Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 146

With this in mind, extensive research is ongoing to develop methods for
eliminating or reducing the effects of these factors on the quality of RCA.
In the following sections, a number of available methods for dealing with
these quality-degrading factors are introduced. The focus is placed on two
novel microwave-assisted technologies that have been shown to effectively
improve the quality of RCA.
4.6 ELIMINATION OF IMPURITIES/ CONTAMINANTS
The general term for contaminants used here refers to all materials, haz-
ardous or nonhazardous, that when present in concrete demolition debris,
the source material used in producing RCAs, may lower the quality of the
RCAs or make them unsuitable for some concrete applications. With this
definition, contaminants may include hazardous (chemical or radioactive)
contaminants, which may pose a health hazard, as well as nonhazardous
contaminants, which may compromise concrete quality (e.g., concrete
surface plasters, gypsum, tiles, glazing, etc.). Minimising the presence of
such impurities and contaminants in the RCA batches produced may be
enhanced through two different routes:
1. Elimination of contaminants at the source (demolition site)
2. Elimination of contaminants at the recycling plant
As discussed previously, elimination at the source is performed during
the soft-strip stage prior to the demolition of a building, whereas elimi-
nation at the recycling plant relies on postdemolition separation methods
to sort and separate the different types of demolition debris delivered at
the recycling site. A number of available techniques relating to the second
strategy were discussed in Section 4.2. In this section, the focus is placed
on the methods available to improve or help in the elimination of the con-
taminants before building demolition.
Ideally, prior to the demolition of the structure, by incorporating a soft-
strip stage, various nonconcrete elements, such as composite roofing, sani-
tary products, doors, window frames, suspended ceilings, raised floors,
carpeting, furnishings, plant and machinery, and so on can be removed in
advance before actual demolition on site. However, even if this is the case,
because of technical difficulties and the lack of suitable surface removal
methods, impurities (such as plasterboard, gypsum, and tiles) present on
the surfaces of typical concrete structural elements may not be completely
removed efficiently. In addition, depending on the building type and use, the
concrete structure may have chemical or physical contaminants (Table 4.7)
infused or infiltrated into the structural elements. Such contamination is
normally limited to a thin layer on the concrete surface, depending on the
age of the structure and length of exposure. Hence, if the contaminated