The cohesion of a soil is essentially assured by its clay content. The extent to which clay will bond depends on its particular mineralogical characteristics. But its capacity to bond decreases significantly when the clay is in direct contact with water. In addition, clay swells in the presence of water, causing movements in the soil structure. Clay is, therefore, a natural binder, which is cheap, but unstable.
Stabilization is a technical process, the object of which is to neutralize or at least restrict the detrimental behaviour of the clay present and thus reduce the natural sensitivity of soil to water, which leads to a loss of strength and cohesion. The degree of sensitivity of a soil to water varies according to the proportion of clay compared with other inert constituents (sand, silt), and according to its mineralogical nature. There are, in fact, several distinct types of clay, the principal ones being:
kaolinite, which is relatively stable,
illite, which is of average stability, and
montmorillonite, which is highly sensitive to water (causing swelling, etc).
The behaviour of each type of clay is linked to its mineralogical structure and more particularly to the layers, which are the basic elements of clay micelles (large clay molecules).The degree of bonding and the extent of swelling of various clays are determined by the structure of these layers.
Modifying the proportion of clay or the mineralogical nature of the clay content can already have a stabilizing effect on the behaviour of a soil. Correcting the characteristics of a clayey soil by adding sand or another soil, which is not very plastic, is common practice and constitutes an elementary form of stabilization.
A second form of stabilization is to reduce the proportion of voids in a soil. The penetration of water into the material, leading to the swelling of the clay layers, depends on the presence of large voids. This proportion can be reduced, either by increasing the density (ie by compacting the soil mechanically), or by adding a material with a complementary granulometric composition, the finer or coarser grains of which will allow a more regular distribution and a better volume occupancy. Adding a "thinning" sand to a clayey soil helps both to improve its compactness and to achieve a quantitative reduction in the proportion of its clay content.
Finally, the characteristics of the clay layers themselves can be affected directly during the manufacturing process, by adding a stabilizer, which has a direct or indirect physico-chemical stabilizing effect on the layers. Stabilizers are very commonly used in the production of compressed earth blocks.
Various groups of products can be classified according to the way they act on clay plates. This classification is important, since for each group there is a particular way of proceeding and each has varying degrees of efficiency, depending on the type of soil. A simplified classification of additives is shown in the chart below.
The Need for Physico-Chemical Stabilization
No single method of stabilization precludes the use of another; on the contrary, the most durable earth blocks result from a rational use of several stabilization methods.
In general, the following points must be remembered
Good technical results can be obtained at the outset by careful soil selection, correcting the granular composition as necessary, and reducing the proportion of voids by adequate compaction.
Stabilization by adding a physico-chemical product is not essential.
Physico-chemical stabilization is necessarily costly, partly because of the cost of the stabilizer, and partly because of the higher quality standards, which need to be respected in preparing and mixing the soil, and in the additional operations it requires.
First and fore most, attention must be paid to the architectural aspects of building with earth in order to limit damage caused by weathering; poor design cannot be compensated for by the indiscriminate stabilization of materials. Stabilization must only be considered in addition to proper building design, and the decision to stabilize should be taken only after comparing the building costs with and without stabilization (ie considering the costs of surface protection, architectural design features, maintenance, etc). It must, however, be recognized that stabilization does provide additional guarantees of durability for buildings and materials. It also facilitates certain operations, such as storing blocks in the open before use.
Principal Requirements of Stabilization
The efficacy of stabilizers is linked to a certain number of conditions:
the stabilizer must be selected to suit the soil type;
the soil must be prepared (ie sifted, crushed, sieved) with particular care, in order to ensure that the stabilizer can be uniformly distributed throughout the material;
the soil-stabilizer mix must be perfectly homogeneous;
in the case of stabilizers which begin to cure quickly, such as Portland cement, the time between mixing and use must be kept to a minimum;
curing conditions - humidity, temperature, etc - must be suited to the type of stabilizer.