Additives to clay - mineral & synthetic
Practical Action
Building Design
The design should take account of the properties of the raw material by appropriate load
distribution and structural dimensions, and by incorporating protective elements against
damp, rain, impact and abrasion. Protection could be achieved by adding more durable but
complimentary materials at places such as the wall base, roof overhang and the copings, and
by using plasters and renders.
Improving the Raw Material Quality
Different treatments or additives, collectively known as stabilisation, can modify the
properties of soils to control their shrinkage and swelling characteristics and so improve the
binding ability of the clay in the soil. These stabilisation methods are described in this leaflet.
Compaction
Compaction increases the soil density and hence its strength and resistance to mechanical
damage. It also reduces its water absorption, although at the expense of reducing porosity, so
possibly reducing durability.
Compaction is done in a mould or form:
statically (i.e. in a single pressing), with cylindrical rollers, wheeled rollers or presses;
dynamically (i.e. repeated), with tampers or rammers, vibrating rammers or pick
hammers;
surface, with a beater – mainly for floors or roofs, although sometimes used on rammed
earth walls before they dry.
Effectiveness of compaction depends on applied pressure or energy, soil type and water
content.
Mineral additives
The first material to be added to a soil, if it has too much clay, is sand and gravel to obtain a
more uniform grainsize distribution. This reduces the shrinkage and swelling characteristics
as well as giving a better pore size distribution and higher density, with improved cohesion
between particles of the soil. Adding an optimum quantity of sand and gravel to a soil often
reduces the amount of other, more expensive, additives needed for effective stabilisation.
Care, however, needs to be taken when mixing sand and gravel with clayey soil because of the
presence of clay lumps. Therefore, if the mixing is carried out in a dry, humid or plastic
state, the soil first needs to be pulverised to break up any lumps. The alternative is to mix
the soil in a liquid (fluid) state.
Pozzolanas, such as certain types of volcanic ash, are another mineral additive which will
improve the properties of soils containing more than the optimum proportion of clay.
Ashes
Ashes often contain calcium carbonate and may have stabilising properties. Ash is normally
added in proportions of between 5 and 10% often achieving improvements in dry compressive
strength, but having less effect on water resistance, although it can also reduce shrinkage and
swelling. Ashes can be somewhat pozzolanic, so when combined with lime, additional
improvements are possible. Ash, when added to soils, has also been known to act as a
termite repellent.
Synthetic additives
Portland Cement
The reaction between clay and cement is thought to be a three-phase mechanism, hence
there is both a long-term and a short-term effect. Cement also bonds with the sandy skeleton
of the soil in the conventional way. The cement does not bond with all of the soil particles,
but helps to form a stable matrix throughout.
The effect of adding cement is to improve the dry compressive strength, but more particularly,
the wet compressive strength. Resistance to erosion, rain and abrasion is much improved. To
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