Transportation Research Record 998 47

The Use of By-Product Phosphogypsum for Road Bases and Subbases

C. A. GREGORY, D. SAYLAK, and W. B. LEDBETTER

ABSTRACT The resulting filter cake containing the hydratea calcium sulfate is called phosphogypsum. Typically, the filter cake slurry and wash solution are pipeo Phosphogypsum is a chemical by-product of to large stockpiles where they are allowed to dis­ phosphoric acid processing plants and con­ perse. sists mainly of calcium sulfate dihydrate Approximately 4.5 to 5.5 tons of phosphogypsum (Caso4 • 2H20). Because of trace impurities, are generated for every ton of phosphoric acid pro­ this material has not been used to replace duced (2). Worldwide demand for phosphoric acid natural gypsum in the housing industry caus­ further -magnifies the problem of efficiently and ing domestic stockpiles to accumulate at economically dealing with growing phosphogypsum in­ rates that could produce an inventory of one ventories. In 1980 phosphogypsum was generated at an billion tons by the year 2000. Environmental annual rate of 840,000 tons in Australia alone !ll. pressures as well as increased land costs In 1979 Japan was producing phosphogypsum at a rate associated with stockpiling phosphogypsum of 2.748 million metric tons per year (4). In Flor­ are causing researchers to look for better ida, more than 334 million tons of phosphogypsum had utilization of this material. One concept been stockpiled as of 1980. At that time, Florida's involves the use of fly ash or portland ce­ phosphogypsum production was 33 million tons per ment stabilized by-product phosphogypsum year. Long-term projections predict that more than 1 mixtures for construction of road bases and billion tons will be stockpiled in Florida by the subbases. The results of a study conducted year 2000 (5). Obviously, these large quantities of by the Texas Transportation Institute for material must be dealt with before they become un­ the Mobil Chemical Company are presented. manageable. Similar problems with utilization and Phosphogypsum from the Mobil Chemical Com­ disposal of this material also exist in Europe and pany's stockpile in Pasadena, Texas, is used Canada (§.l. in the study. use of this locally available Crystals of calcium sulfate can exist in at least material would result in significant energy three forms: anhydrate (CaS04), hemihydrate (CaS04 • savings by reducing expensive transportation l/2H20) and dihydrate (Caso4 • 2H 20) • The percentages charges usually incorporated in the cost of of each crystal type in a given sample determine, conventional construction materials that are both directly and indirectly, the properties of the presently used in the Houston area. Factors phosphogypsum. Goers (7) states that crystal type involving strength development such as de­ and size affect the aiii"ount of phosphoric acid re­ gree of compaction, stabilizer content, op­ maining in the filter cake after filtration, which, timum moisture content, and chemical make-up in turn, may affect the engineering properties of of the phosphogypsum are presented relative the phosphogypsum (!!_). to acceptability for use in base courses and Chemical impurities in phosphogypsum hinder cost­ s ubbases. In particular, excellent compres­ effective utilization. A study presented by Sweeney sive strengths were obtained so long as com­ and May (5) summarizes an analysis of nine different pacted dry densities were at least 97 per­ phosphogypsum storage sites in Florida. Thirty-nine cent of optimum modified Proctor. It was different elements were detected. The same study also found that the pH of the phosphogypsum also examined radiation emissions from the stock­ may have a significant effect on the com­ piles. Results showed that radium concentration av­ pressive strength. Strengths of mixtures eraged 21 picocuries per gram and was greatest in containing highly acidic (pH < 3) phosphogyp­ the finer size fractions. The complexity and variety sum were significantly less than those con­ of components within a stockpile coupled with pos­ taining a less acidic (pH around 5) material. sible radiation hazards further deters the wide­ spread acceptance and utilization of phosphogypsum in building materials and cements. Present attempts to develop beneficial uses of phosphogypsum can be summarized as follows:

Phosphogypsum is a by-product of phosphoric acia production. Of the several different phosphoric acid production processes, the wet process is the most 1. As a cement retarder <1>, frequently used (.!l. In the wet process, finely 2. As a sulfur source through desulfurization ground phosphate rock, ca10F 2 (P04)6, is dissolved in (!Q), phosphoric acid to form a monocalcium phosphate so­ 3. As building plaster (Plaster of Par is) (11), lution (see Equation 1 below). Sulfuric acid, which and is added to the slurry, reacts with the monocalcium 4. In reclamation of sodic soils <11>. phosphate to produce a hydrated calcium sulfate that can then be separated from the phosphoric acid by filtration (see Equation 2): Each of these applications have met with varying degrees of success. However, the common denominator, Ca10F2(P04)6 + 14H3P04 + lOCa(H2P04l2 + 2HF (1) especially concerning uses in building materials and cement, appears to be the degree of purity of the Ca(H2P04)2 + H2S04 "H20 + CaS04 "H20 + 2H 3P04 (2) phosphogypsum. 48 Transportation Research Record 998

ENERGY CONSIDERATIONS the system. Two such materials readily available are high lime fly ash and portland cement. Each have In 1980 Texas was the nation's leading producer of been used successfully as soil stabilizers for many crushed stone, with 177 quarriers producing 76 mil­ years. High lime fly ash (ASTM c 618 Class C) (17) lion tons 113), Texas is the nation's second leading is a pozzolan that contains appreciable amounts of state in t~ production of construction sand and calcium oxide (CaO) and exhibits a cementitious re­ gravel, producing about 47 million tons in 1980 action with water in addition to its pozzolanic re­ (13). The uses of these materials include: activity. This reactivity could be used to create a mixture comparable to fly ash stabilized soils. 1. Aggregate for asphaltic concrete and portland In general, portland cement is more readily cement concrete, available than is high lime fly ash and does not 2. Material for the construction of highways and have the potential problem of variability associated streets, with a by-product such aa fly ash. Addition of suf­ 3. Material for pipe bedding, and ficient percentages of portland cement may produce a 4. Material for load-bearing surfaces. material comparable to soil cement. This report contains the results of a study in While phosphogypsum does not present a viable alter­ which high lime fly ash and portland cement were native to use Number 1, it should be considered for used as stabilizing agents for phosphogypsum. The the remaining categories. phosphogypsum was collected from two different A significant portion of this construction mate­ stockpiles at the same storage site. Design, con­ rial is needed to satisfy the needs of Houston, struction, and evaluation of seven experimental test Texas. Although Houston accounts for only slightly sections resulted from the laboratory study. more than 20 percent of the state's population, it accounts for some 26 percent of anticipated new highway construction (14). Sizable amounts of aggre­ MATERIALS gate are currently used"""as pipe bedding in sewer and wastewater project construction in Houston. Although Phosehogypsum (PG) the quantities of aggregate used in load-bearing surfaces such as parking lots, container storage All phosphogypsum used in this study was furnished yards, and so forth, is presently not definable, it by Mobil Chemical Company {MCC) of Pasadena, Texas. is possible to say that this application is a poten­ Presently, their inventory contains about 25 million tially viable segment for phosphogypsum application tons of PG that are stored in three separate stock­ (.!!_) • piles referred to as Pile Nos. 1, 2, and 3, respec­ Although an aggregate demand exists, an adequate tively. The numbering system reflects the relative supply in the Houston area does not. "There is vir­ age of the pile with Pile No. l being the oldest. tually no crushed stone production near the Houston Pile Nos. 2 and 3 are considered active piles, but market. The production occurs in the San Antonio, only the latter is currently being used for PG dis­ Austin, and Georgetown, Texas, area--approximately charge and therefore has been exposed to the ele­ 200 miles from the Houston market" (14). Some large ments for the shortest time. Initial research in­ shipments have been made to Houston area markets volving the use of Pile No. 3 material produced from as far away as Kentucky. Although little or no mixtures with low compressive strengths. Mixtures crushed stone is produced in Houston, there is a prepared with PG from Pile No, 2 possessed more ac­ significant volume of sand and gravel quarried in ceptable strength characteristics. Therefore, subse­ the same area. One major supplier is located approx­ quent laboratory and field experimentation involved imately 60 miles from Houston city limits. material from Pile No. 2. Chemical and physical It comes as no surprise, then, that transporta­ properties of phosphogypsum from Piles 2 and 3 are tion charges represent a significant share of the given in Table 1. total costs of aggregate materials (including sand and gravel) in the Houston market <.!!> • As stated by one observer, "the major factor, which gives a com­ Fly Ash (FA) mon and nearly ubiquitous mineral resource a measure of intrinsic value, is the cost of transporting the The fly ashes used in this study were furnished from mineral to market• (12_). the W.A. Parrish power plant located in Thompsons, By using a supply of aggregate material located Texas. Laboratory analyses were conducted in which nearer the market, there would be a significant re­ the ash was established to be acceptable for classi­ duction in energy consumption. It is estimated that fication as a Class c fly ash in accordance with approximately 410 BTUs are required to move a ton of ASTM c 618 (!l.). The results of the chemical analy­ aggregate one mile by rail (BTU/TM) (16). Aggregate sis are given in Table 2. Total Cao contents analy­ shipments by truck require 2,280 BTUs/TM (16). It is ses were also performed by use of a heat evolution obvious that by using an aggregate such a-;:- phospho­ test developed at Texas Transportation Institute gypsum located in the Houston area, considerable (18). A total cao content of 25,4 percent was deter­ amounts of energy, both in plant operations and mined. hauling, could be saved. In addition, pH and specific gravity measurements produced values of 11.6 and 2.55, respectively. Par­ ticle size analysis showed the fly ash fineness, POSSIBLE SOLUTIONS which was evaluated on the basis of the amount re­ tained on the Nos. 200 and 325 sieves to be 10 per­ As stockpiles continue to grow and environmental cent and 15 percent, respectively. constraints become more str inge.nt, widespread uses of phosphogypsum must be developed. One such possi­ ble use, especially in aggregate-poor areas such as Portland Cement (PC) Houston, could be as a base material in highway con­ struction. If feasible, large tonnages of stockpiled The cement used in this study was Type I. common material could be used. In an effort to enhance the percentages of cement us d for stabilization range engineering properties of most phosphogypsum, some from 3 to 8 for sands and gravels to 10 to 16 for cementitious or pozzolanic !:finder could be added to plastic clays (19). Gregory et al. 49

TABLE 1 Chemical Breakdown and Physical LABORATORY STABILIZATION PROGRAM Properties for MCC Phosphogypsum General Pile No. 3 Constituenta (Weight Percent) Unconfined compressive strength was chosen as the governing parameter for evaluation. Typically, rela­ tions have been developed relating unconfined com­ Ca 20.8 pressive strength to other material characteristics S0 53. l (flexural strength, modulus, etc.) for stabilized 4 soils and concrete. Although no such correlation Si02 2.5 currently exists for stabilized phosphogypsum mix­ tures, unconfined compressive strength was selected P0 2. l 4 as an appropriate starting point. Mg 0.1

H 0 20.0 2 Exper imental Outline Other l.4 Results from work performed with Pile No. 3 material were instrumental in the evaluation of pile No. 2 pH material (J_Q). In particular, the effects of pH and Pile No. 2 5.0 dry density on compressive strength helped to limit further experimental efforts to phosphogypsum from Pile No. 3 2.6 the surface of the pile compacted at optimum mois­ (below surface) ture contents. The Pile No. 3 material was acidic Specific Gravity 2.3 with a pH of about 3.0. Dry density levels of at least 97 percent optimum modified Proctor resulted Moisture Content, % 20.0 in a dramatic upturn in strength (see Figure 1). (Average)

aSamples were known to contain varying quantities of Li , Al , Fe and F. These were not qualified due to incomplete dilute mineral acids. 20% FA BO% PG (Pile 3) with pH •3.0

TABLE 2 Laboratory Analysis of Fly Ash

Test Results on Dry Basis

Loss on Ignition ~ 0.01

Silicon Dioxide, Si02 38. 01 95 100 PERCENT ORY DENSITY Aluminum Oxide, Al 2o3 21.44 FIGURE 1 Relationship between optimum dry density and Calcium Oxide, CaO 17.66 unconfined compressive strength for fly-ash stabilized Ferric Oxide, Fe?03 6.13 phosphogypsum from Pile No. 3. Magnesium Oxide, MgO 4.25 Sulfur Trioxide, so 2. 33 To analyze other factors affecting mixture 3 strength, a detailed experimental design was devel­ Titanium Dioxide, Ti0 oped for Pile No. 2 (pH around 5) material. Several 2 l.65 percentages for each stabilizer were investigated. Sodium Oxide, Na 2o 3.79 Strength was measured at four ages with three repe­ Potassium Oxide, K 0 0. 79 titions for each sample type. The 4-in. x 4.5-in. 2 specimens, subjected to modified Proctor compaction Manganese Oxide, Mn0 2 0.04

TABLE 3 Summary uf Mix Designs Evaluated with Pile soils of 200 to 300 psi. All 7-day strengths of the No. 2 Phosphogypsum stabilized mixtures evaluated exceeded 200 psi whereas the 6 percent and 10 percent specimens each exceeded the 300 psi level. Optimuma The Texas State Department of Highways and Public PHc~nt Moi5ture Stabilizer Stabilizer Content Transportation (SDHP'l') specifies a minimum strength of 650 psi after 7 days for cement-stabilized bases (CSB) (25). Although mixtures of Pile No. 3 material Fly Ash 0 14 did notliieet this specification, Pile No. 2 material 10 13 stabilized with 30 percent fly ash and 10 percent portland cement materials met these specifications 15 13 .5 (see Figures 2 and 3). Furthermore, 28-day strengths 20 14 of the 20 percent fly ash specimens exceeded this strength level. With this in mind, stage construc­ 30 13 tion of a pavement system may be a viable possibil­ Portland 14 ity when using such materials. This would mean Cement keeping the base free from heavy traffic and well­ 6 14 drained until sufficient strength had been developed. Stabilized Pile No. 2 strengths were much greater 10 14 than those from stabilized Pile No. 3 phosphogypsum. These differences appear to be linked with pH. aBased on total dry weight of mi x. Strengths from Pile No. 3 material (pH around 3)

~2000 °lo FLY ASH Optimum Modified I Proctor Density ·- 30')'. 1- (.'.) z PG pH =5.0 w 0:: I­ V> w > iii wVl 1000 -· 0:: ----· a.. •-20% :t 0 (.) . - 15% 0 wz ,,,,.~ LL z 200 -·----· •-10% 0 (.) ._0% z 0 :::) -·-·-· . 0 2 10 20 30 AGE(DAYS) FIGURE 2 Strength development of Pile 2 phosphogypsum (stabilized with fly ash) with time.

PG pH = 5 0 ~'a CEMENT -s;1000 a. Optimum Modified Proctor Density I i- (.'.) z w 0:: I­ V> 6% w > ------·- ~ 500 w a..0:: 3% :::!: • 0 (.) 0 w z LL z 100 0 (.) ~ 00 2 10 20 30 AGE (DAYS) FIGURE 3 Strength development of Pile 2 phosphogypsum (stabilized with portland cement) with time. Gregory et al. 51

stabilized with 30 percent fly ash were 300 psi REFERENCES after 28 days. Pile No. 2 material (pH around 5) , compacted and stabilized similarly, achieved 1,800 1. W.O. Arnold and F.J. Hurst. Uranium Control in psi at the same age. Mixtures of Pile No. 2 material Phosphogypsum. Proc., International Symposium reached strengths in excess of 900 psi with the on Phosphogypsum, Florida Institute of Phos­ addition of 10 percent portland cement. phate Research, Vol. 2, 1980, pp. 424-441. In any event, data from two different sources 2. Proc., International Symposium on Phosphogypsum. point out that pH, or something associated with pH, Florida Institute of Phosphate Research, Vol. 1, can have a significant effect on compressive 1980, p. iv. strength of mixtures containing phosphogypsum. 3. J. Beretka. Properties and Utilization of By­ product Phosphogypsum in Australia. Proc., In­ Whether pH itself is the key indicator of phospho­ ternational Symposium on Phosphogypsum, Florida gypsum quality or just a flag for some other factor Institute of Phosphate Research, Vol. II, 1980. has yet to be determined. The determination of this 4. M. Miyamoto. Phosphogypsum Uti1ization in fact, perhaps through neutralization of the material, Japan. Proc., International Symposium on Phos­ should be the subject of further research. Such a phogypsum. Florida Institute of Phosphate Re­ determination may result in a process whereby exist­ search, Vol. 2, 1980, pp. 583-614. ing fresh phosphogypsum could, if necessary, be mod­ 5. A. May and J.W. Sweeney. Assessment of Environ­ ified for use. mental Impacts Associated with Phosphogypsum in Florida. Proc., International Symposium on CONCLUSIONS Phosphogypsum, Florida Institute of Phosphate Research, Vol. 2, 1980, p. 481. The strength of stabilized phosphogypsum was evalu­ 6. R. K. Collings. Phosphogypsum in Canada. Proc. , ated in this study from two different stockpiles International Symposium on Phosphogypsum, Flor­ 1ocated within the same storage site. The effects of ida Institute of Phosphate Research, Vol. 1, stabilizer type and content, age, and pH on uncon­ 1980, pp. 1-6. fined compressive strengths were investigated. 7. W.E. Goers. Nissan Hemi Phosphogypsum. Proc., International Symposium on Phosphogypsum, Flor­ Based on the results generated to date, the fol- 1owing conclusions are drawn. ida Institute of Phosphate Research, vol. 1, 1980, pp. 36-52. 8. F.C. Appleyard. Gypsum Industry in the United 1. The potential exists for use of fly ash-phos­ States. Proc., International Symposium on Phos­ phogypsum and portland cement-phosphogypsum mixtures as a base materia1. phogypsum, Florida Institute of Phosphate Re­ search, Vol. 1, 1980, pp. 66-100. 2. Stabilizer content, mixture dry density, and phosphogypsum pH have a major influence on uncon­ 9. G. Erlenstadt. Upgrading of Phosphogypsum for fined compressive strength of the stabilized mix­ the Construction Industry. Proc., International tures. Symposium on Phosphogypsum, Florida Institute 3. The ASTM specification for lime-fly ash sta­ of Phosphate Research, Vol. 1, 1980, pp. 254- bilized material (400 psi) was met with the addition 293. of 20 percent fly ash. 10. T.D. Wheelock. oesulfurization of Phosphogyp­ sum. Proc., International Symposium on Phospho­ 4. Portland Cement Association recommendations for unconfined compressive strength for soil cement gypsum, Florida Institute of Phosphate Re­ (200 to 300 psi) were met by using at least 3 per­ search, Vol. 2, 1980, pp. 376-400. cent portland cement and modified Proctor compaction 11. E. Prandi. Use and Valor ization of Phosphogyp­ procedures. sum in Road Construction and Civil Engineering. 5. Compressive strength of stabilized samples Proc., International Symposium on Phosphogyp­ prepared with Pile No. 3 material (with a phospho­ sum, Florida Institute of Phosphate Research, gypsum pH around 3) did not meet SDHPT specifica­ Vol. 1, 1980, pp. 310-324. tions for cement-stabilized base (650 psi in 7 12. U.N. Mishra. use of Phosphogypsum in Reclama­ days). Compressive strength of stabilized samples tion of Sodic Soils in India. Proc., Interna­ prepared with Pile No. 2 material (with a phospho­ tional Symposium on Phosphogypsum, Florida In­ gypsum PH around 5) did meet this specification when stitute of Phosphate Research, Vol. 1, 1980, mixed with 30 percent fly ash or 10 percent portland pp. 256-282. cement. 13. The Mineral Industry of Texas. Bureau of Eco­ nomic Geology, University of Texas, Austin, Tex., 1980. FUTURE WORK 14. J .T. Lamkin and D.H. Robertson. The Potential for Phosphogypsum as a Competitive Product in Although much as been learned about the properties the Aggregate Industry. Prepared by the Texas of phosphogypsum, many areas are left unexplored. Transportation Institute, College Station, Nov. The actual role played by phosphogypsum as reflected 1982. by its pH must be determined. Successful research 15. J,R, Dunn, et al. How Valuable Are Your Mineral may lead to phosphogypsum modification for more op­ Resources? Rock Products, Sept. 1970. timum utilization. Other engineering properties for 16. A.B. Rose and K.J. Reed. Energy Intensity and stabilized phosphogypsum should also be evaluated. Related Parameters of Selected Transportation These include determination of wet-dry susceptibil­ Modes: Freight Movement. Oak Ridge National ity, fatigue properties, shrinkage characteristics Laboratory, Tenn., June 1979. and a modulus, preferably by a repeated load tri­ 1 7. 1980 Annual Book of ASTM Standards. Part 14, axial test. ASTM, Philadelphia, Pa. 18. w.c. McKerrall, W.B. Ledbetter, and o.J. Teague. Analysis of Fly Ashes Produced in ACKNOWLEDGMENT Texas. Research Rept. 240-1. Texas Transporta­ tion Institute, College Station, 1979. This work was sponsored under contract by the Pasa­ 19. Portland Cement Association. soil-Cement In­ dena Chemical Corporation of Pasadena, Texas--a spector's Manual. PA0505, Skokie, Ill., 1963. wholly owned subsidiary of Mobil Oil Corporation. 20. C.A. Gregory. Enhancement of Phosphogypsum with 52 Transportation Research Record 998

High Lime Fly Ash. Masters thesis. Texas A&M 25. Cement Stabilized Base. Item 274, Standard University, College Station, May 1983. Specifications for Construction of Highways, 21. 1983 Annual Book of ASTM Standards. Section 4, Streets, and Bridges. Texas State Department of Volume 4.08, ASTM, Philadelphia, Pa. Highways and Public Transportation, 1982. 22. 1979 Annual Book of ASTM Standards. Part 13, ASTM, Philadelphia, Pa. 23. D.L. Smith. Phosphogypsum--Fly Ash: A Base Ma­ terial Combination. Texas A&M University, Col­ lege Station, April 1983. Publication of this paper sponsored by Committee on 24. Portland Cement Association. Soil-Cement Lab­ Energy Considerations in Design and Construction of oratory Handbook. Skokie, Ill., 1979. Transportation Facilities.

Construction and Performance of Experimental Base Course Test Sections Built with Waste Calcium Sulfate, Lime, and Fly Ash

MUMTAZ A. USMEN and LYLE K. MOULTON

ABSTRACT In recent years, considerable attention has been given to the utilization of waste materials in high­ way construction to cope with local aggregate short­ Although a modest amount of information is ages, waste disposal costs, and environmental con­ available on the properties of lime-fly straints. Fly ash, a by-product of the electric ash-calcium sulfate mixtures, little is power industry, has been extensively researched over known about their field performance. Conse­ the past few decades, and the technology for its use quently, an investigation was undertaken to in pozzolanic lime-fly ash-aggregate (LFA) base determine the practical feasibility of using courses is fairly well-advanced (1,2). The actual such mixtures as roadway base course materi­ use of such products, however, has -not been very als. This study involved the construction extensive, probably because of incomplete familiar­ and performance evaluation of four exper i­ ity with field performance and relatively slow mental pavement base course sections 100 ft strength gain. Although fly ash utilization in gen­ long and 10 ft wide incorporating predeter­ eral construction (concrete, lightweight aggregate, mined formulations of waste calcium sulfate fills, etc.) increased significantly in the 1970s, (HF residue), lime and fly ash. Mix I was vast quantities are still unused and undisposed of. composed of 30 percent fine calcium sulfate, Of the total 1978 power plant ash production of 68 5 percent lime, and 65 percent rLy ash; Mix million tons in the United States, only 2~.3 percent II consisted of 50 percent fine calcium sul­ was utilized, and it was projected that nearly one­ fate, 5 percent lime, and 45 percent fly half of the total production by 1990 would still ash; Mix III contained 65 percent fine cal­ have to be disposed of Ill . cium sulfate, 5 percent lime, and 30 percent Waste sulfate is another material generated in fly ash; and Mix IV was composed of 75 per­ large amounts and holding promise as a highway base cent coarse calcium sulfate, 5 percent lime, course component. Included in the category of was·te and 20 percent fly ash. The results of a sulfate materials are mainly the products resulting full range of laboratory studies and field from industrial effluent discharges (hydrofloric evaluation of the performance of the experi­ acid and phosphoric acid production, fluidized bed mental base course sections over an 18-month combustion), flue gas desulfur ization in i;>ower period indicated that all of the mixtures plants by scrubbers, and neutralization of acid mine tested generally had adequate strength and drainage with lime. Annual i;>roduction of waste sul­ durability and performed well in service. It fates in the united States have been estimated to be was thus concluded that such mixtures show between 20 and 30 million tons (4,5), and a drastic good promise for use as pozzolanic base or increase is expected with a full implementation of sub-base courses, and relatively large per­ flue gas desulfu.rization processes, and advent o.f centages of waste calcium sulfate can be fluidized bed plants. With the exception of hydro­ successfully used in such mixtures. Mixtures flor ic acid (HF) residue, such as that produced by II and III exhibited slightly higher the Allied Chemical company's Nitro Plant, which is strength and durability in laboratory tests in solid form, all of these sulfate wastes are pro­ and somewhat better performance in the field duced as sludges with varying percentages of solids compared to Mixtures I and IV. that are composed of calcium sulfate, calcium sul-