Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 155

Table 4.10 Efficiency of the thermal-mechanical separation 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 thermal-
mechanical
separation

300ºC ~2.1 3.3 2430 31

500ºC ~2.1 2.1 2480 21

the mechanical stresses generated through the application of mechanical
separation methods to separate the adhering mortar from RCA [28,29].
In a typical combined thermal-mechanical separation method, referred to
as the heating-and-rubbing method, the adhering cementitious mortar is
dehydrated and thus becomes brittle by heating in a vertical furnace. The
heated RCA is then transferred to rubbing equipment, which comprises a
tube-type mill with inner and outer cylinders loaded with a number of iron
balls to remove the adhering mortar. Shima et al . (2005) showed that the
heating-and-rubbing method may increase the quality of RCA significantly,
so it could comply with the quality requirements of the Japanese Concrete
Institute (JCI) for the high-quality H RCA [28]. The results of a study we
conducted (Table 4.10) show that the heating of RCAs in a conventional
oven at 500°C for 2 hours followed by mechanical rubbing using a Los
Angeles testing machine loaded with 10 iron balls for 100 revolutions could
result in 55% reduction in the mortar content of RCA.
However, despite the report of such promising separation rates, this com-
bined method tends to be high in energy consumption and costly as well.
The negative impacts in terms of energy and associated carbon emissions
could easily negate the perceived environmental advantages of concrete
recycling. The separation efficiency of the combined thermal-mechanical
method can also be increased by increasing the duration and temperature
of heating, the number of iron balls, and the number of revolutions of the
drum but at the expense of additional costs and energy consumption. A
detailed economic and environmental analysis is required before this
method can be adopted optimally.

4.7.1.4 Chemical separation
Because of the alkaline nature of cement, cementitious mortar can be eas-
ily corroded by strong acids [34]. The chemical separation method uses the
inherently weak corrosion resistance of the cementitious mortar to sepa-
rate the adhering mortar from NAs in RCA. In this method, RCAs are
immersed in diluted acids for about 24 hours and then washed to remove