Civil Engineering Reference
In-Depth Information
Once in the building, the materials are relatively harmless, and as waste they
are considered inert. The exceptions to this are asbestos and boron substances
which have a pollution risk during their entire life span.
The non-metallic minerals are usually impossible or difficult to recycle as they
are usually in the form of new chemical compounds in the final material. They
nearly always have to be extracted from their raw state, sulphur though is an
exception, which can be smelted out easily.
All glass can be recycled by smelting. But smelting of coloured glass has been
found to be impractical. Also, used glass must be cleaned of all impurities for
smelting.
The most important non-metallic mineral raw materials
in the building industry
Lime
Lime is the starting point for the production of pure lime binders, as well as
cements. It is also an important ingredient in glass. In the production of alu-
minium from nephelin, a great deal of lime is used, which becomes Portland
cement as a by-product.
Most places on the Earth have deposits of lime, either as chalk deposits or coral
and sand formed from disintegrated seashells. The purity of the lime is the deci-
sive factor as far as the end product is concerned. For pure lime binders there has
to be a purity of 90 per cent, preferably 97 per cent. Lime in Portland cement can
be less pure. Chalk is a white or light grey lime originating from the shell of
Foraminifera
organisms.
The production of lime binder from lime ore starts with a burning process,
usually called calcination:
CaCO
3
= CaO + CO
2
- 165.8 kJ
(1)
This dividing reaction is endothermic and continues as long as the energy keeps
the temperature 800-1000°C. Calcination is usually performed by breaking up
the limestone into pieces of 2-8 cm which are then burned in kilns at 900-1200°C.
There are a number of kiln constructions in use. Many are simple both to build
and use, and production rates of 30-150 tons per 24 hours can be reached local-
ly. There are mobile variations that can be used on very small lime deposits.
Wood is the best fuel, as the flames are long-lasting and create a more even burn-
ing of the limestone than other fuels.
