## 3 Pavement Design Process Based on Scientific Principles
### Scientifically Based Material Properties of Naturally Available Materials
#### 3.2.3 Secondary Minerals Defining the Condition of the Stone
It follows that in order to effectively utilise materials with a high clay content, specialised technologies are required to achieve the required hydrophobic nature, expel the water and achieve the strengths required through stabilisation. Clay in a dry state has a high bearing strength. The problem is that clay in nature is usually associated with water \"capturing\", and to make it hydrophobic, water is required as a carrier fluid for the nano-silane modifier to reach the clay particles. The clay particles will release and repel the water molecules only after the characteristics of the clay particles have been changed. After repelling the water particles, the material will have to reach a moisture content not exceeding the Optimum Moisture Content (OMC) of the modified material (in the order of 10 per cent less than the original OMC) before stabilisation, compaction and strength can be achieved. The process of the effective utilisation of materials containing a high clay content presents technological as well as practical challenges. Generally available nano-silane particles are in the order of 2 to 5 nm in size and will be ineffective to penetrate the clay matrix and render clay particles hydrophobic. Figure 12: Ref. [41]: Some typical sizes of clay crystals as measured in Angstrom (where 1 Angstrom (A) = 0.1 nm). The treatment of several clay nano-layers is easily achievable using these technologies. However, any stress/strain put on these samples causes slip-planes within the layers, which allow water to have access to the clay nano-layers that have not been accessed and have not been chemically changed to become hydrophobic. The result is that the material becomes hydrophilic, absorbs water, swells and breaks any bonds created by an applied stabilising agent. Ideally, the area of the particles to be bound together should be matched by the area that can be covered by the stabilising agent. With materials containing a grading with a high percentage of clay (<0.002 mm in size), the stabilising agent should have a grading to match the increase in the area to be covered, creating a stable environment within the material. This concept is illustrated in Figure 13, the practical implementation of which is discussed in the following recommended detailed material design method. Hence, to effectively treat materials containing high percentages of clay, the nano-silane particle must be of the highest quality and purity of a sub-nano (Pico) size in order to penetrate the clay nano-matrix. Only nano-silane particles exhibiting these characteristics will be able to replace the captured water molecules by binding with free cations present in the clay particles. By penetrating the clay matrix, the pico-size silane can render each clay nano-layer hydrophobic, repelling the captured water molecules and preventing any water from penetrating the clay nano-layers. No shearing of the clay nano-layers under loading will now expose the clay matrix to allow water ingress to occur, and a durable  
Figure 13: Illustration of grading of material (particle size) versus area to be covered by an applicable stabilising agent and required particle size of the stabilising agent. stable layer will be created. This process will result in a considerable reduction in volume when the water is repelled from the clay nano-layers, which will normally (in the case of high-swelling clays) be associated with considerable shrinkage, as shown in Figure 14.  
Knowing the properties of the materials, a hydrophobic clay matrix can be achieved with an adequate amount of applicable nano-silane of sub-nano particle size (even for materials commonly referred to as black-cotton soils) in a laboratory environment. The stabilising agent used in conjunction with the nano-silane will also have to be able to actively bind clay particles together.