the primary feedstock [87]. 

A study performed by ICF Northwest Laboratory to produce synthetic aggregate from five 

different FGD materials, including SDA material, using two production processes: 

1) agglomeration followed by sintering and 2) briquetting followed by CO2 environment curing. 

The SDA material and FSI material produced the strongest briquettes, with an average strength 

of 10.71 and 7.43 MPa (1554 and 1077 psi), respectively [34]. 

SDA material was used as an aggregate in asphaltic concrete pavement in a test road. Type 31B 

base course and Type 41A wear course mixes were made in the lab with 5% SDA material and 

5.5% or 5.8% asphalt, respectively. The asphalt content needed to be decreased to meet MnDOT 

air void specifications. However, no conclusions can be drawn from these data, because it was 

difficult for the batch operators to meter the amount of SDA material and other aggregates used. 

An addition of 5% of SDA material caused a decrease in both stability and percent air voids 

compared to the control mixes. It was noted that less compactive effort was required to achieve 

the 95% of maximum density when using the SDA material containing bituminous material [46]. 

Synthetic aggregate production using the Aardelite process has been used commercially to 

manufacture synthetic aggregate. Aardelite is a proprietary technology owned by Danieli Corus 

but licensed to Aarding Lightweight Granulates B.V. in the Netherlands. The technology and 

equipment are sold by Aarding Lightweight Granulates B.V. to power plants. To date, four 

Aardelite plants have been built [88]. To make Aardelite, SDA material can be mixed with water 

and pelletized [65]. The pellets are then embedded in fly ash, so the final pellet has a core of 

SDA material surrounded by fly ash. The composite pellets are hardened with steam at 90°C and 

size-graded by sieving. The four plants use a different embedding material. However, according 

to a company representative, there are no commercial plants using SDA material in operation 
Results and Discussion of the Literature Review 

2-35 

[89]. Aardelite pellets can be used as raw material in a variety of building applications including 

the following [88]: 

• Masonry (building) blocks 

• Ready mix concrete 

• Prefabricated concrete elements 

• Concrete piles 

• Bitumen-bonded asphalt for road construction 

• Paving stones 

It has been suggested that these aggregates could also be used in structural concrete. However, 

research carried out in Denmark by ELSAM indicates that the pellets do not have sufficient 

strength for this application [65]. When mixed with fly ash, the SDA material can form the 

mineral thaumasite, which has poor strength characteristics. Low-grade aggregates and pellets 

may be suitable for use in road base and as filler in asphaltic concrete. 

Research initiated in Denmark has examined using the self-hardening properties of SDA material 

to form pellets [36]. Additives, such as portland cement or hydrated lime, may be added to SDA 

material with a low reactivity so that the pellets acquire adequate strength and density. Pellets 

(5–15 mm) were manufactured in the laboratory using a continuous roll mill pelletizing machine, 

and cured in a water-saturated environment. Despite encouraging results, the demonstration work 

did not result in the commercial production of aggregates. 

It is possible to produce artificial gravel and chippings by mixing SDA material with 10%–20% 

water and compacting it in a pelletizing plant to a density of 2 t/m3

 and curing. A 5% addition of 

cement is required for low-fly ash SDA materials. The artificial gravel and chippings can be used 

as a substitute for natural gravel or chippings for cement or bitumen-bound road surfaces and 

cement-bound building brick. Development work in Denmark has been discontinued because of 

long setting times, reduced frost resistance, and an increase in volume due to thaumasite 

formation [17]. 

Wallboard 

The use of SDA material in wallboard has been demonstrated in Denmark by DURACON ApS 

[65]. The process uses the same production facilities as other fiber–cement boards that are fireresistant. The boards consist of a mixture of SDA material (~40%), fly ash (~10%), recycled 

paper (10%), cement, and additives. The finished panels should be cured in an autoclave. The 

raw materials are low in cost, minimizing production costs compared with similar wallboards. 

The panels are strong enough, with sufficient stiffness, for normal handling and working, and 

fire resistance tests show good performance. The product was undergoing full-scale testing in 

1992, with a potential daily production capacity of about 4000 m2

 of board. Instances of this 

product reaching full commercial implementation were not found. 
Results and Discussion of the Literature Review 

2-36 

Kolar also reported that several authors have described the production of wallboard made of 

SDA material, fly ash, fibers, and portland cement or lime hydrate [17]. The board is autoclaved 

and dried. The boards are claimed to be suitable for interior applications in residential buildings, 

hospitals, schools, and industrial buildings. 

ACAA has not reported dry scrubber FGD material use in wallboard applications since dry 

scrubber FGD materials have been categorized. 

Wet FGD Sorbent 

Dry FGD material is used as a sorbent in the wet FGD process in both Germany and Denmark; 

although it has not reached commercialization in the United States [16, 22]. The excess alkali in 

the dry FGD material serves as a sorbent for the SO2 in the flue gas, and the sulfite in the 

material is simultaneously converted to commercial-grade gypsum. A prerequisite for this use is 

a very low fly ash content. It has been reported that the gypsum quality has improved when 

operating on dry FGD material as compared to using limestone powder and typically contains 

98%–99% calcium sulfate dihydrate (gypsum) [16]. 
3-1 

3

SUMMARY, CONCLUSIONS, AND 

RECOMMENDATIONS 

An extensive literature review was performed that provided information on: 

• SDA systems and their operation and the impacts of system components and operation on the 

SDA material composition. 

• The production and use statistics for SDA material produced in the United States and Europe. 

• SDA material characteristics. 

• Commercial and potential uses of SDA material. 

The literature review brought into focus several issues evident to producers and users of SDA 

material that have an impact on the status of this material in commercial markets. These issues 

are summarized here and include a list of barriers to the commercial utilization of SDA material. 

SDA Production and Material Characteristics 

SDA systems have two primary configurations. In the United States, the most common system 

configuration has the spray dryer chamber located so the flue gas is combined with the sorbent 

before it is introduced into the primary particulate collection system. In this configuration, the fly 

ash and dry SDA material are mixed in the spray dryer chamber, and in cases where the fly ash is 

alkaline, the fly ash actually serves as a sorbent so the fly ash particles are coated with calcium 

sulfite/sulfate. Recycle of this mixture is frequently used to optimize sorbent utilization. 

Variables in this system include the amount of fresh lime sorbent that is introduced, the fly ash 

characteristics, the recycle rate, and the spent sorbent–fly ash ratio. 

The typical European SDA system configuration includes a fly ash precollection unit that 

removes the fly ash from the flue gas before it enters the spray dryer chamber. The variables in 

this system configuration are fewer resulting in a more consistent material between plants. 

The SDA material that results from these two primary system configurations have obvious 

compositional differences. The systems with fly ash precollection produce an SDA material that 

is composed primarily of calcium sulfite, calcium sulfate, and other minor calcium-based 

compounds. The systems that incorporate the fly ash stream into the sorbent stream in the spray 

dryer chamber produce SDA materials that are composed of these same calcium compounds with 

fly ash that may or may not be coated with calcium sulfite or sulfate, depending on the alkalinity 

and composition of the fly ash. Even without accounting for varying chemical composition of the 

fly ash, it is apparent that SDA materials produced from these two configurations will be 
Summary, Conclusions, and Recommendations 

3-2 

significantly different in composition performance. In European literature, SDA materials are 

frequently referred to as dry FGD material. This term is used more generically in the United 

States but does include SDA material. Unfortunately, in most of the literature reviewed, the 

system configuration was not described nor was the percentage of fly ash, if present, noted. 

However, the variability of the materials investigated and/or used is evident in the results of the 

literature review by the variability of the composition and properties reported. Wide variability 

for these materials was documented for every physical property for which literature was found 

and for the chemical composition to include the trace element concentrations. Although 

mineralogical characterizations were limited, it is expected that similar crystalline phases would 

be present in all types of SDA materials, but that those with fly ash present would also contain a 

significant amorphous phase. On review of the characterization data from the literature review, it 

was noted that varying conclusions were drawn by individual authors. The variability of samples 

used in the studies likely contributed to the divergence in conclusions. Another factor that likely 

played a role in the differences was the methodology used to evaluate specific parameters. This 

serves as a caution in relating literature data and conclusions as representative of SDA materials 

without a clear understanding of the specific material(s) referred to in the literature. 

Utilization of SDA Material 

As described in Section 2 under “Current and Potential Uses of SDA Material,” SDA material is 

commercially used in a variety of applications in the United States and Europe. A review of the 

ACAA production and use survey results also provides an indication of utilization in the United 

States. These applications are noted in Table 3-1. 

Table 3-1 

Commercial Applications for SDA Material in the United States and Europe 

European Commercial Applications U.S. Commercial Applications 

Agriculture (e.g., sulfur fertilizer) Agriculture 

Flowable fill Concrete, concrete products, and flowable fill 

Raw material for cement manufacture Soil stabilization 

Raw material for sulfuric acid production Structural fills/embankments 

Wet FGD sorbent Synthetic aggregatea 

 Mining applications 

a

 Although not listed in the 2006 ACAA statistics [22], this is a current SDA material use. 
Summary, Conclusions, and Recommendations 

3-3 

The utilization profiles for SDA material in the United States and Europe are different at least in 

part because of the different chemical composition resulting from the inclusion of fly ash in the 

United States, while in Europe, SDA material typically does not include a fly ash component. 

Additionally, the more stringent European disposal regulations encourage utilization of CCPs, 

and likely encouraged utilization of SDA materials. Some commercial applications, such as mine 

fill and wet FGD sorbent, may have less stringent composition or quality control criteria. 

Based on the commercial applications reported and research on utilization of SDA material, a list 

of applications was assembled, ranking applications as having high, moderate, and low potential 

for the U.S. market relative to the technical achievability of the applications. Key technical 

issues that were considered in ranking these applications were: 

• Variability of SDA materials – Most U.S. current and projected SDA systems incorporate 

some fly ash into the final SDA material. The level of fly ash present and the specific 

chemistry of that fly ash results in a variability in the chemical composition of the final 

SDA material, and that is expected to result in a variability in the physical and 

engineering performance of the material for many of the use applications noted. 

• Sulfur content – Since SDA is used to remove sulfur gases from flue gas, the total sulfur 

content of SDA materials can be significantly higher than fly ash. For some applications, 

this elevated concentration of sulfur may limit the use. Additionally, most of the sulfur 

present in SDA material is present both as sulfite and as sulfate, with sulfite being the 

prevalent form. For some applications, the potential for oxidation of the sulfite to sulfate 

could limit use of SDA materials and needs to be considered. 

• Potential for expansion – SDA material that contains fly ash has the potential to expand, 

especially when exposed to water. The composition of SDA materials containing fly ash 

includes the primary building blocks for ettringite formation, a key secondary hydrated 

phase that can cause expansion if it forms in finished products or final placements. The 

pH of SDA material is not considered conducive to ettringite formation, but free lime 

present in SDA material may provide enough alkalinity to allow ettringite formation to 

occur. Expansion needs to be considered in any use applications for SDA material. While 

expansion can be predicted by use of appropriate tests, many tests currently available for 

determining expansion potential may not be adequate to predict field or full-scale 

performance. This is due to many factors, but laboratory studies indicated that expansion 

may be taking place into material voids. A simple test based on density changes of 

materials that will with great certainty predict expansion under laboratory conditions has 

been developed by the EERC [90]. 

Economic feasibility was not used in this ranking process, but in order for any material to be 

commercially viable in a utilization application, the economics need to be advantageous and 

should be considered in feasibility investigations. For some applications, the environmental 

suitability of SDA materials, or any other CCP or industrial resource, may need to be evaluated. 

Environmental performance of SDA materials was outside the scope of this literature review, but 

the issues associated with use applications that may result in impacts to human health and the 

environment are prevalent in public reports and technical literature. 
Summary, Conclusions, and Recommendations 

3-4 

High-Potential Applications 

High-potential applications for the U.S. market are estimated to be those that take advantage of 

the presence of the fly ash component of the SDA material, can tolerate relatively high sulfur 

content, and have limited susceptibility to expansion or reduce expansion potential in the 

production process. These applications fall into two categories: 1) cementitious products and 

2) mining. 

Cementitious Products 

ACAA’s 2005 utilization rate for dry scrubbing materials in concrete and concrete products was 

~14,000 tons, or ~1% of the 2005 U.S. production. It is unlikely that this utilization rate includes 

significant quantities of sodium-based dry scrubber materials because of the technical and 

performance issues associated with high levels of available alkali in materials used for 

formulation of concrete. Therefore, it is assumed that the 2005 1% utilization rate refers almost 

exclusively to SDA material. Anecdotal evidence suggests that this utilization is likely primarily 

in concrete products such as masonry. ACAA also reported that nearly 10,000 tons of dry 

scrubbing materials were used for flowable fill in 2005 (0.7% of the amount produced) [59]. It is 

also assumed that the bulk of the dry scrubber material reported in flowable fill is also likely 

SDA material. Literature and anecdotal evidence indicated that some of these cementitious 

products or applications have specific requirements for the SDA material to be used most 

effectively. It is predicted that these use applications have high potential for growth as more 

SDA materials become available. Based on the prediction of 13–14 M tons of SDA material to 

be produced annually in the United States by 2017, it is estimated that the utilization rate should 

be maintained at the current approximate 2% level and could reach 5% or higher of production if 

industry develops technical and performance data and markets the product effectively. This 

equates to about 260,000 to 700,000 tons annually. Location of material and markets will have a 

definitive impact on market development and penetration. Much of this growth is anticipated 

after 2012 as installations increase in number. 

Masonry 

Although SDA material has been shown to be an effective component of masonry products, most 

commonly concrete blocks, the SDA material may need to be conditioned prior to use, and 

cementitious additives may be required for unreactive to moderately reactive SDA materials. Use 

of alkaline activation and the elevated-temperature curing of these products has been shown to 

result in good performance of the SDA material in these products. 
Summary, Conclusions, and Recommendations 

3-5 

Flowable Fill 

The basic physical and engineering properties (moisture density, compressive strength 

development, and permeability) of reactive and low-reactivity SDA material indicate that these 

materials should be able to perform acceptably as flowable fill material. Research conducted to 

date supports this claim and shows that SDA material can be an economical alternative to 

conventional materials; however, more research is needed on nonreactive SDA materials. It is 

recommended that more long-term tests be conducted to test long-term strength, stress–strain 

behavior, freeze–thaw properties, swell potential, and corrosivity. 

Synthetic Aggregate 

SDA material is currently being used to manufacture synthetic aggregate in the United States; 

however, there is also evidence that some synthetic aggregate-manufacturing processes 

developed to use SDA material have not had commercial success. Successful aggregate 

production from SDA material indicates the potential for use in concrete block, brick, and other 

shaped compacted product production. Much like the masonry application, it may be necessary 

to activate the SDA or use an elevated-temperature curing to facilitate good product 

performance. 

Mining Applications 

SDA material has been used commercially as a mine fill. Additionally, SDA material has been 

shown to be capable of neutralizing the spoil acidity and reestablishing vegetative cover to 

stabilize soils and reduce erosion. However, excessive application on mine spoils did cause 

excessively high pH and cementation. The technical issues associated with the use of SDA 

material in mine applications can be addressed through an understanding of the experience 

already reported in the literature, characterization of the material, and careful attention to the 

methods for applications and quantities of material applied. Statistics from 2005 indicate that 

approximately 8% of dry scrubber materials produced in the United States were used in mining 

applications [59]. This represented by far the largest current use of dry scrubber materials. It is 

reasonable to assume that much of the dry scrubber materials included in the 2005 statistics were 

SDA materials simply because of the very limited number of other dry scrubber systems 

currently being used as compared to SDA systems reported. Since projected SDA installations 

are expected to be in the Midwest and western United States, it is likely that mining applications 

of SDA materials will remain a good utilization candidate and will increase proportionately to 

the amount of material produced in the western states. Contingent on rules currently under 

development by the Office of Surface Mining, it is anticipated that the utilization rate of 8% can 

be readily maintained and could be expected to increase to 15% during the period from 2007 to 

2012. During that period, most of the new installations will be in western states, where mine 

placement is expected to be economically feasible. Between 2012 and 2017, this use application 

could continue to increase but on a more limited basis because later SDA installations are 

anticipated to be in the Midwest and the advantage of location is not expected to favor mine 

reclamation as much as western installations. It is estimated that a maximum utilization rate of 

20% could be realized by 2017. This would equate to 2.6–2.8 million tons annually, a more than 

10-fold increase over current utilization volumes. 
Summary, Conclusions, and Recommendations 

3-6 

Moderate-Potential Applications 

Cement Replacement in Concrete 

SDA material should be evaluated for suitability as a cement replacement in concrete, especially 

as groups like ASTM International and the American Association of State Highway and 

Transportation Officials (AASHTO) move toward performance-based specifications as opposed 

to prescriptive specifications. Research has shown that except for retardation in setting times, 

concrete in which cement was partially substituted by SDA material showed strength and 

durability performance comparable to superior to traditional concrete. This application is most 

promising when SDA material and fly ash are collected together with a high percentage of fly 

ash, as is common in U.S. installations. The technical issues that require some continued 

evaluation can be readily addressed using protocols similar to the ones developed and used for 

fly ash and other supplementary cementitious materials. It will be possible to determine the 

appropriateness and amounts of specific SDA materials for use in concrete, and there is technical 

evidence that it can offer good performance. Much like fly ash qualification for use in concrete, 

the producer or potential marketer will need to take the responsibility of evaluating their specific 

material, developing the market, and educating the users. This process will require time, so 

although the market has high potential to be technically successful, it is expected to take time to 

develop that market. 

The potential for large volumes of SDA material to be used in this application will likely be 

dependent on a number of factors, including potential reductions in the availability of fly ash for 

use in concrete, which is expected to be impacted by the installation of mercury control 

technologies that use activated carbon and combine the activated carbon with the fly ash stream. 

If even some of the current concrete-quality fly ash being produced becomes unsuitable for use 

in concrete, there may be a market need for alternate materials. Of course, if activated carbon is 

used and mixed with fly ash on a unit or plant where the fly ash is incorporated into the SDA 

material, the SDA material may not meet the required specifications for use in concrete. 

Current data on dry FGD material utilization do not separate use in concrete or concrete 

products. It is estimated that only very low quantities of SDA material (<0.5%) are currently 

being used for cement replacement. This utilization rate is not expected to increase significantly, 

but at the predicted production rate for 2017 (13–14 M tons/year), a use rate of 0.5%, or 65,000–

70,000 tons, would be significant for the overall use of the material. 

Engineering Applications 

SDA material has been used commercially in engineering applications in Europe and with 

limited success in the United States. ACAA reported 2666 tons of dry FGD material were used 

for structural fills and embankments in the United States in 2005 [59]. This utilization rate of 

approximately 2% was likely primarily SDA material and indicates that SDA can be successfully 

utilized in engineered fills. This utilization rate would be expected to be maintained as additional 

SDA materials are generated. 
Summary, Conclusions, and Recommendations 

3-7 

Agriculture 

In Europe, SDA material is typically collected separately from fly ash, whereas in the United 

States, SDA material and fly ash are often intermingled. This difference could be the reason that 

SDA material is used commercially as a sulfur fertilizer in Germany, Denmark, and Austria, but 

not in the United States. There are unanswered engineering and environmental questions as well. 

The potential for SDA material to be used as a liming agent or for soil amendment has received 

mixed results depending on the pH of the soil, crops planted, amount of SDA material used, and 

whether the SDA material was blended with any other material. The leaching of boron could be a 

potential environmental concern. ACAA reported ~19,000 tons of dry scrubber material used in 

agricultural applications in 2005, nearly 1.5% of the total production of ~1.4 million tons [59]. 

This represents the second largest current use of dry scrubber material. It is likely that the dry 

scrubber material used in agricultural applications was SDA material because the sodium-based 

dry scrubber materials have potential detrimental effects on many crops and soil types. An effort 

is currently under way in the United States to demonstrate the effectiveness of FGD gypsum for 

use as an agricultural soil amendment. Even though SDA material contains fly ash and calcium 

sulfite, which makes it different from FGD gypsum, the potential for SDA material to be used in 

agricultural applications may be enhanced by the current efforts on FGD gypsum use. The 

agricultural use of calcium sulfite, which is toxic to plants under certain conditions, appears to be 

less problematic than initially expected. Under normal soil pH and with a limited supply of air 

for oxidation, it would be expected that the oxidation of sulfite to sulfate, which is not toxic to 

plants, would be extremely slow. However, in field studies, this conversion occurs rapidly. This 

anomaly is likely due to microbial transformations of sulfite to sulfate. It has been demonstrated 

that simply placing the sulfite-rich material in the agricultural soils a few weeks in advance of 

planting mitigates any detrimental effects of sulfite. There is moderate potential for SDA 

material to be used in agriculture in the western United States, where FGD gypsum is expected 

to be less available. The trace element concentrations of SDA materials are likely one technical 

and regulatory issue that will need to be addressed. Utilization rates are predicted to be 

maintained at near 1% based on the increased production. 

Soil Stabilization 

SDA material has physical and chemical characteristics that are similar to a lime–fly ash mix 

and, thus, has the potential to be used in soil stabilization even though mixed results have been 

found in the expansion potential of SDA material used as a road foundation. The ability for the 

SDA material to form ettringite without swelling is key to its engineering performance. It also 

needs to be durable enough to withstand potential damage due to freezing and thawing and 

wetting and drying action. Soil stabilization with SDA material has been documented in only a 

few laboratory and field demonstration projects, and no commercial uses were found in the 

reviewed literature even though marketers indicated the successful use of specific SDA materials 

in soil stabilization in certain regions of the United States. There is the potential for the technical 

issues related to the use of SDA material in soil stabilization to be addressed, so this application 

has moderate potential. Currently, there is only anecdotal evidence of SDA material being used 

for soil stabilization in the United States, but if the technical issues are adequately addressed, it is 

projected that the utilization rate could reach 0.5%–1.0% of the production rate. 
Summary, Conclusions, and Recommendations 

3-8 

Wet FGD Sorbent 

Dry FGD material has been used commercially as a sorbent in the wet FGD process in Germany 

and Denmark. Since there is alkalinity still available in SDA material, it is possible that it could 

be a good candidate for use in wet FGD systems in the United States, likely in inhibited or 

natural oxidation systems. The technical issues would need to be addressed, and the actual use 

would likely be dependent on locations of an SDA producer and wet FGD system. Currently, 

there is no SDA material being used in this application, but it could reach a utilization rate of up 

to 1% or greater by 2017. This application warrants investigation to evaluate the impact of the 

presence of fly ash in the SDA material, and it is likely that this application has the highest 

potential for a region where an SDA material producer and wet FGD system user are in close 

proximity. 

Low-Potential Applications 

The following applications are expected to have low utilization potential. Predictions for 

utilization rates have not been made. 

Binder 

SDA material used as a binder for interior plasters was noted in U.S. and German patents. The 

use of SDA material as a flooring binder is considered promising, and other potential 

applications include use in insulating building materials and raw materials for double-floor plates 

where it may be substituted for cement products. This is a promising low-volume, high-value 

application. 

Cement Manufacture 

Although SDA material has been used commercially to manufacture cement in Germany, it has 

not reached the demonstration phase in the United States. A process has been patented in the 

United States to use dry FGD material in place of fly ash and gypsum in the production of 

cement; however, it is not anticipated that the process will reach commercialization. 

Fixating Agent for Hazardous Waste Sludge 

Research has shown that SDA material has the potential to solidify or fixate hazardous wastes if 

proper fixation techniques can be developed. The mechanism for this is likely due to the excess 

alkaline materials present in SDA material. Although SDA material particles are generally 

coated with sulfite and sulfate, the grinding action of a pug mill would likely activate the 

material by exposing alkaline surfaces through abrasion of particles. 
Summary, Conclusions, and Recommendations 

3-9 

Marine Applications 

Research conducted to date has shown that SDA material can be used in marine applications 

such as artificial reefs and offshore sea defenses. SDA materials exposed to marine environments 

appear to show no evidence of surface friability or cracking and are a favorable habitat for 

marine life. The economical and environmental feasibility of this application should be further 

evaluated. 

Mineral Wool 

The production of mineral wool from SDA material is technically feasible if the SDA material 

contains a high percentage of fly ash. However, the market is not demanding this process at this 

time. 

Sulfuric Acid Production 

The use of SDA material in the production of sulfuric acid may be technically feasible; however, 

the authors believe this is not an economical beneficial use option. 

Wallboard 

Using SDA material as a source of gypsum in traditional wallboard is not a promising utilization 

option. However, research conducted in Denmark showed that SDA material, when mixed with 

fly ash, cement, and other additives and cured in an autoclave can produce good-quality fiber–

cement board. The economics of this application need to be considered. 

Barriers to SDA Material Utilization 

The European and U.S. production and utilization statistics for SDA material clearly indicate that 

SDA materials are currently underutilized, especially in the United States, even though this 

report summarized a number of commercial applications and several applications that have high 

potential to become commercial. The literature reviewed brought into focus the barriers that exist 

and limit the use of SDA material in the United States. The barriers identified by the authors are: 

• Inconsistent Terminology Used to Define the Material – In the literature reviewed, there 

was a marked inconsistency with regard to the terms used to describe SDA materials and 

FGD materials in general. The discrepancies were so broad that, in some cases, the authors 

could not determine the specific product to which the reports referred. Inconsistent 

terminology makes it difficult for those in the industry, particularly government entities, to 

define the material and its potential uses properly. In any area, a well-defined vocabulary is 

the cornerstone of effective communication, and this is essential in technical fields. This lack 

of consistent terminology is a barrier to both the commercialization and to the research and 

development of SDA material utilization. 
Summary, Conclusions, and Recommendations 

3-10 

• Lack of Understanding of the Material – The successful management of SDA material 

requires a thorough understanding of the engineering and chemical properties of the material. 

Although a number of references were identified that considered the characterization of the 

material as it relates to potential uses, the engineering and chemical properties of specific 

materials need to be investigated further. As with other CCPs, it is difficult to generalize the 

properties of the material because of differences in coal type, combustion system, collection 

process, and management. As previously noted, SDA materials exhibit a variability that 

results from the system configuration and the percentage of fly ash present in the final SDA 

material. Potential uses that apply to one type of material may not be appropriate for others. 

The natural oxidation of sulfite to sulfate in SDA material is documented and yet the impact 

of this oxidation process on product performance is not well-defined and needs to be 

considered in evaluating utilization applications. 

• Limited Data on Environmental and Health Effects – Although all SDA materials 

encountered in this literature review meet regulatory limits for classification as nonhazardous 

wastes, there are still concerns about surface water and groundwater contamination by runoff, 

seepage, and leachate during disposal or use applications. Many potential uses for SDA 

material fall under the general category of land application, which raises questions about the 

potential for the material to affect the environment and/or human health. 

• Inconsistent Guidelines on Beneficial Ash Use – Many state rules apply to fly ash, bottom 

ash, and boiler slag utilization; however, products from FGD systems are relatively new in 

comparison to other products, and specifications have not been written that deal specifically 

with SDA material. In European countries where regulations have been adopted, most 

progress has been made with regard to SDA material utilization.

• Economics – Economic factors are an overriding issue in utility ash management decisions. 

Currently, the potential to produce revenue from the sale of SDA material is limited; 

therefore, most utilities find it more economically feasible to dispose of the material rather 

than dedicate resources (i.e., employees and infrastructure) to utilize it. The prices received 

for SDA material are simply too low to justify a large financial commitment to SDA material 

marketing. In some countries in Europe, increasing landfill taxes have driven the 

development of SDA material applications. 
Summary, Conclusions, and Recommendations 

3-11 

Conclusions 

The literature assembled and reviewed provided a good representative cross section of the 

technical information available on the utilization of SDA material in the United States and 

Europe. The following conclusions were developed based on the information assembled from the 

review: 

• SDA materials exhibit a broad variability based on the SDA system configuration, the fly ash 

content of the SDA material, the composition of the fly ash in the SDA material, and the use 

of optional sorbent recycle. 

• The presence of significant levels of calcium sulfite and the natural oxidation of sulfite to 

sulfate has the potential to affect the material performance in utilization applications and 

products. 

• European SDA materials, frequently referred to as dry FGD material in the European 

literature, generally do not incorporate fly ash into the final SDA material, and documented 

commercial utilization of SDA material in Europe is higher than in the United States. 

• U.S. SDA systems typically incorporate fly ash into the final SDA material and are most 

commonly used in coal-fired units where alkaline ash is produced so the fly ash can also act 

as an SO2

 sorbent. 

• U.S. SDA material utilization rates are lower than European utilization rates, likely because 

of higher variability of the U.S. material, incorporation of fly ash into the material, and 

current regulations in Europe that promote industrial resource utilization. 

• Numerous utilization applications have high to moderate potential for commercialization in 

the United States, but technical, environmental, and economic evaluations will likely be 

needed before these materials can be successfully introduced into the markets identified. 

Recommendations 

As U.S. coal-fired power plants install additional FGD systems to reduce SO2 emissions in 

coming years, it is anticipated that a number of plants will elect to use SDA systems, resulting in 

an associated increase in the volume of SDA material produced in the United States. Although 

disposal is currently the predominant management option for SDA material in the United States, 

the potential exists that viable commercial options can be developed for this material. In order to 

optimize SDA material use in existing commercial applications and develop the potential 

commercial options in the United States, technical, environmental, and economic evaluations 

will be required. The following recommendations present an outline for the CCP industry to 

maintain existing commercial markets and develop new markets for U.S. SDA materials: 

• Work within existing organizations such as ASTM International and ACAA to develop and 

put into use appropriate terminology and definitions for SDA materials. 

• Develop an understanding of the impact of compositional variability on the performance 

characteristics of SDA materials. 
Summary, Conclusions, and Recommendations 

3-12 

• Develop an understanding of the oxidation profiles of SDA materials and evaluate the 

impacts of oxidation on product performance. 

• Educate potential users, regulatory representatives, and other stakeholders about SDA 

materials. 

• Address quality, compositional, environmental, and performance criteria in research, 

development, and demonstration efforts. 
4-1 

4

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A-1 

A

BULK CHEMICAL COMPOSITION – REPORTED AS 

OXIDES – OF SDA MATERIAL REPORTED IN 

LITERATURE REVIEWED 
Bulk Chemical Composition – Reported as Oxides – of SDA Material Reported in Literature Reviewed 

A-2 

Table A-1 Bulk Chemical Composition of SDA Materials Containing Fly Ash Reported in Literature Reviewed 

Source [2] [14] [30] [31] [33] [34] [35] [40] [44

No. of 

Samples 3 2 – 5 7 Unknown 2 Unknown 

Up To 7 Of 

24 References 

For Each 

Unknown 1 

Units wt%, Dry Basis wt%, Dry Basis wt% % wt%, Dry Basis % % % wt% 

SiO2 38.78 – 46.26 22.05 – 33.10 16 – 26 21.1 – 32.4 24.05 – 30.22 16 – 32 6 – 68 21.1 26.5Al2O3 17.51 – 20.57 9.84 – 18.26 6.3 – 13 8.5 – 23 11.52 – 15.89 6 – 23 4 – 44 8.5 8.2Fe2O3 2.83 – 3.63 2.55 – 5.82 1.6 – 7.7 4.2 – 6.8 2.21 – 3.79 2 – 8 1 – 44 6.8 4.9CaO 13.12 – 18.34 21.59 – 29.27 18 – 31 19.7 – 32.2 25.66 – 34.13 18 – 32 0.2 – 52 18.1 27.1SO3 3.35 – 10.07 5.96 – 20.05 — — 7.92 – 11.98 6 – 22 0 – 32 3.4 16.4MgO 1.34 – 1.47 2.18 – –4.33 0.65 – 4.6 5.8 – 6.0 0.89 – 3.90 0.6 – 6 0.1 – 14 5.8 4.6Na2O 1.02 – 1.39 0.49 – 1.52 0.9 – 46 2.7 – 4.7 0.13 – –1.82 0.9 – 5 0.2 – 28 4.7 2.6K2O 0.83 – 0.97 0.36 – 0.50 0.2 – 0.84 0.37 – 0.5 0.43 – 1.10 0.2 – 0.8 0.1 – 6.37 0.5 0.8P2O5 0.68 – 1.18 0.15 – 1.10 — 0.4 0.03 – 1.01 — 0.2 – 1.2 0.4 0.0TiO2 0.54 – 0.70 0.41 – 1.16 0.27 – 0.92 — 0.57 – –1.19 — 0.5 – 1.0 0.5 0.4BaO — — — — — — — — 0.8MnO2 0.03a 0.00 – 0.12 — — — — — — 0.0SrO 0.11 – 0.19 — — — — — — — 0.4Moisture — — — 1 – 5 1.22 – 1.71 — <0.1 – 13.2 1.3 1.1LOI or C — 0.19 – 6.90 — 0.8 — — 0.32 – 20.5 — 1.6Unaccounted — — — — — — — 1.7 CaSO3 — — — — 11.89 – 17.96b — — 14.6 — 
Bulk Chemical Composition – Reported as Oxides – of SDA Material Repor

Table A-1 (continued) Bulk Chemical Composition of SDA Materials Containing Fly Ash Reported in Literature Reviewed 

Source [2] [14] [30] [31] [33] [34] [35] [40] [44

No. of 

Samples 3 2 – 5 7 Unknown 2 Unknown 

Up To 7 Of 

24 References 

For Each 

Unknown 1 

Units wt%, Dry Basis wt%, Dry Basis wt% % wt%, Dry Basis % % % wt% 

CaSO3 · 1⁄2 H20 —

—

—

—

—

— 13 – 59 

—

—

CaSO4 —

—

—

—

—

— 6 – 43 8.9 

—

CaCO3 —

—

—

—

—

— 2.1 2.1 

—

Ca(OH)2 —

—

—

— 8.5 – 25.0c — 4 – 21 

—

Free Lime 

—

—

—

—

—

—

— 1.6 

—

Ca as Available CaO 

—

—

—

—

—

—

0 – 5.4 

—

—

SO4 —

— 12 – 22 

—

—

—

—

—

—

SO2 —

—

—

—

—

— 0.1 – 20.5 

—

— 

CO2 — 1.77 – 7.54 

—

—

—

— <0.1 – 15d —

—

Hydroxide 

—

— 0.45 – 8.2 

—

— 0.5 – 10 <0.1 – 9.5 

—

—

Cl 

—

—

—

—

—

—

—

—

— 

a Mn3O4 b Based on total sulfur content in ash c Based on thermogravimetric analysis (TGA) or lime index measurement d Carbonate, as CO2

Bulk Chemical Composition – Reported as Oxides – of SDA Material Reported in Literature Reviewed 

A-5 

Table A-2 

Bulk Chemical Composition of SDA Materials with Precollection of Fly Ash Reported in 

Literature Reviewed 

Source [41] [45] [49] 

No. of 

Samples 3a

 4 Unknown 

Units wt%, 

Dry Basis wt% wt% 

SiO2 7.9 – 10.6 1.4 – 11.1 <2.5 

Al2O3 4.1 – 4.7 0.8 – 5.4 <1.5 

Fe2O3 1.9 – 4.4 0.4 – 1.8 <0.7 

CaO 33.6 – 37.8 32.9 – 46.9 35–60 

SO3 17.4 – 30.0 — 2–17 

MgO 1.0 – 1.9 0.4 – 1.7 <0.8 

Na2O 0.2 – 0.3 — <0.1 

K2O 0.3 – 0.6 — <0.2 

TiO2 — 0.0 – 0.2 <0.1 

LOI or C — 1.3 – 2.1 —

Na2O/K2O — 0.2 – 0.8 —

CaSO3 · 1⁄2 H20 — 50.0 – 62.0 —

CaSO4 · 2 H2O — 4.5 – 10.0 —

CaCO3 — 0.5 – 12.5 —

CaCl2 — 0.6 – 7.2 —

Ca(OH)2 — 2.4 – 18.9 —

Free Lime <0.5 – 4.0 — —

SO4 7.0 – 24.6 — —

SO2 — — 9 – 40 

Total Sulfur 9.3 – 15.7 — —

CO2 10.6 – 20.1 — —

C (org.) 0.1 – 0.2 — —

SO3

–2

— 29.7 – 38.8 —
Bulk Chemical Composition – Reported as Oxides – of SDA Material Reported in Literature Reviewed 

A-6 

Table A-2 (continued) 

Bulk Chemical Composition of SDA Materials with Precollection of Fly Ash Reported in 

Literature Reviewed 

Source [41] [45] [49] 

No. of 

Samples 3a

 4 Unknown 

Units wt%, 

Dry Basis wt% wt% 

SO4

–2

— 2.5 – 6.9 —

Cl 0.93 – 3.0 — 0.5 – 6 

Cl–

— 0.4 – 4.6 —

CO3

–2

— 0.3 – 7.5 —

a

 May contain up to 20% fly ash. 
B-1 

B

TOTAL ELEMENTAL COMPOSITION OF SDA 

MATERIAL REPORTED IN LITERATURE REVIEWED 

Table B-1 Total Elemental Composition of SDA Materials Containing Fly Ash Reported in Literature Reviewed 

Source [12, 13] [17] [30] [30] [31] [35] [37] [44] [46] 

No. of Samples 8 1a 8 8 (by AA) Unknown Unknown 13 1 1 

Units mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg 

Aluminum (Al) 33,300–66,700 — 33,300–66,700 — 18,000–110,000 18,000–230,000 10,000–86,000 — 92,100 Antimony (Sb) 5.6b–29 — 5.6b–29 — 7.5–8 0.8–20 — — — Arsenic (As) 3.6–48 0.4–380 <12–22b 3.6–48 14.2–47 2.3–1200 6.9–165 28.7 4 Barium (Ba) 190–3090 — 190–6060 — 100–2000 20–12,000 0.76 3200 Beryllium (Be) 1.4–3.1 — 0.8–3.1 — 4–63 0.94–63 0.7–9.6 — 3 Boron (B) <10–230 — <10–230 — 500 10–1300 292–948 1460 650 Bromide (Br) — — — — 0.3–21 0.3–21 — — — Cadmium (Cd) 0.2b–0.63 0.1–5 <0.4–5.8 <0.2–0.63 0.5–70 0.01–70 1.1–40.7 0.88 <4 Calcium (Ca) 14,300–223,000 — 126,200–223,000 — — 7100–360,000 178,000–401,000 — 159,400 Cesium (Cs) — — — — — 1–22 — — — Chloride (Cl) — — — — — <0.1–10,200 — — — Chromium (Cr) 36–210 — 36–210 — 27–130 3.6–1000 11.4–56.3 14 62 [Cr(VI): <6] Cobalt (Co) 9.9–26 <0.5–62 9.9–26 — 4.8–80 4.8–172 8.0–20.8 — <20 Copper (Cu) 20–160 — 20–160 — 8.1–170 7.1–655 13.3–73.2 30 61 Fluoride (F) — — — — 0.4–1000 0.4–1000 — — — Iodine (I) — — — — — 0.1–0.6 — — — Iron (Fe) 10,900–53,700 — 10,900–53,700 — 10,000–367,000 6300–367,000 19,000–51,000 — 29,300 Lead (Pb) <0.3–19 4–550 <16–44b <0.3–14 4.4–150 3.1–800 6.5–139 16.3 <20 Lithium (Li) 18–42 — 18–42 — — 48–530 15.1–90.5 — 110 Magnesium (Mg) 3880–27,700 — 3880–27,700 — — 3000–22,000 3800–151,000 — 17,900 Manganese (Mn) 55–680 — 55–680 — 45–630 24.5–1432 43–501 — 680 Mercury (Hg) <0.05–0.39 <0.1–10 — <0.05–0.39 BDL–0.5 <0.005–2.5 — 0.631 <0.02 Molybdenum (Mo) 2.7–514 — 2.7–514 — 0.5–33 0.5–110 <0.02–50.2 — <20 Nickel (Ni) 17–110 1.4–125 17–110 — 13–460 1.8–460 12.4–58.9 13 <20 

Table B-1 (continued) Total Elemental Composition of SDA Materials Containing Fly Ash Reported in Literature Reviewed 

Source [12, 13] [17] [30] [30] [31] [35] [37] [44] [46] 

No. of Samples 8 1a 8 8 (by AA) Unknown Unknown 13 1 1 

Units mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg 

Phosphorus (P) — — — — — — 21–1752 — 2200 Potassium (K) 1760–7020 1760–7020 — — 1700–9300 1600–5000 — 4700 Rubidium (Rb) — — — — — 48–530 — — Selenium (Se) <0.4–7.3b 3–34 71b–283 <0.4–7.3b 4.7–20 0.6–760 4.9–23.0 5.4 <4 Silicon (Si) 72,800–120,000 — 72,800–120,000 — — 70,000–99,000 22,000–137,000 — 157,200 Silver (Ag) 1.6b–5.8 — 1.6b–5.8 — 0.2–0.5 0.04–8 — 0.20 <4 Sodium (Na) 6540–34,000 — 6540–34,000 — — 710–240,000 — — 13,500 Strontium (Sr) 184–3040 — 184–3040 — 84–2500 30–13,000 153–1370 — 4300 Sulfur (S) — — — — — — 42,000–163,000 — — Thallium (Tl) <18 <10 <18 — 20–30 0.1–42 — — <1 Tin (Sn) <19–349 — <24–349 — 30–36 0.01–962 — — Titanium (Ti) 1610–5360 — 1610–5360 — — 1050–6700 — — 3700 Uranium (U) <5–140 — <25–140 — — 0.8–30 — — — Vanadium (V) 72–180 — 72–180 — 0.4–610 0.4–950 22.5–83.9 49.7 86 Zinc (Zn) 22–110 — <6–79 — 12–330 12–9000 52–266 25 70 

aHigh fly ash. bIndicates less than 5 times the detection limit. cMultiplying or dividing by 2 can give the range. dCombination of LECO and ICP concentrations.

B-7 

Table B-2 

Total Elemental Composition of SDA Materials with Precollection of Fly Ash Reported in 

Literature Reviewed 

Source [41] [49] 

No. of Samples 3a Frequent Conc.b

Units mg/kg mg/kg 

Arsenic (As) 4.8–11 —

Barium (Ba) 270–4000 —

Boron (B) 100–150 —

Cadmium (Cd) 0.9–6.9 1 

Chromium (Cr) 34–50 60 

Copper (Cu) 43–47 80 

Lead (Pb) 28–110 100 

Manganese (Mn) 170–420 —

Mercury (Hg) <0.3 <0.1 

Molybdenum (Mo) 2.7–5.4 —

Nickel (Ni) 32–36 80 

Selenium (Se) 3.9–6.5 —

Vanadium (V) 60–70 —

Zinc (Zn) 94–380 120 

a

 May contain up to 20% fly ash. 

b

 Multiplying or dividing by 2 can give the range. 


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