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(lime kiln dust)

Code L

     Beelman Truck Co. is a supplier of lime kiln dust (Code L) which is a by-product of the lime manufacturing industry.  Lime kiln  dust's chemical makeup consists of quicklime, silicates, and other reactive and inert materials  making it uniquely suitable for soil stabilization and soil modification.

Production Process

Selecting Lime Products for Soil Improvement

     Quicklime, hydrate limed, lime slurry, lime kiln dust, blended lime products, and agricultural lime are used for soil improvement.  The choice of lime product depends on a number of factors, including:

  • Project specifications

  • Jurisdiction/ agency requirements

  • Desired outcome for the treated soil

  • Soil properties

  • Soil moisture content

  • Contractor preferences and capabilities

  • Material availability

  • Total delivered cost

     As many of these factors can vary from job-to-job, it is often beneficial to contact soil stabilization specialists in the area for information and opinions.

Quicklime for Soil Improvement

     According to information reported by the United States Geological Service (USGS), quicklime is the widely used lime product for soil treatment in the U.S.

     Quicklime production begins by extracting limestone from quarries and mines.  The limestone must be of suitable chemical composition to produce quicklime - not all limestone will do.  The limestone is crushed to the proper size and then heated in vertical or rotary kilns to 1700°-2450°F.  This heating process, called "calcination," converts the limestone into quicklime and carbon dioxide.  The quicklime, composed calcium oxide (CaO) and magnesium oxide (MgO), is then cooled, stores, and shipped for use or processed further to produce hydrated lime.

Specifications for Quicklime for Lime Stabilization

     ASTM and AASHTO have developed material specifications for lime (quicklime and hydrated lime) for soil stabilizations:

  • ASTM C977 - Standard Specification for Quicklime and Hydrated Lime for Soil Stabilization

  • AASHTO M 216 - Standard Specification for Lime for Soil Stabilization

     In the U.S., most state DOT's and other specifying agency use these reference specifications for quicklime and hydrated lime for soil drying, modification and stabilization.  For quicklime, several state DOTs further modify these standards to limit the maximum particle size to 3/8 inch to better insure that the quicklime is fully hydrated and dispersed into the soil before final compaction.

Forms of Quicklime

     The National Lime Association identifies three forms of quicklime available in the U.S.

  • High calcium quicklime - derived from limestone containing 0 to 5% magnesium carbonate

  • Magnesian quicklime - derived from limestone containing 5 to 35% magnesium carbonate

  • Dolomitic quicklime - derived from limestone containing 35 to 46% magnesium carbonate

     All three forms of dry quicklime can be used for soil drying, modification and stabilization providing the lime meets the requirements of ASTM C977 or AASHTO M216 and all project-specific requirements established in the project documents, including all mix design criteria.

Rotary Lime Kiln

Quicklime for Soil Improvement

Hydrated Lime for Soil Improvement

     Hydrated lime, often called "hydrate," is produced by combining a controlled amount of quicklime and water to chemically convert quicklime into hydrated lime.  This is done in specially manufactured equipment called "hydrators."  The resulting hydrated lime is a fine, dry powder, with most particles less than 75 microns (#200 sleve opening) in size.  ASTM C977 and AASHTO M 216 require no more than 25% be retained on a #200 sleve.

Specifications for Hydrated Lime for Soil Stabilization

     ASTM and AASHTO have developed material specifications for lime (quicklime and hydrated lime) for soil stabilization:

  • ASTM C977 - Standard Specification for Quicklime and Hydrated Lime for Soil Stabilization

  • AASHTO M 216 - Standard Specification for Lime for Soil Stabilization

Lime Slurry for Stabilization

     Lime slurry is a suspension of calcium hydroxide in water.  Slurry lime can be produced from high calcium quicklime or hydrated lime.  It can be delivered from a central mix plant or produced on site.  Slurry preparation facilities should be approved by the project engineer.  Regardless of location, slurry created from quicklime is hot because the chemical reaction between quicklime and water is exothermic.  Slurries created by mixing hydrated lime and water are not hot.

     Solids in the slurry generally range between 30 and 35%, although technology exists to increase the solids well above 40%.

Lime Kiln Dust (LKD) for Soil Improvement

     Lime kiln dust (LKD) is a co-produce of quicklime production.  It is obtained from the air pollution control systems of rotary kilns used in lime manufacturing.  LKD is a dry mixture of dust from the limestone fed into the kilns, calcinated lime particles (quicklime) and fly ash from the fuel - primarily coal - used to heat the kilns.

     In the U.S., some companies market and sell lime kiln dust under propriety names such as EnviroLime and Calciment.  In their construction specifications, some specifying agencies also referred to LKD by other names, including "Lime By-Products" (Indiana DOT), "Lime Pozzolan" (Pennsylvania (DOT), and "By-Product, Non-Hydrated Lime for Soil Modification and Soil Stabilization"

Hydrated Quicklime

Lime slurry spread on soil prior to mixing

Lime kiln dust

Blended Lime Products

     In some parts of the U.S. and in some other countries, lime is blended to create a mixture that is particularly beneficial to the soil in the area.  These blended products can be:

  • A blend of lime products - such as quicklime and LKD - created by the lime producer and shipped to the job site

  • A blend of lime and other beneficial materials - such as lime and fly ash or lime and Portland cement - created by the lime producer or other parties and shipped the the job site

  • A blend of materials created by the soil mixing operations.  For instance, lime can be mixed into an overly-wet, low-plasticity soil of little clay content (to dry and condition the soil), followed by Portland cement to create soil cement.

     Local stabilization specialty contractors are excellent sources of information regarding the fasibility and availability of blended lime products.

Agricultural Lime (Ag-Lime)

Agricultural lime, also called aglime, agricultural limestone, or garden lime, is a soil additive made typically from pulverized limestone or chalk.  It is not used for modification or stabilization of soil, but strictly to augment the soil chemistry for improved plant growth.  The active component is calcium or magnesium carbonate.  Additional chemicals vary depending on the mineral source.  The effects of agricultural lime on soil are:

  • It increases the pH of acidic soil (the higher the pH, the less acidic the soil);

  • It provides a source of calcium and magnesium for plants;

  • It permits improved water penetration for acidic soils;

  • It improves the uptake of major plant nutrients (nitrogen, phosphorus, and potassium) of plants growing on acid soils.

     Aglime is not in that same class as the other forms of lime describe herein (Quicklime, Hydrated Lime, LKD, etc.) because it is simply crushed Calcium Carbonate that has never been introduced to a kiln and converted to the chemical Calcium Oxide.  The only reason for its inclusion is because sometimes there is confusion as to its use.

Features of Different Lime Products for Soil Improvement

Pound for pound, quicklime is the most effective of the lime products for drying, modifying and stabilizing soil.  This is because quicklime contains high amounts of calcium oxide (CaO) - typically from 70% - 93%, depending on the form of the quicklime.  When mixed into moist soil, the CaO "reacts" with soil moisture:

CaO + H2O → CA(OH)2 + Heat

Calcium Oxide + Water → Calcium Hydroxide + Heat

     Then, when hydrate by the soil moisture, each pound of CaO becomes 1.32 pounds of calcium hydrate in tehs oil-lime mixture.  When hydrated lime is used, the lime is in hydrate form (calcium hydroxide) before it is mixed into the soil.  Therefore, the soil moisture will not be reduced as much as when quicklime is used.

     Lime slurry typically consists of 65% - 70% water, although some newer, more advances devices can produce lime and lime kiln dust slurry with significantly higher solids content.  Therefore, traditional lime slurry is not as effective as quicklime, hydrated lime or lime kiln dust when soil drying is the goal, but may be beneficial for stabilizing a relatively dry soil.  It can also be used where dust is intolerable.

     Lime kiln dust contains lesser amounts of CaO than quicklime; the CaO content of most suitable LKDs range from about 10% - 40%.  Due to its small particle size, the CaO portion of LKD hydrates rapidly, often eliminating the need for a second mixing during construction.  Lime kiln dust contains pozzolanic materials (primarily silica and alumina compounds) which will contribute tot he lime stabilization chemical reaction.  Additionally, the delivered cost of LKD can be lowest of all of the lime products, depending on location and availability.

Advantages and Disadvantages for Different Lime Products

     The following information has been extracted, with permission, from the National Lime Association's publication "Lime Treated Soil Construction Manual," Bulletin 326.  The entire document can be downloaded at:

http://www.lime.org/documents/publications/free_downloads/construct-manual2004.pdf

     The type of lime stabilization technique used on a project should be based on multiple considerations, such as contractor experience, equipment availability, location of project (rural or urban), and availability of an adequate nearby water source.  Some of the advantages and disadvantage of different lime application methods follow:

  • Dry Hydrated Lime:

    • Advantages​:  Can be applied more rapidly than slurry.  Dry hydrated lime can be used for drying clay, but it is not as effective as quicklime.

    • Disadvantages:  Hydrated lime particles are fine.  Thus, dust can be a problem and renders this type of application generally unsuitable for populated areas.

  • Dry Quicklime:

    • Advantages:  ​Economical because quicklime is a more concentrated form of lime than hydrated lime, containing 20 - 24% more "available" lime oxide content.  Thus about 3% quicklime is equivalent to 4% hydrated lime when conditions allow full hydration of the quicklime with enough moisture.  Greater bulk density requires smaller storage facilities.  The construction season may be extended because the exothermic reaction caused with water and quicklime can warm the soil.  Dry quiclime is excellent for drying wet soils.  Larger particle sizes can reduce dust generation.

 
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