## 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
The size of the molecule of the stabilising agent;
* The size of the particle of the nano-silane modification of the stabilising agent;
* The practical considerations during construction. The \"clay problem\" is a subject that in itself requires extensive discussion, which will not be covered in any detail within this article. In order to appreciate the considerable challenges offered by materials containing a high percentage of clay, the properties of some of the most commonly found clays are summarized in Table 5 (based on [40]). The specific surface area of the different clays (in m\({}^{2}\)/g) shown in Table 5 is of high importance when stabilising materials containing high percentages of clay using a nano-silane modifications and the selection of an applicable stabilising agent. In order to achieve a high level of hydrophobicity, the total area of all the particles in the material needs to be covered. It is seen that the specific surface area of clay could vary from about 700 to 800 m\({}^{2}\)/g (Smectite) to 7 to 30 m\({}^{2}\)/g (Kaolinite), a considerable difference that will determine the percentage of a nano-silane of appropriate size to be required to achieve a durable hydrophobic effect that will be lasting under the effect of loading. It follows that the smaller the particle size of the nano-silane, the less volume will be required to penetrate the clay matrix to achieve the required hydrophobic effect and vice-versa. However, at the same time, the Cationic Exchange Capacity (CEC) of the specific clay is just as important. The higher the CEC, the more suspectable to chemical interaction the clay will be. Smectite, for example, has a comparatively very high specific area in combination with a high CEC. Hence, Smectite is much more susceptible to chemical alteration than, for example, Kaolinite, which has a relatively low specific area but a similarly low CEC (7 to 15 meg/100 g). A low CEC in combination with relatively small particle sizes may prove difficult to chemically alter and firmly bind together through any modification. In all definitions, Kaolinite can be considered as a \"dead\" clay with little attraction to nutrients and water for agricultural purposes. Similarly, materials containing relatively high percentages of Kaolinite present considerable challenges towards the effective strong formation of chemical bonds during the stabilisation process. The challenge is to penetrate the clay matrix, activate the surface of the clay with an applicable HCT and to create the chemical bonds that will enable the material to be stable and durable under loading conditions. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Property**} & \multicolumn{5}{c}{**Type of Clay**} \\ \cline{2-6}  & **Smectite** & **Vermiculite** & **Illite** & **Chlorite** & **Kaolinite** \\  & **(Montmorillonite)** & & & & \\ \hline
**Specific Area** & 700–800 & 500–700 & 50–200 & 25–40 & 5–30 \\ (m\({}^{2}\)/g) & & & & & \\ \hline
**External surface** & Very High & Very High & Medium & Medium/low & Low \\ \hline
**Basal spacing (Å)** & 9.6–20 & 10–15 & 10 & 14 & 7.2 \\ \hline
**Internal surface** & Very High & High & Medium & Medium & None \\ \hline
**Expanding** & Yes & Slightly & No & No & No \\ \hline
**Swelling capacity** & High & Medium High & Medium & Low & Low \\ \hline
**Caionic exchange** & 80–100 & 100–150+ & 10–40 & 10–40 & 3–15 \\
**capacity (CEC)** & & & & & \\ (meq/100g) & & & & & \\ \hline
**Similar clays** & Beidellite & Mica & Halloysite & \\  & Nontronite & Fe\({}_{2}\)O\({}_{3}\) & Anauxite Dickite \\  & Saponite & Fe(OH)\({}_{3}\) & Nacrite \\  & Bentonite & Al(OH)\({}_{3}\) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Clay characteristics of importance (Based on [40]). Clay crystals are usually in the order of 1 to 2 nm in size (Figure 12[41]). These crystals form nano-plates that (depending on the type of clay) are susceptible to water \"capturing\", resulting in the considerable swelling of the material (e.g., the Smectite group of clays).