Production of light expanded clay

All clays can be expanded, though some expand more easily than others. And melting must not occur before the clay has expanded -this mainly depends upon the minerals in the clay. The ideal clay is very fine, with a low lime and high iron content.

Clay used for the production of expanded clay pellets needs to air for about a year before being used. It is then ground, mixed with water and made into pellets. Medium-quality clay can have chemicals added, mostly ammonia sulphite in a proportion of 3% volume of the dry clay, and sodium phosphate in a proportion of 0.1%.

Expansion can occur in a vitrifying kiln where sawdust, oil or coal can be mixed with the clay and then fired. Alternatively, the more efficient rotating kiln can be fired with coal dust, oil vapour, natural gas or biogas. The rotating kiln usually consists of a metal cylinder with a diameter of 23 m and a length of 12-60 m. There are also smaller, mobile models (see Figure 8.12). The kiln temperature is about 1150 °C and the firing time from clay pellets to expanded clay pellets is approximately seven minutes.

For the manufacture of a light clay block Zytan moulds are filled with light expanded clay, then heated at temperatures of about 1000 °C (Brien et al., 1978). The light clinker expands even more. The spaces within the mould are filled and the material becomes a solid block. Once the moulds have cooled down, the result is a homogeneous and highly insulating thermal block which can be used immediately. The density of

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8.11

Examples of English patterns for tiles from around AD 1200.

8.11

Examples of English patterns for tiles from around AD 1200.

the blocks can vary from 200 kg/m3 to 1200 kg/m3 depending on the firing temperature. All blocks are loadbearing. Holes can be sawn and drilled, just as in other light clay blocks. At present these blocks are not produced commercially.

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