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• User Guidelines for Waste and Byproduct Materials in Pavement Construction

User Guidelines for Waste and Byproduct Materials in Pavement Construction

ORIGIN

Processing of crushed stone for use as construction aggregate consists of blasting, primary and secondary crushing, washing, screening, and stockpiling operations.⁽¹⁾ Quarry by-products are produced during crushing and washing operations. There are three types of quarry by-products resulting from these operations: screenings, pond fines, and baghouse fines.

Screenings

Screenings is a generic term used to designate the finer fraction of crushed stone that accumulates after primary and secondary crushing and separation on a 4.75 mm (No. 4) sieve. The size distribution, particle shape, and other physical properties can be somewhat different from one quarry location to another, depending on the geological source of the rock quarried, the crushing equipment used, and the method used for coarse aggregate separation. Screenings generally contain freshly fractured faces, have a fairly uniform gradation, and do not usually contain large quantities of plastic fines.⁽²⁾

Settling Pond Fines

Pond fines refer to the fines obtained from the washing of a crushed stone aggregate. During production, the coarser size range (greater than No. 30 sieve) from washing may be recovered by means of a sand screw classifier. The remainder of the fines in the overflow are discharged to a series of sequential settling ponds or basins, where they settle by gravity, sometimes with the help of flocculating polymers. Pond clay is a term usually used to describe waste fines derived from the washing of natural sands and gravels.⁽³⁾

Baghouse Fines

Some quarries operate as dry plants because of dry climatic conditions or a lack of market for washed aggregate products. Dry plant operation requires the use of dust collection systems, such as cyclones and baghouses, to capture dusts generated during crushing operations. These dusts are referred to as baghouse fines.

It is estimated that at least 159 million metric tons (175 million tons) of quarry by-products are being generated each year, mostly from crushed stone production operations. As much as 3.6 billion metric tons (4 billion tons) of quarry by-products have probably accumulated.⁽³⁾

CURRENT MANAGEMENT OPTIONS

Recycling

The exact quantity of quarry by-products that are being recycled is not known. Very little of the 159 million metric tons (175 million ton) produced annually is thought to be used, especially the pond fines. In a recent survey, three states (Arizona, Illinois, and Missouri) indicated that quarry by-products have been used as an embankment material and three other states (Florida, Georgia, and Vermont) indicated some use of quarry by-products in base or subbase applications.⁽⁴⁾ Some use has been made of limestone screenings as agricultural limestone, and baghouse fines from quarry sources have been used as mineral filler in asphalt paving.

Disposal

Virtually all of the quarry by-products generated are disposed of at the quarry source. Screenings are stockpiled in a dry or damp form. Pond fines are conveyed in slurry form to settling ponds. Baghouse fines are usually sluiced into settling ponds.

MARKET SOURCES

Quarry by-products are available at over 3,000 stone quarry operations located in every state except Delaware. Screenings are readily available at most quarries, especially limestone quarries. Although large quantities of pond fines are produced, they must be reclaimed from the ponds and adequately dewatered before they can be considered suitable for use. Baghouse fines are only produced at dry processing plants in areas where there is a lack of market for washed aggregate products. These areas are usually in the more arid regions of the country in the western states.

Screenings

At most quarries, screenings are stockpiled separately at or near the primary or secondary crushing equipment. Screenings are essentially a damp, silty, sand-sized material, usually with an estimated 5 to 10 percent moisture content, depending on the length of time the screenings have been stockpiled. The material is easily recovered by using standard excavation equipment and dump vehicles for loading and hauling.

Pond Fines

Pond fines are usually cleaned or removed from settling ponds by draglines, stockpiled for several months to allow for natural dewatering, then truck hauled, usually to another location on the quarry site. Larger production operations may use equipment such as hydrocyclones, clarifiers, thickeners, or belt presses for dewatering the fines. When dredged from settling ponds, the moisture contents of the fines can be as high as 70 to 80 percent. Final moisture contents in the 20 to 30 percent range are attainable, with the degree of dewatering dependent on the mineralogy and gradation of the fines, equipment used, as well as climatic conditions. Unless sufficiently dewatered, pond fines are too wet to be considered a useable material. The physical and chemical properties of settling pond fines can vary widely with aggregate type and source, but are relatively consistent within a single quarry.⁽³⁾

Baghouse Fines

The properties of baghouse fines vary with rock type, processing equipment, dust collection equipment, and point of collection within the plant. Baghouse fines, because they are generated as a fine, dry powder, are usually less of a handling problem than either the screenings or the pond fines. For a given aggregate and production source, the physical and chemical properties tend to be relatively uniform over time.⁽⁵⁾

HIGHWAY USES AND PROCESSING REQUIREMENTS

Portland Cement Concrete, Asphalt Concrete, and Flowable Fill Aggregate

Screenings have properties that are suitable for use as an aggregate substitute in Portland cement concrete, flowable fill, and asphalt paving applications. Baghouse fines and/or pond fines could potentially replace much of the fines in flowable fill mixes, depending on strength requirements, which are usually fairly low.

Granular Base

If properly blended, screenings can potentially be used in granular base courses.

Mineral Filler

Quarry baghouse fines have been successfully used as a mineral filler in asphalt paving. Dewatered pond fines have the potential for use as a mineral filler in hot mix asphalt paving, depending on the clay content of the pond fines.

The only quarry fines by-product that would require significant processing for any of the foregoing applications are the pond fines, which would have to be adequately dewatered before use. Pond fines would require a greater degree of dewatering for use as mineral filler in asphalt than for use in flowable fill.

Physical Properties

• Screenings

  Screenings are a uniformly sized, fine, sandy material with some silt particles. Screenings commonly range in particle size from 3.2 mm (1/8 in) down to finer than 0.075 mm (No. 200 sieve). Normally, the percentage of particle sizes finer than 0.075 mm (No. 200 sieve) is 10 percent or less by weight. Stockpiles of screenings may contain some particles up to 4.75 mm (No. 4 sieve) in size, which is usually the screen size used for separation. Some weathered rock or overburden material may be present in the screenings from certain processing operations.

  Table 12-1 compares the particle size distribution of the fines fraction (finer than No. 4 sieve) of screenings from several different aggregate sources. Different types of crushers were used to produce these screenings. Despite differences in rock types and crushing machinery, the gradings of the resultant screenings are quite similar.

Table 12-1. Average particle size distribution of screenings from processingof different quarry sources.⁽³⁾

• Pond Fines

  Pond fines, when initially recovered from the pond, consist of a low solids content, fine-grained slurry, usually with 90 to 95 percent of the particles finer than 0.15 mm (No. 100 sieve) and 80 percent or more of the particles finer than 0.075 mm (No. 200 sieve).

  Table 12-2 compares the average particle size distribution of pond fines sampled from two quarries using two different dewatering processes (a sand screw and a U.S. Bureau of Mines dewatering unit). Table 12-2 also includes a listing of gradation ranges for each of the pond fines samples. The table records particle sizing from 0.6 mm (No. 30 sieve) down to the 0.045 mm (No. 325 sieve) size. Within this size range, the grading of the samples is fairly consistent.

• Baghouse Fines

  Although particle sizing may vary somewhat with fines from different types of stone, the range in particle size is from 0.075 mm (No. 200 sieve) down to 0.001 mm or even finer.

  Table 12-3 compares the particle size distribution of 10 different samples of quarry baghouse fines from the processing of four different types of stone. All are predominantly finer than 0.05 mm (No. 270 sieve) and most have a substantial percentage of particles finer than 0.01 mm, although there is some variation in the finer sizes.⁽⁵⁾

Chemical Properties

There is very little difference in the chemistry or mineralogy of screenings and pond fines from the same quarry or rock source, and also very little difference in the chemistry within the size fractions of the pond fines.⁽⁵⁾ Table 12-4 provides a listing of the chemical and mineralogical constituents of screenings (minus 4.75 mm (No. 4 sieve) material), and pond fines from the same granite quarry. The pond fine components are further subdivided into the overall bulk sample and the portions of the sample that are both coarser and finer than 0.106 mm (No. 140 sieve).

Mechanical Properties

Published data on the mechanical properties (unit weight, compacted density, California Bearing Ratio (CBR), shear strength, etc.) of either screenings, settling pond fines, or baghouse fines are not readily available. The mechanical properties of quarry by-products can be expected to vary according to the type of rock from which the by-products were derived.

Table 12-2. Particle size distribution of pond fines from dewatering systems adolomitic limestone and diabase quarries (percent passing). ⁽³⁾

Table 12-3. Particle size distribution of baghouse dusts from processing of several different rock types.⁽³⁾

Table 12-4. Chemical composition and mineralogical identity of screenings and pond fines from a granite quarry.⁽³⁾

REFERENCES

1. Tepordei, Valentin V. Crushed Stone. U.S. Bureau of Mines Annual Report, Washington, DC, April, 1992.

2. Kalcheff, I.V. and C.A. Machemehl, Jr. "Utilization of Crushed Stone Screenings in Highway Construction." Presented at the 59th Annual Meeting of the Transportation Research Board, Washington, DC, January, 1980.

3. Wood, Sandra A. and Charles R. Marek. "Recovery and Utilization of Quarry By-Products For Use in Highway Construction." Proceedings of the Symposium on Recovery and Effective Reuse of Discarded Materials and By-Products for Construction of Highway Facilities, Federal Highway Administration, Denver, Colorado, October, 1993.

4. Collins, Robert J. and Stanley K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice No. 199, Transportation Research Board, Washington, DC, 1994.

5. Marek, C.R. "Realistic Specifications for Manufactured Sand." Proceedings of the Materials Engineering Congress, Materials Performance and Prevention of Deficiencies and Failures (T. D. White, Editor), Atlanta, Georgia, August, 1992.

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