Additives to clay - mineral & synthetic
achieve good results it is important that the cement is well mixed with the soil and that the
product is cured properly.
Ordinary Portland Cement is normally preferred, but Portland-pozzolana cement could also be
used. Some soils require only a 3% addition of cement, but more usually 5 to 8% cement is
added – the higher the shrinkage of the soil the higher the proportion of cement.
This produces similar effects to Portland cement, but of lesser magnitude. Natural hydraulic
lime does not dilute the natural colour of clays.
Principally two effects operate in the reaction between clay and hydraulic lime; a short-term
flocculation effect, making the clay more cohesive and a longer-term effect which is a
combination of the pozzolanic reaction between lime and clay and a reaction between lime
and carbon dioxide in the air. Stabilisation is particularly effective with soils containing
kaolinitic clay. Significant increases in compressive strength are then possible. Quicklime
addition can also be used if the soil is wet as this will help to dry it and break up lumps of
Lime should be added to a soil at least 2, but preferably 8 to 16 hours before use to allow
short-term stabilisation to take place making the soil easier to work and compact. Lime is
suited to soils with a relatively high proportion of clay and is added in proportions of 3 to
10% but usually at the upper end of this range. Local artisanal lime can be used but might
not be as effective as industrial lime. In addition, a small quantity of Portland cement can be
added to give additional stabilisation and reduce hardening time. Lime-pozzolana cement can
also be used and the degree of stabilisation is likely to be similar to using hydraulic lime.
Excellent results have been obtained with high lime contents (one part lime to three or four
parts soil) for thin (approximately 5mm) finishing coats applied on mud plasters and floated
This is not a common additive to soils. It is not moisture resistant, therefore not suited to
exposed wet conditions. Usually it is added in proportions of between 10 and 20% to sandy
soils. Because it sets quickly it is best used by mixing small batches of soil and binder.
Soap is used to waterproof soil because it reduces its sensitivity to water by about 25%. It
can either be mixed with the soil or applied as a surface finish. It is generally applied in very
small quantities (0.1 or 0.2%) and because it is washed out by water, it needs periodic re-
application if used as a surface finish.
Bitumen is added in the form of an emulsion that is in suspension in water or another liquid
medium. On drying the bitumen forms a thin film which coats the soil particles. The main
effect of adding bitumen is to improve cohesion and water resistance. However, if it is added
in excess the compressive strength can be reduced.
Normally the quantity of bitumen to be added (excluding the solvent or water) is 2 to 3% for a
cutback (solvent-based), or in the following proportions for an emulsion:
- 4 to 6% for a soil with a high sand content;
- 7 to 12% for low sand content soils;
- 13 to 20% for clayey soils.
The bitumen needs to be mixed with a small quantity of soil before mixing it with the
remainder; the mixing needs not to be excessive to avoid breaking down the emulsion and
making stabilization less effective.