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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Both fresh cement kiln dust (CKD) and lime kiln dust (LKD) can be used as mineral filler in hot mix asphalt paving.(1) The use of low addition levels of both fresh CKD and LKD as mineral filler in hot mix asphalt surface course mixes (approximately 5 percent by weight of aggregate) reportedly exhibit mix properties comparable to that of natural filler such as hydrated lime and stone dust. Gradation, organic impurities, and plasticity requirements normally associated with mineral filler specification requirements can usually be met without difficulty.
Although research has shown the feasibility of using CKD, and especially LKD, in lieu of commercial hydrated lime as a mineral filler, few agencies have been involved in testing or field trials.(2,3) Three states have reported some research on the use of CKD for use as mineral filler (Arizona, New York, and Utah), and only one state (Utah) has investigated the use of LKD for this application.(4)
Some use of CKD as a mineral filler in highway asphalt concrete has been reported in Italy where both the performance and Marshall properties of asphalt mixes with CKD filler were reported to be good, at addition rates that are comparable to that for commercial hydrated lime (about 6 percent).(1)
The lime components of the CKD and LKD can assist in promoting stripping resistance. In this application, these dusts can be used to replace hydrated lime or liquid antistripping agents.(2,3)
MATERIAL PROCESSING REQUIREMENTS
The specification requirements for mineral fillers (AASHTO M17),(5) are shown in Table 8-3. The grading requirements for mineral fillers are such that CKD can, with minimal screening, readily comply with the specification requirements. Some LKD can be somewhat coarse, which will necessitate screening to remove oversize particles.
Table 8-3. AASHTO M17 specification requirements for mineral filler for use in bituminous paving mixtures.
Because only fresh, dry kiln dust should be used, the CKD and LKD must be stored in silos or cement trucks. When stored for extended periods in silos, kiln dusts may have a tendency to clump or bridge together at the silo feed opening, due to their fine nature. Consequently, bins or silos should be equipped with suitable vibration devices at the feed opening to mitigate this potential problem.
Some of the properties of kiln dust that are of particular interest when kiln dust is used as a mineral filler in asphalt paving applications include particle size and specific gravity.
Particle Size: The maximum particle size of most kiln dusts is about 0.3 mm (No. 50 sieve), which conforms to mineral filler top size requirements outlined in AASHTO M17. Commercial kiln dusts are highly angular. The Blaine fineness(6) ranges from about 4600 (coarser) cm2/g to 14000 (finer) cm2/g.(7) LKD is generally somewhat more coarse than cement kiln dusts, having a top size of about 2 mm (No. 10 sieve) and Blaine fineness ranging between about 1300 and 10000 cm2/g.
Specific Gravity: The specific gravity of CKD is typically in the range of 2.6 to 2.8. LKD exhibits specific gravities in the range of 2.6 to 3.0.(7)
Some of the properties of the asphalt mix that are of interest when kiln dusts are used as a mineral filler are mix durability and binder properties.
Durability: CKD fillers have been observed to increase the durability of hot mix asphalt, but not to the same extent as hydrated lime (due to the lower free lime content of CKD). CKD is not very effective in reducing moisture-related damage (stripping) in asphalt concrete mixes, but LKD (especially highly reactive LKD) can provide benefits similar to that of commercial hydrated lime.(3)
Binder Properties: At high addition levels, CKD acts as an extender of the asphalt cement binder, allowing up to about 25 percent by volume replacement of the asphalt cement component. CKD mineral filler results in increased stability and stiffening (lower penetration, higher softening point, and higher kinematic viscosity).(8)
Asphalt mixes containing kiln dusts can be designed using standard laboratory procedures. The mineral filler should satisfy the requirements of AASHTO M17.(9) Immersion-compression tests should be conducted to supplement Marshall mix design testing to determine the moisture sensitivity of paving mixtures containing different types and concentrations of mineral fillers.(6)
Conventional AASHTO pavement structural design methods are appropriate for asphalt concrete incorporating kiln dust as mineral filler.
There are no additional or special construction procedures involved for the use of CKD or LKD beyond those required for conventional mineral fillers.
Kiln dusts for use as mineral filler can be precoated with asphalt emulsion (approximately 10 percent by mass) to facilitate handling and transportation. The emulsion value is largely recovered when incorporated in hot mix asphalt, but the cost nevertheless increases without apparent improvement in mix properties.
Placing and Compacting
The same methods and equipment used for conventional pavements are applicable to asphalt pavements containing kiln dusts.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing kiln dusts. Mixes should be sampled in accordance with AASHTO T168,(10) and tested for specific gravity in accordance with ASTM D2726,(11) and in-place density in accordance with ASTM D2950.(12)
Some kiln dust sources have been used successfully as mineral filler in asphalt paving applications; however, there are adequate data on the different properties of specific sources of kiln dust and its economic viability for use as a mineral filler.
The addition of larger quantities of kiln dust to the binder (stone mastic asphalt) and its properties should be investigated to assess its potential for use in special applications.
The effectiveness CKD and LKD as antistripping agents should be evaluated at different rates of addition.
The environmental properties of CKD and LKD should be better defined and management practices delineated to ensure that CKD and LKD are managed in an environmentally acceptable manner.