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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
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 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)
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
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.
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.
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.
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 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)
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.
If properly blended, screenings can potentially be used in granular base courses.
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.
Table 12-1. Average particle size distribution of screenings from processingof different quarry sources.(3)
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).
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)