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Reuse of Lime Sludge from Water Softening in Road Construction and Other Applications

Compacting the test embankment

Vibrating pad foot compaction machine compacts the test embankment


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Project status


Start date: 09/01/02
End date: 12/31/04


Report: Reuse of Lime Sludge from Water Softening in Road Construction and Other Applications (351k pdf) December 2004



About the research

Abstract: Disposal of lime sludge remains a major challenge to cities in the Midwest. Disposal of lime sludge from water softening adds about 7-10% to the cost of water treatment. Having effective and safe options is essential for future compliance with the regulations of the State of Iowa and within budget restrictions. Dewatering and drying are essential to all reuse applications as this affects transportation costs and utility.

Feasibility tests were conducted on some promising applications like SOx control in power generation facilities that burn coal, replacement of limestone as an ingredient in portland cement production, dust control on gravel roads, neutralization of industrial wastewater pH, and combination with fly ash or cement in construction fill applications. A detailed report and analysis of the construction fills application is presented in the second half of the report. A brief discussion of the results directly follow.

1. Dewatering and drying

Various tests were performed on lime sludge samples to observe any restrictions on drying the sludge. The tests indicated that 98% of the water in the sludge is free water. This provides for the potential to use low temperature and open air-drying techniques. Drying the sludge is important for two reasons. First, drying the sludge meets pneumatic transport requirements, and second, for it reduces bulk transportation costs.

2. Use in SOx control at power plants

Lime sludge was tested for control of SOx in flue gases at the Iowa State University (ISU) Power Plant and found effective, but only over short test runs. Since it was difficult for the plant to handle a calcium carbonate feed with a moisture content of greater than 2%, clogged feeding mechanisms prevented longer runs. Long term testing is required to adequately assess the operational impact of using lime sludge in place of ground limestone. Other plants may feed the treatment chamber differently and may accommodate the lime sludge at the moisture content that it is currently dried to.

3. Use in cement production

Tests were performed using 20 tons of lime sludge in the manufacture of about 80 tons cement. Plant personnel at Lehigh Cement in Mason City stated that the quality of the product was satisfactory. However, due to the long transportation distances for lime sludge and the on-site availability of limestone at Lehigh Cement, this application was considered uneconomical.

4. Use for dust control on unpaved roads

A truckload of lime sludge was applied to each of two test locations in Story County. The amount of dust generated was recorded, but the results were inconclusive. Dust control is being investigated by other researchers in the Geotechnical Division, Department of Civil and Construction Engineering at Iowa State. We hope to gain some more insights from that research.

5. Use as a construction fill material

The construction fill application was selected for a more thorough investigation. This was based on the initial test results and the potential for use in Iowa?s road construction projects. Strength and durability testing have demonstrated that lime sludge can be combined with fly ash and portland cement to produce a useful construction fill material.

Lime sludge does not act like cement. Mixes containing lime sludge, bottom ash, and fly ash were designed and tested. According to Ferguson and Levorson (1999), if 50 psi (345 kPa) compressive strength can be achieved in soils stabilized with fly ash, the potential for settlement in deep fills is significantly reduced. The tests described in this report show that adding about 50% fly ash (dry fly ash to dry lime sludge, by weight), the unconfined compressive strength results are 1380 kPa or greater. Higher amounts of stabilizer in the lime sludge mixtures resulted increased unconfined compressive strength, higher internal friction angles in drained direct shear, and higher California Bearing Ratio (CBR) values.

Since particle size analysis and Atterburg Limits tests classify lime sludge as a silty material with low plasticity, durability was a concern. Freeze/thaw and wet/dry durability tests were performed. The freeze/thaw tests were the most aggressive. Results mandated that lime sludge mixtures should be used underneath a sufficient layer of soil that is not frost susceptible to protect it against freezing and thawing volume changes and strength loss.

To demonstrate the construction techniques of the mixtures in the field, a test embankment about 20 feet wide, 3.5 feet tall at center, and about 37 ft long was constructed over the summer using full-size equipment. Air-dried lime sludge was mixed with the fly ash in windrowed piles. Subsequent in-situ testing needs to be conducted to report density, stiffness, temperature, and signs of durability problems during the winter and spring.

Further testing regarding the effects of the mixes on the environment, as mandated by state regulations, needs to be completed to have this option ready to implement. In addition, geotechnical testing is still in progress. Peer-reviewed technical papers will follow. In the end, this work will move the State of Iowa closer to lower water treatment costs and an alterative source for construction fill materials.