Utilization of Recycled Materials in Illinois Highway Construction
- Limestone (calcium carbonate) heated in a kiln, drives off carbon dioxide and forms lime (calcium oxide). The kiln's exhaust gasses-filtered using electrostatic precipitators, baghouses, or other such methods-are collected and sold as by-product lime. Lime Kiln Dust (LKD) can vary chemically depending on the type of lime being manufactured. It can be categorized according to reactivity, which is based on the amount of free lime and magnesia content-corresponding to lime types: calcitic (chemical lime, quicklime, etc.) or dolomitic (1).
- Physical Properties:
- By-product lime is a very fine, white powdery material of uniform size containing calcium and magnesium carbonates as its principle mineral constituents. Much of LKD's properties are determined in the plant production: feedstock, kiln design, fuel type, and type of dust control/collection method employed (1).
- Engineering Value:
- By-product lime is valued as both a modifying and stabilizing agent in soil treatment. It generally increases the workability of clayey soils by reducing the plasticity index and increasing the optimum moisture content. On the other hand, high levels of free lime content in LKD have been shown to result in poorer dimensional stability (shrinkage, expansion) (1).
- Present Application:
- By-product lime provides a stable, working platform for paving operations. This material aids in the reduction of high moisture borrow soils in embankment construction (2, 3).
- Quantity Used:
- 46,760 tons (2001 MISTIC estimate).
- Economic Impact:
- By-product lime usage is one of the least expensive remedial actions for unstable subgrade soils. Using this material, the Department spent approximately $701,500 in 2001.