Civil Engineering Reference
In-Depth Information
mortar has a higher attenuation factor and is therefore heated faster than
natural stone aggregates when placed in a microwave field.
Another important point to be taken into account is the considerable
increase in the attenuation factor of mortar with an increase in its water
content. As a result, an increase in the water content of mortar could con-
siderably increase the differences between the attenuation factor and thus
the heating rate of the mortar and natural stone aggregates. On the other
hand, mortar is of a more porous nature and thus has higher water absorp-
tion than NAs. Therefore, saturating the adhering mortar through soak-
ing the RCA particles in water for a few minutes can be used to increase
the differences in the water content and thus the differences between the
microwave heating rate of the adhering mortar and NAs. In the microwave-
assisted separation method, these inherent differences between the dielec-
tric properties and water absorption rates of the mortar and NAs are used
to generate a localised field of differential thermal stresses in the mortar,
especially at the interface between mortar and NA present in the RCA in a
relatively short duration (a few seconds to a few minutes, depending on the
microwave power and the volume of the RCA processed) without causing a
significant temperature rise in the NA itself [31,38].
To elaborate, let us revisit the microwave power dissipation formula-
tions discussed in Chapter 1. The microwave power dissipation in a dielec-
tric material may be estimated using Lambert's law. The simple form of
Lambert's law may be stated as
=−
∂
Ix
x
()
−
2
β
x
PL x
()
=
2
β
Ie
(4.1)
0
∂
Considering an RCA particle exposed to microwaves, here
PL
(
x
) is the
microwave energy dissipated at a distance
x
from the microwave-exposed
surface of RCA,
I
0
is the microwave power transmitted into RCA, and β
is the attenuation factor of RCA. As can be seen in Equation 4.1, a higher
attenuation factor results in higher microwave power dissipation and thus
faster decay of the microwave energy in the material. Consider a material
comprising two layers with two different attenuation factors, β
1
and β
2
,
and two different thicknesses,
L
1
and
L
2,
respectively, exposed to uniform
microwave power from its top surface. The microwave power dissipation in
the upper layer (layer 1) may be calculated as
−
2
β
x
PL x
()=
2
β
I e
(4.2)
1
1
1 0
The dissipated microwave power at distance
x
from the top surface of the
next layer (layer 2) is calculated through
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