Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 154

Table 4.9 Efficiency of the mechanical separation (mechanical rubbing) method

Properties of RCA

Process
duration
(h)

24-h water
absorption
(%)

Bulk density
(OD)
(kg/ m 3 )

Mortar
content (%)
by mass Type of RCA

Before separation 0 4.2 2370 47

After
mechanical
separation

100 revolutions ~0.1 3.5 2410 34

200 revolutions ~0.2 3.0 2420 28

300 revolutions ~0.3 2.8 2440 17

as eccentric-shaft rotor, RCAs are passed through rubbing equipment
comprising an inner and outer cylinder that rotate eccentrically at a high
speed. The RCAs are rubbed against the cylinder walls and each other,
breaking the adhering mortar into a fine powder, which is collected after
passing through the 2- to 4-mm sieves provided on the surface of the inner
cylinder. In another mechanical separation method referred to as the grind-
ing method, a drum containing iron balls is used to provide the required
impact and the friction forces required to separate mortar from RCA. In
this method, RCAs are rubbed against each other and the iron balls housed
in the rotating portioned sections [33].
The results of a study we conducted showed that 100, 200, or 300 revo-
lutions of a mechanical rubbing drum equipment filled with 10 steel balls
resulted in about 28%, 40%, and 64% reduction in the mortar content
of RCA, respectively (Table 4.9). As can be seen, relatively high mortar
removal rates can be achieved using the mechanical separation method,
and the removal rate tends to increase with an increase in the number
of iron balls and drum revolutions. The rate, however, is achieved at the
expense of higher energy consumption and costs; therefore, a trade-off has
to be made between the costs incurred in adopting particular regimes of the
separation process and the quality of RCA produced at the end of the pro-
cess. Mechanical separation is easy to use and more efficient in the removal
of mortar compared to the thermal separation method. However, it comes
with environmental impacts arising from the relatively high energy con-
sumption required and inherently high noise pollution. This method also
has a relatively lower yield of coarse aggregates compared to other methods
because mechanical rubbing and impact also tend to break a considerable
portion of the coarse NA particles into finer particles.

4.7.1.3 Thermal- mechanical separation
The thermal-mechanical class of methods uses a combination of thermal
stresses generated through conventional heating at high temperatures and