Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 152

be reduced considerably by increasing the number of crushing stages. The
reduction in the mortar content by incorporating an additional crushing
stage varies from 10% to 40% depending on the size of the RCA particles,
strength of the mortar, and type of crusher used [15]. However, despite the
considerable mortar content reduction achievable, one major disadvantage
of incorporating additional crushing stages is the significant decrease in
the overall yield of the coarse RCAs produced. This is because, besides
the adhering mortar, a considerable portion of weaker embedded original
aggregates is also broken into finer aggregate particles. Also, any additional
crushing stage introduced adds considerably to the costs of recycling; thus,
a trade-off between costs and RCA quality exists.
Alternatively, the second route or strategy for reducing the mortar content
of RCAs is postrecycling separation of the mortar present in the RCAs. As the
name implies, this form of separation is used to separate the adhering mortar
from the RCAs after some or all of the stages of the conventional recycling
processes are completed. A number of postrecycling mortar separation meth-
ods have been proposed in the available literature. These include thermal
separation, mechanical separation, thermal-mechanical separation, chemi-
cal separation, and microwave-assisted separation. In the following sections,
the working principles as well as the advantages and disadvantages of these
methods are discussed briefly. The focus is then placed on the microwave-
assisted postrecycling mortar separation method, which has been shown to
result in considerable improvements in the properties of RCAs.
4.7.1 Conventional mortar separation techniques
4.7.1.1 Thermal separation
Thermal separation invests on the differences between the thermal expan-
sion rates of mortar and NAs to cause thermal stresses within RCA. In
thermal separation, RCA particles are heated to temperatures ranging from
300°C to 600°C, depending on the strength of mortar and NA type, for
about 2 hours to break up and separate the mortar [28,29]. Mortar in
RCA typically has a higher thermal expansion rate than NAs and thus
expands faster than NA when heated. Therefore, considerably higher ther-
mal stresses develop in the mortar than in the NA when RCA is heated.
Moreover, the differences in the expansion rate of NA and mortar is
expected to lead to considerable differential thermal stresses at their inter-
face. These mechanisms together with the intrinsically weaker nature of
typical mortar compared to typical NA are mobilised to break the adhering
mortar into fine powder, thereby separating mortar present in the RCA. A
number of studies have recommended that presoaking the RCA in water to
saturate the adhering mortar can result in higher separation efficiency. This
is deemed to be because of higher internal pore water pressures developing