Stabilizing aggregate and other additives

In certain situations it may be necessary to add stabilizers. These usually have three functions:

• To bind the earth particles together strongly. These are substances such as lime, Portland cement, pozzolana cements and natural fibres. These strengtheners are necessary for buildings more than two storeys high, whatever the quality of the earth.

• To reduce water penetration. Lime, Portland cement, pozzolana cement, bitumen and waterglass are examples. Traditionally whey, casein and oxblood have also been used. In areas where there is a great deal of driving rain it is advisable to have one of these additives in the earth mix, and in extreme conditions a separate external cladding as well.

• To avoid shrinkage. This is mainly achieved by natural fibres, even though cement and lime can also be used.

Lime and cement. Lime is the stabilizer for earth rich in clay. Both slaked and unslaked lime can be used. The lime reacts with the clay as a binder and can be added in a proportion of 6-14% by weight.

Portland cement is often used as stabilizer for earth rich in sand or containing little clay. The proportion of cement to earth is 4-10% by weight. Mixtures with Portland cement are also used in foundation walls. The humus in the earth can attack the cement, so this construction technique is assumed to have low durability.

Pozzolanic cement can be used in both types of earth, either lacking or containing a lot of clay. It has to be added in slightly larger quantities than with Portland cement.

Adding cements or lime based additives increases the environmental impact and energy use. All lime and cement constituents also reduce the possibility of recycling the earth after demolition or decay.

Natural fibres are best used in earth containing a lot of clay to increase thermal insulation and reduce shrinkage. A mixture of even 4%

by volume of natural fibre will have a very positive effect on shrinkage and strength. Common proportions in the mixture are 10-20% by volume. Larger amounts than this will reduce its strength. In non-structural walls that are primarily for thermal insulation, fibre content is sometimes increased up to 80%, see Chapter 14.

Straw chopped into lengths of about 10 cm, preferably from oats or barley, is normally used. Pine needles are also good binders; alternatively stalks from corn, flax, hemp, dried roots, animal hair, twigs, sawdust, dried leaves and moss can be used.

If large amounts of organic material are used, mould can be a problem. These walls must thus dry out properly and should not be covered until the moisture content has been reduced to around 18%.

Expanded mineral products such as exfoliated vermiculite, expanded clay pellets and perlite can be used as aggregate. There is no chance of mould, and higher thermal insulation is achieved. However, mineral aggregates require much more energy to extract and produce than natural fibres, with large emissions of carbon dioxide as a result.

Waterglass. An earth structure can be made more water resistant by brushing a solution of 5% waterglass on the surface of the wall. This can also be used for dipping earth blocks before mounting them.

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