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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Both cement kiln dust (CKD) and lime kiln dust (LKD) can be used as activators in pozzolanic stabilized base mixtures. Depending on its reactivity, CKD may have potential for use as the sole cementitious material in stabilized base mixtures. Most work with kiln dusts, however, has involved combinations of kiln dusts and coal fly ash as the cementitious component of the mixture. The strength of kiln dust-fly ash-aggregate base and subbase materials is dependent on three factors: the kiln dust, the coal fly ash, and the aggregate. Either pozzolanic or self-cementing fly ash can be used as part of stabilized base (PSB) mixtures, although precautions may be necessary when using a self-cementing fly ash. With self-cementing fly ashes, no activators are needed.
There is no widespread commercial use of kiln dusts in stabilized base mixtures in the United States today. Only four states are reported to have research data on the use of CKD for stabilized base.(1) Research conducted in Kentucky found that LKD performed better in stabilized base applications than CKD.(2) Trial LKD sections constructed in Ohio and tested over a period of 6 months exhibited no cracking or surface damage, and deflections were observed to decrease with time as the pavement cured.
In the Ohio project, six test sections were constructed in November 1997 as part of a research study by the University of Toledo. The fly ash/CKD/Portland cement blends (percent by mass dry ingredients) utilized in this study were: 6/6/0, 8/8/0, 10/10/0, 12/12/0, 8/8/0.5, and 8/8/1. The total traffic during this period was 25,820 equivalent 8,165 kg (18,000 lb) single-axle loads. CKD sections were reported to exhibit dimensional (volumetric) stability problems, which were attributed to the presence of sulfates and alkalis in the dust.
MATERIAL PROCESSING REQUIREMENTS
Where possible, aged stockpiled CKD and LKD should not be used (due to hydration and the loss of reactivity) as a component of stabilized highway base material unless conditioned by the addition of commercial lime or other caustic material to enhance short-term strength development.(3)
If fresh kiln dust is being used, it must be stored in enclosed bins or silos to keep out moisture and prevent dusting. CKD and LKD can also be used with fly ash, provided the dust is conditioned with moderate amounts of water. The use of moisture-conditioned kiln dust (to suppress dusting) and fly ash allows greater quantities of both materials to be set aside for future use without the need for enclosed storage. Such moisture conditioning will, however, cause full or partial hydration of the available free lime in the dust that may necessitate the later addition of a commercial hydrated lime to improve early strength development.
The properties of kiln dusts that are of particular interest when kiln dusts are used in stabilized base applications are particle size and specific gravity.
Particle Size: Maximum particle size of most kiln dusts is about 0.3 mm. According to AASHTO T153-86(4) the Blaine fineness ranges from about 4600 (coarser) to 14000 (finer) cm2/g.(3) 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.
Specific Gravity: The specific gravity of CKD is typically in the range of 2.6 to 2.8, similar to Portland cement. LKD exhibits specific gravities in the order of 2.6 to 3.0.(3)
Kiln dusts have been used, primarily as a pozzolanic activator, in kiln dust-fly ash-aggregate base mixtures. Some of the properties of these mixtures that are of interest include strength, durability, volume stability, and autogenous healing properties. These properties are dependent on the properties of the three major components: the kiln dust, the fly ash, and the aggregate.
Strength: Kiln dust-fly ash stabilized base mixtures containing dolomitic limestone aggregates generally exhibit higher densities and compressive strengths than siliceous aggregate blends. Similarly, the strength of the mixtures will be somewhat dependent upon the aggregate shape (higher strength with angular crushed aggregates).
The effect of fly ash on strength development of kiln dust-fly ash-aggregate mixes depends on the type of fly ash used in the mix. Table 8-4 lists typical 7-day compressive strength results that were achieved for kiln dust-fly ash-aggregate mixes.(3) Mixes employing self-cementing fly ash tend to have higher 7-day strengths than those with pozzolanic fly ash.
Table 8-5 presents a listing of kiln dust-fly ash-aggregate mixes test results for splitting tensile strength and resilient modulus versus comparative stabilized base mixes using commercial lime.(3)
Table 8-4. Compressive strength of Kiln dust-fly ash-aggregate mixes.
Table 8-5. Splitting tensile strength and resilient modulus of Kiln dusts-fly ash-aggregate mixes.
Durability: The durability of kiln dust-fly ash-aggregate mixtures assessed using the AASHTO T136(5) procedure tends to be very good, with freeze-thaw losses of 2 percent or less measured.(3) This compares favorably with conventional lime-fly ash-aggregate mixes.
Volume Stability: Most kiln dust-fly ash-aggregate mixtures are dimensionally stable (no volumetric expansion) over long periods of time, and again compare favorably to conventional lime-fly ash-aggregate mixtures. CKD that is high in sulfates and alkalis, and LKD that has higher free lime content tends to exhibit poorer dimensional stability.(3)
Autogenous Healing: Kiln dust-fly ash-aggregate mixtures in field performance demonstrate autogenous healing properties, although not to the same extent as commercial lime-fly ash-aggregate blends. However, kiln dust systems re-cement across crack boundaries and regain a significant portion of their initial compressive strength.(3)
Chemical Composition: CKD has lower free lime content (approximately 4.4 percent) compared to LKD which can be as high as 40 percent.(6) The pH of water in contact with CKD and LKD mixtures is typically about 12. Due to its relatively high pH, some corrosion of metals (e.g., aluminum) in direct contact with CKD and LKD may occur.
Mix designs for kiln dust-fly ash-aggregate mixtures for stabilized base should follow similar mix design procedures that have been recommended for lime-fly ash-aggregate mixtures.(3,7) The moisture-density relations of soil cement/stabilized base mixtures are determined using the AASHTO T134 method, (8) with the durability of the mixtures evaluated using the AASHTO T135 wetting and drying test(9) and AASHTO T136 freezing and thawing test.(5)
The suitability of the kiln dust for stabilized base use should be established by a laboratory prescreening test of trial mixes. Due to the difference in free lime content between LKD and CKD, for CKD, a kiln dust:fly ash ratio between 2:1 and 1:1 has been recommended. For LKD, a kiln dust:fly ash ratio between 1:1 and 1:2 has been recommended.(3)
Test specimens should be prepared in accordance with ASTM C593(10). A seven-day compressive strength of 2760 kPa (400 lb/in2) is considered to be the minimum value for acceptance of the kiln dust with Class F fly ash and 4140 kPa (600 lb/in2) if Class C fly ash is used.
It is important to maintain a condition of excess moisture for highly reactive LKD and CKD to permit complete hydration of any free lime. A separate slaking test can be completed in advance to estimate the range of slaking water required.
After determining the optimum moisture content for molding, specimens should be prepared and tested for compressive strength, durability (vacuum saturation method as per ASTM C593(10) and freeze-thaw test in accordance with AASHTO T136,(5)) and volume stability (autoclave expansion test (AASHTO T107)(11) or mortar bar expansion test (AASHTO M210).(12) At least three kiln dust-fly ash proportions should be tested in compression to determine the optimal mix for durability and volume stability testing. High-strength mixes may not require durability testing.
Structural design procedures for stabilized base containing kiln dusts are the same as design procedures for conventional stabilized base materials.
Material Handling and Storage
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 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 similar fashion to conventional cement or lime - pneumatically loaded and unloaded from cement tanker trucks and stored in silos.
Kiln dust-fly ash mixtures produced using self-cementing fly ash must handled somewhat differently than mixes with pozzolanic fly ash because self-cementing fly ash mixtures (especially those high in free lime) set up much more rapidly when exposed to water.
Due to the fine nature of kiln dusts, they may have a tendency to clump or bridge together at the feed opening when stored in silos. Consequently, bins or silos should be equipped with suitable vibration devices at the feed opening to mitigate this problem.
Mixing, Placing and Compacting
Kiln dust-fly ash-aggregate mixtures are mixed and placed in uniform lifts and compacted using conventional construction equipment. The mixtures must be placed and compacted continuously and cannot be stockpiled for more than a couple of hours, after which time they will set.
The same test procedures used for conventional aggregate base courses should be used for mixes containing kiln dusts. Various field and laboratory tests for compacted density and field measurement of compaction are given by AASHTO test methods T134,(8) T191,(13) 205,(14) and T238.(15)
There is a need for additional research to assess the suitability of using LKD and particularly CKD as a pozzolan activator in stabilized base applications. Specifications are required to define both the physical and chemical requirements needed to ensure acceptable field performance.
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.