Page 644 - Materials Chemistry, Second Edition

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of new drywall. The paper content of drywall waste limits the amount of recycled gypsum allowed
in new drywall because the paper content affects the fire rating. Other potential markets for drywall
waste include:

● Cement plants. The gypsum is added to the clinker to control the setting time.
● Stucco additive.
● A bulking material for sludge drying and composting.
● Water treatment. Recycled gypsum can settle suspended clay particles in turbid water.
● Manure treatment. Recycled gypsum can be mixed with animal wastes to react with
ammonia and reduce odor.
● Combine with wood shavings for animal bedding. Gypsum can substitute for sawdust or
sand to absorb moisture.
● Grease absorption. Recycled gypsum can be sprinkled on shop floors to absorb oil and
grease.
● Athletic field marker. Gypsum is used to mark lines on athletic fields.
● Agriculture.

New construction drywall can be recycled into a soil amendment. Specific applications include
(CIWMB, 2002e):
● General agriculture
● Mushroom culture
● Forestry and mine land reclamation
● Nurseries
● City parks and recreation areas
● Residential lawns (sod)
● Golf courses
● Compost additives

Gypsum provides a number of practical benefits to soils; for example, it neutralizes some soil
acidity and adds plant nutrients such as calcium and sulfur. Gypsum also improves water penetra-

tion and workability of an impermeable sodic (i.e., high Na ) soil. Recycled gypsum could be used
to leach out sodium salts from soil along roadways where salt is applied during winter. Gypsum can
also reduce the plasticity of soils with high clay content. Regardless of reuse option, however, the
following contaminants should be removed:

● Nails
● Tape
● Joint compound. This compound is made primarily of limestone or gypsum; however, if
the structure was built before the mid-1970s, asbestos may occur.
● Paint usually covers demolition drywall. Structures built before 1978 may contain lead-
based paint.

Drywall waste from new construction sites is freer of contaminants than demolition drywall,
and it is the most commonly recycled. Grinding equipment can range from a large plant to a small
mobile chipper. A hammermill is often used. The machinery grinds the drywall, producing about
93% gypsum powder and 7% (by wt) shredded paper. The gypsum can be sold as a powder, with or
without paper, or molded into pellets. Drywall recycling produces dust, which is often collected via
a baghouse or similar filter system. The amount of drywall waste can be reduced in construction by
constructing standard-sized walls and flat ceilings, and by ordering custom-sized sheets for non-
standard walls. Wastes are also reduced by finding substitutes that are reusable, such as modular
“demountable partitions” for commercial buildings (CIWMB, 2002e).