CLAYS AND CLAY-WATER SYSTEMS

Professor D.d. Van Reenen

INTRODUCTION

The material small change in the Earth crust can or it cannot be worn away. The standard classification for the size of the grain of grounds is as it follows:

The clay term does not have any genetic meaning. It is used for the material that is the product of the wearing down by the atmospheric action or it has been deposited as sediment, and he is always present in grounds. A property of the clay is the plasticity.

By plasticity one talks about the property of the materials to become deformed itself under the application of the pressure, and the obtained deformation that is conserved when the pressure takes off.

The finish of argillaceous material is an expression for any granular, natural, dirt material fine that includes the clay. The minerals of the clay are the crystalline phases that dominate fine (< to the 0.002 mm) fraction (clay) of stones and ground.

The knowledge of the clay mineralogy, therefore, is essential if one wishes to predict or to modify the properties of the ground engineering. Certain materials of the clay such as smectite, that it only can be present in small amounts in the ground, will be able to exert an enormous influence in the properties of the ground.

MINERALOGY OF THE CLAY

The minerals of the filosilicatos clay are mainly leaf or, and like such, they are seaplane aluminum silicates that contain certain amount of magnesium and of iron and in some of them the alkaline metals are component essential.

The structure of these silicate leaves can be derived by the combination of a leaf of silicon oxygen (a tetrahedron of the silicone, SiO-4, consisting of a silicon, Si+4), surrounded by four oxygen atoms, (fig ç and d) with a consistent octahedral cation leaf for example Al+3, Mg+2, Fe+2, Fe+3 etc. surrounded by six groups by oxhidril (OH)

The structure that it is possible to be derived this way is the one of the group of the caolinita of minerals of the clay that are made up of single octahedron (Or-leaf) combined with a single tetrahedral leaf (T-leaf) through the substitution of oxygen atoms in the tetrahedral leaf for oxhidril in the octahedral leaf to form a structural unit of the common layer

The loads within this unit are balanced and the structural formula is Al2 (SiÒ5)(OH)4.

The main types of layers (of smectite) group of which montmorillonite is an important member, consist of a combination of two tetrahedral leaves (T-leaves) combined with a central octahedral leaf (Or-leaf). The inferior part of these layers is of such nature that exists a very weak connection among them.

This characteristic is responsible for the most important property of the structure of smectite (montmorillonite) which the polar water and other molecules can enter between the layer of the unit, doing the grids to extend itself (the calls expansive clays).

The theoretical formula of smectite, except the substitution of the grid is Al2(SiÔ10)(OH)2, nHÒ (nHÒ is water of the interlayer). There are changeable cations (Ca+2, Na+, etc.) between the layer of silicate (fig 1f), of the changeable cation in a given pressure of the water-steam.

The changeable cations are absorbed between the layers of the unit and around its edges to balance the deficiencies of the load due to the substitution of Al+3 for Si+4 in the tetrahedral leaf, and Mg+2, Fe+2, etc. for Al+3 in the octahedral leaves.

CAPACITY OF INTERCHANGE OF IONS IN MINERALS OF THE CLAY

Minerals of clay are highly reactive due to his great area superficial and because commonly they take a load that must to the ionic substitution in the tetrahedral and octahedral leaves, according to the described thing above.

The existence of the load in minerals of the clay is the base for the interchange capacity, which combined to the property of expansion of the clay, is two extremely important characteristics for the civil engineer. The capacity of ion interchange of the clay is the characteristic of minerals of the clay to absorb cations and anions (due to the deficiencies of the load of the interlayer) and to conserve them in a changeable state, e.g. these ions are changeable by other cations and anions by means of the treatment of ions in a water solution. These reactions of the ion interchange do not affect generally the structure of the mineral of the clay.

The ion interchange is of great importance because the physical characteristics (e.g. plasticity) of the argillaceous materials are frequently employees of changeable ions taken by the clay.

The plastic characteristics of the clay, for example, are highly employees of the changeable cation Na+ or Ca+2. The engineer of the ground, therefore, can change the plastic characteristics of many grounds to solve his necessities, making a reaction of the ion interchange.

CLAY-WATER SYSTEMS

The clay characteristics in normal temperatures to a large extent are determined by the interaction of minerals of the clay with the water. The distortion of the laying of foundations of the way is been from the exit or the loss of water in the ground, and the magnitude of the change in volume are determined to a large extent by the mineral presence specific of the clay.

This clay interaction with water only depends on the water that can be retained by clays to relatively low temperatures (less than 100 - 150° C). The understanding of the nature of the low temperature of the water is of great importance, since it determines to a large extent the plastic one, the connection, the suspension, the compaction and other characteristics of minerals of the clay.

The water that is lost in the low temperatures can classify in three general categories.  (Fig. 2):

1.- The water in the volume of pores and capillaries (absorbed water).

2.- The water in the surfaces and around the edges of particles of minerals of the clay as well as in the surfaces of pores (water fixed by absorption).

3.- The water of the interlayer (water fixed by absorption) that causes the expansion of montmorillonite.
The absorbed water (type 1) requires very little energy to be removed (e.g. the curing in a temperature slightly superior to the room temperature).

The water fixed by absorption (types 2 and 3) requires defined energy to be removed completely. The particle of the clay when it is suspended in water could be surrounded by a hydrosphere of the water fixed by adsorption, within which they are the soluble ions of diverse loads.

Around the last clay particle there is an ion layer of negative load (due to the fact that oxygen forms the composition of silicates) and these are balanced by a cluster of the cations that spread through the hydrosphere.

These "opposite" cations provide connections between particles with the clay obtaining with it plasticity. The plasticity is associated with the formation of water films absorbed with certain thickness around each particle, and is therefore a function determined by the water content.

The Maximum plasticity of the clay is obtained in a specific water content that corresponds to a film of density around each particle of approximately 2 000 Å.

For most of clays, this would be in the operational range of 15 - 25 percent of the weight. An attempt to compact so material will cause a total reduction in the graduation, which will give rise to a later increase of the plasticity. This is due to the development of the hydrostatic pressure within the material during the compaction.

THE QUESTION IS, how can be removed permanently the absorbed water?

THE ANSWER IS with the use of CBR PLUS.