Sulfate Wastes - Material Description

ORIGIN

Fluorogypsum and phosphogypsum are sulfate-rich by-products generated during the production of hydrofluoric and phosphoric acid, respectively.

Fluorogypsum

Fluorogypsum is generated during the production of hydrofluoric acid from fluorspar (a mineral composed of calcium fluoride) and sulfuric acid. Fluorogypsum is discharged in slurry form and gradually solidifies into a dry residue after the liquid has been allowed to evaporate in holding ponds. When removed from the holding ponds (if it is to be used), the dried material must be crushed and screened. This produces a sulfate-rich, well-graded sandy silt material with some gravel-size particles. Approximately 90,000 metric tons (100,000 tons) of fluorogypsum are generated annually in the United States, mostly in Delaware, New Jersey, Louisiana, and Texas.

Phosphogypsum

Phosphogypsum is a solid by-product of phosphoric acid production. The most frequently used process for the production of phosphoric acid is the "wet process," in which finely ground phosphate rock is dissolved in phosphoric acid to form a monocalcium phosphate slurry. Sulfuric acid is added to the slurry to produce phosphoric acid (H₃PO₄) and a phosphogypsum (hydrated calcium sulfate) by-product.

Phosphogypsum is generated as a filter cake in the "wet process" and is typically pumped in slurry form to large holding ponds, where the phosphogypsum particles are allowed to settle. The resulting product is a moist gray, powdery material that is predominantly silt sized and has little or no plasticity.

Approximately 32 million metric tons (35 million tons) of phosphogypsum are produced annually, mostly in central Florida, but also in Louisiana and southeastern Texas. Total accumulations of phosphogypsum are well in excess of 720 million metric tons (800 million tons) and are expected to approach 900 million metric tons (1 billion tons) by the year 2000.⁽¹⁾ As a general rule, 4 to 5 metric tons (4.5 to 5.5 tons) of phosphogypsum are generated for every ton of phosphoric acid produced.

CURRENT MANAGEMENT OPTIONS

Fluorogypsum

Recycling

Fluorogypsum is not being used in any commercial applications; however, fluorogypsum has been evaluated for use as a road base material.⁽²⁾ It has also been proposed for use in the production of impure plasterboard.⁽³⁾

Disposal

All fluorogypsum is currently landfilled or disposed of in holding ponds.

Phosphogypsum

Recycling

Phosphogypsum is a calcium sulfate hydrate that is pumped into ponds, eventually dewatered, and ultimately disposed of in large stockpiles called stacks. Phosphogypsum has been recovered and reused with some success in stabilized road bases,⁽⁴⁾ unbound road bases,⁽⁵⁾ and roller-compacted concrete.⁽⁶⁾ Phosphogypsum can be used for agricultural purposes, if the radium-226 concentration of the source material is less than 10 pCi/g.⁽⁷⁾ At the present time, a petition to the EPA is required if phosphogypsum is planned for use in highway applications.⁽⁷⁾

Disposal

At the present time all phosphogypsum is stockpiled in large stacks, some of which may occupy several hundred hectares of land.

MARKET SOURCES

Fluorogypsum

Fluorogypsum may be obtained by contacting the chemical companies that produce hydrofluoric acid in a number of industrialized states, including Delaware, New Jersey, Louisiana, and Texas.⁽⁸⁾

Fluorogypsum that has solidified in sedimentation ponds is removed by blasting, crushing, and screening, much like rock is obtained from a quarry. The recovery and processing of solidified fluorogypsum results in a coarse or "crusher run" type of aggregate with a 38 mm (1-1/2 in) top size and fine calcium sulfate or "natural fines," which is predominantly a sand- and silt-sized material.⁽²⁾ The recovery and processing of solidified fluorogypsum may be accomplished by commercial aggregate producers who are under contract to the chemical companies.

Phosphogypsum

Phosphogypsum may be obtained by directly contacting phosphate producers located mainly in Florida, Louisiana, or Texas, since the companies that mine phosphate rock and produce fertilizers from it also have ownership rights to the phosphogypsum stacks.

There are, however, environmental concerns regarding radon emanation from phosphogypsum stacks. Special petition requirements, which are defined by the EPA for any commercial use or research activity, must be followed before phosphogypsum can be used.⁽⁷⁾

HIGHWAY USES AND PROCESSING REQUIREMENTS

Fluorogypsum

Embankment, Fill, and Road Base Material

Limited local use has thus far been made of reclaimed dried fluorogypsum. This material has been previously used in West Virginia as a fill material, as a subbase material, and as aggregate in a lime-fly ash stabilized base. The solidified fluorogypsum was blasted, removed, crushed and screened prior to being used as a coarse and fine aggregate material in base course applications.⁽¹⁾

Phosphogypsum

Stabilized Base Filler

To date, phosphogypsum has been successfully demonstrated as a road base material in stabilized and unbound base course installations and in roller-compacted concrete mixes. The only processing required for the phosphogypsum is the use of a vibrating power screen to break up lumps prior to mixing with a binder.

MATERIAL PROPERTIES

Fluorogypsum

Physical Properties

As previously noted, fluorogypsum solidifies in holding ponds and must be removed, crushed, and graded, if it is to be used as an aggregate substitute material. In the process of size reduction, coarse 38 mm (1-1/2 in) top size material and fine, minus 2.0 mm (No. 10 sieve), sulfate-rich material is produced. The coarse sulfate is a well-graded sand and gravel size material, while the fine sulfate is a silty-clay type material.

Table 1 presents some typical fluorogypsum particle size ranges, moisture content and specific gravity values. The average moisture content of the coarse sulfate material reportedly ranges from 6 to 9 percent, while the average moisture content of the fine sulfate material ranges from 6 to 20 percent. The average specific gravity of the coarse and fine sulfate is approximately 2.5, indicating that fluorogypsum is slightly lighter in weight than naturally occurring aggregates, such as crushed limestone or sand and gravel.⁽¹⁾ Table 1. Typical physical properties of fluorogypsum.

Property	Value
Size Range	Coarse Fraction - minus 38 mm  (1-1/2 in)Fine Fraction - minus 2.0 mm (No. 10 sieve)
Moisture Content	Coarse Fraction   6 to 9%Fine Fraction    6 to 20%
Specific Gravity	2.5 (Coarse and fine fractions)

Chemical Composition

Table 2 presents an average chemical analysis of samples of coarse and fine fluorogypsum.⁽¹⁾ Fluorogypsum is primarily calcium sulfate with approximately 1 to 3 percent fluoride present. It exhibits slightly acidic properties.

Mechanical Properties

Fluorogypsum particles, although solidified in a holding pond, are relatively soft. The results of Los Angeles Abrasion tests performed on a composite sample of coarse fluorogypsum indicate a relatively high abrasion loss of 84 percent.⁽¹⁾

Table 2. Typical chemical composition of fluorogypsum

Constituent	Coarse Sulfate	Fine Sulfate
Sulfate (CaSO4)	71.0	65.6
Fluoride (F)	1.6	2.5
Free Water	8.6	10.4
Combined Water	14.9	15.2
Acidity (H2SO4)	.06	.06
pHa	4.5	4.6
a. Values of pH expressed in pH units.

Phosphogypsum

Physical Properties

As previously noted, phosphogypsum is a damp, powdery, silt or silty-sand material with a maximum size range between approximately 0.5 mm (No. 40 sieve) and 1.0 mm (No. 20 sieve) and between 50 and 75 percent passing a 0.075 mm (No. 200) sieve size. The majority of the particles are finer than .075 mm (No. 200 sieve), and the moisture content usually ranges from 8 to 20 percent. The silty size range of phosphogypsum would classify it as an A-4 soil in the AASHTO soil classification system.⁽¹⁰⁾

There are two predominant forms of phosphogypsum: dihydrate phosphogypsum (CaSO₄×2H₂O) and hemihydrate phosphogypsum (CaSO₄×½H₂O). Dihydrate phosphogypsum is generally more finely graded than hemihydrate phosphogypsum.

Table 3 presents some typical physical properties of phosphogypsum.

Table 3. Typical physical properties of phosphogypsum.⁽⁵⁾

Property	Value
Specific Gravity	2.3 - 2.6
Compactive Characteristics	1470 - 1670 kg/m3
Maximum Dry Density	(92 - 104 lb/ft3)
Optimum Moisture	15 - 20%

The specific gravity of phosphogypsum ranges from 2.3 to 2.6. The optimum moisture content of either type of phosphogypsum can normally be expected to fall within the range of 15 to 20 percent. The maximum dry density is likely to range from 1470 to 1670 kg/m³ (92 to 104 lb/ft³), based on standard Proctor compaction.⁽⁵⁾

These are typical values for phosphogypsum produced in Florida. Values for phosphogypsum produced in other states (such as Texas or Louisiana) may vary somewhat. The addition of fly ash or Portland cement to phosphogypsum yields slightly higher maximum dry density and optimum moisture content values for stabilized phosphogypsum mixtures, in comparison with unstabilized phosphogypsum blends.⁽¹¹⁾

Chemical Composition

The major constituent in phosphogypsum is calcium sulfate and, as a result, phosphogypsum exhibits acidic properties. Table 4 presents a listing of some typical chemical analyses of phosphogypsum samples from different production areas.⁽¹¹⁾ Phosphogypsum often contains small residual amounts of phosphoric acid and sulfuric acid and also contains some trace concentrations of uranium and radium, which result in low levels of radiation.

Mechanical Properties

Table 5 presents some typical mechanical property values of phosphogypsum. The shear strength of unconsolidated-undrained specimens of unstabilized phosphogypsum has exhibited average internal friction angles of 32 degrees and cohesion values of 125 kN/m² (18 lb/in²). Cement-stabilized specimens have exhibited internal friction angle values ranging from 28 to 47 degrees, and cohesion values from 76 to 179 kN/m² (11 to 26 lb/in²).⁽¹²⁾ Coefficient of permeability values for unstabilized phosphogypsum have been found to range from 1.3 ´10^-4 cm/sec down to 2.1 ´ 10^-5cm/sec.

Table 4. Typical chemical composition of phosphogypsum (percent by weight).⁽¹¹⁾

Constituent	Louisiana	Texas	Florida
CaO	29 - 31	32.5	25 - 31
SO4	50 - 53	53.1	55 - 58
SiO2	5 - 10	2.5	3 - 18
Al2O3	0.1 - 0.3	0.1	0.1 - 0.3
Fe2O3	0.1 - 0.2	0.1	0.2
P2O5	0.7 - 1.3	0.65	0.5 - 4.0
F	0.3 - 1.0	1.2	0.2 - 0.8
pHa	2.8 - 5.0	2.6 - 5.2	2.5 - 6.0
a. Values of pH expressed in pH units.

Table 5. Typical mechanical properties of phosphogypsum.⁽¹²⁾

Property	Value
Friction Angle	32°
Cohesion Values	125 kPa
Coefficient of Permeability	1.3 x 10-4 to 2.1 x 10-5 cm/sec

REFERENCES

1. U.S. Environmental Protection Agency, Office of Solid Waste. Report to Congress on Special Wastes from Mineral Processing. Report No. EPA 530-SW-90-070B, Washington, DC, July, 1990.
2. Usmen, Mumtaz A. and Lyle K. Moulton. "Construction and Performance of Experimental Base Course Test Sections Built with Waste Calcium Sulfate, Lime, and Fly Ash," Transportation Research Record No. 998, Transportation Research Board, Washington, DC, 1984.
3. Clifton, James R., Paul W. Brown and Geoffrey Frohnsdorff. Survey of Uses of Waste Materials in Construction in the United States. National Bureau of Standards, Report No. NBSIR 77-1244, Washington, DC, July, 1977.
4. Gregory, C.A., D. Saylak, and W.B. Ledbetter. "The Use of By-Product Phosphogypsum for Road Bases and Subbases," Transportation Research Record No. 998. Transportation Research Board, Washington, DC, 1984.
5. Chang, Wen F., David A. Chin and Robert Ho. Phosphogypsum for Secondary Road Construction. Florida Institute for Phosphate Research, Publication No. 01-033-077, Bartow, Florida, June, 1989.
6. Chang, Wen F. A Demonstration Project: Roller Compacted Concrete Utilizing Phosphogypsum. Florida Institute for Phosphate Research, Publication No. 01-068-072, Bartow, Florida, December, 1988.
7. Code of Federal Regulations. "National Emission Standards for Hazardous Air Pollutants," 40 CFR Part 61. July 1, 1996.
8. Smith, L.M. and H.G. Larew. User's Manual for Sulfate Waste in Road Construction. Federal Highway Administration, Report No. FHWA-RD-76-11, Washington, DC, December, 1975.
9. American Coal Ash Association. Coal Combustion By-Product Production and Use: 1966-1993. Arlington, Virginia, 1995.
10. ASTM D3282. "Standard Practice for Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.08, ASTM, West Conshohocken, Pennsylvania, 1994.
11. Taha, Ramzi and Roger Seals. "Engineering Properties and Potential Uses of By-Product Phosphogypsum." Proceedings of Utilization of Waste Materials in Civil Engineering Construction. American Society of Civil Engineers, New York, NY, September, 1992.
12. Lopez, Alfred M. and Roger K. Seals. "The Environmental and Geotechnical Aspects of Phosphogypsum Utilization and Disposal,"Environmental Geotechnology. Rotterdam, The Netherlands, 1992.

Sulfate Wastes - Stabilized Base

INTRODUCTION

Sulfate wastes, in particular, fluorogypsum and phosphogypsum, can be used in stabilized base and/or subbase applications. These materials are stabilized using either lime and pozzolanic fly ash, Portland cement, or self-cementing fly ash. The stabilized base material is produced, placed, and compacted in essentially the same manner as other lime-fly ash or cement-stabilized base materials. Stabilized base mixtures containing sulfate wastes have strength development and durability characteristics that are comparable to those of conventional stabilized base materials. Any proposed use of phosphogypsum requires special petition to the EPA to demonstrate that the proposed practices will not result in any adverse health or environmental impacts.⁽¹⁾

PERFORMANCE RECORD

There have been a limited number of stabilized base installations in which either fluorogypsum or phosphogypsum have been used. The most extensive experience to date with each of these materials has been in or around Houston, Texas.

Fluorogypsum

The earliest known application in which fluorogypsum was used in stabilized base compositions occurred during the construction of the TRANSPO’72 International Transportation Exposition held in 1972 at Dulles Airport near Washington, D.C. A portion of the new 40 hectare (100 acre) parking area was used by the Federal Highway Administration (FHWA) to demonstrate the potential for use of base course mixtures containing a number of calcium sulfate wastes including fluorogypsum.

A portion of the parking area was paved with a 127 mm (5 in) thick compacted mixture of 2.5 percent lime, 62 percent fly ash, 18 percent bottom ash, 13 percent crushed limestone, and 2.5 percent fluorogypsum.⁽²⁾ The overall results from the TRANSPO’72 demonstration project indicated that lime-fly ash-sulfate mixtures were potentially useful for highway base course construction. However, the combination of poor weather and saturated subgrade conditions at the time of construction eventually resulted in the removal and replacement of the experimental base materials.⁽³⁾

Fluorogypsum use in stabilized road base has been limited to one demonstration project in West Virginia and several construction projects in Texas. In August 1981, four test sections of lime-fly ash-fluorogypsum, each approximately 3.0 meters (10 feet) wide by 30 meters (100 feet) long, were constructed as side by side pairs within the base course of a plant haul road at the Allied Chemical Company plant in Nitro, West Virginia. Each test section had an increased percentage of fluorogypsum and a decreased percentage of fly ash, while using 5 percent lime. One test section contained 75 percent coarse fluorogypsum sulfate, while the other three contained from 30 to 65 percent fine fluorogypsum.

The 28-day compressive strengths of laboratory test mixes from each of these four test sections ranged from 5090 kPa (738 lb/in²) to 7625 kPa (1,105 lb/in²). Retained strengths after freeze-thaw durability testing ranged from 73 to 109 percent of control. All four test sections exhibited satisfactory performance during the first 18 months of exposure and service.⁽⁴⁾

Gulf States Materials, in the Houston area, has produced a number of cement and/or self-cementing fly ash stabilized base materials in which fluorogypsum was used. These compositions have been used in lower volume applications such as parking lots and access roads for businesses and shopping centers, as well as base course layers for residential streets. There are no documented instances of unsatisfactory performance of stabilized fluorogypsum in these applications.⁽⁵⁾

Phosphogypsum

Prior to 1989, when EPA began its regulation of phosphogypsum, a number of stabilized base installations containing phosphogypsum were placed in Florida and Texas. Also, as recently as 1992, the Institute of Recyclable Materials at Louisiana State University had applied to the Louisiana Department of Environmental Quality for a permit to construct a phosphogypsum cement-stabilized road base.⁽⁶⁾

The use of phosphogypsum in stabilized base installations placed in Florida has been limited to only a few experimental pavements involving either phosphogypsum-sand blends sealed with asphalt or roller-compacted concrete mixtures containing Portland cement, sand, and phosphogypsum.⁽⁷⁾

In September 1986, a 2.3 km (1.4 mi) section of roadway in central Polk County was stabilized by placing from 76 mm (3 in) to 152 mm (6 in) of dihydrate phosphogypsum and then mixing the phosphogypsum with the underlying sandy subgrade to a depth of 250 mm (10 in) to 380 mm (15 in) by means of a pulvi-mixer. The base surface was sealed with an RC-70 cut back asphalt, then overlaid with a 25 mm (1 in) layer of asphalt surface.⁽⁸⁾

In late 1986, approximately 2.7 km (1.7 mi) of a section of White Spring Road in northern Columbia County was reconstructed using a blend of one part dihydrate phosphogypsum to two parts subgrade sand by weight as a stabilized base. Approximately 127 mm (5 in) of phosphogypsum was mixed by pulvi-mixer to a depth-approximately 350 mm (14 in) with the sandy soil. The base surface was sealed with an RC-30 cut back asphalt and covered with 25 mm (1 in) to 50 mm (2 in) asphalt concrete surface.⁽⁸⁾

The Polk County roadway was compacted to between 92 and 98 percent of a modified Proctor density. On-site CBR (California Bearing Ratio) measurements taken 1 to 2 months after construction indicated CBR values ranging from 43 to over 100 percent. The Columbia County roadway was compacted to between 92 and 100 percent of the modified Proctor density. The Clegg impact tester was used in lieu of on-site CBR testing for the Columbia County roadway. The Clegg impact values (CIV) ranged from 14 to 38 with an average CIV of 24.3. This CIV corresponds with a CBR value of approximately 40 percent. Minimal distress was observed in these two pavements, indicating the suitability of phosphogypsum as a potential component of stabilized base material.⁽⁸⁾

In February 1988, a demonstration roller-compacted concrete pavement was constructed in the service driveway and parking areas of the Florida Institute for Phosphate Research (FIPR), located in Bartow, Florida. Approximately 1700 m² (2,000 yd²) of pavement were involved. The roller-compacted concrete mix consisted of 13 percent dihydrate phosphogypsum, 14 percent Type II (sulfate resistant) Portland cement, and 73 percent coarse and fine limestone aggregate, proportioned by dry weight. A power screen was used to break up the lumps in the phosphogypsum. The screened phosphogypsum, Portland cement, and limestone aggregates were blended together in a pugmill mixer.⁽⁹⁾

The roller-compacted concrete mix was designed for a compressive strength of 17 MPa (2,500 lb/in²). Compressive strengths of laboratory Proctor specimens after 74 hours averaged 12 MPa (3,000 lb/in²). No cracking was observed in the pavement after 3 months in service. Limited observations of performance have led to the conclusion that phosphogypsum-based-roller- compacted concrete is suitable for the construction of parking lot facilities.⁽⁹⁾

From 1983 to 1989, cement-stabilized (or self-cementing fly ash-stabilized) phosphogypsum was frequently used as a roadbase material for city streets, parking lots, truck terminals, shopping centers, and loading platforms in and around the Houston area. The lone supplier of the stabilized base with phosphogypsum (Mobil Chemical Company) has reported greater than a 95 percent success rate on more than 100 projects.⁽⁵⁾

The only unsatisfactory stabilized phosphogypsum projects in Texas resulted from attempts to extend the use of these types of base course installations to state or federally funded roadways. One project using a 10 percent cement-stabilized phosphogypsum base on Texas State Highway 146 proved unsuccessful. Two other projects using varying amounts of self-cementing fly ash and/or Portland cement as stabilization reagents had to be replaced after less than a year in service.⁽¹⁰⁾

When construction difficulties were encountered, problems were related to either excessive moisture, overstabilization (accompanied by swelling), incomplete mixing, insufficient compaction and sealing, or incompatible stabilizers and prime coats.⁽⁵⁾

MATERIAL PROCESSING REQUIREMENTS

Fluorogypsum

Excavation

Fluorogypsum solidifies in holding ponds and must be excavated and removed, which normally results in both a fine and a coarse fraction. The resultant products are referred to as coarse and fine sulfate.

Crushing or Screening

Besides the initial excavation and breakdown into different size pieces, further processing will usually require some type of crushing and/or screening, much like a conventional aggregate. This can be achieved using standard construction and/or mineral processing equipment.

Phosphogypsum

Drying

Phosphogypsum is a fine, damp, silty material. Construction of stabilized base mixtures in which phosphogypsum is to be dumped and spread, then mixed in place with other additives (sand, cement, and/or self-cementing fly ash) using a pulvi-mixer, essentially requires no processing unless the phosphogypsum is excessively wet. In such cases, the material should be allowed to dry somewhat before being used. If unable to be satisfactorily dried in a stockpile, the wet phosphogypsum may need to be spread out in fairly thin layers for a few days.

Screening

If the phosphogypsum is to be blended with the other additives using a mixing plant, the phosphogypsum should first be passed over a power screen in order to break up any lumps prior to being fed into the plant.

ENGINEERING PROPERTIES

Fluorogypsum

Some of the engineering properties that are of particular interest if fluorogypsum is used as an aggregate in stabilized base and subbase applications include gradation, moisture content, specific gravity, and Los Angeles Abrasion loss.

Gradation: Coarse fluorogypsum typically consists of a combination of gravel and sand size particles, with a top size of 50 mm (2 in), a range of 18 to 50 percent passing the 4.75 mm (No. 4) sieve, and between 10 and 20 percent passing the 0.075 mm (No. 200) sieve. Fine fluorogypsum is a predominantly sand and silt size material with a rather narrow range of gradation, having a top size of 2 mm and from 25 to 50 percent passing a 0.075 mm (No. 200) sieve.⁽⁴⁾

Moisture Content: Coarse fluorogypsum has a moisture content from 6 to 9 percent with an average of 7.5 percent. Fine fluorogypsum has a moisture content that ranges from 6 to 20 percent, with an average of 11 percent.

Specific Gravity: The specific gravity of coarse and fine material is quite uniform and averages 2.50, indicating the fluorogypsum is slightly lighter in unit weight than sand, gravel, or crushed stone.⁽⁴⁾

Los Angeles Abrasion Loss: The Los Angeles abrasion test (ASTM C131)⁽¹¹⁾ was performed on a composite sample of coarse fluorogypsum. This material corresponded to grading "A" in the test method and had an abrasion loss of 84 percent, which is normally considered high for a base course aggregate. However, the fines generated were nonplastic and potentially reactive, which could enhance the binding of the stabilized matrix (develop higher strength).⁽⁴⁾

Phosphogypsum

Some of the engineering properties of phosphogypsum that are of particular interest in stabilized base and subbase applications include gradation, moisture content, specific gravity, moisture-density relationship, and unconfined compressive strength. It should also be noted that phosphogypsum is usually slightly radioactive and that the level of radioactivity must be determined prior to making any use of the material.

Gradation: The grain size of phosphogypsum typically ranges from approximately 0.50 mm (0.02 in) to 0.02 mm (0.0008 in), with between 60 and 80 percent of all particles being finer than a 0.075 mm (No. 200) sieve. The gradation is generally determined by the degree of grinding received by the ore during the beneficiation process. Phosphogypsum is generally classified as a silt or sandy silt.⁽¹²⁾ Material that has been stacked for some time tends to develop agglomerations, thus reducing its fineness or percentage passing the 0.075 mm (No. 200) sieve.

Moisture Content: The moisture content or amount of free moisture in phosphogypsum may range from 3 to 20 percent.

Specific Gravity: The specific gravity of solids ranges from 2.30 to 2.50, with an average in the 2.35 to 2.40 range.⁽¹²⁾ Phosphogypsum has little or no plasticity.

Moisture-Density Relationship: Using the modified Proctor compaction method (ASTM D1557),⁽¹³⁾ maximum dry density values ranged from 1440 to 1650 kg/m³ (90 to 103 lb/ft³) with optimum moisture content values varying from 13 to 18 percent. Maximum dry density values using the standard Proctor compaction method (ASTM D698)⁽¹⁴⁾ were always lower, usually by approximately 5 to 10 percent, with optimum moisture content values for stabilized phosphogypsum blends comparable to unstabilized phosphogypsum.⁽¹²⁾

The addition of Portland cement or fly ash to phosphogypsum yields slightly higher maximum dry density and optimum moisture content values for stabilized phosphogypsum blends, in comparison to unstabilized phosphogypsum.⁽¹²⁾

Unconfined Compressive Strength: The unconfined compressive strength of phosphogypsum when compacted in a standard Proctor⁽¹⁴⁾ mold at an optimum moisture content of 16.7 percent is typically 96 kPa (14 lb/in²) when soaked and about 1690 kPa (245 lb/in²) when air dried.⁽⁷⁾ These are values for compacted phosphogypsum without the addition of a stabilization reagent.

DESIGN CONSIDERATIONS

Mix Design

Fluorogypsum

The fluorogypsum materials used in the experimental road base sections in West Virginia consisted of a coarse and a fine-graded sulfate aggregate. These materials were blended with varying percentages of lime and pozzolanic fly ash to produce four trial mixes. In each mix, 5 percent lime was used. Fly ash percentages ranged from 20 to 65 percent. One mix contained coarse calcium sulfate at 75 percent. The three other mixes contained fine calcium sulfate at 30 to 65 percent.⁽⁴⁾

The maximum dry density and optimum moisture content conditions for each of these mixes were initially established using standard Proctor (ASTM D698)⁽¹⁴⁾ test procedures. The moisture content identified as optimum is the recommended moisture content for molding test specimens for unconfined compressive strength testing. Testing for unconfined compressive strength and durability should be done in accordance with ASTM C593⁽¹⁵⁾ procedures.

Instead of curing for 7 days at accelerated (38^oC (100^oF)) temperatures, test specimens may be cured for 7 days at ambient (23^oC (73^oF)) temperatures. Additional test specimens should also be prepared and cured for 28, 56, and 90 days at ambient temperatures. In blends containing fluorogypsum in mixes outlined above, the unconfined compressive strength criteria of 2760 kPa (400 lb/in²) may not be achieved after 7 days of curing, but should be exceeded after 28 days of curing.⁽⁴⁾

Phosphogypsum

The parameter that usually governs mix design proportions for stabilized base compositions containing phosphogypsum is the unconfined compressive strength. Unless local or state specifications require otherwise, the criteria for mix proportioning is typically based on the compressive strength and freeze-thaw durability requirements of ASTM C593.⁽¹⁵⁾ This specification, which was originally developed for lime-fly ash-aggregate base course mixtures, requires that stabilized base mixtures attain 2760 kPa (400 lb/in²) unconfined compressive strength after 7 days of curing at 38^oC (100^oF) and also after vacuum saturation. The fly ash normally used in such mixtures is pozzolanic fly ash.

The optimum moisture content for cement-stabilized base mixes containing phosphogypsum can be determined in accordance with the test procedures of ASTM D558.⁽¹⁶⁾ Some states (such as Texas) have imposed more stringent strength criteria than ASTM C593 where cement-stabilized base mixtures are designed to achieve a minimum compressive strength of 4500 kPa (650 lb/in²) after 7 days of curing at 23^oC (73^oF). Other states (such as Louisiana) specify that soil-cement mixtures have a minimum compressive strength of only 1725 kPa (250 lb/in²) after 7 days of ambient temperature curing.

Depending on weather conditions, it may be advisable to evaluate durability (wet-dry) testing in accordance with ASTM D559.⁽¹⁷⁾ Since phosphogypsum is only produced in Florida, Louisiana, and Texas, it is unlikely that freeze-thaw durability criteria in these locations will be of significance in determining mix design proportions.

It is recommended that stabilized base mixes containing phosphogypsum be designed as close as possible to optimum moisture content and maximum dry density conditions, as determined by either the modified⁽¹³⁾ or standard⁽¹⁴⁾ Proctor test method. The development of compressive strength is directly related to moisture-density characteristics, with the highest strength development occurring at or near the maximum density.⁽¹⁸⁾

To properly perform a moisture-density test for phosphogypsum, the raw phosphogypsum should be initially dried at a temperature of 55^oC (131^oF) to a constant weight. When using Portland cement as the stabilization reagent, type II (sulfate resistant) cement should be selected.⁽¹⁸⁾

Trial mixes of phosphogypsum and reagent should be prepared at optimum moisture content using increasing reagent percentages. Test specimens should be sealed in plastic bags, cured, and then tested for 7 day unconfined compressive strength. Longer term (28, 56, and 90 days) curing and compressive strength testing is also recommended to ensure that the design mix does not lose strength over time.

In preparing trial mixes, grinding of the phosphogypsum lumps insures the homogeneity of the mix.⁽⁹⁾ An added caution in designing trial mixes with phosphogypsum is not to add excessive reagent in order to obtain strength because swelling may result. It should also be noted that mixtures containing highly acidic (pH less than 3) phosphogypsum were found to result in significantly lower strengths than mixtures containing less acidic (pH around 5) material.⁽¹⁹⁾

Structural Design

The structural design of stabilized base or subbase mixtures containing sulfate waste (fluorogypsum or phosphogypsum) can be undertaken using the structural equivalency design method described in the AASHTO Design Guide.⁽²⁰⁾

Table 6 lists recommended structural layer coefficient values for stabilized base or subbase mixtures. These coefficient values are based on the use of a₁ = 0.44 (used for a bituminous wearing surface) and a value of a₃ = 0.15 (used for a crushed stone base). The following structural layer coefficient values are derived from studies of pozzolanic and crushed stone base materials performed at the University of Illinois.⁽²¹⁾

Table 6. Recommended structural layer coefficient values for stabilized base mixtures.⁽²¹⁾

Quality	Compressive Strength (7 days AT 38°C (100°F)) kPa (lb/in2)	Recommended Structural Layer Coefficient
HighAverage Low	Greater than 6900 (1,000)4500 to 6900 (650 to 1,000) 2800 to 4500 (400 to 650)	a2 = 0.34a2 = 0.28 a2 = 0.20

Structural layer coefficient values of 0.30 to 0.35 have been recommended for Portland cement-stabilized bases.⁽²⁰⁾

The main factors influencing the selection of the structural layer coefficient for thickness design using the AASHTO method are the compressive strength and modulus of elasticity of the stabilized base material. The value of compressive strength recommended for determination of the structural layer coefficient is the field design compressive strength. The field design compressive strength is simulated by the compressive strength developed in the laboratory after 56 days of moist curing at 23^oC (73^oF).⁽²²⁾ However, other curing conditions may be required by various specifying agencies.

CONSTRUCTION PROCEDURES

Construction procedures for stabilized base and subbase mixtures in which sulfate wastes (fluorogypsum and phosphogypsum) are used are essentially the same as those used for more conventional pozzolanic stabilized bases and subbases.

Material Handling and Storage

Fluorogypsum, once it has been crushed and sized, can be stockpiled much like any conventional aggregate. However, lengthy stockpiling is not recommended because the particles may begin to weather or break down over an extended period of time.

Phosphogypsum must be evaluated for its radiation level prior to use. If suitable for use (i.e., radiation level less than 10 pCi/g), phosphogypsum can be stockpiled in the same way as soil. The material should not be stockpiled for an extended period under rainy conditions or it may become excessively wet.

Mixing, Placing, and Compacting

The blending or mixing of sulfate waste in stabilized base mixtures can be done either in a mixing plant or in place. Plant mixing is recommended because it provides greater control over the quantities of materials batched and also results in the production of a more uniform mixture. Mixing in place does not usually result in as accurate a proportioning of mix components as plant mixing.

To develop the design strength of a stabilized base mixture, the material must be well compacted and must be as close as possible to its optimum moisture content when placed. Plant-mixed materials should be delivered to the job site as soon as possible after mixing and should be compacted as soon as possible after placement. This is particularly the case with mixtures in which self-cementing fly ash is used as an activator.

Stabilized base materials containing sulfate waste should not be placed in layers that are less than 100 mm (4 in) nor greater than 200 to 225 mm (8 to 9 in) in compacted thickness. These materials should be spread in loose layers that are approximately 50 mm (2 in) greater in thickness prior to compaction than the desired compacted thickness. The top surface of an underlying layer should be scarified prior to placing the next layer. Smooth drum, steel-wheeled static rollers are most frequently used for compaction of base materials containing sulfate waste.

Curing

After placement and compaction, stabilized base materials containing sulfate waste must be properly cured to protect against drying and assist in the development of in-place strength. An asphalt emulsion seal coat should be applied to the top surface of the stabilized base or subbase material within 24 hours after placement. Placement of asphalt paving over the 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 over the material until it has achieved an in-place compressive strength of at least 2415 kPa (350 lb/in²).⁽²²⁾

Special Considerations

Late-Season Construction

Stabilized base materials containing sulfate waste 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 in order to withstand the disruptive forces of such cycles. For northern states, many state transportation agencies have established construction cutoff 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.⁽²²⁾ These cutoff dates are more applicable to fluorogypsum than phosphogypsum, since phosphogypsum is only produced in Florida, Louisiana, and Texas, where winter weather is usually mild.

Use of Self-Cementing Fly Ash

When self-cementing fly ashes are used as a cementitious material in stabilized base mixtures, compaction should be accomplished as soon as possible after mixing. Otherwise, delays between placement and compaction of such mixtures may be accompanied by a significant decrease in the strength of the compacted stabilized base material.⁽²³⁾

Crack-Control Techniques

Stabilized base materials containing sulfate waste, especially those in which Portland cement is used as the reagent, may be subject to crack development. The cracks are almost always shrinkage related and are not the result of any structural weakness or defects in the stabilized base material. Unfortunately, shrinkage cracks eventually reflect through the overlying asphalt pavement and must be sealed at the pavement surface to prevent water intrusion and subsequent damage due to freezing and thawing. Cracking is also likely to occur when sulfate wastes are blended or stabilized with lime and pozzolanic fly ash, or with self-cementing fly ash.

One approach to controlling or minimizing reflective cracking associated with shrinkage cracks in pozzolanic stabilized base materials has been 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 spacings of 9 m (30 ft) have been suggested.⁽²²⁾ For parking lots, the joints should be cut in two directions, perpendicular to each other at approximately the same spacing. The joints should all be sealed using a hot-poured asphaltic joint sealant.

UNRESOLVED ISSUES

Fluorogypsum

Because fluorogypsum is the only sulfate waste that dehydrates to form a solid instead of remaining a slurry or sludge, it has somewhat unique properties or characteristics, especially when crushed and sized into coarse and fine fractions. More information is needed concerning the properties or characteristics of fluorogypsum, along with its long-term field performance in stabilized base applications.

Phosphogypsum

Prior to the use of phosphogypsum, a petition to the EPA must be made in accordance with the provisions outlined in 40 CFR Part 61 of the Code of Federal Regulations.⁽¹⁾

To date, there is no known petition that has been made for the use of phosphogypsum in highway applications under the referenced regulation. The primary issue is the radon content and emissions from products containing phosphogypsum both during and after the useful service life of the product. Additional effort is required to determine what concentration and emission levels would be acceptable, and whether there are suitable applications in which phosphogypsum may be used under the terms of these regulatory provisions.

REFERENCES

1. Code of Federal Regulations. "National Emissions Standards for Hazardous Air Pollutants," 40 CFR Part 61, July 1, 1996.
2. Brink, Russell H. "Use of Waste Sulfate on Transpo'72 Parking Lot," Proceedings of the Third International Ash Utilization Symposium. U.S. Bureau of Mines, Information Circular No. 8640, Washington, DC, 1974.
3. Smith, Lloyd M. and Gordon Larew. "Technology for Using Waste Sulfate in Road Construction," Proceedings of the Fourth International Ash Utilization Symposium, Energy Research and Development Administration. Report No., MERC/SP-76/4, Morgantown, West Virginia, 1976.
4. Usmen, Mumtaz A. and Lyle K. Moulton. "Construction and Performance of Experimental Base Course Test Sections Built with Waste Calcium Sulfate, Lime, and Fly Ash," Transportation Research Record No. 998, Washington, DC, 1984.
5. Saylak, Donald, Scullion, Tom, and D.M. Golden. "Applications for FGD By-Product Gypsum," Proceedings of the Symposium on Recovery and Effective Reuse of Discarded Materials and By-Products for Construction of Highway Facilities. Federal Highway Administration, Publication No. FHWA-PD-94-025, Washington, DC, October, 1993.
6. Lea, Reid, Faschan, Adam, and Marty Tittlebaum. "Environmental Monitoring Plan for a Pilot Study Using Phosphogypsum as a Roadbed Material." Proceedings of Conference on Utilization of Waste Materials in Civil Engineering Construction, American Society of Civil Engineers, New York, New York, September, 1992.
7. Kumbhojkar, A.S. "Utilization of Phosphogypsum as a Construction Material," Proceedings of the Mediterranean Conference on Environmental Geotechnology, Rotterdam, The Netherlands, 1992.
8. Chang, Wen F., Chin, David A., and Robert Ho. Phosphogypsum for Secondary Road Construction. Florida Institute for Phosphate Research, Publication No. 01-033-077, Bartow, Florida, June, 1989.
9. Chang, Wen F. A Demonstration Project: Roller Compacted Concrete Utilizing Phosphogypsum. Florida Institute for Phosphate Research, Publication No. 01-068-072, Bartow, Florida, December, 1988.
10. Wong, C. and M. K. Ho. The Performance of Cement-Stabilized Base on State Highway 146, La Porte, Texas. Texas State Department of Transportation, Research Section, Austin, Texas, October, 1988.
11. ASTM C131. "Standard Test Method for Resistance to Degradation of Small-Size Coarse Aggregate by Abrasion and Impact in the Los Angeles Machine." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.02, West Conshohocken, Pennsylvania, 1996.
12. Taha, Ramzi and Roger Seals. "Engineering Properties and Potential Uses of By-Product Phosphogypsum," Proceedings of Conference on Utilization of Waste Materials in Civil Engineering Construction. American Society of Civil Engineers, New York, NY, September, 1992.
13. ASTM D1557. "Standard Test Methods for Moisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 10-lb. (4.54 Kg) Rammer and 18-in. (457 mm) Drop." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.08, West Conshohocken, Pennsylvania, 1996.
14. ASTM D698. "Standard Test Methods for Moisture-Density Relations of Soils and Soil-Aggregate Mixtures Using 5.5 lb (2.49 Kg) Rammer and 12-in (305 mm) Drop." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.08, West Conshohocken, Pennsylvania, 1996.
15. ASTM C593. "Standard Specification for Fly Ash and Other Pozzolans for Use with Lime." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.01, West Conshohocken, Pennsylvania, 1996.
16. ASTM D558. "Standard Test Methods for Moisture-Density Relations of Soil-Cement Mixtures." American Society for Testing and Materials,Annual Book of ASTM Standards, Volume 04.08, West Conshohocken, Pennsylvania, 1996.
17. ASTM D559. "Standard Methods for Wetting-and-Drying Tests of Compacted Soil-Cement Mixtures." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.08, West Conshohocken, Pennsylvania, 1996.
18. Taha, Ramzi, Roger K. Seals, Willis Thornsberry, and James T. Houston. "The Use of By-Product Phosphogypsum in Road Construction," Presented at the 71st Annual Meeting of the Transportation Research Board, Washington, DC, January, 1992.
19. Gregory, C. A., D. Saylak, and W. B. Ledbetter. "The Use of By-Product Phosphogypsum for Road Bases and Subbases," Transportation Research Record No. 998, Transportation Research Board, Washington, DC, 1984.
20. AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, DC, 1986.
21. Ahlberg, Harold L. and Ernest J. Barenberg. Pozzolanic Pavements. University of Illinois, Engineering Experiment Station, Bulletin 473, Urbana, Illinois, February, 1965.
22. American Coal Ash Association. Flexible Pavement Manual. Alexandria, Virginia, 1991.
23. Thornton, Samuel I. and David G. Parker. Construction Procedures Using Self-Hardening Fly Ash. Federal Highway Administration, Report No. FHWA/AR/80-004, Washington, DC, 1980.