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Utilization of Recycled Materials in Illinois Highway Construction

Wet-Bottom Boiler Slag

Wet-Bottom Boiler Slag (WBBS or "black beauty") is a by-product of coal burning in wet-bottom boilers. Slag tap boilers burn pulverized coal and retain up to 50 percent of the accumulated ash as slag-the rest being fly ash. Cyclone boilers burn crushed coal, and retain as much as 80 percent as boiler slag. In both cases, the bottom ash is held at the bottom of the furnace in a molten liquid state, hence the name wet-bottom (1).
Physical Properties:
When molten boiler slag comes into contact with water, it immediately fragments becoming coarse, angular, glassy particles. WBBS is a porous, glassy granular particle that is primarily regarded as a single-sized coarse to medium sand. This material is essentially composed of silica, alumina, and iron with small amounts of calcium, magnesium, and sulfates. As long as it is collected from wet-bottom boilers (otherwise it would be considered bottom ash), the composition of the material is governed by the coal source not by the type of furnace (1).
Engineering Value:
WBBS is generally a somewhat durable material of uniform size that can be blended with other fine aggregates to meet gradation requirements. This material exhibits less abrasion and soundness loss than bottom ash as a result of its glassy surface texture and lower porosity. In Illinois, WBBS is usually limited to use as a seal coat aggregate on very low volume highways or as an abrasive mixed with deicing salt.
Present Application:
WBBS is incorporated as an aggregate in top surface dressing of bituminous surfaces, embankments, trench backfills, sand backfills for underdrains, bedding, porous granular backfills, membrane water proofing, snow and ice control. Department use of WBBS varies greatly from year to year. Also, when used for ice control, a material inspection is not required, thus little documentation exists regarding its use in this fashion. Outside of the Department-local agencies especially-WBBS has been utilized as an aggregate in blasting grit, roofing shingle granules, asphalt paving, and in roadway base and subbase applications (3, 4).
Quantity Used:
0 tons (2001 MISTIC estimate).
Economic Impact:
The Department's records do not indicate utilization of WBBS in 2001, although it was used extensively by local agencies.
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John Bukowski
Office of Asset Management, Pavements, and Construction
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Updated: 02/20/2015

United States Department of Transportation - Federal Highway Administration