Introduction - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 21

electrostatically to the electric field and tend to align themselves with the
field mechanically (Figure 1.7). Because the electric field is altering with
time, the dipoles will attempt to realign each time the field reverses and so
are in a constant state of mechanical oscillation in tandem with the micro-
wave frequency. Frictional forces generated within the molecules cause heat
to be developed because of the motion of the dipoles.
The amount of microwave energy dissipated through dielectric loss var-
ies significantly with the polarisability and other dielectric properties of
the material heated. Water is the most popular material displaying such
polar characteristics and is the main component in most of the dielectrics
that show good microwave power absorption. Concrete, cementitious mor-
tar, and some aggregates are also porous materials with pores that may
be filled with water. Water absorbs EM energy very strongly when highly
pure, although it also has a high DC electrical resistivity of approximately
105 Ω. However, when small quantities of solid are dissolved in water, the
DC resistivity falls, and conduction via movement of the ionic charge carri-
ers may become a significant component in heat dissipation.
It is interesting to note that, unlike dipolar heating, conduction heating
tends to fall away when raising the frequency into the microwave domain.
This is mainly because the mass of the ions is such that their movement is
curtailed. Therefore, polarisation caused by the alternating field may be
considered the main heating mechanism involved in the microwave heating
of dielectric materials such as concrete and its constituent materials.
1.6 ELECTROMAGNETIC POWER TRANSFER
We discussed previously that exposure to microwave power may lead to
heating of the dielectric materials through dipolar losses. However, it is
important to understand how the power generated in the microwave gen-
erator unit is transferred to the material. In microwave heating, a hollow
metallic tube of either rectangular or circular cross section, made of alu-
minum, copper, or brass of various sizes, is usually used to transmit the
generated power. Such a structure is commonly known as a waveguide
(Figure 1.14). Waveguides may be used to transfer the microwave power
directly to the material surface or to a microwave heating chamber (cavity)
in which the material is placed. Microwave cavities are normally in the
form of a metallic chamber similar to those used in domestic microwave
ovens. There are sets of standard waveguide dimensions for the range of
microwave frequencies and waveguide cross sections used. For instance, the
standard dimensions of rectangular waveguides at the common frequencies
used in microwave heating are listed in Table 1.1.