• Introduction
• Matrix New!
• Glossary New!
• Notice

• Environmental and Engineering Evaluation New!
• State Contacts New!
• Cost Issues
• Standards/Specifications New!

• Baghouse Fines
• Blast Furnace Slag
• Coal Bottom Ash/Boiler Slag Updated!
• Coal Fly Ash Updated!
• FGD Scrubber Material Updated!
• Foundry Sand Updated!
• Kiln Dusts
• Mineral Processing Wastes
• MSW Combustor Ash
• Nonferrous Slags
• Quarry Byproducts
• Reclaimed Asphalt Pavement Updated!
• Reclaimed Concrete Material Updated!
• Roofing Shingle Scrap Updated!
• Scrap Tires
• Sewage Sludge Ash
• Steel Slag
• Sulfate Wastes
• Waste Glass

• General Descriptions
• Asphalt Concrete Pavement
• Portland Cement Concrete Pavement
• Granular Base
• Embankment or Fill
• Stabilized Base
• Flowable Fill

[ Material Description ] - [ Stabilized Base ] - [ Flowable Fill ]

FGD SCRUBBER MATERIALUser Guideline

Stabilized Base

INTRODUCTION

Fixated or stabilized flue gas desulfurization (FGD) scrubber material can be used as a stabilized base or subbase material in the same manner as lime-fly ash or cement-stabilized base materials. Fixated FGD scrubber material may be used in an "as produced" condition, provided the material meets specifications, or the FGD scrubber material can be modified with additional reagents such as Portland cement, lime, fly ash, etc. to improve characteristics. In addition to adding fixation reagents, an aggregate material (sometimes coal bottom ash) can be blended with the fixated FGD scrubber material to improve material performance. Properly designed fixated FGD scrubber material has comparable strength development and durability characteristics to that of conventional stabilized base materials.

PERFORMANCE RECORD

According to a 2005 coal combustion product use survey, 12 states have specifications for soil stabilization, which includes stabilized base and subbase materials.⁽¹⁾ FGD scrubber material has been employed as a component in stabilized base and subbase material since the 1970's. Typically dewatered FGD material is combined with fly ash or cement, lime, and aggregate, which is often coal bottom ash. The FGD base mixture is placed and compacted to thicknesses between 150 to 300 mm (6 to 12 in) and then overlaid with a wearing surface. With proper construction, FGD material used in stabilized base applications has performed successfully.

In Florida, crushed lime rock is commonly used as a road base construction material. Laboratory results indicated that base course compositions of lime, fly ash, and dewatered FGD scrubber sludge (referred to as Poz-o-Tec) had better bearing strength characteristics than lime rock base.⁽²⁾ Between 1977 and 1989, 12 FGD base course projects were constructed in central Florida. The base course thicknesses for these projects varied from 150 to 300 mm (6 to 12 in). Performance was reported as highly satisfactory.⁽²⁾

In 1977, a 244 m (800 ft) long section of pozzolanic road base material composed of lime, fly ash, bottom ash, and dewatered FGD scrubber material was placed as a demonstration on a state secondary road in southwestern Pennsylvania. The entire base was placed in one day in a single lift at a compacted thickness of 250 mm (10 in). The base was covered by a 50 mm (2 in) bituminous concrete binder course and a 25 mm (1 in) bituminous concrete wearing surface. Test data and visual observations over a 7 year period following installation demonstrated successful performance. Despite severe freeze-thaw cycles, the road exhibited no potholes or broken areas and the base remained intact as verified by periodic coring. The average unconfined compressive strength of road base cores was 4960 kPa (720 lb/in²) after 1 year, 5800 kPa (840 lb/in²) after 3 years, and 6990 kPa (1014 lb/in²) after 7 years.⁽³⁾

In 1995, a 77 m (250 ft) test section of cement, bottom ash, and fixated FGD scrubber sludge base material was placed as part of a demonstration haul road at the Gavin power plant in Cheshire, Ohio. The base course mix design involved a blend of 40 percent bottom ash and 60 percent fixated FGD scrubber sludge to which 7 percent Portland cement was added. The base course mix was produced in a pugmill mixing plant.⁽⁴⁾ The FGD material base course was compacted to a thickness of 200 mm (8 in) and overlaid with 75 mm (3 in) of asphalt paving. The haul road was used extensively by heavy trucks on a daily basis and was evaluated more than al year after installation and showed no noticeable signs of distress.⁽⁴⁾

Two demonstration test sections, each 92 m (300 ft) long and 300 mm (12 in) thick, were constructed in 1993 at the Riverside Campus of Texas A&M University. The first test section consisted of a blend of self-cementing fly ash and dewatered FGD sludge (fixated FGD materail), coal bottom ash and additional self-cementing fly ash. The second test section consisted of the fixated FGD material blended with type II Portland cement.⁽⁵⁾ The test sections were constructed by spreading the component materials with a motor grader and mixing in place with a pulvi-mixer. A control section, also 93 m (300 ft) long, was also placed between the two test sections. The control section consisted of 300 mm (12 in) of crushed iron ore gravel. After 2 years of monitoring, both of the FGD scrubber sludge base sections demonstrated significantly higher strength and stiffness than the control section.⁽⁵⁾

An additional FGD base course test section was constructed at the Riverside Campus of Texas A&M University in 1999. This research focused on reducing sulfate attack and swelling by using high volume fly ash cement (58 percent Class F fly ash to 42 percent Type I Portland cement) in a FGD stabilized base mix. The FGD base mix consisted of 50 percent bottom ash, 40 percent FGD scrubber material, and 10 percent high volume fly ash cement. This research demonstrated that this FGD base course could be employed under high volume traffic conditions and the use of the high volume fly ash cement reduced the cement demand by 58 percent while at the same time reducing sulfate attack and expansion.⁽⁶⁾

MATERIAL PROCESSING REQUIREMENTS

Material from dry FGD systems is handled in a similar manner as fly ash. Dry FGD material may need moisture content conditioning (to approximately 10 percent) prior to transportation to reduce dusting.⁽⁷⁾ Wet FGD material requires dewatering and fixation, which typically occur at the power plant, before transport and use in stabilized base material.

Dewatering

The calcium sulfite scrubber material from wet FGD systems is generally of toothpastelike consistency (from 25 to 50 percent solids) after dewatering. Dewatering is usually accomplished by belt filter presses or centrifuges. Centrifuges are normally able to dewater the sludge to a higher solids content than filter presses.

Stabilization/Fixation

Dewatered FGD material must be stabilized or fixated by the addition of dry reagents (quicklime and Class F fly ash, Class C fly ash, or Portland cement) before being use in stabilized base compositions. The fixated wet FGD sludge should have a solids content of at least 65 percent prior to compaction. The material can then be placed and compacted as a base material or blended with additional reagent, natural aggregate, or bottom ash to meet strength and durability requirements.

ENGINEERING PROPERTIES

FGD scrubber material properties of interest when using FGD material in stabilized bases and subbases include particle size distribution, moisture content, wet and dry density, hydraulic conductivity, compaction characteristics, compressive strength, resilient modulus, and durability.

Particle Size Distribution :

For most dry FGD systems, the FGD by-product is a fine material of uniform consistency with between 56 and 90 percent of the particles having a diameter smaller than 0.025 mm.⁽⁸⁾ Grain size uniformity coefficients for dry FGD material are in the range of 1.8 to 2.4, which represent a uniform particle size. The particle size distribution and uniformity indicates that dry FGD material can be classified as a silt-sized material.^(7;8)

Dewatered and unstabilized calcium sulfite FGD material consists of fine silt-clay sized particles with approximately 50 percent finer than 0.045 mm (No. 325 sieve). Calcium sulfate FGD material is more course and is classified as a silt with sand. At temperatures above 100 c, calcium sulfate particles have been observed to degrade; therefore, oven drying FGD calcium sulfate can effect the particle size distribution.⁽⁹⁾ Particle size information for wet FGD material (both calcium sulfite and sulfate) is given in Table 8.⁽¹⁰⁾

Table 8. Typical physical properties of wet FGD scrubber material. ⁽¹⁰⁾

Moisture Content : Dry FGD material is collected in a particle control device as a dry product and typically needs to be water conditioned (to approximately 10 percent) to reduce dusting during transport.⁽⁷⁾ Dewatered calcium sulfate FGD scrubber material is typically stabilized or fixated before transport and use as a stabilized road base material. Prior to dewatering, the calcium sulfate slurry has a moisture content of approximately 60 percent. After dewatering, the moisture content is reduced to roughly 35 percent. Following stabilization or fixation, the FGD scrubber material has a moisture content of approximately 15 percent.⁽¹⁰⁾

Wet and Dry Unit Weight : The degree to which FGD scrubber material is treated (i.e. dewatered, stabilized, and fixated) influences the wet and dry unit weight. Table 9 shows the wet and dry unit weight for typical calcium sulfate FGD scrubber material in a dewatered, physically stabilized, and fixated condition.

Table 9. Physical characteristics of typical calcium FGD scrubber material.^(10;11)

Hydraulic Conductivity: As the moisture content of calcium sulfate FGD scrubber material is reduced through dewatering and stabilization, the hydraulic conductivity of the material is also reduced. Prior to dewatering, calcium sulfate slurry has a hydraulic conductivity of approximately 8 × 10^-5 cm/sec. After dewatering, the filter cake has a hydraulic conductivity of approximately 1 × 10^-5 cm/sec; after stabilization the hydraulic conductivity is about 1 × 10^-6 cm/sec; and after fixation the hydraulic conductivity may be as low as 5 × 10^-8 cm/sec.⁽¹⁰⁾ Compacted stabilized FGD material demonstrated a decrease in hydraulic conductivity with an increase in curing time.⁽¹²⁾

Compaction Characteristics : Depending on the amount of fly ash in a stabilized blend, maximum dry unit weight values of fixated FGD scrubber material can range from 10.4 to 15.2 kN/m³ (66 to 97 lb/ft³) at optimum moisture contents ranging from 12 to 37 percent when tested using the standard Proctor (ASTM D698)⁽¹³⁾ test method.^(12;14) Higher proportions of fly ash yield a higher maximum density.⁽¹²⁾

Compressive Strength : The compressive strength of most fixated FGD material base course mixtures continues to increase over time. Strength gain is more gradual with pozzolanic stabilized mixtures than mixtures stabilized with Portland cement or Class C fly ash. After 7 days, compressive strengths can range from 1400 to 2800 kPa (200 to 400 lb/in²) for pozzolanic stabilized mixtures and from 4100 to 6200 kPa (600 to 900 lb/in²) for mixtures stabilized with Portland cement or Class C fly ash. These strengths continue to increase over time and can exceed 13.8 MPa (2000 lb/in²) after one year. There is an increase in the unconfined compressive strength with an increase in the cement content of FGD stabilized mixes.⁽¹⁴⁾ Compactive effort has a significant influence on the compressive strength of stabilized FGD mixtures.⁽¹⁴⁾

Durability : Durability testing of fixated FGD scrubber material may involve either freeze-thaw or wet-dry testing. Freeze-thaw testing should be performed in accordance with ASTM D560.⁽¹⁵⁾ Wet-dry testing should be performed in accordance with ASTM D559.⁽¹⁶⁾ A freeze-thaw study on stabilized calcium sulfite FGD material stabilized with fly ash and lime showed a relationship between higher sample water contents and decrease in strength with freeze-thaw cycling.⁽¹⁷⁾ Increased cure time prior to first freeze improved freeze-thaw durability and a resistance to water infiltration. This study recommends a minimum of 60 days curing time be allowed before freezing temperatures are expected when using FGD material.⁽¹⁷⁾

DESIGN CONSIDERATIONS

Mix Design

Base and subbase mix design for FGD scrubber sludge involves blending fixated FGD sludge with one or more stabilization reagents (lime, fly ash, or Portland cement). The stabilized mix design may also include coal bottom ash or aggregate. Using a series of trial mixtures, final mix proportions are selected on the basis of the results of both strength and durability testing according to ASTM C593 procedures.⁽¹⁸⁾

A minimum unconfined compressive strength of 2800 kPa (400 lb/in²) is recommended after ambient curing for between 14 and 28 days. If Portland cement is used as the stabilization reagent, some states require 4500 kPa (650 lb/in²) unconfined compressive strength after curing for 7 days.

Mixes containing fixated FGD scrubber material should be tested for moisture-density relationships and molded as close as possible to optimum moisture content and maximum dry density. If bottom ash is used as an aggregate, the ratio of FGD scrubber sludge to bottom ash is often in the 1.5:1 to 1:1 range. The addition of bottom ash has been shown to enhance the strength development of stabilized base mixes.

When using Portland cement as a stabilization reagent, type II (sulfate resistant) cement should be used. When a pozzolanic fly ash and quicklime are used to stabilize the FGD scrubber material, adequate strength can usually be achieved by the addition of up to 7 to 8 percent cement by weight of dry solids, or by adding more quicklime If a self-cementing fly ash is used as the FGD material fixation reagent, then adding a lower percentage of cement (possibly 3 to 4 percent) or the addition of more fly ash may be needed to achieve the required strength.

Structural Design

Designing pavement structures that include stabilized base or subbase layers that include FGD material typically follow AASHTO pavement design methods provided in the Guide for Design of Pavement Structures.⁽¹⁹⁾ or the Guide for the Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures.⁽²⁰⁾ The AASHTO methods account for the predicted loading (the number of 80 kN equivalent single axle loads), required reliability (degree of certainty that a design will function properly during the design life), serviceable life (ability to maintain quality during the pavement life), the pavement structure (characterized by the structural number), and subgrade support (related to the resilient modulus of the subgrade).⁽¹⁹⁾

A hierarchical approach in the mechanistic-empirical design method allows for varying levels of material characterization depending on project criteria. Mechanistic material properties such as dynamic modulus, resilient modulus, and Poisson's ratio are employed to evaluate pavement performance. The levels in the hierarchical system directly measure strength characteristics (level 1), use correlations to develop strength characteristics (level 2), or use typical material property default values (level 3). Chemically or cement stabilized base materials are included in the mechanistic-empirical design method.

Pavement design employing a structural number accounts for the relative strength of the constructed materials. The total structural support from the surface course, base course, and any subbase course equals the required structural number. Layer thicknesses are calculated using layer coefficients that define the structural support. The layer coefficients can be obtained from the relationship provided by AASHTO based on CBR or MR.⁽¹⁹⁾

When a Portland cement concrete roadway surface is to be designed with a stabilized base or subbase, the AASHTO structural design method for rigid pavements can be used.⁽¹⁹⁾

CONSTRUCTION PROCEDURES

Construction procedures for stabilized base and subbase mixtures in which fixated FGD scrubber material is used are the same as conventional pozzolanic stabilized bases and subbases.

Material Handling and Storage

Fixated FGD scrubber material is typically stockpiled on a concrete pad at the power plant to allow for initial set of the material, typically less than a few days. The material can then be blended with additional reagent (such as lime or Portland cement), blended with bottom ash, boiler slag, or other aggregate, or transported and placed at a job site in the fixated condition.

Mixing, Placing, and Compacting

Plant mixing of FGD material for stabilized bases is recommended because of the greater control over the quantities of materials batched, which results in a more uniform mixture. Mixing in place of dewatered FGD scrubber material is not recommended because of the toothpastelike consistency.

To develop the design strength of a stabilized base mixture, the material should be well-compacted at the optimum moisture content. Fixated FGD scrubber materials should be delivered, placed, and compacted as soon as possible after mixing. This is particularly the case with mixtures in which Class C fly ash is used as an activator.

Fixated FGD scrubber base materials should be placed in layers that result in a compacted thickness between 10 and 22 cm (4 and 9 in). These materials should be spread in loose layers that are approximately 5 cm (2 in) thicker than the desired compacted thickness. The top surface of an underlying layer should be scarified prior to placing the next layer. Smooth drum, steelwheeled vibratory rollers are most frequently used for compaction, although satisfactory compaction results have also been obtained using smooth drum, steelwheeled static rollers. The smooth drum roller also seals the surface of the road base to minimize adverse impacts from rainfall.⁽²⁾

Curing

After placement and compaction, fixated FGD scrubber base material should be properly cured to protect against drying and assist in the development of in-place strength. An asphalt emulsion seal coat can be applied to the top surface of the stabilized base or subbase material. For most types of stabilized base materials, seal coat is applied within 24 hours after placement. Placement of asphalt paving over stabilized base is recommended within 7 days after the base has been installed. Unless an asphalt binder and/or surface course has been placed over the stabilized base material, vehicles should not be permitted to drive on the stabilized base until an in-place compressive strength of at least 2400 kPa (350 lb/in²) is achieved.⁽²¹⁾

Special Considerations

Cold Weather Construction
Stabilized base materials containing fixated FGD scrubber material that are subjected to freezing and thawing conditions must be able to develop a certain level of cementing action and in-place strength prior to the first freeze-thaw cycle. For northern states, many state transportation agencies have established construction cut-off dates for stabilized base materials. These cutoff dates ordinarily range from September 15 to October 15, depending on the state, or the location within a particular state, as well as the ability of the stabilized base mixture to develop a minimum desired compressive strength within a specified time period.(21) A laboratory study on calcium sulfite FGD material recommends a minimum of 60 days curing time between compaction of the FGD material and the first freeze.⁽¹⁷⁾

Use of Self-Cementing Fly Ash with FGD material
Stabilized base mixtures using self-cementing fly ash as an activator should be placed and compacted as soon as possible after mixing. Delays in placement and compaction of self-cementing fly ash mixes may significantly decrease the strength of the compacted stabilized base material.⁽²²⁾

Crack Control Techniques
Stabilized FGD base layers constructed with fly ash are less likely to produce reflection cracking in overlying pavement as is sometimes the case with Portland cement stabilized base layers. This is most likely due to a less stiff bond. Approaches for controlling or minimizing the potential effects of reflective cracking associated with stabilized base layers have been recommended by the ACAA.⁽²¹⁾

An approach to controlling or minimizing reflective cracking associated with shrinkage cracks in stabilized base materials is to saw cut transverse joints in the asphalt surface that extend into the stabilized base material to a depth of 75 to 100 mm (3 to 4 in). Joint spacing of 9 m (30 ft) have been suggested.⁽²¹⁾ Joints should be sealed with a material such as hot poured asphaltic joint sealant.

ENVIRONMENTAL CONSIDERATIONS

The use of FGD material in base material requires good management and care to ensure that the FGD material does not result in a negative impact on the environment. In particular, areas with sandy soils possessing high hydraulic conductivities and areas near shallow groundwater or drinking aquifers should be given careful consideration. An evaluation of groundwater conditions, applicable state test procedures, water quality standards, and proper construction are all necessary considerations in ensuring a safe final product.⁽²³⁾

WiscLeach is a modeling program specifically developed for coal combustion by-product reuse in highway applications. WiscLeach is available in the public domain and uses analytic methods to simulate two-dimensional flow and transport.⁽²⁴⁾ Factors found to have the greatest influence on concentrations in groundwater are depth to the groundwater table, thickness of a by-product layer, hydraulic conductivity of the least conductive layer in the vadose (unsaturated) zone, hydraulic conductivity of the aquifer, and initial trace element concentrations in the by-product layer.⁽²⁴⁾

REFERENCES

A searchable version of the references used in this section is available here.
A searchable bibliography of FGD scrubber material literature is available here.

1. Dockter BA, Jagiella DM. Engineering and environmental specifications of state agencies for utilization and disposal of coal combustion products. In: 2005 world of coal ash conference, Lexington, KY. 2005.
2. Smith CL. First 100,000 tons of stabilized scrubber sludge in roadbase construction. In: Second international exhibition and conference for the power generation industries - POWER-GEN '89, New Orleans, LA. 1989.
3. Smith CL. FGD sludge C coal ash road base: Seven years of performance. In: Proceedings of the 8th international coal and solid fuels utilization conference, Pittsburgh, PA. 1985.
4. Amaya PJ, Booth EE, Collins RJ. Design and construction of roller compacted base courses containing stabilized coal combustion by-product materials. In: Proceedings of the 12th international symposium on management and use of coal combustion by-products. Electric Power Research Institute; Palo Alto, CA: 1997.
5. Prusinski JR, Cleveland MW, Saylak D. Development and construction of road bases from flue gas desulfurization material blends. In: Proceedings of the eleventh international ash utilization symposium. Electric Power Research Institute; Palo Alto, CA: 1995.
6. Saylak D, Estakhri CK. Stabilization of road bases containing coal combustion by-products sulfates and sulfites using high volume fly ash cement. In: 1999 international ash utilization symposium, Center for applied energy research. University of Kentucky; 1999.
7. Berland TD, Pflughoeft-Hassett DF, Dockter BA, Eylands KE, Hassett DJ, Heebink LV. Review of handling and use of FGD material. Report nr 2003-EERC-04-04, Energy & Environmental Research Center, University of North Dakota; Grand Forks, ND: 2003.
8. Kost DA, Bigham JM, Stehouwer RC, Beeghly JH, Fowler R, Traina SJ, Wolfe WE, Dick WA. Chemical and physical properties of dry flue gas desulfurization products. Journal of Environmental Quality 2005;34:676.
9. Taha R, Saylak D. The use of flue gas desulfurization gypsum in civil engineering applications. In: Proceedings of utilization of waste materials in civil engineering construction. American Society of Civil Engineers; 1992.
10. Smith CL. FGD waste engineering properties are controlled by disposal choice. In: Proceedings of conference on utilization of waste materials in civil engineering construction, New York, NY, American Society of Civil Engineers; 1992.
11. Patton RW. Disposal and treatment of power plant wastes. In: Presented at the society of mining engineers fall meeting, salt lake city, UT. 1983.
12. Butalia TS, Wolfe WE. Evaluation of permeability characteristics of FGD materials. Fuel 1999;78:149-52.
13. ASTM D698-07e1 standard test methods for laboratory compaction characteristics of soil using standard effort (12 400 ft-lbf/ft³ (600 kN-m/m³)). In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2007.
14. Taha R. Environmental and engineering properties of flue gas desulfurization gypsum. Transportation Research Record 1993;1424.
15. ASTM D560-03 standard test methods for freezing and thawing compacted soil-cement mixtures. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2003.
16. ASTM D559-03 standard test methods for wetting and drying compacted soil-cement mixtures. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2003.
17. Chen X, Wolfe WE, Hargraves MD. The influence of freeze-thaw cycles on the compressive strength of stabilized FGD sludge. Fuel 1997;76(8):755-9.
18. ASTM C593-06 standard specification for fly ash and other pozzolans for use with lime for soil stabilization. In: Annual book of ASTM standards. ASTM; West Conshohocken, Pennsylvania: 2006.
19. AASHTO. Guide for design of pavement structures.: American Association of State Highway and Transportation Officials; Washington, DC, 1993.
20. NCHRP 1-37A. Guide for mechanistic – empirical design of new and rehabilitated pavement structures. National Cooperative Highway Research Program, Transportation Research Board; 2004.
21. American Coal Ash Association (ACAA). Flexible pavement manual: Recommended practice - coal fly ash in pozzolanic stabilized mixtures for flexible pavement systems. 1991:128 p.
22. Thornton SI, Parker DG. Construction procedures using self-hardening fly ash. Report nr FHWA/AR/80, 004 Federal Highway Administration; Washington, DC: 1980.
23. Environmental Protection Agency (EPA), Federal Highway Administration (FHWA). Using coal ash in highway construction - A guide to benefits and impacts. ; Report nr EPA-530-K-002:ID: 151. 2005.
24. Li L, Benson CH, Edil TB, Hatipoglu B. Groundwater impacts from coal ash in highways. Waste and Management Resources 2006;159(WR4):151-63.

[ Material Description ] - [ Stabilized Base ] - [ Flowable Fill ]