Page 643 - Materials Chemistry, Second Edition

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614 Waste Management Practices: Municipal, Hazardous, and Industrial
materials including crushed stone, sand, gravel, slag, or rock dust. Cement and asphalt serve as
binders. Aggregate is used as a replacement for road-base gravel or it is incorporated directly into
asphalt or concrete.
A typical roadway is constructed in several layers: pavement, base, and sometimes subbase. The
pavement is the surface layer and is composed of Portland cement concrete or asphalt concrete. The
base layer supports the pavement, and is composed of aggregate base. The subbase layer supports
the base and is made of aggregate subbase (Figure 21.5). The subbase layer has less strength, but is
useful and cost-effective for bringing the road up to grade. The subbase layer includes a larger pro-
portion of sand, silt and clay than the aggregate base layer (CIWMB, 2002c).
Recycled aggregate originates primarily from Portland cement concrete and asphalt concrete
from road maintenance and demolition. Aggregate is recycled by crushing and sorting operations.
Portland cement concrete and asphalt concrete are delivered to the processor in large chunks. Heavy
crushing equipment is required to break the chunks into aggregate of manageable size. A crushing
plant may include a hopper to receive the material, a jaw to break it into smaller pieces, an impact
crusher to reduce further its size, a vibrating screen to sort to the required specifications, and a con-
veyor belt with a rotating magnet to remove metal contamination such as rebar (CIWMB, 2002).
Aggregate is recycled used for many purposes, including:

● In paved roads as aggregate base, aggregate subbase, and shoulders
● In gravel roads as surfacing
● As base for building foundations
● As fill for utility trenches

The primary market for aggregate, however, is as a road base and subbase.

21.4.2 DRYWALL (SHEETROCK, GYPSUM)
Drywall, also referred to as gypsum board, wallboard, plasterboard and rockwall, is the primary
material used in the United States for interior wall construction. It is composed of a sheet of gyp-
sum covered on both sides with a paper facing and a paperboard backing. Gypsum is a naturally
occurring mineral composed of calcium sulfate dihydrate (CaSO ·2H O). Drywall sheets are man-
2
4
ufactured in sizes from 4 8 to 4 16 ft and range in thickness from 1/4 to 1 in.
Most drywall waste is generated from new construction (64%), followed by demolition (14%),
manufacturing (12%), and renovation (10%). Approximately 15 million tons of new drywall are
manufactured per year in the United States. About 12% of new construction drywall is discarded
during installation.
Drywall is recycled by first separating the paper backing from the gypsum material. The paper
can be recycled into new paper backing, and the gypsum is remixed and used in the manufacture

Asphalt concrete pavement
Cement
walkway
Natural
surface
soil

Trench for Subgrade
drainage

Asphalt concrete base
FIGURE 21.5 Cross section of a road showing the various base layers.