Civil Engineering Reference
In-Depth Information
cause splitting of the brick when it is exposed to damp. The clay can contain
lime, but it has to be evenly distributed. It is an advantage if the clay is well
mixed with sand. Clay with too little sand is not easy to shape, but has the
advantage of not shrinking so much when drying or being fired. Sand can be
added to clays that are too 'fatty'. An idea of the quality of a clay can be found
through some simple tests. It must easily form into a ball and keep the prints
made by the fine lines of the hand. During drying it must become hard without
too many fine cracks.
One thousand square metres of clay can produce about 650 000 bricks per
metre of depth. The clay does not usually lie too deep in the ground, so it is
relatively easy to extract. This is usually done by first scraping away the soil,
then extracting the clay and, after re-planning the area, placing the soil back
again.
After the clay has been extracted from the ground, it is covered with water. It
then used to be worked by hand with a special hoe or by ramming. The latter
method was preferred because it made small stones in the clay obvious. This
operation is now carried out by a machine which grinds the clay down to a fine
consistency. Additives to reduce its fattiness can be put in the clay and the mix-
ture is then well kneaded. If the clay is stored for between one and three months
in an out-house it becomes more workable and produces a better quality final
result.
Sand can be used to make the clay leaner, but slag, fly ash and pulverized glass
are also suitable. These not only reduce the amount of shrinkage, but make the
clay easier to form. The porosity of brick can be increased by adding materials
which burn out when the stone is fired, leading to higher insulating values and
better moisture regulation. Materials that can be used for this are sawdust, dried
peat, chopped straw or pulverized coal. Porosity can also be increased by adding
15-20 per cent of materials that evaporate through heating, such as ground lime,
dolomite or marble, which produce carbon dioxide when fired. These additives
bind the released sulphur and fluorine into harmless compounds such as gyp-
sum.
Insulating materials such as fossil meal can be added in parts of up to 90
per cent. Fossil meal is a form of earth which consists of air-filled fossils from
silica algae. The resulting block has very good insulation value and high
porosity. Around Limfjorden in Denmark there is a clay containing fossil
meal (about 85 per cent ) which occurs naturally. It is called molere, and has
a complete brick industry based around it. The resources, however, are very
limited.
Forming
Clay needs a water content of approximately 25 per cent in order to be formed.
The forming is carried out mechanically by forcing the clay through a die or by
