Page 198 - Materials Chemistry, Second Edition

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Sustainability of Municipal Solid Waste Management
However, foam glass also has inherent strength properties. Foam glass charac-
teristics of low flammability, thermal stability, and high chemical durability
are a distinct advantage over polymeric materials, which have poor fire resist-
ance. Foam glass also has the advantage that it contains no fibrous material.
Fibrous insulation materials such as fiberglass require special handling proce-
dures to protect the user from inhalation of fibers and skin irritation.
The desirable properties of foam glass are high strength, low density, and
low thermal conductivity. Generally these properties are achieved by having
a large number of small, evenly sized bubbles, with thin walls in between. As
the product is made of glass it is naturally inert in most environments with
respect to biological, thermal, chemical, and environmental degradation.

Foam glass mechanism
The principle of the foam glass process is that between 700°C and 900°C the
glass powder forms into a viscous liquid and then the foaming agent decom-
poses to form a gas that in turn forms bubbles. The glass needs to have suf-
ficient viscosity not to allow the gas bubbles to rise through the mass of the
body but remain in position during the foaming heat cycle. If the tempera-
ture is too high the bubbles will rise and the body will collapse and not form
a foam body. The control of the heating rate is one of the most important
factors in optimizing the foam glass product. Rapid heating can cause the
foam glass feedstock to crack, whilst slow heating will lead to early release
of the gas from the foaming agent before the viscosity of the glass is low
enough to allow the glass to expand.
A further complication is that the foam glass feedstock is relatively
insulating due to a pack density of 80% and as the feedstock expands from
the top surface this further insulates the materials below. Therefore, there is
the potential to overheat the top surface in order to heat the bottom of the
feedstock. This overheating can cause the top cells to collapse resulting in
an inferior product.
The finely ground glass powder is mixed with the foaming agent which is
the feedstock for the foaming furnace. Suitable foaming agents can be calcium
sulphate (CaSO 4 ), coal, glass water, aluminum slag, or calcium carbonate
(CaCO 3 ). The thermal conductivity will be lower if CaSO 4 is used: however,
CaCO 3 is easier to work with. This is due to the production of sulfur gases from
CaSO 4 during the foaming process. SO 2 has a lower thermal conductivity than
CO 2 ; however, the formation of SO 2 requires more control as it is a noxious
gas. Silicon carbide (SiC) is also used as a foaming agent, which gives con-
trolled and precise cell sizes. It is thought that SiC is the most commercially
used foaming agent for the reasons of control and reproducibility. The SiC
reacts with the SO 3 within the glass structure to form CO 2 and S.
Gypsum is a readily available source of CaSO 4 and limestone is a readily
available source of CaCO 3 . If the air in the furnace at the foaming zone is
replaced with either SO 2 or CO 2 then this will lower the thermal conductivity