Microwave-assisted concrete recycling - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 160

L

1

− ∫

− 2 β x

PL () x = 2 β I PL xe () (4.3)

2

2 2 0 1

0

Now, consider that layer 1 has a considerably higher attenuation factor
compared to layer 2. In this case, according to the equations, more energy
tends to be dissipated in layer 1; thus, layer 1 would be heated much faster
than layer 2.

ββ
1

> (4.4)

2

and

L L

1 1

>− () →

∫

− ()

∫

→ ()

I I PL x 2 β I  2 β I Lx PL x Lx  ( )

0 0 1 1 0 2 0 1 1 2

0 0

Hence, in this case, microwave heating leads to differential heating, espe-
cially at the interface of the two layers. Now, consider RCA as a composite
material comprising two layers: the adhering mortar and the recycled NA.
For a similar moisture condition, the attenuation factor and thus the micro-
wave energy absorption rate of mortar is higher than for NAs (Figure 4.9).
In addition, as mentioned, the difference in the microwave absorption rate
of NA and mortar could be considerably increased by increasing the water
content of mortar. Hence, if RCA is exposed to microwaves, the mortar
layer would be heated much faster than the NA; thus, significant differen-
tial thermal stresses may develop at the mortar-aggregate interface.
In the microwave-assisted separation method, these differential thermal
stresses may be harnessed to remove the adhering cementitious mortar
from RCA and thereby increase the quality and yield of the RCA products.
Moreover, it is well known that the water-to-cement (w/c) ratio increases
at the interfacial transition zone (ITZ) and can be significant if bleeding
is predominant; hence, the ITZ normally has higher porosity and water
absorption than the bulk cementitious mortar. As a result of the higher
water content, the ITZ is expected to heat up even faster when RCA is
exposed to microwaves, leading to higher differential thermal stresses at
the ITZ. Besides the differences between the water absorption and the EM
properties of mortar and aggregate, the differences in the thermal proper-
ties, such as the specific heat, thermal conductivity, and expansion coef-
ficient, may also contribute to the generation of differential stresses at the
mortar-aggregate interface when RCA is subjected to microwave heating.
However, the effects of these properties are considered to be much less sig-
nificant compared to the effects of the different EM properties [31].