Civil Engineering Reference
In-Depth Information
The alternative is a cement industry based on medium- or small-sized busi-
nesses. Setting up takes little time, and investment is small enough to be covered
by local demand. These smaller plants can be placed where the cement is to be
used and the raw materials extracted. The local infrastructure should be able to
support them, and as changes in market forces will be local, they will be less dev-
astating. The technology is relatively straightforward and could be adequately
served by local small workshops and services.
Hydraulic binders
Hydraulic binders include lime pozzolana cements, hydraulic lime, Portland
cement, Portland pozzolana cement and mortar cement - a mixture of lime and
Portland cement.
A hydraulic binder can harden with dampness, even under water, but it must
contain an acid. The most suitable are silicium dioxide and aluminium silicates,
which are plentiful in clay. Argillaceous ingredients, pozzolanas such as broken
up brick, can be added with other silicium-rich additives such as fossil meal and
volcanic earths. Ashes from silica plants can also be used, (see 'Silicates', p.185).
The hardening reaction is:
2(2CaO
SiO
2
)+4H
2
O = 3CaO
2SiO
2
3H
2
O + Ca(OH)
2
(3)
At the outset one may think that quartz sand, which is almost pure SiO
2
, would be
usable. However, quartz sand in principle cannot form silicic acid under normal
pressure and temperature conditions. It can in a damp, warm atmosphere and
under pressure - a method used in the manufacture of lime sandstone. In many of
the castles of the Middle Ages on the European continent a mixture of lime and
quartz sand was used as a cold mix: we must assume that the silicic acid has been
released from the sand, thus forming a durable binder, as these buildings are still
solid today.
Pozzolana cements are low energy because the pozzolana undergo only a mod-
erate warming. For the same reason there is very little gaseous pollution during pro-
duction. Heavy metals such as nickel and thallium need a much higher temperature
for vaporizing. Pozzolana cements can also be produced more economically than
Portland cement, but they are often weaker. A ton of Portland cement is equivalent
to 1.7 tons of lime pozzolana cement.
The following hydraulic binders are the most common.
Lime pozzolana cements
Fossil meal/slaked lime
Fossil meal is an earth rich in SiO
2
which consists of shells of petrified silica algae.
Pure fossil meal reacts with slaked lime in its natural state even in weak frost,
