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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Kiln dusts are fine by-products of Portland cement and lime high-temperature rotary kiln production operations that are captured in the air pollution control dust collection system (e.g., cyclones, electrostatic precipitators, and baghouses).
Cement Kiln Dust
Cement kiln dust (CKD) is a fine powdery material similar in appearance to Portland cement. Fresh cement kiln dusts can be classified as belonging to one of four categories, depending on the kiln process employed and the degree of separation in the dust collection system.(1) There are two types of cement kiln processes: wet-process kilns, which accept feed materials in a slurry form; and dry-process kilns, which accept feed materials in a dry, ground form. In each type of process the dust can be collected in two ways: (1) a portion of the dust can be separated and returned to the kiln from the dust collection system (e.g., cyclone) closest to the kiln, or (2) the total quantity of dust produced can be recycled or discarded. A simplified schematic of a Portland cement manufacturing operation is presented in Figure 8-1.
The chemical and physical characteristics of CKD that is collected for use outside of the cement production facility will depend in great part on the method of dust collection employed at the facility. Free lime can be found in CKD, and its concentration is typically highest in the coarser particles captured closest to the kiln. Finer particles tend to exhibit higher concentrations of sulfates and alkalis. If the coarser particles are not separated out and returned to the kiln, the total dust will be higher in free lime (since it will contain some coarse particles). CKD from wet-process kilns also tends to be lower in calcium content than dust from dry-process kilns.
Approximately 12.9 million metric tons (14.2 million tons) of CKD are produced annually.(2)
Lime Kiln Dust
Lime kiln dust (LKD) is physically similar to cement kiln dust, but chemically quite different. LKD can vary chemically depending on whether high-calcium lime (chemical lime, hydrated lime, quicklime) or dolomitic lime is being manufactured.
Fresh LKD can be divided into two categories based on relative reactivity, which is directly related to free lime and free magnesia content. Free lime and magnesia content are most dependent on whether the feedstock employed is calcitic or dolomitic limestone. LKD with a high free lime content IS highly reactive, producing an exothermic reaction upon addition of water. This "quick" LKD is of greatest commercial interest as a direct replacement or substitute for hydrated lime.
Figure 8-1. Portland cement manufacturing operations.
Approximately 1.8 to 3.6 million metric tons (2 to 4 million tons) of LKD are generated each year in the United States.(3)
In addition to fresh CKD and LKD production, it is estimated that the total amount of kiln dust currently stockpiled throughout the country exceeds close to 90 million metric tons (100 million tons). These stockpiles are usually located relatively close to the cement and lime manufacturing plants, and vary in age and composition, with exposure to the elements (moisture in particular) reducing the chemical reactivity of the dusts.
CURRENT MANAGEMENT OPTIONS
Most of the CKD produced is reused within the cement plant. About 64 percent of the total CKD generated (or about 8.3 million metric tons) is used in this fashion.(2) Approximately 6 percent of the total CKD generated is utilized off-site. The most common beneficial use of CKD is its use as a stabilizing agent for wastes, where its absorptive capacity and alkaline properties can reduce the moisture content, increase the bearing capacity, and provide an alkaline environment for waste materials.
Both cement and lime kiln dusts have been used as stabilizing and solidifying agents in the treatment of soft or wet soils for engineering purposes(4) and for environmental remediation.(5) Both dusts have also been used as pozzolan initiators,(6) as a pelletized lightweight aggregate material, as a mineral filler in asphalt pavements, and as a fill material in earth embankments.
A significant potential market for CKD and LKD exists for its use as a soil conditioner for agricultural purposes (in lieu of agricultural lime) and as an acid-neutralizing agent in agricultural and water treatment applications. However, at the present time, the EPA is evaluating the possible need to regulate the use of CKD in this application.
In recent years hazardous waste has been used as a fuel in cement kiln operations. The use of waste materials in cement kiln operations has raised concerns regarding the accumulation of heavy metals (e.g., lead, cadmium, and chromium) in CKD generated by plants that use these alternative materials.(2) In addition, runoff and precipitation that contact CKD storage piles have exhibited pH levels above 12.5, which can be highly corrosive.(7) The EPA has expressed concern over uncontrolled transport, storage, and disposal of large volumes of CKD (in uncovered and unlined piles), which are easily removed by wind and eroded by water.
In a recent regulatory determination, the EPA committed to the development of revised standards for managing CKD.(7) In this regulatory determination, EPA stated, with respect to beneficial uses, that "for most off-site uses (e.g., waste stabilization or certain construction uses) EPA's current record indicates that there are no significant risks." This would not preclude the need to examine the chemical quality of CKD prior to its use.
At the present time, approximately 80 percent of the surplus CKD remaining after reuse in cement manufacturing is stockpiled or landfilled.(2) Most of the LKD generated in the United States is currently disposed of in stockpiles or landfills.(3)
Kiln dusts may be obtained directly from Portland cement or hydrated lime producers. Waste management firms retained by the manufacturers may also supply cement and lime kiln dusts.
The specific characteristics of the CKD and LKD vary from plant to plant depending on the feedstock employed at the cement or lime production plant, the major products being manufactured, kiln design and operation, fuel type, and the type of dust control/collection systems employed.
The primary value of cement and lime kiln dusts is their cementitious properties. Depending on the concentration of hydratable oxides present in the CKD and LKD, primarily unreacted or free lime (CaO) and free magnesia (MgO) respectively, cement kiln dust and lime kiln dust can be highly cementitious.
Fresh CKD and LKD are generally difficult to handle in bulk because of their fine, dry, powdery nature and caustic characteristics. The addition of water to mitigate blowing and dusting problems during transport is common, but this practice causes premature hydration of the free lime or magnesia and significantly reduces the cementitious potential of the CKD or LKD. Where the CKD or LKD must be kept dry to preserve its cementitious potential, it must be handled in a fashion that is similar to conventional cement or lime (pneumatically loaded into and unloaded from cement tanker trucks and stored in silos).
The processing of stockpiled CKD and LKD can be difficult. Typically, very large, above- ground stockpiles or backfilled quarries (source of raw product for cement manufacture) are involved, representing many years of cement or lime production. The surface of the stockpile or fill site usually crusts over and becomes hard, while the interior of the stockpile can stay relatively loose and can contain some unhydrated material even after many years if exposure to moisture is limited. Processing of hardened stockpiled kiln dusts requires crushing and screening equipment to remove oversize pieces as well as any litter or garbage (wood, etc.) that may have become mixed with the kiln dusts.
HIGHWAY USES AND PROCESSING REQUIREMENTS
Asphalt Concrete Mineral Filler
CKD and LKD have been used as mineral filler in asphalt concrete mixes. The blending of CKD into the asphalt cement binder prior to incorporation with the hot mix aggregate results in a binder (mastic) that can significant-ly reduce asphalt cement requirements (between 15 and 25 percent by volume).(8) Further, the lime components of the CKD and LKD can assist in promoting stripping resistance (preventing moisture-related damage resulting from the separation of the asphalt cement film from the aggregate at its interface in the presence of moisture that is most common in siliceous aggregates). In this application, these dusts can be used to replace hydrated lime or liquid antistripping agents.
CKD can also be used as a replacement for Portland cement or hydrated lime in slurry seals (mix of fine aggregate and emulsified asphalt). Slurry seal mixes with 2 percent kiln dust prepared in the laboratory, using a stripping fine aggregate gave excellent results in abrasion resistance testing.(9)
Asphalt Concrete Aggregate
CKD and LKD can also be agglomerated or pelletized to produce an artificial aggregate for special applications. In Japan an oil-absorbing artificial aggregate is reportedly manufactured using CKD that is used to improve the rutting resistance of asphalt concrete pavements by absorbing the lighter fractions of excess asphalt cement binder during hot weather.
Asphalt Cement Modifier
CKD can be added to asphalt binder to produce a low ductile mastic asphalt. Mastic asphalt is a mixture of asphalt binder and fine mineral material. When mastic asphalt is produced using CKD mixed 50/50 with an asphalt cement binder, a potential exists for a relatively large volume replacement of asphalt cement (between 15 and 25 percent by volume). The European use of mastic asphalts, with low ductility, for bridge deck waterproofing and protection is well documented, and this could represent a potential application for kiln dusts in the United States.(10,11,12)
Stabilized Base or Flowable Fill Cementitious Materials
CKD can be used as a cementitious material or a pozzolan activator in stabilized base or flowable fill applications. LKD has potential for use as a pozzolan activator in each respective application. As a cementitious material, CKD can replace or be used in combination with Portland cement. As a pozzolan activator, both CKD and LKD can replace or be used in combination with Portland cement or hydrated lime.
CKD and LKD are fine, powdery materials of relatively uniform size. Table 8-1 lists some typical physical properties of both cement and lime kiln dusts.
Table 8-1. Typical range of physical properties of cement and lime kiln dusts.(7)
Approximately 75 percent of the kiln dust particles are finer than 0.030 mm (No. 450 sieve). The fineness of kiln dust, as Portland cement, can be determined using the Blaine air permeability apparatus in accordance with ASTM C204.(13)
The maximum particle size of most CKD is about 0.30 mm (No. 50 sieve), with the Blaine fineness ranging from about 4600 (coarser) to 14000 (finer) cm2/g.(1) LKD is generally somewhat more coarse than CKD, having a top size of about 2 mm (No. 10 sieve) and Blaine fineness ranging between about 1300 and 10000 cm2/g. In comparison, the Blaine fineness of type Portland cement is about 3500 to 3800 cm2/g.(14)
The specific gravity of CKD is typically in the range of 2.6 to 2.8, less than that of Portland cement (specific gravity of 3.15). LKD exhibits specific gravities ranging from 2.6 to 3.0.(1)
Chemically, CKD has a composition similar to conventional Portland cement. The principal constituents are compounds of lime, iron, silica and alumina. Table 8-2 lists typical compositions for fresh and stockpiled CKD and LKD.
The free lime content of LKD can be significantly higher than that of CKD (up to about 40 percent), with calcium and magnesium carbonates as the principal mineral constituents.
There is very little, if any, free lime or free magnesia content in stockpiled CKD and LKD that has been exposed to the environment for long periods.(1)
The pH of CKD and LKD water mixtures is typically about 12. Both materials contain significant alkalis, and consequently are considered to be caustic. Due to the caustic nature of CKD and LKD, some corrosion of metals (e.g., aluminum) that come in direct contact with CKD and LKD may occur.
Trace constituents in CKD (including certain trace metals such as cadmium, lead, and selenium, and radionuclides) are generally found in concentrations less than 0.05 percent by weight. Because some of these constituents are potentially toxic at low concentrations, it is important to assess their levels (and mobility or leachability) in CKD before considering its use.(7)
CKD has a loose density of only about 480 kg/m3 (30 lb/ft3), but can be compacted to about 1350 to 1500 kg/m3 (85 to 95 lb/ft3) using conventional soils compaction practices.(15)
Table 8-2. Typical chemical compositions of cement kiln dust and lime kiln dust.(1)
TRT Terms: Waste products as road materials--Handbooks, manuals, etc, Pavements, Asphalt concrete--Design and construction--Handbooks, manuals, etc, Pavements, Concrete--Design and construction--Handbooks, manuals, etc, Pavements--Additives--Handbooks, manuals, etc, Fills (Earthwork)--Design and construction--Handbooks, manuals, etc, Roads--Base courses--Design and construction--Handbooks, manuals, etc, Wastes, Environmental impacts, Recycling