Page 74 - Highway Engineering Handbook Building and Rehabilitating the Infrastructure

ENVIRONMENTAL ISSUES 57

suitable for highway use because they are hazardous or because leachate from these mate-
rials are a threat to the environment. Through treatment, some industrial wastes otherwise
deemed a threat to the environment may be rendered usable. Petroleum-contaminated soils,
for instance, once thermally treated, can be used as fill material and have been used in
asphalt mixtures as fine aggregates. Petroleum-contaminated soils are not currently being
recycled into highway projects but have been used on road and street construction at the
local level. The principal recoverable wastes from industrial activities are described below.
Coal Ash By-Products. NCHRP Synthesis 199 cites an American Coal Ash
Association publication, (Coal Combustion By-Product Production and Consumption.
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1992) when noting that 66 million tons (60 10 kg) of coal ash is produced annually
from the 420 coal-burning power plants across the country. Coal is either anthracite, bitu-
minous, or lignite (subbituminous); the particular form has a bearing on the characteris-
tics of the by-products.
Fly Ash. ASTM divides fly ash into two classes: class F, from anthracite coal; and
class C, from lignite coal. Class F fly ash reacts with calcium and water at ordinary tem-
peratures to form a cementlike compound. Class C fly ash has a higher lime content than
class F fly ash and can be self-setting. To be usable as a cementitious substitute for Portland
cement, fly ash must meet quality standards established by ASTM (Standard C-618).
Approximately 25 percent of the fly ash produced meets this standard, yet only about half
of the viable resource is being used.
Bottom Ash and Boiler Slag. Bottom ash and boiler slag are also by-products of coal
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burning, amounting to approximately 18 million tons (16 10 kg) of waste produced
annually. These by-products are being researched for use in embankments, unbound aggre-
gate, and asphalt paving and antiskid material.
Blast-Furnace Slag. Slag that is the by-product of producing iron in a blast furnace is
nonferrous and consists of silicates and aluminosilicates of lime. Of the three types of slag
produced from blast furnaces (expanded, granular, and air cooled), about 90 percent of that
recovered for use in construction is air cooled. Air-cooled slag is porous and suitable for
use as aggregate in lightweight concrete, in asphalt, in roadway bases, and in fill material.
Granulated slag can be finely ground as slag cement, and expanded slag can be used as
aggregate in lightweight concrete. The primary barrier to use of slag is that it was not sep-
arated into homogeneous piles and it was mixed with steel slag.

Steel Slag. Steel slag is the product of lime flux reacting with products in a steel furnace
such as pig iron. Steel slag consists of calcium, iron, unslaked lime, and magnesium. It can
be very expansive if not properly “aged” through treatment with water. Because of its
characteristics of being very hard, stable, and abrasion-resistant, it is used in paving mate-
rial and snow control. It is heavier than most aggregate and has been used as fill material
and as railroad ballast. However, some concern has developed recently that the leachate
from these two uses clogs drains and can affect receiving waters. About 7.9 million tons
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(7.2 10 kg) of steel slag is sold in the United States annually.
Nonferrous Slag. Slag from smelting operations for other ores such as copper, lead, zinc,
nickel, and phosphates is grouped together under a single heading. Each must be evaluated
and treated separately because of the varying properties these slags possess. Phosphate
slag, copper oxide blasting slag, and zinc slag have been used as aggregate in paving mix-
tures. Aluminum slag has been used experimentally for asphalt paving aggregate, but the
material proved not to be durable and is no longer used.